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.