Electroorganic syntheses of macrocyclic lactones mediated by vitamin B-12 model complexes Part 17. Hydrophobic vitamin B-12

The electrolyses of bromoalkyl acrylates were carried out in N,N-dimethylfo rmamide in the presence of a catalytic amount of heptamethyl cobyrinate per chlorate under various reaction conditions. A large-membered cyclic lactone was obtained in a moderate yield. A photo-sensitive intermediate having a cobalt-carbon bond formed during the electrolysis has been characterized us ing electronic and electrospray ionization mass spectroscopies in the dark. The formation of a cyclic lactone was inhibited by the addition of the spi n-trapping reagent, a-phenyl N-(t-butyl)nitrone. Based on various spectrosc opic results. the electrolysis proceeds as follows: the Co(II) complex as a catalyst is electrochemically reduced to the Co(I) species, and the corres ponding alkylated complex is generated by the reaction of the supernucleoph ilic Co(I) species with a bromoalkyl acrylate. The alkylated complex is sub sequently decomposed to form the substrate radical, and the intramolecular addition of the radical affords the cyclic product. During the catalytic re action, the cobalt complex repeatedly acts as a mediator. (C) 2001 Elsevier Science BN. All rights reserved.