Enantioselective Lactonization of 3,3,6-Trimethyl-4(E)-heptenoic Acid Esters

Abstract

Studies on the use of lactonization in the asymmetric synthesis of 6,6-dimethyl-4-isopropyl-3-oxabicyclo[3.1.0]hexan-2-one were described. An asymmetrically induced lactonization reaction was performed on 3,3,6-trimethyl-4(E)-heptenoic acid esters (1) and enantiomerically pure alcohols such as (−)-menthol (a), (+)-menthol (b), (−)-borneol (c), (+)-isomenthol (d), (−)-isopinocampheol (e), and (S)-(−)-1-(2-bornylphenyl)-1-ethanol (f). The enantiomerically pure alcohols that were used as ancillary chiral substances were characterized by markedly different values of induction power; menthol (a, b), borneol (c), and phenetyl alcohol (f) performed better in asymmetric δ-lactonization, whereas isomenthol (d) and isopinocampheol (e) tended to favor asymmetric γ-lactonization.