purified by column chromatography on silica gel with hexane ethyl acetate 95 5 to give the aldehyde 3 364 mg 81 yield as an oil 1 h nmr 500 mhz cdcl 3 u03b49 55 1h d j 3 1 hz cho 4 00 1h s 8 u03b1 h 2 33 1h m 20 h 1 89 1h dm j 12 4 hz 1 07 3h d j 6 8 hz 21 h 3 0 939 3h s 18 h 3 0 862 9h s si t bu u2212 0 009 and u2212 0 026 each 3h each s each sime 13 c nmr 125 mhz u03b4 205 37 d cho 68 99 d c 8 52 28 d 51 58 d 49 15 d 42 58 s c 13 40 35 t 34 29 t 26 16 t 25 74 q sic me 3 23 27 t 17 96 s si c me 3 17 52 t 14 04 q c 21 13 28 c 18 u2212 4 85 and u2212 5 23 each q each si me the aldehyde 3 364 mg 1 12 mmol was dissolved in methylene chloride 15 ml and a 40 aq n bu 4 noh solution 1 47 ml 1 45 g 2 24 mmol was added the resulting mixture was stirred under argon at room temperature for 16 h diluted with methylene chloride 20 ml washed with water dried na 2 so 4 and concentrated under reduced pressure a residue was chromatographed on silica gel with hexane ethyl acetate 95 5 to afford a mixture of aldehyde 3 and its 20 epimer 292 mg 80 yield in ca 1 2 ratio by 1 h nmr this mixture of aldehydes 292 mg 0 9 mmol was.