Syntheses of terpenes

The diastereosclectivity in this particular cyclization was exploited in a number of direct syntheses of terpenes from the eudesmane type (see Table 2). By this technique tricyclic compounds can be synthesized generating three new asymmetric centers55. 

Terminal attack occurs with water, methyl iodide, and trimethylchlorosilane, whereas central attack was preferred with alkenyl halides, aldehydes, and ketones at low temperatures under kinetic control [Eq. (5)]. The Et3SiO group is readily removed from 6 by potassium fluoride in isopropanol to give the vinyl ether RCH2CH2COCH=CH2 (61). Some of these reactions have also been used in elegant syntheses of terpenes (99-102). 

The hydrolysis step could be avoided and the ring expansion could be achieved directly from the trimethylsiloxy derivatives by treatment with sodium hydroxide in methanol. This approach was successfully applied to the production of a range of polycyclic ketones, many of which were intermediates in multistage syntheses of terpenic natural products (Table 5). 

Hydroboration and asymmetric reductions proceed with high stereoselectivity because of the small atom radius and the large electronegativity of boron [22]. Hence, organoboron compounds are used for asymmetric reactions, especially for asymmetric reduction [9,22,36]. Both the yield and the optical yield of asymmetric reactions with organoboron compounds are high, and the isolation of chiral products is easy. Therefore, these reactions are exploited for syntheses of terpenes [22]. 

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