Total synthesis of ent-cholesterol via a steroid C,D-ring side-chain synthon

For the first time, one of the two enantiomers of cholesterol (ent-cholesterol) has been synthesized by a synthetic route that starts from a precursor containing the D-ring and entire side chain of cholesterol. As part of the reported synthetic route, a method of general utility for the large scale (> 10 g) preparation of each enantiomer of [1α(R*),7aα]-1-(1,5-dimethylhexyl)-1,2,3,6,7, 7a-hexahydro-7a-methyl-5H-inden-5-one, C,D ring-side chain synthons that can be used for the synthesis of enantiomers of vitamin D₃, cholesterol, and their analogues was also developed. Using the enantiomer of the C,D-ring side-chain synthon that leads to ent-cholesterol, the A- and B-rings were elaborated from a linear fragment that is sequentially cyclized to form the steroid B- and A-rings. Using this route, ent-cholesterol was prepared in 23 steps from the methyl ester of (1α,5α,6α)-(±)-6-methyl-2-oxo-bicyclo[3.1.0] hexane-1-carboxylic acid in a total yield of 2.6%.