Pathways to Allylic and Cyclic Compounds

The catalytic enantioselective desymmetrization of meso compounds is a powerful tool for the construction of enantiomerically enriched functionalized products." Meso cyclic allylic diol derivatives are challenging substrates for the asymmetric allylic substitution reaction owing to the potential competition of several reaction pathways. In particular, S 2 and 5n2 substitutions can occur, and both with either retention or inversion of the stereochemistry. In the... 

The reaction between octyl iodide and Et2Zn in the presence of PdCbCdppf) (1.5 mol%) in THF at 25 °C produces OctZnI within 2 h in 75-80% yield. Such palladium- or nickel-catalyzed reactions proceed by a radical pathway. By using a precursor such as 253 as a 1 1 diastereomeric mixture, the palladium-catalyzed cyclization provides in a stereoconvergent way the cyclopentylmethylzinc derivative 254, which after allylation produces the diastereomerically pure cyclic ester 255 in 71% yield. The intermediate radical cyclizes via a transition state A where all the substituents are in an equatorial position. The similar reaction using Ni(acac)2 as a catalyst allows the preparation of heterocyclic compounds such as 256. The relative stereochemistry of up to three contiguous centers can be controlled by using this cyclization (Scheme 2-99). 

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