Of pyrrolidines by nitrogen radical cyclization

In common with the general increasing interest in synthetic applications of free-radical cyclizations, this year has seen a high level of interest in the synthesis of nitrogen heterocycles by such processes. Padwa et al. have reported the synthesis of a range of N-benzenesulphonyl-pyrrolidines (220) and -piperidines... 

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

Azides are highly valuable radical acceptors that form nitrogen-centered radicals after addition onto them, as in the case of the transformation of 31 into 32. This reaction opens new possibilities for making pyrrolidines. Murphy, for instance, disclosed the synthesis of ( )-horsfihne and ( )-coerulescine by tandem cyclization of iodoaryl alkenyl azides such as 29 [42]. By the same strategy and using precursor 33, formal syntheses of ( )-vindohne [43] and ( )-aspidospermidine [44] have been rendered possible (Scheme 10). 

A few reactions of nitrogen compounds interesting from a stereochemical aspect have been reported. Tokuda and coworkers [372] reported the stereochemically clear result that lithium alkeynylamides undergo a highly stereoselective cyclization by anodic oxidation to give C75-1-methyl-2,5-disubstituted pyrrolidines in reasonable yields, as in Eq. (59). The cyclization seems to proceed via neutral amino radicals. 

Pyrrolidines. Homolysis of benzotriazol-l-ylmethylamines leads to a-amino radicals. This Sml -mediated reaction is followed by cyclization when the amino nitrogen atom is located at the 8-carbon of an a,P-unsaturated ester. 

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