Naod Kebede - Academia.edu (original) (raw)

Papers by Naod Kebede

Nutrition Research, Oct 1, 1993

ABSTRACT

Journal of the American Chemical Society, Jun 1, 1999

Irradiation of dimethylpyridine vapors (2-5 Torr) at 254 nm results in the formation of two sets ... more Irradiation of dimethylpyridine vapors (2-5 Torr) at 254 nm results in the formation of two sets of isomerization products. One set, formed in the larger yield, is substantially quenched when the irradiations are carried out in the presence of 15-21 Torr of nitrogen and is not formed when the irradiations are carried out with light of λ > 290 nm. In addition, a second set of reactions, which involve the interconversion of 2,3-and 2,5-dimethylpyridines, is enhanced by the addition of nitrogen, and these reactions are the only photoisomerization reactions observed when the irradiations are carried out with light of λ > 290 nm. In addition to the photoisomerizations, four of the dimethylpyridines also undergo demethylation to yield monomethylpyridines, and 2,6-dimethylpyridine undergoes methylation to yield a trimethylpyridine product. A variety of crossover experiments confirmed that the photoisomerizations are intramolecular. Based on the major phototransposition products, the six dimethylpyridines can be divided into two triads. Interconversion of the three members of each triad results in the major phototransposition products. These intra-triad interconversions are suggested to occur via 2,6-bonding, originating in a vibrationally excited S 2 (π,π*) state of the dimethylpyridine, followed by nitrogen migration and rearomatization. This allows nitrogen to insert within each carbon-carbon bond. Phototransposition of 2,6-dideuterio-3,5-dimethylpyridine to a mixture of 5,6-dideuterio-2,4-dimethylpyridine and 3,4-dideuterio-2,5-dimethylpyridine is consistent with this mechanism. In addition to these intra-triad reactions, 2,5-dimethylpyridine, a member of triad 1, was observed to interconvert with 2,3-dimethylpyridine, a member of triad 2. These inter-triad reactions are suggested to occur via interconverting Dewar pyridine intermediates, formed from the triplet state of the dimethylpyridines. These Dewar pyridine intermediates were also observed by 1 H NMR spectroscopy after irradiation of the dimethylpyridines in CD 3 CN at-30°C.

Arkivoc, Dec 16, 2018

Irradiation of the six isomeric dimethylpyridines or six isomeric trideuteropyridines in the vapo... more Irradiation of the six isomeric dimethylpyridines or six isomeric trideuteropyridines in the vapor phase at 254 nm leads to two triads of three interconverting compounds. Monosubstituted pyridines undergo phototransposition in the vapor phase at 254 nm to yield one triad of the three interconverting compounds. Thus the three isomeric methylpyridines, the three isomeric cyanopyridines, and the three isomeric tetradeuteropyridines each undergo phototransposition to three interconverting compounds. These ring transposition reaction take place by photo-ring closure followed by N-migration around the five sides of the cyclopentadienyl and rearomatization. This mechanism allows nitrogen to insert between each pair of ring atoms. The mechanism predicts the observe regioselectivity of the transposition reactions.

ChemInform, Aug 3, 2010

ChemInform Abstract Sowohl in verdünnten deuterierten Säuren als auch in schwerem Wasser findet a... more ChemInform Abstract Sowohl in verdünnten deuterierten Säuren als auch in schwerem Wasser findet an den 2-und 6-ständigen Methylgruppen der Hexadienyleisentricarbonyl-Komplexe (I) ein H-D-Austausch statt, dessen Mechanismus undÜbergangszustände diskutiert erden.

Journal of Heterocyclic Chemistry, Mar 1, 1997

Dimethylpyridines undergo deuterium‐hydrogen exchange when heated in deuterium oxide containing p... more Dimethylpyridines undergo deuterium‐hydrogen exchange when heated in deuterium oxide containing potassium carbonate at ring positions 2 and 6 when these positions are unsubstituted and at methyl groups located at ring positions 2,4, and 6 exclusively.

Journal of Organic Chemistry, Dec 1, 1995

Phytochemistry, May 1, 1994

The diketopiperazine suggested to be the intermediate during the spontaneous isomerization of B-O... more The diketopiperazine suggested to be the intermediate during the spontaneous isomerization of B-ODAP and a-ODAP was synthesized. Its behaviour was studied at selected pH values and provided evidence that its natural occurrence is unlikely. 2-Hydroxy-imidazolidine-2&dicarboxylic acid (for the rearrangement /?-ODAP-0DAP) or 2-hydroxy-pyrimidine-2Jdicarboxylic acid (for the rearrangement y-ODAB e+a-ODAB) are suggested to be the unstable intermediates. INTRODUCIION The neurotoxin /?-ODAP (3-N-oxalyl-2,3-diaminopropanoic acid) occurs as a free non-protein amino acid in the seeds of grass-pea (Luthyrus sat&s), a hardy crop in regions in Africa and Asia. The seeds are a common food for several hundred million people, but, in the case of drought-triggered famine, overconsumption of seeds containing /?-ODAP can give rise to irreversible spastic paraparesis, called lathyrism, in epidemic proportions. Bell and O'Donovan [l] have shown that in mild acidic solution j?-ODAP undergoes a rearrangement to the non-toxic 2-N-oxalyl isomer (a-ODAP) until a pHdependent equilibrium between both compounds is established. The same equilibrium position is reached from a-ODAP (Fig. 1). An cc,/?diketopiperazine (DKP) was proposed to be the intermediate in this isomerization. Bell and O'Donovan [l] also studied the rearrangement of y-ODAB (4-Noxalyl-2,4-diamino-butanoic acid) to its a-isomer. Abegaz et al. [Z] have studied similar rearrangements in a series of higher homologues of /l-ODAP, namely 4-N-oxalyl-2,4-diamino-butanoic acid, 6-N-oxalylomithine and Eoxalyl-lysine. They suggested an intramolecular rearrangement, probably involving a cyclic intermediate. From these data we can conclude that the equilibrium occurs easily when a five-membered or six-membered ring can be established as in the case of ODAP. It is less easy when only a six-membered ring can be the intermediate, as in the case of ODAB. A seven-membered ring *Parts of this paper were presented during the XIVth Intema

Journal of Chromatography A, Dec 1, 1994

A simple procedure for the precolumn derivatization of toxic and non-toxic non-protein amino acid... more A simple procedure for the precolumn derivatization of toxic and non-toxic non-protein amino acids occurring in the legume crop Lathyrus sativus and other Lathyrus species with phenyl isothiocyanate and an HPLC method for the separation of the derivatives in the nano-and picomole range are reported. The results are compared with those of conventional ion-exchange chromatography for the separation of non-protein amino acids with postcolumn ninhydrin reaction. The relative standard deviations for the two methods are compared.

Analytical Biochemistry, Feb 1, 1993

Journal of Heterocyclic Chemistry, Mar 1, 2005

Oxazole derivatives R 0220 Photochemistry of 4-and 5-Phenyl Substituted Isoxazoles.-Substituted i... more Oxazole derivatives R 0220 Photochemistry of 4-and 5-Phenyl Substituted Isoxazoles.-Substituted isoxazoles (I), (IV), (VI), and (IX) undergo phototransposition via a pathway involving only interchange of the N-2 and C-3 ring position. Additionally, the phototransposition of isoxazoles (I), (VI), and (IX) is accompanied by a photo-ring cleavage. The effect of a trifluoromethyl substituent on the photochemistry of 5-phenylisoxazole (I) is demonstrated with compound (XII). Irradiation of (XII) in methanol leads to a substantial decrease in the yield of phototransposition product (XIV) and to the formation of a mixture of the diastereomeric aziridines (XV) and (XVI). However, upon irradiation in acetonitrile, the only product obtained is (XIV).-(PAVLIK*,

Journal of the American Chemical Society, May 15, 2001

We have found O 2-substituted diazeniumdiolates, compounds of structure R 2 N-N(O)dNOR′ that are ... more We have found O 2-substituted diazeniumdiolates, compounds of structure R 2 N-N(O)dNOR′ that are under development for various possible pharmaceutical uses, to be rather photosensitive. With R) ethyl and R′) methyl, benzyl, or 2-nitrobenzyl, the observed product distributions suggest that two primary pathways are operative. A minor pathway involves the extrusion of nitrous oxide (N 2 O) with simultaneous generation of R 2 N • and R′O • , which may then form amines, aldehydes, and alcohols. The major reaction pathway is an interesting photochemical cleavage of the NdN bond to form a nitrosamine (R 2 NNdO) and an oxygensubstituted nitrene (R′ON). The intermediacy of the O-nitrene was inferred from the production of abundant oxime, via rearrangement of the O-nitrene to a C-nitroso compound (R′ON f OdNR′), and subsequent tautomerization to the more stable oxime. Involvement of the O-nitrene was confirmed by trapping with 2,3dimethyl-2-butene to form the aziridine and with oxygen to generate the nitrate ester. 2-Nitro substitution on the benzyl derivative had surprisingly little effect on the reaction course. For each compound examined, minor amounts of nitric oxide (NO), presumably produced by secondary photolysis of the nitrosamine, were observed. Time-resolved infrared experiments provided additional support for the above reaction pathways and confirmed that the nitrosamine is a primary photoproduct. We have also found that the relative contributions of the reaction pathways can be altered in certain derivatives. For example, when R′) 2,4-dinitrophenyl, the contribution of the nitrosamine/O-nitrene-forming pathway was diminished. Pharmacological implications of these results are discussed.

Arkivoc, 1970

Arkivoc, 2017

Based on their phototransposition chemistry, the three dimethylpyrazines and four dimethylpyrimid... more Based on their phototransposition chemistry, the three dimethylpyrazines and four dimethylpyrimidines can be arranged into two groups. 2,5-Dimethylpyrazine, 2,5-dimethylpyrimidine, and 4,6-dimethylpyrimidine constitute a photochemical triad. Irradiation of any one member of the triad in the vapor phase results in the formation of the other two members. The other four isomers, 2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 2,4dimethylpyrimidine, and 4,5-dimethylpyrimidine constitute a photochemical tetrad. Irradiation of any one member results in the formation of the other three. In addition, 2,4-dimethylpyrimidine and 2,6-dimethylpyrazine also photoisomerize to 3,6-dimethylpyridazine. Irradiation of the last in the vapor state resulted in the four members of the tetrad.

Toxicon, 1994

From soil to brain: zinc deficiency increases the neurotoxicity of Lathyrus sativus and may affec... more From soil to brain: zinc deficiency increases the neurotoxicity of Lathyrus sativus and may affect the susceptibility for the motorneurone disease neurolathyrism. Toxicon 32, 461-466, 1994.-Zinc deficiency and oversupply of iron to the roots of grass pea (Lathyrus sativus) induce increases in the content of the neurotoxin ß-L-ODAP (3-oxalyl-L-2,3-diaminopropanoic acid) in the ripe seeds. The transport of zinc to the shoots is enhanced by the addition of ß-L-ODAP. The neurotoxin of L. sativus is proposed to function as a carrier molecule for zinc ions. Soils, depleted in micronutrients from flooding by monsoon rains (Indian subcontinent) or otherwise poor in available zinc and with high iron content (Ethiopian vertisols), may be responsible for higher incidence of human lathyrism, one ofthe oldest neurotoxic diseases known to man. A role for brain zinc deficiency in the susceptibility for lathyrism is postulated .

Phytochemistry, 1994

Nutrition Research, 1993

... The pulse is also used as snacks in the roasted and boiledforms, mainly in Shina, the village... more ... The pulse is also used as snacks in the roasted and boiledforms, mainly in Shina, the village where lathyrism is endemic at a ... Aregay Waktola, Principal Investigator of the Lathyrism Project in Ethiopia, Valerie Palmer of TWMRF and Penny Jendenof Band-Aid are thanked for ...