Cyclization of vinyl radicals

Cyclization of vinyl radicals. Vinyl radicals formed by reduction of vinyl halides with tri-n-butyltin hydride (AIBN, hv) can undergo intramolecular cyclization to a suitably situated double bond. When either a five- or a six-membered ring can be formed, the former ring is favoured. When cyclization could lead to a six- or seven-membered ring, only the former is formed, as in the cyclization of 1 to 2. The cyclization of 3 to 4 is particularly interesting because the exocyclic double bond is formed at a predictable and useful position, and because a quaternary centetis formed without difficulty. 

Gomez, A M, Danelon, G O, Valverde, S, Lopez, J C, Regio- and StereocontroUed 6-endo-trig radical cyclization of vinyl radicals a novel entry to carbasugars from carbohydrates, J. Org. Chem., 63, 9626-9627, 1998. 

The five-ring annulation also controls the steric course of the cyclization of vinyl radicals. The tributyltin hydride initiated ring-closure of an (iodobutenyl)cyclohexene, yields a civ-fused bicycle46. The double bond in the product serves as the site for further synthetic modifications. 

Vinyl radicals are useful in radical coupling and addition reactions. Stork used the cyclization of vinyl radical 163 (derived from vinyl bromide 162) to complete a synthesis of norseychellanone (164). The cyclization step proceeded in 70% yield. 

The reaction of diphenyl disulfide with a variety of substituted acetylenes, promoted by thermal decomposition of di-rert-butyl peroxide, provides l,2-bis(phenylthio)ethylene adducts together with the corresponding benzothiophenes in reasonable yield ". The mechanism conceived for these transformations is depicted in Scheme 3, where the key step is the intramolecular cyclization of vinyl radicals. 

In the cyclization of vinyl radicals. Here, the 6-ring product is formed by rearrangement of the kinetic 5-ring, via a cyclopropyl methyl radical. The ratio of 5-exo to G-endo products can be controlled to some extent by altering the concentration of BUjSnH. 

Cyclization on to unsaturated C=N bonds has been reported by a number of workers to proceed in the unusual 6-endo fashion preferentially. Generation of the initial radical by addition of either a tin-centred or a sulfiir-centred radical to an alkyne or allene has been demonstrated. Thus cyclization of vinyl radicals (produced by the BusSnH-mediated addition to alkynes) on to imines proceeds in a 6-endo manner to give functionalized piperidines (Scheme 21). In a similar manner, allyl radicals (generated by Ts addition to allenes) add in a 6-endo manner to hydroxylamine derivatives. EUmination of the RO leads to the formation of dihydropyridines (Scheme 22). Functionalized bicyclic j8-lactams can be produced by an unusual 1-endo cyclization triggered by a similar addition of a tin-centred or sulfur-centred radical to substituted Af-propargyl )3-lactams. ... 

Denes F, Beaufils F, Renaud P. Preparation of five-membered rings via the transposition-cyclization of vinyl radicals. Synlett 2008 (16) 2389-2399. 

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