Stereoselective synthesis and molecular modeling of chiral cyclopentanes (original) (raw)
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Stereoselective synthesis and molecular modelling of chiral cyclopentanes
Carbohydrate Research, 2015
The reaction of 3-methyseleno-2-methylselenomethyl-propene with benzyl 2,3-anhydro-4-O-triflylβ-L-ribopyranoside provides a major convenient enantiomeric product of 1-methylene-(benzyl3, 4-dideoxy-α-D-arabinopyranoso)-[3,4-c]-cyclopentane, with benzyl-2,3-anhydro-4-deoxy-4-C-(2methyl-propen-3-yl)-α-D-lyxopyranoside as a minor product. While the reaction of 3-methyseleno-2-[methylselenomethyl]-propene with benzyl 2,3-anhydro-4-O-triflyl-α-D-ribopyranoside produces a good yield of benzyl-2,3-anhydro-4-deoxy-4-C-(2-methylpropen-3-yl)-α-D-lyxo-pyranoside. Molecular modeling and molecular dynamics simulations indicate that the intermediate in the reaction of the β-L sugar frequently occupies an optimal conformation that leads to the formation of cyclopentane, while the intermediate in the reaction of the α-D sugar has a very small probability. The results point to the dominant role of the β-L sugar intermediate in controlling the cyclopentane formation.
J Org Chem, 1996
The iodohydrins 2, 4, and 5 were prepared by the ring opening of benzyl 2-O-p-tosyl-3,4-anhydro-L-arabinopyranoside (1) or benzyl 2,3-anhydro-4-O-acetyl-R-D-ribopyranoside (3), respectively, using sodium acetate, sodium iodide, and acetic acid in acetone which on treatment with POCl 3 in pyridine yielded the unsaturated sugars 6 and 7. After deacetylation of 7 with MeOH/H 2 O/Et 3 N (3:2:1) and treatment of 8 with tosyl chloride/pyridine at 50°C 9 was obtained. The reaction of benzyl 2-O-p-tosyl-3,4-dideoxy-R-D-glycero-pent-3-enopyranoside (6) and benzyl 2,3,4-trideoxy-4chloro-L-glycero-pent-2-enopyranoside (9) with the sodium enolate of dimethyl propargylmalonate in the presence of catalytic amounts of tetrakis(triphenylphosphine)palladium(0) afforded the branched-chain sugars 10 and 11. The isomer 10 was obtained as a minor product from 6 with retention of configuration around C-2, and the major isomer 11 as a result of allylic rearrangement in a ratio of 1:9. On the other hand, compound 9 afforded 10 as a major product and its regioisomer 11 as a minor product in a ratio of 8:2; formation of the above mentioned isomeric mixture involves cis and trans diastereomeric intermediates during the reaction. Treatment of these precursors with Co 2 (CO) 8 in benzene followed by oxidative decomplexation with DMSO yielded in a stereoselective manner the polysubstituted bis-cyclopentanoids 12 and 13. The stereochemistry of 13 was assigned with the help of X-ray analysis. Attempts were made to prepare the tetracyclic systems 15 and 17 using 12 and 13 with 3-acetoxy-2-[(trimethylsilyl)methyl]-1-propene (14); however, the alkylation products 16 and 18 were obtained.
Asymmetric Diels-Alder reactions of chiral (E)-2-cyanocinnamates with cyclopentadiene
The Journal of Organic Chemistry, 1992
High and complementary diastereoselectivities were obtained in the asymmetric Diels-Alder reactions of chiral (E)-2-cyanocinnamates with butadiene when (SI-ethyl lactate and (R)-pantolactone were used as chiral auxiliaries in the presence of TiC14. The most selective reactions allowed the synthesis of the cycloadducts 2a and 3b, whose absolute configurations were assigned by a n X-ray diffraction study of diastereoisomer 3b. The hydrolysis and subsequent hydrogenation of the stereoisomers gave the corresponding enantiomeric cyanocarboxylic acids. From these products the four 1-amino-2-phenyl-1-cyclohexanecarboxylic acids were synthesized in enantiomerically pure form following a protocol with stereocontrolled and stereodivergent transformations. Cativiela, C.; Dfaz-de-Villegas, M. D.; Mayoral, J. A.; Peregrina, J. M. Tetrahedron 1993, 49, 677. (b) Avenoza, A.; Cativiela, C.; Diaz-de-Villegas, M. D.; Peregrina, J. M. Tetrahedron 1993,49, 10987. (4) (a) Avenoza, A.; Cativiela, C.; Mayoral, J. A.; Peregrina, J. M.; Sinou, D. Tetrahedron: Asymmetry 1990, I, 765. (b) Avenoza, A.; Cativiela, C.; Mayoral, J. A.; Peregrina, J. M. Tetrahedron: Asymmetry 1992, 3, 913. (c) Avenoza, A.; Cativiela, C.; Gimeno, S.; Lahoz, F. J.; Mayoral, J. A.; Peregrina, J. M. J. Org. Chem. 1992, 57, 4664. 0022-3263/94/1959-7774$04.50/0 synthesis of chiral products containing quaternary carbon atoms and also that (8)-ethyl lactate and (R)-pantolactone are excellent and complementary chiral auxiliaries.
Synthesis and evaluation of new chiral diols based on the dicyclopentadiene skeleton
Tetrahedron Letters, 2003
The resolution by Lipase PS of rac-5 (from reduction of ketone 6, obtained from dicyclopentadiene with a new environment-friendly synthesis) gives (2S)-5, which was further reduced to the endo (2R)-1a alcohol. The endo (2S)-1b alcohol was obtained from camphor with a multistep synthesis. Pinacol couplings of 3a,b, carried out with Mg/Hg or Corey's general procedure respectively, afforded with high diastereoselectivity the C 2 symmetry diols (2R,2%R)-2a and (2S,2%S)-2b, with endo oriented OH functions. The enantiogenic power of the endo alcohol (2R)-1a and (2S)-1b and of the diols (2R,2%R)-2a and (2S,2%S)-2b was tested towards the LiAlH 4 reduction of acetophenone. The C 2 symmetry appears to play a fundamental role.
This is the complementing part of the previously published: “Mechanistic views of stereoselective synthesis of tri- and tetra-substituted alkenes, part I; The Organic Chemistry Notebook Series, a Didactical Approach, Nº 3”. As underlined in three previous papers the presentation of synthesis works in a verbal and graphical succinct manner, needs a didactical approach. Isomerically pure tri- and tetra-substituted alkenes are difficult to obtain as shown in several publications. We used a series of reactions to synthesize tri- and tetra-substituted alkenes as reviewed by W. Carruthers, and we have proposed didactical and mechanistic views for the reviewed reactions. These two latest approaches are included in the synthetic methods reviewed by W. Carruthers with respect to the “Formation of carbon-carbon double bonds”. Spanish title: Vistas mecanísticas de síntesis estereoselectivas de alquenos tri y tetrasubstituidos, Parte II, Serie: El cuaderno de notas de química orgánica, un enfoque didáctico, Nº4.
Journal of the American Chemical Society, 2013
Asymmetric olefin isomerization of β,γto α,β-unsaturated butenolides catalyzed by novel cinchona alkaloid derivatives was investigated in-depth using density functional theory (M05-2x and B2PLYP-D). Three possible mechanistic scenarios, differing in the binding modes of the substrate to the catalyst, have been evaluated. Computations revealed that both the protonated quinuclidine and the 6′-OH of catalysts may act as the proton donor in the stereocontrolling step. Variation of the catalytic activity and enantioselectivity by tuning the electronic effect of catalyst was well reproduced computationally. It suggested that, for certain acid−base bifunctional chiral catalysts, the acid−base active sites of catalysts may interconvert and give new catalyst varieties of higher activity and selectivity. In addition, the noncovalent interactions in the stereocontrolling transition-state structures were identified, and their strength was quantitatively estimated. The weak nonconventional C−H···O hydrogen-bonding interactions were found to be crucial to inducing the enantioselectivity of the cinchona alkaloid derivatives catalyzed asymmetric olefin isomerization. The computational results provided further theoretical evidence that the rate-limiting step of this bioinspired organocatalytic olefin isomerization is inconsistent with that of the enzyme catalyzed olefin isomerization.
Tetrahedron, 2001
AbstractÐDihydroxylated cyclopentenones can be readily synthesised via a base mediated isomerisation reaction. These species and their derivatives can be used to provide a wide range of synthetically useful intermediates by participation in a variety of stereoselective transformations. These include: nucleophilic and conjugate addition, intermolecular cycloaddition, intramolecular free-radical cyclisation and palladium mediated coupling reactions. q Scheme 1. Biologically important molecules possessing the cyclopentanol ring.
An intramolecular Pauson-Khand approach to the synthesis of chiral cyclopentadienes
Tetrahedron letters, 2002
A procedure for the synthesis of chirally-substituted cyclopentadienes has been developed by employing the intramolecular Pauson-Khand reaction of chiral amides derived from 8-nonen-2-ynoic acid. Hydride-mediated reduction of the resulting cyclopentenone adducts followed by acid-catalysed dehydration leads to the formation of the corresponding cyclopentadienes in good overall yield.