Showing papers by "Kirpal S. Bisht published in 1996"

Schiff Bases of Amino Acid Esters as New Substrates for the Enantioselective Enzymatic Hydrolysis and Accompanied Asymmetric Transformations in Aqueous Organic Solvents1,2

Abstract: The enzyme (lipases and chymotrypsin)-catalyzed hydrolysis of Schiff bases derived from racemic amino acid esters and aromatic aldehydes has been investigated. The reactions were successfully carried out in different aqueous organic solvents at ambient temperature, but the aqueous acetonitrile (5.4% water content by volume) was the solvent of choice. The l-amino acid (ee 98%) precipitated out from the solution as the reaction progressed, and the liberated aldehyde and unhydrolyzed d-ester (ee 40−98%) remained in the solution. The range of substrates included amino acids having different types of side chains. The addition of an organic base (DABCO) into the solution resulted in the racemization of the remaining d-ester and the additional hydrolysis of the substrate, thus leading to the effective asymmetric transformation of the initial ester. Upto 87.5% of the initial racemate was converted into the l-enantiomer.

Preparative and mechanistic aspects of inter-estification reactions on diols and peracetylated polyphenolic compounds catalysed by lipases

Abstract: The lipases from porcine pancreas and Cundida cylindracea have been successfully used in carrying out simple, efficient and potentially useful biotrans- formations on diols and peracetylated polyphenolic esters. It has been observed that the esterification reactions on 1,Zdiols are highly regiaselective and the primary hydroxyl group is exclusively esterified over the one at the secondary position. The results on lipase-mediated deacylation reactions on the mixed diesters of 1,2-diols suggest that during deacylation, the migration of acyl group takes place. Addition- ally, the lipase-catalysed deacetylations on peracetates of polyhydroxy aromatic es- ters substantiate the Schiff's base complex hypothesis proposed earlier by us for the regiospecific deacetylations on peracetates of polyhydroxyaryl-alkyl ketones and

Chiral discrimination by hydrolytic enzymes in the synthesis of optically pure materials

Abstract: Enantioselection effected by commercially available hydrolytic enzymes reported in recent years from our own laboratory and those published by other groups is described. The discussion is confined to enantioselection in symmetric diols, amino acids and benzopyran derivatives only. The paper describes a variety of substrates accepted by these hydrolytic enzymes to produce compounds in high enantiomeric excess which can be used as chiral building blocks for the synthesis of compounds of pharmaceutical and synthetic interest.

Novel biotransformations on peracylated polyphenolics by immobilized lipases in microemulsion-based gels and on carbohydrates by Candida antartica lipase

Abstract: The lipases from porcine pancreas (PPL) and Curzdidu cylindruceu (CCL), solubilized in AOT (sodium bis-(2-ethylhexyl) sulphosuccinate) water-in-oil (w/o) microemulsions and supported on gelatin have been used for selective deacylations of perpropanoates and peracetates of different classes of polyphenolics. In addition, lipase from Curzdidu arzturticu (Novozym 435) has been used for regioselective propanoylation of 2-deoxy- D-ribose and D-ribose, the products on subsequent acetylation with acetic anhydride/pyridine afforded the corresponding tri- and tetraacyl derivatives, universal synthones for the synthesis of 2'-deoxyribonucleosides and ribo- nucleosides, respectively. An attempt has also been made to synthesise acylated erythrooxetanose using the same methodology.

3,5-Dichloro-2,6-dimethoxycyclohexa-2,5-diene-1,4-dione

Abstract: A new synthesis of the title compound, C 8 H 6 Cl 2 O 4 , is described. The molecule has a mirror plane and the six-membered carbon ring assumes a slight boat conformation. The ring substituents are arranged in an alternating 'up and down' pattern with respect to the best plane through the ring C atoms.