Compounds related to provitamin D3 (III)): Homocholesterol and the corresponding provitamin (3‐homoprovitamin D3)

3-Homocholesterol (III), obtained by action of LiAlH4 on cholesteryl-3-carboxylic methyl ester (II), which has been prepared from cholesteryl chloride by Grignard reaction, has the same steric configuration (β) at carbon atom 3 as cholesterol. Bromination of its acetate (IV) by means of bromosuccinimide, followed by elimination of HBr, leads to the acetate of 3-homoprovitamin D3 (Vα). This 3-homoprovitamin D3 (IX) can be synthesized with a far better yield by introducing first the double bond 7 and then the hydroxymethyl group 3. The magnesium derivative of 7-dehydrocholesteryl chloride is transformed by CO2 into the carboxylic acid (VII), which is readily methylated with diazomethane. Finally the 7-dehydrocholesteryl-3-carboxylic methyl ester (VIII) is reduced with LiAlH4 to 3-homoprovitamin D3 (IX). It is pointed out that the ultraviolet absorption characteristic for the conjugated system of double bonds 5-7 is subjected to an influence of molecular size.