Enzyme catalysed irreversible transesterifications with vinyl acetate. Are they really irreversible?

• Published in: Tetrahedron: asymmetry Vol. 6; no. 9; pp. 2237 - 2244
• Main Authors: Lundh, Mårten, Nordin, Ove, Hedenström, Erik, Högberg, Hans-Erik
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 1995; Elsevier
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Chemistry, Physical; Enzyme Catalysed; Kemi; NATURAL SCIENCES; NATURVETENSKAP; Physical Sciences; Science & Technology; ALCOHOLS; QUANTITATIVE-ANALYSES; LIPASE; ORGANIC-SOLVENTS; KINETIC RESOLUTION; ENANTIOMERS
• ISSN: 0957-4166; 1362-511X
• DOI: 10.1016/0957-4166(95)00298-4

When racemic β-methyl-(2-thiophene)propanol ( 1) was resolved via transesterification catalysed by lipase from Pseudomonas fluorescens (PFL) using an excess vinyl acetate in chloroform the enantioselectivity was high E ≈ 200 (conversion, c = 39%) when calculated from the ee of the ( S)-ester S-1Ac. However, based on the ee of the remaining ( R)-alcohol R-1, assuming irreversibility and when measured at c = 57 %, E = 37 was obtained. Similarly, when the 5-(1-ethoxyethoxy)-3-pentyn-2-ol ( 2) was resolved using immobilised lipase B from Candida antarctica (Novozym SP435) under similar conditions, the ( R)-ester product R-2Ac at c = 22% gave E ≈ 140 compared with that of the remaining alcohol S-2 at c = 65 % which gave E = 15. These results are interpreted as a consequence of a reversible process occurring in transesterifications of this type, which are commonly referred to as irreversible. The apparent E-values decreased with increasing conversion if irreversiblility is assumed in transesterification of racemic chiral alcohols with various enzymes and vinyl acetate in an organic solvent. These results are best explained in terms of a reversible reaction occurring in the system.