Isomerization alkyne-allene base-catalyzed

Now that we have the structures of allenes and cumulenes under control, it is time to look at reactivity. Our brief look at allenes permits us to understand one of the most astonishing reactions of alkynes the base-catalyzed isomerization of an internal triple bond to the terminal position (Fig. 12.7). 

The base-catalyzed isomerization of disubstituted to terminal alkynes takes place through allene intermediates (Fig. 12.13). Addition reactions of HX orX2 to conjugated dienes can give products of both 1,2- and 1,4-additions (Fig. 12.38). 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

Especially in THF, methylphenylacetylene was found to undergo a base-catalyzed rearrangement to its isomeric alkyne and allene ... 

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