Regioselective eliminations in reactions of carbohydrate derivatives with superoxide, or with borohydride in 2-propanol

Publisher Website
Google Scholar
Abstract
The reaction between 2,3,4,6-tetra-O-benzyl-1,5-di-O-mesyl-D-glucitol (2) and potassium superoxide resulted in the loss of H-4 and the 5-mesyloxy (as well as 1-mesyloxy) substituent, and an almost quantitative conversion into enol ether 4, i.e., 1,3,4,5-tetra-O-benzyl-3-dehydro-2-deoxy-L-threo-hex-2-enitol. When the reaction was performed with 8, in which the 1-O-mesyl group (of 2) is replaced by O-(methoxy)trityl, the outcome was wholly different: an olefin was formed through the removal of a primary (H-6) rather than a secondary (H-4) proton, and nucleophilic displacement also took place. Results similar to those for 8 were obtained with the 2-epimer of 2, i.e., 2,3,4,6-tetra-O-benzyl-1,5-di-O-mesyl-D-mannitol (12). It is suggested that selective displacement of the 1-O-mesyl group of 2 by superoxide generates a 1-peroxy anion (6) that abstracts H-4 intramolecularly, promoting concomitant loss of the 5-mesyloxy group. The transition state for proton abstraction within anion 6 appears to be more stable than that of the corresponding anion in the manno series, accounting for the different reaction routes for 2 and 12. Elimination occurred also in the reduction of 2,3,4,6-tetra-O-benzyl-D-glucopyranose with sodium borohydride in 2-propanol, en route to 2, affording 2,4,6-tri-O-benzyl-2-dehydro-3-deoxy-D-threo-hex-2-enitol (21). The lH nmr spectra of alditol derivatives 2, 8, and 12 show that these molecules depart substantially from extended zigzag conformations, in contrast to configurationally related peracetylated acyclic derivatives.

This publication has 0 references indexed in Scilit: