Stereocontrolled Synthesis of Cyclic Ethers by Intramolecular Hetero-Michael Addition. 5. Synthesis of All Diastereoisomers of 2,3,5,6-Tetrasubstituted Tetrahydropyrans

A systematic approach to the enantiomeric synthesis of all possible diastereoisomers of 2,6-dialkyl-3,5-dioxytetrahydropyrans is described. The key step in the described methodology is the intramolecular cyclization of enantiomerically enriched (≥95% ee) 7-hydroxy-4-(benzoyloxy)-2,3-unsaturated esters. In fused systems, six of the eight diastereoisomers for one enantiomeric series were synthesized using this procedure as a key step. Using those with the suitable stereochemistry, the two left were synthesized by simple chemical transformations:  in one case by the basic isomerization of the carbon with the (methoxycarbonyl)methyl substituent or by a Mitsunobu inversion of a secondary alcohol available from the benzoyloxy group, in the remaining one by a consecutive sequence of oxidation and reduction reactions again over the free secondary alcohol. The stereochemistry of the intramolecular hetero-Michael addition leading to 2,3-disubstituted tetrahydropyrans is highly predictable when kinetic conditions (l...