Triquinane terpenes

Briggs et al. proposed a new strategy for the synthesis of tricyclic structures using acyl xanthates as precursor for acyl radicals [121]. Irradiation with visible light of a solution of acyl xanthate in presence of 1,6-diene 126 afforded czs-fused bicyclic compound 127 in a good yield (Scheme 38). Radical reduction of xanthate and subsequent aldol condensation leads to the formation of [5.5.5]-fused ring systems similar to those of the triquinane terpene family. 

The capnellenols, e.g. (24) [cf. (10)], are a unique family of sesquiterpenes, found in the soft coral Capnella imbricata. They are related structurally to the hirsutane family of triquinane terpenes, e.g. (18), but with the three methyl groups distributed differently about the tricycle. A special feature of the capnellenols is the presence of an unusual Wj-allylic alcohol functionality associated with ring c. The 8-epimer of (24) has now been synthesized by a route (Scheme 3) in which this novel functionality is introduced by annulation to (23) via reductive cyclization of the keto-acetylene precursor (22), followed by treatment of the resulting allylic alcohol (23) with t-butyl hydroperoxide in the presence of catalytic selenium dioxide. ... 

The synthesis of triquinane acids, initiated by the preparation of isocomenic acid [22], thus provided a general method for control of the stereochemistry of secondary methyl groups in these terpenes. The [4+1] annulation based on the dienes of type 23 then laid the groundwork for the first-generation design and a model study for the approach to retigeranic acid [23]. 

Among the variety of terpenes which were synthesized with an intramolecular meta photocycloaddition as key step, triquinane derivatives were particularly well studied. Two isomers of these compounds are readily accessible via intramolecular meta cycloaddition in position 1,3 (Sch. 9). The following cyclopropanation reaction of intermediate O controls which of the angular or linear isomers is formed (Sch. 17) [67]. As this step is almost always unselective, both isomers are concomitantly formed. However, one of the isomers can be obtained predominantly via an additional photochemical equilibration step (compare Sch. 11). The triquinane frame is obtained by rupture of the distant C-C bond of the cyclopropane in connection to the 10-membered ring moiety. 

Hudlicky T, Short RP (1982) Terpenic Acids by Cyclopentane Annulation of Exocyclic Dienes. Synthesis of Triquinane Portion of Retigeranic Acid. J Org Chem 47 1522... 

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