Selected Syntheses of Terpenes

The special potential for constructing double bonds stereoselectively, often necessary in natural material syntheses, makes the Wittig reaction a valuable alternative compared to partial hydrogenation of acetylenes. It is used in the synthesis of carotenoids, fragrance and aroma compounds, terpenes, steroides, hormones, prostaglandins, pheromones, fatty acid derivatives, plant substances, and a variety of other olefinic naturally occurring compounds. Because of the considerable volume of this topic we would like to consider only selected paths of the synthesis of natural compounds in the following sections and to restrict it to reactions of phosphoranes (ylides) only. 

Numerous syntheses of methyicyclopentanoid monoterpenes have been reported, but we will restrict our discussion here to a selected number of the more general and stereoselective syntheses of the basic skeletons, giving particular attention to the stereoselective syntheses of optically active forms of these terpenes. 

In their efforts to study the mechanism of terpene synthases, Coates and coworkers synthesized aza derivatives of ent-kaurene. The two heterocyclic analogs, 16-aza-trachylobane (107) and 16-aza-beyerane (108), were synthesized froment-beyeran-16-one (103), which was obtained from the natural product stevioside (102, Scheme 5.24T These analogs were assessed for their utility as active site probes for various ent-diterpene cyclases and as selective inhibitors of gibberellin biosynthesis in plants. 

The Barton nitrite photolysis reaction also had an important role in terpene synthesis and the elucidation of terpene stmcture. For example, in Corey s syntheses of azadiradione and perhydrohistrionicotoxin " selective functionalization by means of the nitrite photolysis reaction was the crucial... 

The main products are monolitic terpenes such as alpha-terpineol 32. Minor amounts of bicyclic compounds (34), e.g. borneol 31, can be observed. Some of these products are of commercial importance as fragrances with lilac and nutmeg odor (monocyclic alcohols like alpha-terpineol or 1-terpinene-4-ol) or with campher-like and extremely delicate pepper odor (bicyclic compounds). In current industrial practice, the bicyclic alcohol borneol 31 is synthesized by a multi-step procedure. Much research has been done to develop clean processes which have high selectivity towards one of the products starting directly from alpha-pinene 30. 

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