A mixture of 6a (9.36 g, 27 mmol), triethyl orthoacetate (34.75 mL, 189 mmol) and propionic acid (0.2 mL, 2.7 mmol) was heated at 138° C. for 20 h with removal of ethanol. The reaction mixture was cooled and concentrated under vacuum, and the residue was purified by flash column chromatography over silica gel with hexane:EtOAc (19:1) as eluant to give 7a (8.77 g, 78%) as an oil; IR (neat) 1737, 1471 cm-1 ; 1H NMR (CDCl3) δ 0.01 (br s, 12 H, SiCH3), 0.85 (br s, 18 H, C(CH3)3), 1.21 (t, 3 H, CH3CH2), 2.42 (s, 2 H, CH2COEt), 3.61 (AB q, J=9.3 Hz, 4 H), 4.06 (q, 2 H, CH2CH3), 5.06 (m, 2 H, CH=CH2), 5.84 (dd, J=19.9, 11.3, 1 H, CH=CH2); 13C NMR (CDCl3) δ -5.58, 14.24, 18.23, 25.84, 36.81, 45.97, 59.84, 64.63, 114.47, 139.73, 171.87. Anal. Calcd for C21H44O4Si2 : C, 60.52; H, 10.64. Found: C, 60.46; H, 10.62.
