Allenic alkyllithiums cyclization

Although similar chemistry was observed with iodide 34 (R = Me, Scheme 7-26) which possesses a methyl substituent at the internal allenic carbon, the alkyl substitution at the remote end of the allene did not lead to the cyclic product [25]. The same methodology was applied to systems in which the reactive functions are separated by longer chains, as y-allenic alkyllithium 35. The latter cyclizes at low temperature to produce, after reaction with an electrophile, a mixture of cyclic products [27] (Scheme 7-28). 

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. 

The rhodium-cataiyzed intramoiecuiar [5+2] cycioaddition of an allene and vinylcyclopropane was the key step in the asymmetric total synthesis of the trinorguaiane sesquiterpene (+)-dictamnol by P.A. Wender and co-workers.The cyclization precursor allene-cyclopropane was assembled starting from commercially available cyclopropane-carbaldehyde. Using the HWE oiefination, the Weinreb s amide moiety was installed and subsequently reacted with a primary alkyllithium that was generated via lithium-halogen exchange. 

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