Steric course of some cyclopropanation reactions of L-threo-hex-4-enopyranosides

Completely protected 4-deoxy-α-L-threo-hex-4-enopyranosides undergo the dichlorocarbene addition affording exclusively diastereomeric adducts with the cyclopropane ring anti to the C-3 alkyloxy substituent, while the reaction with 3-unprotected derivatives affords a mixture of syn and anti derivatives. Under the Simmons–Smith cyclopropanation adducts with a syn stereochemistry are obtained. Starting from , the cyclopropanated sugar is obtained by reduction with LiAlH4, thus the two diastereomers and can be stereoselectively obtained through the two different pathways. For a useful comparison, 4-deoxy-β-L-threo-hex-4-enopyranoside was also subjected to the above two cyclopropanation methods affording the expected cycloadduct and a diastereomeric mixture of dichlorocycloadducts and (/=2.8:1).