| 摘要 |
Described herein is a process for the preparation of an acetal of formula (I) in which R1 is an alkyl, R2 is H or an acyl, X is beta -OH and Y is H, comprising the acetalization with control of the epimeric distribution at the C-22 position, for treatment with R1 CHO in aqueous HBr or HI of a compound of formula (II) in which R2 has the meaning specified above, R3 and R4 are both H, or, taken together, are -C(R5)(R6)-, where R5 and R6, which may be the same or different from one another, are alkyl groups, and X and Y are chosen from among the following: A) X and Y, taken together, are an additional bond between C-9 and C-11; B) X is beta -OH, and Y is H or an alpha -halogen; C) X is beta -OR7 where R7 is an acyl, and Y is H or an alpha -halogen; and D) X and Y, taken together, are -O-. <CHEM> t |
