Olivanic acid analogues. Part 10. X-Ray crystallographic study of the stereochemistry of some 7-heteroatom-substituted 7-acetyl-8-oxo-3-oxa-1-azabicyclo[4.2.0]octane-2-spirocyclohexanes: functional group control of the stereoselectivity of their reduction products using borohydride reagents

Abstract

(6RS, 7SR)-7-Acetyl-7-azido-8-oxo-3-oxa-1-azabicyclo[4.2.0]octane-2-spirocyclohexane 3 was obtained by reaction of mesyl azide with the trans-ketone 2 in the presence of aqueous base, and the stereochemistry of its major sodium borohydride reduction product, (6RS,7SR,9SR)-7-azido-7-(1-hydroxyethyl)-8-oxo-3-oxa-1-azabicyclo[4.2.0]octane-2-spirocyclohexane 4, was determined by X-ray crystallography. X-Ray studies of the keto sulfide 17 and the chloro ketone 20 confirmed their (6RS,7SR) relative stereochemistry. Reduction of 17 gave the alcohol 18 also with (6RS,7SR,9SR) stereochemistry. In contrast, reduction of 20 with trialkylborohydride reagents gave the (6RS,7SR,9RS)-chloro alcohol 21, and a mechanism is proposed to account for this reversal of C-9 stereoselectivity.