A highly stereoselective intramolecular aldol condensation. Part I. Nuclear magnetic resonance spectroscopic investigation of the stereochemistry of the products derived from the reaction of 2,2′-O-methylene-bis-D-glycerose with base

Abstract

The intramolecular aldol condensation of 2,2′-O-methylene-bis-D-glycerose has been found to yield only two of the four possible diastereomeric products. At higher base concentration or prolonged reaction times, these products undergo a Cannizzaro reaction. The stereochemistry of the products derived from the aldol condensation has been established by means of labelling experiments and by examination of the products by 1Hmr and 13Cmr spectroscopy. The major and minor products of the reaction have been identified as being 2-C-hydroxymethyl-2,4-O-methylene-D,L-ribose (7) and 2-C-hydroxymethyl-2,4-O-methylene-D,L-xylose (8), respectively. The preferential stabilization of these products in the reaction by means of intramolecular hemiacetal formation is proposed as a driving force for the high degree of stereoselectivity; this possibility is supported by the isolation of the bicyclic systems expected from such a reaction. Conditions have been chosen such that 7 is formed exclusively.