STEREOCONTROLLED SYNTHESIS OF CYCLIC ETHERS BY INTRAMOLECULAR HETERO-MICHAEL ADDITION .4. ENANTIOMERIC SYNTHESIS OF SUBSTITUTED AND FUSED OXEPANES

The use of an intramolecular hetero-Michael addition of enantiomerical ly enriched 7-alkoxy4-benzoyloxy-2,3-unsaturated esters for the synthe sis of endo-substituted oxepanes is described, The stereochemistry in the cyclization step is governed by the geometry of the double bond in the unsaturated ester and the presence of a Z-double bond in the line ar chain is required in order to achieve good yields. The synthesis of the fused oxepane-tetrahydropyran nucleus of marine polyether toxins with absolute control of all the stereocentres is described.