OXIDATIVE RADICAL CYCLIZATION TO PYRROLES UNDER REDUCING CONDITIONS -REDUCTIVE DESULFONYLATION OF ALPHA-SULFONYLPYRROLES WITH TRI-N-BUTYLTIN HYDRIDE

1-(2-Bromobenzyl)-2-alkanesulfonylpyrrol (1c, 1d) and 1-(4-bromobutyl) -2-methylsulfonfylpyrrols (8) undergo oxidative radical cyclization wi th 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 S(RN )1 process involving radical addition to the alpha position of the pyr role nucleus not bearing the sulfonyl group. Reductive removal of the alkylsulfonyl moiety is proposed to occur in a second process after co mpletion 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-alkysulfonylpyr roles 11 are reductively desulfonylated under the same conditions that effect the oxidative radical cyclizations.