Of -P-santalene

A number of cycloaddition reactions involving allene derivatives as dienophiles have been recorded. Allene itself reacts only with electron-deficient dienes but allene carboxylic acid or esters, in which a double bond is activated by conjugation with the carboxylic group, react readily with cyclopentadiene to give 1 1 adducts in excellent yield. For example, the allene 12 gave, with very high yield and selectivity, the cycloadduct 13, used in a synthesis of (-)-P-santalene (3.19). An allene equivalent is vinyl triphenylphosphonium bromide, which is reported to react with a number of dienes to form cyclic phosphonium salts. These can be converted into methylene compounds by the usual Wittig reaction procedure (3.20). 

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See also in sourсe #XX -- [ , , ]

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