5-hexenyl radical, diastereoselective cyclization

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

A diastereoselective cyclization of a 5-hexenyl radical linked to a carbohydrate scaffold was reported by Enholm et al. [171]. The authors used (+ )-isosorbit and ( —)-D-xylose as the chiral auxiliaries. The a,p-unsaturated bromo ester 262 derived from (-h )-isosorbit was reacted with tributyltinhydride and Lewis acids. The influence of the reaction temperature and of the solvent on the yield and stereoselectivity of the cyclization were also examined. Best results were obtained when ZnCl2 was used as Lewis acid at — 78°C (Scheme 10.85). The cyclization furnished the ester... 

Enholm, E J, Cottone, J S, Allais, F, Highly diastereoselective 5-hexenyl radical cyclizations with Lewis acids and carbohydrate scaffolds, Org. Lett., 3, 145-147, 2001. 

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