Y. Antonio et al., OXIDATIVE RADICAL CYCLIZATION TO PYRROLES UNDER REDUCING CONDITIONS -REDUCTIVE DESULFONYLATION OF ALPHA-SULFONYLPYRROLES WITH TRI-N-BUTYLTIN HYDRIDE, Canadian journal of chemistry, 72(1), 1994, pp. 15-22
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.