Allylation Reactions Exhibiting a-Chelation Control

Aldehydes that contain a heteroatom substituent at the a-carbon often display high stereoselectivity in reactions with ally lie stannanes. This behavior is particularly the case for heteroatom substituents permitting effective chelation with a Lewis acid. Internal activation of the carbonyl oxygen provides a five-membered chelation complex with Lewis acids, which minimally offer two coordination sites. The stability of the metallocycle may account for high diastereoselection, as nucleophilic approach of the stannane occurs to the less hindered face of the carbonyl. 

Kecki described studies of chelation-controlled additions of enriched mixtures of E- and Z-crotyl tri-n-butylstannanes. Complexation of the a-benzyloxyaldehyde 51 with MgBr2 OEt2 led to selective 

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Scheme 5.2.12 Reinforcing features for asymmetric induction leading to 49 5.2.4 Allylation Reactions Exhibiting a-Chelation Control...

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