The preparation of exo- and endo-6-bromo-3-oxabicyclo[3.1.0]hexanes and their stereospecific reactions with butyl-lithium

Abstract

6,6-Dibromo-3-oxabicyclo[3.1.0]hexane (8a) has been reduced by zinc–acetic acid, zinc–ethanolic potassium hydroxide, tributyltin hydride, and n-butyl-lithium followed by water to give mixtures of exo- and endo-6-bromo-3-oxabicyclo[3.1.0]hexanes (8d and e). The reaction of (8a) with tributyltin hydride is not stereoselective [(8d) : (8e) 48 : 52]. Reduction with zinc causes selective removal of the endo-bromine [(8d) : (8e) 62 : 38 with acetic acid and 3:1 with ethanolic potassium hydroxide]. Reduction with butyl-lithium followed by water is even more stereoselective [(8d) : (8e)ca. 9 : 1]. Both monobromides undergo stereospecific reactions with butyl-lithium followed by carbon dioxide (>99·5% retention of configuration). The n.m.r. spectra of the mono-bromides and carboxylic acids are reported and discussed.