Enantioselective synthesis radical allylation

A study of the synthesis of chromans from allylic carbonates involving Pd-catalysed asymmetric allylic alkylation has established that the addition of acetic acid results in a pronounced increase in enantioselectivity. Furthermore, (E) allylic carbonates afford (R) chromans and the (Z) substrates the (S) heterocycle (Scheme 13) <03JA9276>. This approach to chromans has been combined with a radical epoxide cyclisation in a total synthesis of (-)-siccanin <03AG(E)3943>. 

Finally, Crich [31] and Danishefsky [32] demonstrated that enantioselective hexahydropyrroloindole formation could precede C3-quatemary center formation. The carbon-carbon bond formation could be accomplished through a stereoretentive cationic or radical process as demonstrated through the reverse-prenylation reaction in Danishefsky s synthesis of amouramine [32a, b] or the allylation reaction en route to Crich s synthesis of (+)-debromofhistramine [33], respectively. 

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