Regiochemical control of the ring opening of 1,2-epoxides by means of chelating processes. XII. Synthesis and ring-opening reactions of cis- and trans-oxides derived from cis 2-(benzyloxy)-6-methyl-3,6-dihydro-2H-pyran

The regiochemical outcome under standard (non-chelating) and chelating conditions of the ring opening of 1,2-epoxides bearing remote polar functionalities has been verified in a conformationally semirigid branched O-benzyl pyranosidic oxirane system, namely the cis and trans diastereoisomeric epoxides 3 and 4, respectively. While the cis epoxide 3 obtained in a mixture with epoxide trans-4 gave a complete C-4 regioselectivity under any opening protocol, the regioselectivity observed with the trans epoxide 4 (prepared in a completely stereoselective way) turned out to depend to a certain extent on the opening reaction conditions, and a regioalternating process was obtained in some cases. The structure of mesylate 30 was determined by single crystal X-ray diffraction analysis.