Olefinic alkyllithiums cyclization

B. Cyclization Reaction of Heteroatom-substituted Olefinic Alkyllithiums. 334... 

III. CARBOLITHIATION REACTION OF UNSATURATED ALKYLLITHIUMS A. Cyclization Reaction of Olefinic Alkyllithiums... 

Most, perhaps all, of the reactions that simple alkenes undergo are also available to allenes. By virtue of their strain and of the small steric requirement of the sp-hybrid-ized carbon atom, the reactions of allenes usually take place more easily than the corresponding reactions of olefins. Because the allenes can also be chiral, they offer opportunities for control of the reaction products that are not available to simple alkenes. Finally, some reaction pathways are unique to allenes. For example, deprotonation of allenes with alkyllithium reagents to form allenyl anions is a facile process that has no counterpart in simple alkenes. These concepts will be illustrated by the discussion of cyclization reactions of allenes that follows. 

It has been found that enantio-enriched a-(homoallyloxy)alkyllithiums (84), formed from the corresponding stannanes (83) by stereoretentive transmetallation, cyclize with complete retention of configuration at the lithium-bearing -carbon to yield enantio-enriched Q ,jS-disubsfituted tetrahydrofurans (85 and 86). This is consistent with earlier theoretical calculations which suggested that the lithium might coordinate with the olefinic bond in the transition state. 

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