Vitamin B12-mediated electrochemical reactions in the synthesis of natural products

Abstract

Vitamin B12is an efficient catalyst in electroorganic synthesis since it acts as a mediator in the transfer of electrons from the cathode to electrophilic organic substrates. At the surface of the cathode B12 is fast reduced to Co1at an electrode potential, at which organic substrates remain electroinactive. Co1reacts with electrophilic substrates R-Y as e.g. alkyl- and vinyl halides, α-halo ethers and acyl derivatives with formation of organocobalamins Co-R. As a result of further reduction, thermal or photochemical excitation, the Co-R bond is cleaved; R undergoes follow-up reactions via a radical- or carbanion pathway and the catalyst is recycled by reduction. Synthetically usefull B12-catalyzed reactions are the reductive β-elimination and the conjugate addition of R-Y to activated olefins. The reductive β-elimination has been applied in the removal of β-halo ethyl protecting groups and the stereoselective synthesis of olefins like trans-5-decen-l-yl acetate and trans-10-propyl-trideca-5,9-dien-l-yl acetate. The B12-catalyzed addition of R-Y to activated olefins has been applied in radi-cal-type cyclisations and intermolecular additions of prim. and sec. alkyl-halides leading to: trans-9-oxo-2-decenoic acid ethyl ester, (1R,5S)-(+)- frontalin, (1S,5R)-(-)-frontal in, (1R,5S,7S)-(+)-endo-brevicomin, (1R,5S,7R)-(+)-exo-brevicomin as well as to (1s,2r,4s,5r)-(-)-α-, (1S, 2S, 4R, 5R)-(-)-β-, (1S,2R, 4R, 5R)-(-)-ϒ- and (1S,2S,4S,5R)-(-)-δ-multistriatin. The C-glycosides, 3-(2,3,4,6-tetra-0-acetyl-α-D-glucosyl)propionitrile and 3-(2,3,5-tri-0-ace-tyl -β-D-ribofuranosyl)propionitrile are obtained from the l-bromo-sugars and acrylonitrile. The conjugate addition of acid anhydrides to α,β-unsaturated aldehydes and ketones affords 1,4-dioxo compounds, the precursors of 2-cyclo-pentenones. © 1987 IUPAC