Dienol Rearrangements Catalyzed by Nickel Chloride

Article abstract of DOI:10.1021/jo00180a025

The rearrangement of dienols R2C(OH)CH=CHC(X)=CR2 which possess a nonterminal, arylthio or alkylthio substituent was effected by using nickel chloride in aqueous tert-butyl alcohol at 60 deg C to furnish the vicinally substituded dienol R2C=CHCH=C(X)CR2(OH).Application of this rearrangement to 24-(methylthio)cholesta-5,22,24-trien-20-ol provided 24-(methylthio)cholesta-5,20(22),23-trien-25-ol and formed the basis for a new, five-step synthesis of (20R)-25-hydroxycholesterol from pregnenolone tetrahydropyranyl ether.The rearrangement process appeared to be driven by the relief of unfavorable steric interactions between the quaternary C-13 and C-20 centers in steroids and influenced by the nature of the nickel(II) catalyst.The sulfur-containing substituent was not a prerequisite for the rearrangement since the desulfurized dienols underwent an analogous rearrangement.Preparation of these desulfurized dienols involved the use of a Raney nickel catalyst deactivated by heating in decalin.This catalyst selectively desulfurized vinyl thioethers to olefins.