Regiochemical control of the ring opening of 1,2-epoxides by means of chelating processes.: Part 15:: Regioselectivity of the opening reactions with MeOH of remote O-substituted regio- and diastereoisomeric pyranosidic epoxides under condensed- and gas-phase operating conditions

The regiochemical behavior of pairs of regio- and diastereoisomeric epoxides derived from the 3,4,5,6-tetrahydro-2H-pyrane system, bearing an acetal group as the remote functionality, was determined in the acid methanolysis in the condensed phase (cd-phase) and in the reaction with MeOH in the gas-phase using a gaseous acid (D-3(+)), as the promoting agent. With only one exception, the results obtained in the opening process of these epoxides indicate the incursion in the gas-phase of D+-mediated chelated bidentate species able to modify the regiochemical result found in the methanolysis in the cd-phase. (C) 2003 Elsevier Science Ltd. All rights reserved.