Oxidative radical cyclization to pyrroles under reducing conditions. Reductive desulfonylation of α-sulfonylpyrroles with tri-n-butyltin hydride

Abstract

1-(2-Bromobenzyl)-2-alkanesulfonylpyrroles (1c, 1d) and 1-(4-bromobutyl)-2-methylsulfonfylpyrrols (8) undergo oxidative radical cyclization with partial or complete reductive desulfonylation to the pyrrolizidine derivatives 5 and 9 by an AIBN initiated reaction with tri-n-butyltin hydride. These cyclizations are suggested to proceed via a pseudo SRN1 process involving radical addition to the α position of the pyrrole nucleus not bearing the sulfonyl group. Reductive removal of the alkylsulfonyl moiety is proposed to occur in a second process after completion of the oxidative radical cyclization. The site of the radical addition is supported by deuterium labelling studies. Consistent with the timing of the loss of the sulonyl group is that 2-alkysulfonylpyrroles 11 are reductively desulfonylated under the same conditions that effect the oxidative radical cyclizations.