Isoprenyl radicals

A useful solution to the latter problem, introduced by Keck and Byers,141 makes possible crotylation and isoprenylation. Irradiation of an allyl sulfide (28 3 equiv.), a reactive radical precursor (29 1 equiv.) and hexabutylditin (1.5 equiv.) produces substitution products (30) in good yields (Scheme 41). The... 

Simple stilbenes (Phe-CH=CH-Phe) include the Vitis vinifera (grape) (Vitaceae) cytotoxic resveratrol (4,3, 5 -trihydroxystilbene), the mitochondrial electron transport inhibitor oxyresveratrol (3,5,2, 4 -tetrahydroxystilbene) and the protein kinase inhibitor piceatannol (3,4,3, 5 -tetrahydroxystilbene), all these compounds having antifungal activity. The isoprenylated stilbene chlorophorin (4-geranyl-3,5,2, 4 -tetrahydroxystilbene) is an antioxidant free radical scavenger (AO/FRS). 

The halogenation reaction proceeds in darkness and is reasonably considered as ionic. It has been shown that if chlorine gives primarily free radical addition on mono- and di-substituted alkenes, it gives ionic substitution products with tri- and tetra-substituted alkenes [73], A model compound study, together with NMR analysis of commercial chloro and bromobutyl samples, confirmed that the reaction on isoprenyl unit leads predominantly to the exomethylene-substituted structure A, and this is explained by steric hindrance due to the tetra-substituted carbon in f3-position which favors proton elimination rather than the nucleophilic attack of halide counter ion in the second phase of addition (Fig. 11, Table 1) [74,75]. 

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