of 3 dihydroxy 1 10 methylenedioxy ginkgolide b is obtained 1 h nmr 300 mhz cf 3 cood 6 21 s 1h h 12 5 71 s 1h h 10 5 40 brs 1h h 6 5 38 d 1h h methylene 5 17 t 1h h 2 5 15 d 1h h methylene 4 36 d 1h h 1 3 28 m 1h h 4 3 13 t 1h h 3 2 48 d 2h h 7 2 20 t 1h h 8 1 47 d 3h ch 3 1 21 s 9h t bu 5 g of 3 dehydroxy 1 10 methylenedioxy 14 epi ginkgolide b which is obtained above example 6 is stirred with 50 ml of 2n hcl solution at 100 u00b0 c for 3 hours the reaction mixture is cooled at room temperature the reaction mixture is diluted with 200 ml of water and extracted twice with 200 ml of dichloromethane the combined dichloromethane extracts are concentrated by evaporation in a vacuum and recrystallized from 30 ml of ethylalcohol 4 7 g 95 of 3 dehydroxy 14 epi ginkgolide b is obtained 1 h nmr 300 mhz cf 3 cood 6 18 s 1h h 12 5 77 s 1h h 6 5 43 s 1h h 10 5 37 s 1h h 2 5 12 d 1h h 1 3 55 t 1h h 3 3 32.