Synthesis of 2,6-cis-disubstituted 4-methylenetetrahydropyrans by oxy-Michael addition

The combination of an 'ene' reaction between a 2-(2-trialkylsilyloxyalkyl)prop-2-enyl(trimethyl)silane and an alk-1-yn-3-one mediated by zinc(II) iodide, and an intramolecular oxy-Michael reaction, provides an efficient synthesis of cis-2,6-disubstituted 4-methylenetetrahydropyrans of interest in the context of a synthesis of bryostatins. The stereoselective formation of (E)-vinylsilanes in the 'ene' reaction is of interest.