1 was separated by flash chromatography on silica elution with hexane ethyl acetate 85 15 resulted in partial separation of products 29 mg of 5b mixture of 5a and 5b 85 mg and pure 5a 176 mg total yield 77 rechromatography of the mixed fractions resulted in almost complete separation of the products 5a u03b1 24 d u2212 63 u00b0 c 0 60 chcl 3 1 h nmr 500 mhz cdcl 3 u03b4 0 074 6h s 2 u00d7 sich 3 0 914 9h s si t bu 2 13 3h s och 3 2 00 1h br t j 11 2 hz 2 u03b1 h 2 10 1h d j 10 8 hz 8 u03b1 h 2 34 1h ddd j 11 7 7 0 2 9 hz 2 u03b2 h 2 38 and 2 43 1h and 1h each m u2550 c u2014 ch 2 3 31 1h ddd j 10 8 6 5 2 9 hz 8 u03b2 h 3 35 3h s o u2014 ch 3 3 54 and 3 60 1h and 1h each m ch 2 u2014 c h u2014 o 4 41 1h t j 8 2 hz 3 u03b2 h 4 60 2h s o u2014 ch 2 u2014 o 5 52 1h d j 6 5 hz 5 u03b1 h 5 71 1h.