priyabrata das - Academia.edu (original) (raw)

Papers by priyabrata das

ChemInform, 2010

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Phosphorus, Sulfur, and Silicon and the Related Elements, 2015

Wittig reactions of short-chain trialkylphosphine derived ylides [1] will be presented under aque... more Wittig reactions of short-chain trialkylphosphine derived ylides [1] will be presented under aqueous and non-aqueous conditions as well as applications in natural product [2] and fine chemicals synthesis. [3] Recent advances in the synthesis and reactivity of alpha-[4] and beta-functionalized [5] ylides will also be described. Finally the development of a mild organocatalytic Wittig process [6] and application to bioorthogonal chemistry [7] will be presented.

Tetrahedron Letters, 2008

An efficient, stereoselective method for the synthesis of α-phosphonoenamines based on a modified... more An efficient, stereoselective method for the synthesis of α-phosphonoenamines based on a modified Peterson olefination is described. The carbanion derived from isolatable intermediate 2 reacts with aromatic or aliphatic aldehydes selectively eliminating in Peterson fashion to deliver functionally rich α-phosphonoenamines 3. The synthetic utility of these enamines is demonstrated by their hydrolysis yielding the homologous carboxylic acids in good yield.

Tetrahedron Letters, 2010

An efficient, stereoselective method for the synthesis of (+)-caparratriene based on an aqueous W... more An efficient, stereoselective method for the synthesis of (+)-caparratriene based on an aqueous Wittig reaction has been developed. A functionalized triethylallyl ylide reacted under various conditions with (+)-citronellal to deliver (+)-caparratriene in only three steps with excellent overall yield. The Wittig reaction proceeded with exclusive (4E)-selectivity and an interesting cationic effect was uncovered with good stereoselectivity at the isomerizable allylic position being observed in the presence of lithium salts.

Tetrahedron, 2009

A highly efficient method is described for the one-carbon homologation of aldehydes to carboxylic... more A highly efficient method is described for the one-carbon homologation of aldehydes to carboxylic acid derivatives employing the reaction of a 1,1-bis-dimethylphosphonate derivative with the aldehyde and controlled acid hydrolysis of the derived a-phosphonoenamine intermediate.

European Journal of Organic Chemistry, 2010

The discovery of dichotomous reactivity in the reaction of trialkyl‐ vs. triphenylphosphane HBr s... more The discovery of dichotomous reactivity in the reaction of trialkyl‐ vs. triphenylphosphane HBr salts with acetals allows entry to functionalized α‐methoxy phosphonium salts and a novel process for tertiary phosphane methylation. The new protocol opens a general entry to the synthesis of vinyl ethers and differentially substituted 1,3‐dienes via Wittig reactions of the functionalized ylides derived from the α‐methoxy phosphonium salts.

Chemistry - A European Journal, 2008

Reactions were carried out under an Argon atmosphere in oven-dried glassware. All fine chemicals ... more Reactions were carried out under an Argon atmosphere in oven-dried glassware. All fine chemicals were obtained from Aldrich except tributylphosphane which was obtained from Cytec. THF was distilled from sodium metal with benzophenone indicator. Dichloromethane and ethyl acetate were distilled over calcium hydride. CIMS were run on a Micromass Quattro Ultima spectrometer fitted with a direct injection probe (DIP) with ionization energy set at 70 eV and HRMS (EI) were performed with a Micromass Q-Tof Ultima spectrometer. 1 H, 13 C and 31 P spectra were recorded on a Bruker 200 or AV 600 spectrometer in CDCl 3 with TMS as internal standard, chemical shifts (d) are reported in ppm downfield of TMS and coupling constants (J) are expressed in Hz. Signal assignments were accomplished via analysis of HMBC, HMQC, COSY, NOSEY experiments where necessary. The (E) to (Z) ratios are reported from the relative integration of the 1 H spectra for the olefinic protons and comparison to the 31 P NMR spectra. Trimethylsilyl-methyl-tributylphosphonium iodide (3): (Bu) 3 P I TMS Into a flame-dried flask, containing a magnetic stirring bar, was weighed (iodomethyl)trimethylsilane (200 µL, 1.346 mmol) under argon and dry THF (2.7 mL) was added to make a 0.5 M solution. The flask was stirred for 15 min. at room temperature whereupon tributylphosphine (353 µL, 1.413 mmol) was added slowly to the reaction flask. The flask was maintained at room temperature for 13 hrs. Solvent was removed under vacuum to yield the title compound, 555 mg, (99%) as a white crystalline solid.

Chemistry - A European Journal, 2010

The Wittig olefination reaction [1] is regarded as one of the most strategic, widely applicable c... more The Wittig olefination reaction [1] is regarded as one of the most strategic, widely applicable carbon-carbon doublebond-forming processes available in organic synthesis. [2-4] The reaction has had an enormous impact on the sophistication of the total synthesis of organic molecules. [5] Some drawbacks of the reaction are the lack of stereocontrol achieved in certain cases, for example, in the synthesis of stilbenes from semistabilised ylides, [6] and the practical issue of phosphane oxide side-product removal. Also, protecting groups are usually required on any acidic protons (OH, NH, etc.) on both the ylide and carbonyl components. Water is a desirable solvent for organic reactions for environmental, economical, safety and chemical processing reasons. [7, 8] It has been used as the reaction medium for Wittig reactions of stabilised ylides to give unsaturated esters. [9] Recently, we reported the first examples of aqueous Wittig reactions of semistabilised ylides derived from trialkylbenzyl and trialkylallyl phosphonium salts. [10a,b] Semistabilised tri-A C H T U N G T R E N N U N G ethylbenzylidenyl and triethylallylidenyl ylides were shown to be formed chemoselectively in water by using sodium or lithium hydroxide and to react with aromatic, unsaturated, aliphatic and even enolisable aliphatic aldehydes in water, yielding a wide array of olefinic products (Scheme 1). These reactions proceeded with high (E)-olefin selectivity. The triethylphosphane oxide side-product is readily removed from these processes due to its water solubility and, hence, the Wittig reactions of triethylphosphane-derived, semistabilised ylides encapsulate a single solution to two outstanding problems with Wittig olefinations leading to (E)-olefins. This method was applied to the synthesis of valuable transstilbenes, such as resveratrol and trans-3,4,5,4'-tetrameth-A C H T U N G T R E N N U N G oxystilbene (DMU-212). [11] High-purity trans-stilbenes are also the central component in light-emitting diodes (LEDs) [12] and organic-based photovoltaic solar cells. [13] In our original work, the phosphonium salts were prepared in the usual fashion, by direct substitution of benzylic or allylic halides with triethylphosphane. Triethylphosphane is a highly odoriferous lachrymator that undergoes rapid oxidation in air and is considered pyrophoric. Allyl and benzyl halides are also known lachrymators and are hydrolytically unstable, generally toxic, alkylating agents. We have now developed a direct alkylation strategy that circumvents these issues, allowing a safe, "off-the-shelf" approach to achieving the above Wittig chemistry. Triethylallyl and triethylbenzyl phosphonium salts are directly available from the reaction of a benzylic or allylic alcohol and air-stable triethylphosphane hydrobromide. We also uncovered a pronounced "microwave effect" in the aqueous olefination reaction, leading to successful Wittig reactions by using weak bases, such as potassium carbonate. The innate reactivity of these ylides in water drew our attention to chemoselectivity issues. We report the unprecedented protecting-group-free, aqueous Wittig reactions of phenols, indoles, pyrroles and ketones, including enolisable substrates. The chemistry employed in the direct synthesis of trieth-A C H T U N G T R E N N U N G ylallyl and triethylbenzyl phosphonium salts is outlined in Scheme 2. The synthesis of allylic triphenylphosphonium salts from allylic alcohols and acidic Ph 3 P-HBr was first re-[a] Dr.

Chemical Communications, 2008

Food Chemistry, 2009

ABSTRACT Fractionation of the methanolic and total oligomer flavonoid enriched extracts from Rham... more ABSTRACT Fractionation of the methanolic and total oligomer flavonoid enriched extracts from Rhamnus alaternus leaves resulted in the isolation of three flavonoids: kaempferol 3-O-isorhamninoside (1), rhamnocitrin-3-O-isorhamninoside (2) and rhamnetin-3-O-isorhamninoside (3), along with apigenin, kaempferol and quercetin. The structures were determined using data obtained from FAB–MS, 1H and 13C NMR spectra, as well as by various correlation experiments (COSY, HMQC and HMBC). The antioxidant activities of the isolated compounds were evaluated by measuring their ability to scavenge the DPPH radical and superoxide anions, to inhibit H2O2-induced lipid peroxidation in human K562 cells, and to inhibit xanthine oxidase activity. Compound 3 was a strong scavenger of DPPH and superoxide anion radicals, and a potent inhibitor of H2O2-induced lipid peroxidation, with respective IC50 values of 1.5, 35 and 106 μg/ml, whereas compound 1 showed the better activity in the inhibition of xanthine oxidase activity with an IC50 value of 18 μg/ml, showing some structure–activity relationships.

Turkish Journal of Biology

Antioxidant responses in the estuarine mysid, Mesopodopsis zeylanica, were used to assess mercury... more Antioxidant responses in the estuarine mysid, Mesopodopsis zeylanica, were used to assess mercury (Hg) metal contamination in relation to salinity. Natural mysids acclimatized at 5 psu were subjected to laboratory exposures of 5, 15, and 25 psu salinity singly and under the sublethal Hg concentration of 5 µg/L (one-fifth of 24 h LC 50 value). Lipid peroxidation in the test species increased with salinity variation from 5 to 15 psu and retained the same up to 25 psu with no change after Hg addition. CAT increased significantly from 5 to 15 psu before Hg influence, and GST increased after addition of the metal. However, a reduction in both these enzymes is evident at higher salinities (25 psu) with or without Hg. The energy cost involved in adjusting ionic concentration with hemolymph during salinity deviation from an isotonic environment could be a cause for the reduction of antioxidants and LPX accumulation at 25 psu. Insignificant changes after Hg addition, however, indicate that Hg 2+ free ions did not produce much toxicity to the cellular system. The results suggest that M. zeylanica is comfortable scavenging oxyradicals within the salinity range of 5−15 psu to protect itself from anthropogenic contaminants as evidenced by elevated enzyme activity.

Turkish Journal of Biology

Oxidative stress responses were evaluated in the digestive gland tissue of Perna viridis exposed ... more Oxidative stress responses were evaluated in the digestive gland tissue of Perna viridis exposed to wateraccommodated fractions (WAFs) of petrol and diesel at 0.5% and 5% concentrations over 5-, 10-, and 15-day periods. Increased lipid peroxidation, protein carbonyl, DNA integrity, and hydrogen peroxide in WAF-exposed specimens are indicative of oxidative damage due to dissolved petroleum extracts. A consistent increase in superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, glutathione-S-transferase, reduced glutathione, and ascorbic acid in WAF-exposed specimens signify the importance of antioxidants in protecting the cell against the oxidative damage arising from petroleum contamination. Th e responses of fi eld specimens collected from petroleum-contaminated sites support the observed laboratory experimental results. Th is research recommends the use of these oxidative stress indices in the digestive gland tissue of Perna viridis as biomarkers of petroleum contamination.

Geological Society, London, Memoirs, 2015

In the last two decades multiproxy studies involving process-based sedimentology, geochronology o... more In the last two decades multiproxy studies involving process-based sedimentology, geochronology of interbedded tuff units from different stratigraphic levels, sediment geochemistry including stable isotope signatures and documentation of structural grains within selective stratigraphic intervals from the Chhattisgarh Basin, central India have resulted in a perception change on various aspects of the basin fill including its time frame, stratigraphic framework and depositional architecture in the space–time domain. In addition to establishing a Mesoproterozoic ( c. 1450–1000 Ma) time frame for the basin on a strong foothold, these studies also proposed revision of its stratigraphy by introducing new stratigraphic units at ‘formation’ and ‘group’ level. From collation of available data, their critical evaluation and presentation of new data, the present work proposes a four-tier lithostratigraphy for the Chhattisgarh Supergroup, namely, Singhora Group, Chadarpur Group, Raipur Group an...

The Mesoproterozoic Singhora basin constituting the southeastern part of the Chhattisgarh basin, ... more The Mesoproterozoic Singhora basin constituting the southeastern part of the Chhattisgarh basin, central India offers a unique scope to study evolution of a craton-margin basin in the proximity of a Proterozoic mobile belt i.e. Eastern Ghat Mobile Belt (EGMB). Studies including process-based paleoenvironment analyses, Nd-isotope within the Singhora Group of rocks suggest evolution of early Chhattisgarh sedimentation in an overall compres-upward transgressive depositional motif is postulated across the Rehtikhol-Saraipalli Formation transition in the early Singhora sedimentation history. Abrupt basin-ward shift of facies tract, incision on shelf and shift in rip-up mud clasts) directly above the highstand Saraipalli shelf with ~10 m incision and is inferred as the signal for tectonically triggered forced regression, and development of Type-I sequence boundary. The establishment of the Chuipali shelf system at the terminal Singhora depositional history bears signature of basin-scale t...

Precambrian Research, 2012

ABSTRACT A paleo-environmental and sequence stratigraphic study of the Mesoproterozoic Bhalukona ... more ABSTRACT A paleo-environmental and sequence stratigraphic study of the Mesoproterozoic Bhalukona Formation, Singhora Group, central India provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the Proterozoic siliciclastic ramp settings under falling and lowstand sea level conditions. Beside delineation of two diachronous surfaces, process-based facies analysis identified fourteen facies types that are grouped under five different facies associations. Paleo-environments range among continental fluvial, beach-foreshore, upper shoreface, lower shoreface and wave-dominated delta front. From the paleocurrent measurement within the fluvial channel sandstones and measurement of crest line orientations of wave generated swash bedforms, it is inferred that the Bhalukona Sea had NNE-SSW shoreline trajectory and the south-easterly flowing river system carried sediment on the shoreline from a source in the west-northwest. Such inference though in clear discordance with the earlier proposed south-southeastern sediment source for the Singhora basin, finds support in the shift in epsilon(t)(Nd) (t = 1.42 Ga) values (from -3.5 +/- 3.3 to -9.3 +/- 2.2) indicating change in sediment provenance at the early Bhalukona sedimentation history; sediments in the Bhalukona Formation derived from more evolved or older continental crustal sources in comparison to those of the underlying Saraipalli Formation. A tectonic forcing behind the shift in sediment provenance and fall in relative sea level is inferred that established the forced regressive and lowstand shoreline in the Singhora basin during the Bhalukona time. Abrupt basin-ward shift of facies tract and incision on shelf is exhibited by the occurrence of poorly sorted, coarse granular Bhalukona fluvial system (carrying rip-up mud clasts) directly above the argillaceous highstand Saraipalli shelf with similar to 10 m incision and is inferred as the signal for forced regression and development of Type-I sequence boundary. In low-gradient Proterozoic ramp settings without shelf-slope break, we interpret that the Bhalukona fluvial system incised the coastal prism developed on the Saraipalli highstand coastline. The low-gradient of the ramp, however, prompted long distance (similar to 15 km) regression represented by the offlapped and detached delta front lobe away from the shoreline. The slow, steady rise in sea level, onset of lowstand and establishment of a wave-dominated coastline caused reworking of fluvial sediments in the basinal part (within the wave base) and resulted development of ravinement deposit. Basin-ward, the surface grades into correlative conformity. With aggradational and weak ret-rogradational stacking the beach-foreshore, upper- and lower-shoreface, in order of superposition, record the lowstand depositional history. The basin-scale transgression is witnessed with formation of Transgressive surface of erosion (TSE) and establishment of the Chuipalli shelf system, dominantly beyond storm wave base. Taking into consideration similar to 23 m preserved shoreface succession, 1 m per year eustatic rise consistent with present day rate and average rate of shoreface retreat 0.5 m per year, similar to 11.5 km retreat for the Bhalukona shoreline is estimated in its lowstand history. (C) 2012 Published by Elsevier B.V.

Precambrian Research, 2009

... a Rajiv Gandhi Institute of Petroleum Technology, Rae Bareli, Uttar Pradesh 229006, India. b ... more ... a Rajiv Gandhi Institute of Petroleum Technology, Rae Bareli, Uttar Pradesh 229006, India. b Institute of Petroleum Technology, Gandhinagar, Pandit Deendyal Petroleum University, Gujarat 382009, India. c Department of Geology ...

Precambrian Research, 2013

ABSTRACT Studies in Proterozoic sedimentary basins are generally fraught with continuous and conf... more ABSTRACT Studies in Proterozoic sedimentary basins are generally fraught with continuous and conformable hypothesis. Unlike many such undeformed and unmetamorphosed Proterozoic sedimentary basins around the Globe, the Mesoproterozoic Singhora basin, a so called “proto-basin” for the Chhattisgarh basin of central India, registers signatures of pervasive deformation documented in both field and anisotropic magnetic susceptibility (AMS) based study. While three constituent Formations of the Singhora Group viz. Saraipali, Bhalukona and Chuipali record outcrop-scale deformation in presence of folds having plane non-cylindrical to non-plane cylindrical geometry, the signatures of deformation within the coarse granular sandstones and conglomerates of the basal Rehtikhol Formation are shown principally from angular relation between the magnetic foliation and bedding plane under AMS study. Systematic formation-wise structural analysis reveals that the basin, as a whole, is deformed in the form of a regional-scale non-plane non-cylindrical fold; whereas non-planarity can be visualized in the regional scale, non-cylindricity is more prominent in the outcrop-scale. Penetrative fabric is only observed in the south-eastern corner of the basin. Compression in two directions is inferred, one being broadly N−S and the other E−W, with resultant overall compressional direction of NW−SE. Additionally, a narrow high strain zone is developed at the southern boundary of the basin in contact with gneissic basement, evident from the sheared metabasite band.Basic rocks (sheared and massive), metamorphosed at greenschist facies, present at the basin margin, and unaltered basaltic intrusives within the basin allowed us to constrain the broad time frame of deformation. Despite variation in mineralogy and major element geochemistry between the metamorphosed and unaltered variety, the intrusives are identified as cogenetic from the similarity in REE pattern and trace element character. Considering the ∼1420 Ma emplacement age of the intrusive into the Saraipali Formation, the deformation age is constrained as <1420 Ma.Signatures of compressional deformation traced within all formations of the Singhora Group suggest that the Group is pervasively deformed and thereby indicates an unconformable relationship with its overlying undeformed Chandarpur Group of sediments. The present study warrants necessity of further work in the area to establish causal relationship, if any, between the operative tectonics within the frontal thrust belt in the immediate eastern proximity of the basin and the compressional deformation event/s recorded within the basin.

Journal of Earth System Science, 2011

Besides offering significant clues towards tracking the geochemical evolution of the mantle and a... more Besides offering significant clues towards tracking the geochemical evolution of the mantle and architectural reconstruction of different 'supercontinent', geochronological and geochemical appraisal of igneous inputs are also important to bracket the depositional time frame of any lithopackage, particularly, the unfossiliferous sedimentary successions. The present study deals with diabasic intrusive within Mesoproterozoic Saraipalli Formation, which is an argillaceous constituent present at the basal part of nearly 400 m thick four-tiered unmetamorphosed but deformed sedimentary succession of Singhora Group, Chhattisgarh Supergroup, central India. The SE-NW trending intrusive comprises mainly of plagioclase and augite together with minor orthopyroxene, biotite and opaque minerals. Though some plagioclase laths are partially sericitized, the ophitic-to-subophitic texture of the rock is well preserved. Major and trace element geochemical data indicate that this intrusive is basalt-to-basaltic andesite in character and of subalkaline basalt affinity. Multi-element plot shows overall LILE-enrichment and enrichment of Pb and slight depletion of Nb and P, coupled with moderate La/Nb and Th/Nb ratios. Zr, Y and Nb ternary diagrams plot in the fields of within plate basalt. Selected HFSE ratios indicate a non-plume source with crustal assimilation/sediment mixing. Sm-Nd and Rb-Sr isotope data show that the intrusive has Sr initial and Nd initial of 0.709377-0.706672 and 0.510919-0.510815, respectively. Positive ε t Nd [t = 1420 Ma] values (+0.3 to + 2.3) indicate depleted isotopic nature of their protolith. The calculated T DM age is 1.7-1.9 Ga. The mineral-whole rock isochron data (Sm-Nd systematics) of the intrusive implies an emplacement age of ca. 1420 Ma. Considering synchronous terrain boundary shear zone development in Bastar craton on the southeastern part of the Singhora basin, mafic magmatism in Eastern Ghats and large-scale basic intrusion in Sausar mobile belt, a major tectono-thermal event around 1400 Ma is surmised that affected eastern Indian craton. Moreover, geochronology of a bedded porcellanite unit (ca. 1500 Ma) at the base and a discordant basic intrusive (ca. 1420 Ma) allowed a unique opportunity to qualitatively offer an upper bound of time bracket for the deposition of Saraipalli Formation, i.e., ∼80 Ma.

ChemInform, 2010

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Phosphorus, Sulfur, and Silicon and the Related Elements, 2015

Wittig reactions of short-chain trialkylphosphine derived ylides [1] will be presented under aque... more Wittig reactions of short-chain trialkylphosphine derived ylides [1] will be presented under aqueous and non-aqueous conditions as well as applications in natural product [2] and fine chemicals synthesis. [3] Recent advances in the synthesis and reactivity of alpha-[4] and beta-functionalized [5] ylides will also be described. Finally the development of a mild organocatalytic Wittig process [6] and application to bioorthogonal chemistry [7] will be presented.

Tetrahedron Letters, 2008

An efficient, stereoselective method for the synthesis of α-phosphonoenamines based on a modified... more An efficient, stereoselective method for the synthesis of α-phosphonoenamines based on a modified Peterson olefination is described. The carbanion derived from isolatable intermediate 2 reacts with aromatic or aliphatic aldehydes selectively eliminating in Peterson fashion to deliver functionally rich α-phosphonoenamines 3. The synthetic utility of these enamines is demonstrated by their hydrolysis yielding the homologous carboxylic acids in good yield.

Tetrahedron Letters, 2010

An efficient, stereoselective method for the synthesis of (+)-caparratriene based on an aqueous W... more An efficient, stereoselective method for the synthesis of (+)-caparratriene based on an aqueous Wittig reaction has been developed. A functionalized triethylallyl ylide reacted under various conditions with (+)-citronellal to deliver (+)-caparratriene in only three steps with excellent overall yield. The Wittig reaction proceeded with exclusive (4E)-selectivity and an interesting cationic effect was uncovered with good stereoselectivity at the isomerizable allylic position being observed in the presence of lithium salts.

Tetrahedron, 2009

A highly efficient method is described for the one-carbon homologation of aldehydes to carboxylic... more A highly efficient method is described for the one-carbon homologation of aldehydes to carboxylic acid derivatives employing the reaction of a 1,1-bis-dimethylphosphonate derivative with the aldehyde and controlled acid hydrolysis of the derived a-phosphonoenamine intermediate.

European Journal of Organic Chemistry, 2010

The discovery of dichotomous reactivity in the reaction of trialkyl‐ vs. triphenylphosphane HBr s... more The discovery of dichotomous reactivity in the reaction of trialkyl‐ vs. triphenylphosphane HBr salts with acetals allows entry to functionalized α‐methoxy phosphonium salts and a novel process for tertiary phosphane methylation. The new protocol opens a general entry to the synthesis of vinyl ethers and differentially substituted 1,3‐dienes via Wittig reactions of the functionalized ylides derived from the α‐methoxy phosphonium salts.

Chemistry - A European Journal, 2008

Reactions were carried out under an Argon atmosphere in oven-dried glassware. All fine chemicals ... more Reactions were carried out under an Argon atmosphere in oven-dried glassware. All fine chemicals were obtained from Aldrich except tributylphosphane which was obtained from Cytec. THF was distilled from sodium metal with benzophenone indicator. Dichloromethane and ethyl acetate were distilled over calcium hydride. CIMS were run on a Micromass Quattro Ultima spectrometer fitted with a direct injection probe (DIP) with ionization energy set at 70 eV and HRMS (EI) were performed with a Micromass Q-Tof Ultima spectrometer. 1 H, 13 C and 31 P spectra were recorded on a Bruker 200 or AV 600 spectrometer in CDCl 3 with TMS as internal standard, chemical shifts (d) are reported in ppm downfield of TMS and coupling constants (J) are expressed in Hz. Signal assignments were accomplished via analysis of HMBC, HMQC, COSY, NOSEY experiments where necessary. The (E) to (Z) ratios are reported from the relative integration of the 1 H spectra for the olefinic protons and comparison to the 31 P NMR spectra. Trimethylsilyl-methyl-tributylphosphonium iodide (3): (Bu) 3 P I TMS Into a flame-dried flask, containing a magnetic stirring bar, was weighed (iodomethyl)trimethylsilane (200 µL, 1.346 mmol) under argon and dry THF (2.7 mL) was added to make a 0.5 M solution. The flask was stirred for 15 min. at room temperature whereupon tributylphosphine (353 µL, 1.413 mmol) was added slowly to the reaction flask. The flask was maintained at room temperature for 13 hrs. Solvent was removed under vacuum to yield the title compound, 555 mg, (99%) as a white crystalline solid.

Chemistry - A European Journal, 2010

The Wittig olefination reaction [1] is regarded as one of the most strategic, widely applicable c... more The Wittig olefination reaction [1] is regarded as one of the most strategic, widely applicable carbon-carbon doublebond-forming processes available in organic synthesis. [2-4] The reaction has had an enormous impact on the sophistication of the total synthesis of organic molecules. [5] Some drawbacks of the reaction are the lack of stereocontrol achieved in certain cases, for example, in the synthesis of stilbenes from semistabilised ylides, [6] and the practical issue of phosphane oxide side-product removal. Also, protecting groups are usually required on any acidic protons (OH, NH, etc.) on both the ylide and carbonyl components. Water is a desirable solvent for organic reactions for environmental, economical, safety and chemical processing reasons. [7, 8] It has been used as the reaction medium for Wittig reactions of stabilised ylides to give unsaturated esters. [9] Recently, we reported the first examples of aqueous Wittig reactions of semistabilised ylides derived from trialkylbenzyl and trialkylallyl phosphonium salts. [10a,b] Semistabilised tri-A C H T U N G T R E N N U N G ethylbenzylidenyl and triethylallylidenyl ylides were shown to be formed chemoselectively in water by using sodium or lithium hydroxide and to react with aromatic, unsaturated, aliphatic and even enolisable aliphatic aldehydes in water, yielding a wide array of olefinic products (Scheme 1). These reactions proceeded with high (E)-olefin selectivity. The triethylphosphane oxide side-product is readily removed from these processes due to its water solubility and, hence, the Wittig reactions of triethylphosphane-derived, semistabilised ylides encapsulate a single solution to two outstanding problems with Wittig olefinations leading to (E)-olefins. This method was applied to the synthesis of valuable transstilbenes, such as resveratrol and trans-3,4,5,4'-tetrameth-A C H T U N G T R E N N U N G oxystilbene (DMU-212). [11] High-purity trans-stilbenes are also the central component in light-emitting diodes (LEDs) [12] and organic-based photovoltaic solar cells. [13] In our original work, the phosphonium salts were prepared in the usual fashion, by direct substitution of benzylic or allylic halides with triethylphosphane. Triethylphosphane is a highly odoriferous lachrymator that undergoes rapid oxidation in air and is considered pyrophoric. Allyl and benzyl halides are also known lachrymators and are hydrolytically unstable, generally toxic, alkylating agents. We have now developed a direct alkylation strategy that circumvents these issues, allowing a safe, "off-the-shelf" approach to achieving the above Wittig chemistry. Triethylallyl and triethylbenzyl phosphonium salts are directly available from the reaction of a benzylic or allylic alcohol and air-stable triethylphosphane hydrobromide. We also uncovered a pronounced "microwave effect" in the aqueous olefination reaction, leading to successful Wittig reactions by using weak bases, such as potassium carbonate. The innate reactivity of these ylides in water drew our attention to chemoselectivity issues. We report the unprecedented protecting-group-free, aqueous Wittig reactions of phenols, indoles, pyrroles and ketones, including enolisable substrates. The chemistry employed in the direct synthesis of trieth-A C H T U N G T R E N N U N G ylallyl and triethylbenzyl phosphonium salts is outlined in Scheme 2. The synthesis of allylic triphenylphosphonium salts from allylic alcohols and acidic Ph 3 P-HBr was first re-[a] Dr.

Chemical Communications, 2008

Food Chemistry, 2009

ABSTRACT Fractionation of the methanolic and total oligomer flavonoid enriched extracts from Rham... more ABSTRACT Fractionation of the methanolic and total oligomer flavonoid enriched extracts from Rhamnus alaternus leaves resulted in the isolation of three flavonoids: kaempferol 3-O-isorhamninoside (1), rhamnocitrin-3-O-isorhamninoside (2) and rhamnetin-3-O-isorhamninoside (3), along with apigenin, kaempferol and quercetin. The structures were determined using data obtained from FAB–MS, 1H and 13C NMR spectra, as well as by various correlation experiments (COSY, HMQC and HMBC). The antioxidant activities of the isolated compounds were evaluated by measuring their ability to scavenge the DPPH radical and superoxide anions, to inhibit H2O2-induced lipid peroxidation in human K562 cells, and to inhibit xanthine oxidase activity. Compound 3 was a strong scavenger of DPPH and superoxide anion radicals, and a potent inhibitor of H2O2-induced lipid peroxidation, with respective IC50 values of 1.5, 35 and 106 μg/ml, whereas compound 1 showed the better activity in the inhibition of xanthine oxidase activity with an IC50 value of 18 μg/ml, showing some structure–activity relationships.

Turkish Journal of Biology

Antioxidant responses in the estuarine mysid, Mesopodopsis zeylanica, were used to assess mercury... more Antioxidant responses in the estuarine mysid, Mesopodopsis zeylanica, were used to assess mercury (Hg) metal contamination in relation to salinity. Natural mysids acclimatized at 5 psu were subjected to laboratory exposures of 5, 15, and 25 psu salinity singly and under the sublethal Hg concentration of 5 µg/L (one-fifth of 24 h LC 50 value). Lipid peroxidation in the test species increased with salinity variation from 5 to 15 psu and retained the same up to 25 psu with no change after Hg addition. CAT increased significantly from 5 to 15 psu before Hg influence, and GST increased after addition of the metal. However, a reduction in both these enzymes is evident at higher salinities (25 psu) with or without Hg. The energy cost involved in adjusting ionic concentration with hemolymph during salinity deviation from an isotonic environment could be a cause for the reduction of antioxidants and LPX accumulation at 25 psu. Insignificant changes after Hg addition, however, indicate that Hg 2+ free ions did not produce much toxicity to the cellular system. The results suggest that M. zeylanica is comfortable scavenging oxyradicals within the salinity range of 5−15 psu to protect itself from anthropogenic contaminants as evidenced by elevated enzyme activity.

Turkish Journal of Biology

Oxidative stress responses were evaluated in the digestive gland tissue of Perna viridis exposed ... more Oxidative stress responses were evaluated in the digestive gland tissue of Perna viridis exposed to wateraccommodated fractions (WAFs) of petrol and diesel at 0.5% and 5% concentrations over 5-, 10-, and 15-day periods. Increased lipid peroxidation, protein carbonyl, DNA integrity, and hydrogen peroxide in WAF-exposed specimens are indicative of oxidative damage due to dissolved petroleum extracts. A consistent increase in superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, glutathione-S-transferase, reduced glutathione, and ascorbic acid in WAF-exposed specimens signify the importance of antioxidants in protecting the cell against the oxidative damage arising from petroleum contamination. Th e responses of fi eld specimens collected from petroleum-contaminated sites support the observed laboratory experimental results. Th is research recommends the use of these oxidative stress indices in the digestive gland tissue of Perna viridis as biomarkers of petroleum contamination.

Geological Society, London, Memoirs, 2015

In the last two decades multiproxy studies involving process-based sedimentology, geochronology o... more In the last two decades multiproxy studies involving process-based sedimentology, geochronology of interbedded tuff units from different stratigraphic levels, sediment geochemistry including stable isotope signatures and documentation of structural grains within selective stratigraphic intervals from the Chhattisgarh Basin, central India have resulted in a perception change on various aspects of the basin fill including its time frame, stratigraphic framework and depositional architecture in the space–time domain. In addition to establishing a Mesoproterozoic ( c. 1450–1000 Ma) time frame for the basin on a strong foothold, these studies also proposed revision of its stratigraphy by introducing new stratigraphic units at ‘formation’ and ‘group’ level. From collation of available data, their critical evaluation and presentation of new data, the present work proposes a four-tier lithostratigraphy for the Chhattisgarh Supergroup, namely, Singhora Group, Chadarpur Group, Raipur Group an...

The Mesoproterozoic Singhora basin constituting the southeastern part of the Chhattisgarh basin, ... more The Mesoproterozoic Singhora basin constituting the southeastern part of the Chhattisgarh basin, central India offers a unique scope to study evolution of a craton-margin basin in the proximity of a Proterozoic mobile belt i.e. Eastern Ghat Mobile Belt (EGMB). Studies including process-based paleoenvironment analyses, Nd-isotope within the Singhora Group of rocks suggest evolution of early Chhattisgarh sedimentation in an overall compres-upward transgressive depositional motif is postulated across the Rehtikhol-Saraipalli Formation transition in the early Singhora sedimentation history. Abrupt basin-ward shift of facies tract, incision on shelf and shift in rip-up mud clasts) directly above the highstand Saraipalli shelf with ~10 m incision and is inferred as the signal for tectonically triggered forced regression, and development of Type-I sequence boundary. The establishment of the Chuipali shelf system at the terminal Singhora depositional history bears signature of basin-scale t...

Precambrian Research, 2012

ABSTRACT A paleo-environmental and sequence stratigraphic study of the Mesoproterozoic Bhalukona ... more ABSTRACT A paleo-environmental and sequence stratigraphic study of the Mesoproterozoic Bhalukona Formation, Singhora Group, central India provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the Proterozoic siliciclastic ramp settings under falling and lowstand sea level conditions. Beside delineation of two diachronous surfaces, process-based facies analysis identified fourteen facies types that are grouped under five different facies associations. Paleo-environments range among continental fluvial, beach-foreshore, upper shoreface, lower shoreface and wave-dominated delta front. From the paleocurrent measurement within the fluvial channel sandstones and measurement of crest line orientations of wave generated swash bedforms, it is inferred that the Bhalukona Sea had NNE-SSW shoreline trajectory and the south-easterly flowing river system carried sediment on the shoreline from a source in the west-northwest. Such inference though in clear discordance with the earlier proposed south-southeastern sediment source for the Singhora basin, finds support in the shift in epsilon(t)(Nd) (t = 1.42 Ga) values (from -3.5 +/- 3.3 to -9.3 +/- 2.2) indicating change in sediment provenance at the early Bhalukona sedimentation history; sediments in the Bhalukona Formation derived from more evolved or older continental crustal sources in comparison to those of the underlying Saraipalli Formation. A tectonic forcing behind the shift in sediment provenance and fall in relative sea level is inferred that established the forced regressive and lowstand shoreline in the Singhora basin during the Bhalukona time. Abrupt basin-ward shift of facies tract and incision on shelf is exhibited by the occurrence of poorly sorted, coarse granular Bhalukona fluvial system (carrying rip-up mud clasts) directly above the argillaceous highstand Saraipalli shelf with similar to 10 m incision and is inferred as the signal for forced regression and development of Type-I sequence boundary. In low-gradient Proterozoic ramp settings without shelf-slope break, we interpret that the Bhalukona fluvial system incised the coastal prism developed on the Saraipalli highstand coastline. The low-gradient of the ramp, however, prompted long distance (similar to 15 km) regression represented by the offlapped and detached delta front lobe away from the shoreline. The slow, steady rise in sea level, onset of lowstand and establishment of a wave-dominated coastline caused reworking of fluvial sediments in the basinal part (within the wave base) and resulted development of ravinement deposit. Basin-ward, the surface grades into correlative conformity. With aggradational and weak ret-rogradational stacking the beach-foreshore, upper- and lower-shoreface, in order of superposition, record the lowstand depositional history. The basin-scale transgression is witnessed with formation of Transgressive surface of erosion (TSE) and establishment of the Chuipalli shelf system, dominantly beyond storm wave base. Taking into consideration similar to 23 m preserved shoreface succession, 1 m per year eustatic rise consistent with present day rate and average rate of shoreface retreat 0.5 m per year, similar to 11.5 km retreat for the Bhalukona shoreline is estimated in its lowstand history. (C) 2012 Published by Elsevier B.V.

Precambrian Research, 2009

... a Rajiv Gandhi Institute of Petroleum Technology, Rae Bareli, Uttar Pradesh 229006, India. b ... more ... a Rajiv Gandhi Institute of Petroleum Technology, Rae Bareli, Uttar Pradesh 229006, India. b Institute of Petroleum Technology, Gandhinagar, Pandit Deendyal Petroleum University, Gujarat 382009, India. c Department of Geology ...

Precambrian Research, 2013

ABSTRACT Studies in Proterozoic sedimentary basins are generally fraught with continuous and conf... more ABSTRACT Studies in Proterozoic sedimentary basins are generally fraught with continuous and conformable hypothesis. Unlike many such undeformed and unmetamorphosed Proterozoic sedimentary basins around the Globe, the Mesoproterozoic Singhora basin, a so called “proto-basin” for the Chhattisgarh basin of central India, registers signatures of pervasive deformation documented in both field and anisotropic magnetic susceptibility (AMS) based study. While three constituent Formations of the Singhora Group viz. Saraipali, Bhalukona and Chuipali record outcrop-scale deformation in presence of folds having plane non-cylindrical to non-plane cylindrical geometry, the signatures of deformation within the coarse granular sandstones and conglomerates of the basal Rehtikhol Formation are shown principally from angular relation between the magnetic foliation and bedding plane under AMS study. Systematic formation-wise structural analysis reveals that the basin, as a whole, is deformed in the form of a regional-scale non-plane non-cylindrical fold; whereas non-planarity can be visualized in the regional scale, non-cylindricity is more prominent in the outcrop-scale. Penetrative fabric is only observed in the south-eastern corner of the basin. Compression in two directions is inferred, one being broadly N−S and the other E−W, with resultant overall compressional direction of NW−SE. Additionally, a narrow high strain zone is developed at the southern boundary of the basin in contact with gneissic basement, evident from the sheared metabasite band.Basic rocks (sheared and massive), metamorphosed at greenschist facies, present at the basin margin, and unaltered basaltic intrusives within the basin allowed us to constrain the broad time frame of deformation. Despite variation in mineralogy and major element geochemistry between the metamorphosed and unaltered variety, the intrusives are identified as cogenetic from the similarity in REE pattern and trace element character. Considering the ∼1420 Ma emplacement age of the intrusive into the Saraipali Formation, the deformation age is constrained as <1420 Ma.Signatures of compressional deformation traced within all formations of the Singhora Group suggest that the Group is pervasively deformed and thereby indicates an unconformable relationship with its overlying undeformed Chandarpur Group of sediments. The present study warrants necessity of further work in the area to establish causal relationship, if any, between the operative tectonics within the frontal thrust belt in the immediate eastern proximity of the basin and the compressional deformation event/s recorded within the basin.

Journal of Earth System Science, 2011

Besides offering significant clues towards tracking the geochemical evolution of the mantle and a... more Besides offering significant clues towards tracking the geochemical evolution of the mantle and architectural reconstruction of different 'supercontinent', geochronological and geochemical appraisal of igneous inputs are also important to bracket the depositional time frame of any lithopackage, particularly, the unfossiliferous sedimentary successions. The present study deals with diabasic intrusive within Mesoproterozoic Saraipalli Formation, which is an argillaceous constituent present at the basal part of nearly 400 m thick four-tiered unmetamorphosed but deformed sedimentary succession of Singhora Group, Chhattisgarh Supergroup, central India. The SE-NW trending intrusive comprises mainly of plagioclase and augite together with minor orthopyroxene, biotite and opaque minerals. Though some plagioclase laths are partially sericitized, the ophitic-to-subophitic texture of the rock is well preserved. Major and trace element geochemical data indicate that this intrusive is basalt-to-basaltic andesite in character and of subalkaline basalt affinity. Multi-element plot shows overall LILE-enrichment and enrichment of Pb and slight depletion of Nb and P, coupled with moderate La/Nb and Th/Nb ratios. Zr, Y and Nb ternary diagrams plot in the fields of within plate basalt. Selected HFSE ratios indicate a non-plume source with crustal assimilation/sediment mixing. Sm-Nd and Rb-Sr isotope data show that the intrusive has Sr initial and Nd initial of 0.709377-0.706672 and 0.510919-0.510815, respectively. Positive ε t Nd [t = 1420 Ma] values (+0.3 to + 2.3) indicate depleted isotopic nature of their protolith. The calculated T DM age is 1.7-1.9 Ga. The mineral-whole rock isochron data (Sm-Nd systematics) of the intrusive implies an emplacement age of ca. 1420 Ma. Considering synchronous terrain boundary shear zone development in Bastar craton on the southeastern part of the Singhora basin, mafic magmatism in Eastern Ghats and large-scale basic intrusion in Sausar mobile belt, a major tectono-thermal event around 1400 Ma is surmised that affected eastern Indian craton. Moreover, geochronology of a bedded porcellanite unit (ca. 1500 Ma) at the base and a discordant basic intrusive (ca. 1420 Ma) allowed a unique opportunity to qualitatively offer an upper bound of time bracket for the deposition of Saraipalli Formation, i.e., ∼80 Ma.