Humulene 4,5-epoxide transannular cyclization

Two transannular cyclizations have been reported which lead to isomers of 677. Thus, treatment of 677 or 678 with boron trifluoride etherate gives rise to capnellene (679) Also, conversion of humulene 6,7-oxide (680) to tricyclic epoxide 681 has provided the opportunity for trimethylsilyl triftate-promoted... 

Finally, transannular cyclization involving the A > - rather than the A -double bond in humulene-4,S-epoxide (150) can be achieved by first converting (150) to the corresponding aldehyde (159). Treatment of (159) with TMSOTf is then shown to lead to the fused S,7-ring system (160) in good yield (equation... 

Shirahama H, Hayano K, Kanemoto Y, Misumi S, Ohtsuka T, Hashiba N, Furusaki A, Murata S, Noyori R, Matsumoto T. Conformationally selective transannular cyclizations of humulene 9,10-epoxide. Synthesis of the two skeletally different cyclohumulanoids dl-bicyclohumulenone and dl-africanol. Tetrahedron Lett. 1980 21 4835-4838. 

When the C1-C2 double bond is deactivated, for instance by oxidation of the methyl group to an aldehyde group as in 11, opening of the oxirane ring by trimethylsilyl triflate provided a homoallyl carbenium ion which then underwent transannular Ti-cyclization but with participation of the C8-C9 double bond leading to a mixture of secoprotoilludane derivatives 12 and 13. In this case the rearrangement took place from a different C-C conformation, where in an intermediate step an additional Z/ isomerization of the C1-C2 double bond occurred. Reaction of the 8,9-epoxide of humulene with tin(IV) chloride led initially to formation of a cationic center at C9, but internal 7r-cyclization yielded an alcohol with a hydroazulene structure. ... 

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