Africanol

Intramolecular Friedel-Crafts acylations of olefins also give cycHc a,P-unsaturated cycHc ketones. Cyclopropane fused bicyclo[5.3.0]octenones, thus obtained, were used in the preparation of the marine sesquiterpenes, africanol [53823-07-7] and dactjlol [58542-75-9] (174). 

A chirally modified Raney Ni catalyzes the hydrogenation of 1,3-diketones selectively to give the anti 1,3-diols in about 90% ee (Fig. 32.19) [67]. Natural compounds such as africanol and ngaione are synthesized via this method [68]. 

Anionic29,314 and thermal315 oxy-Cope rearrangements were reported as steps in the syntheses of various bicyclic systems 586 from divinylcycloalkanes 585 (see Section IV.C.2) (equation 236). The same anionic scheme was applied to prepare ( )-africanol and ( )-isoafricanol (hydroazulene systems)316 as well as ajmaline-related alkaloids (equation 237)317. 

In another attempt to mimic the in vivo cyclization of humulene, Mlotkiewicz et have shown that treatment of humulene 4,5-epoxide (272) with boron trifluoride etherate leads to the formation of the two tricyclic alcohols (273) and (274) in 70% yield. The carbon skeleton of these two compounds is exactly that found in africanol (276) and the more recently isolated keto-angelate (275). Further elaboration of the alcohol (273) has in fact resulted in a biomimetic synthesis of the keto-alcohol corresponding to (275). This work constitutes the first example of the direct conversion of a humulene derivative into a naturally occurring compound. 

Cyclization humulene-4,S epoxide. Treatment ol this epoxide (1) with BF, etherate results in cyclization to equal amounts of the two isomeric alcohols 2 and 3, related to the sesquiterpene africanol 4, of marine origiia. Such a reaction could be involved in biogenesis of 4. ... 

Paquette s synthesis of africanol advanced through the a,3-unsaturated acid (41). Addition of 2.2 equiv of methyllithium followed by standard work-up gave a 71% yield of the methyl ketone (42 equation 31). ... 

Sometime later, Matsumoto and Shirahama demonstrated that when humulene-4,5-epoxide (150) was treated with BF3-OEt2 in HOAc at -50 C, the only product isolated was the bicyclohumulenediol (156), in 70% yield. The formation of (156) was rationalized as occurring via the CC-conformation (155 equation 70) of the epoxide, whereas the production of the africanol system (152) occurs via the corresponding CT-conformation (151) of (150). 

Appropriate manipulations of the tricycle (152a) resulting from transannulation of humulene-4,5-epoxide permitted Roberts and Shirahama and their respective collaborators to access africanol (154) and 8-oxysenoxyn-4-en-3-one (157). Shirahama etalP have also described the in vitro conversion of africanol (154) to dactylol (158), found in the sea hare Aplsysia dactyloma, via the novel and unusual cyclopropane-sliding reaction, summarized in Scheme 19. 

The last step in the synthesis of sesquiterpenoids africanol (44) and isoafricanol (45) employed the Simmons-Smith cyclopropanation with diethylzinc/diiodomethane. The addition occurred from the less hindered convex face bearing a hydroxy group to give the sesquiterpenes in good yield. ... 

Reaction of the 4,5-monoepoxide of humulene 8 with boron trifluoride-diethylether complex or trimethylsilyl triflate yielded two isomeric alcohols 6,6,9-trimethyltricyclo[6.3.0.0 ]un-dec-9-en-5-ol 9 and 6,6,9-trimethyltricyclo[6.3.0.0 ]undec-8-en-5-ol 10, in a 1 1 proportion with yields of 70 and 80%, respectively. The isomeric alcohols have the structure of the naturally occurring sesquiterpenes africene and africanol and of some of their derivatives. 

Alternatively, the positive charge appearing at C-l can be attacked by the 8,9-double bond according to Markovnikov s rule followed by nonregioselective proton elimination so that tricycles 8 and 9 were isolated73. Several further reactions led to natural products such as africanol or dactylol74. 

Sinoacutine, sinomenine see morphinan alkaloids. Sinularene see africanol. 

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