The 3,4-dihydro-2H-pyran approach to (+)-milbemycin β3. Part 2. An improved synthesis of (2R,4S,6S,8R,9S)-2-[(5R)-(2E)-3-methyl-5-formyl-hex-2-en-1-yl]-8,9-dimethyl-4-(dimethyl-t-butylsilyloxy)-1,7-dioxaspiro[5.5]undecane

Abstract
A more efficient synthesis of the title compound (2), previously used in a total synthesis of (+)-milbemycin β3(1), is described. The key step in the sequence involves a nucleophilic cleavage of the oxirane (4) by the organocuprate derived from metallation of (2R,3S)-2,3-dimethyl-3,4-dihydro-2H-pyran (5).