Terpenoid intermediates alkylation

IPP and DMAPP are reactive hemiterpene intermediates in the pathways leading to more complex terpenoid structures. They are also used as alkylating agents in the formation of meroterpenoids as indicated above, but examples of these structures are discussed under the section appropriate to the major substructure, e.g. alkaloids, shiki-mate, acetate. Relatively few true hemiterpenes are produced in nature, with isoprene, a volatile compound which is released by many species of plants, especially trees, being the notable example. Isoprene is formed by loss of a proton from the allylic cation (Figure 5.7). 

Addition of the a,3 unsaturated anion (21) to the Michael acceptor (22), in which either alkylation or 1,4-addition is possible, affords only the Michael product. Internal alkylation of the intermediate ester enolates leads to cyclopropyl derivatives (equation 7). Terpenoid polyenes are prepared through conjugate addition of the lithiated protected cyanohydrins (23) to dienyl sulfoxide (24 equation 8). ... 

Acetylenic ketones are useful intermediates in terpenoid synthesis. Coke and co-workers have described a novel synthesis of acetylenic ketones, inspired by, but different from, the Eschenmoser fragmentation. The procedure is illustrated in Scheme 34 by its application to the synthesis of exo-brevicomin (134) the pheromone from Dendroctonus brevicomis. Hydroboration of the double bond of oct-l-en-4-yne with 9-BBN, followed by the acetylene zipper procedure, and base-induced alkyl transfer to t-butyl bromo-methyl ketone offers an efficient route to acetylenic ketones in greater than 70% yield [(135)->(136)]. °... 

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