An Efficient Stereoselective Synthesis of a Racemic CD-Intermediate of Vitamin D

A highly efficient four-step approach to the racemic CD-intermediate 8 from the racemic hydrindenone 10 has been developed. In the key step, the hydrindenone 10 is converted stereoselectively into the (±)-bromohydrindenone 17. Oxygen transposition from C-9 to C-8 is effected by meass of conversion of 17 to the epoxide 20 and regioselective reductive ring opening of 20 with DIBALH. The resulting (±)-hydrindanol 8 was converted by the Horner-Wittig reaction with ±-methacrolein into the diene 24, hydrogenation of which provided the racemic hydrindanol 25. This is a direct precursor of racemic Grundmann's ketone 9 and bears the alkyl side chain and the correct relative configuration at C13, C14, C17, and C20 of Vitamin D.