Elimination-rearrangement acetylene derivs

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

Fragmentation Decarbonylation (Am 28) and loss of CHO (Am 29) followed by elimination of acetylene. In alkyl derivatives [M-l] and, if at least two alkyl carbons are present (dimethyl or ethyl), [M-15] become important. Elimination of CO from the primary fragments. [M-IS]" " mainly with or/Ao-alkylphenols. In derivatives with a longer alkyl chain, benzylic cleavage and alkene elimination (McLafferty rearrangement) are the dominant primary fragmentation processes. The fragments then lose CO (Am 28). 

In the hydrogenolysis of acetylenic Mannich bases, rearrangement with double bond migration, accompanying the elimination of the amine, may take place with formation of an alicne derivative instead of the expected methyl alkyne. 

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