Cationic cyclization, transannular reactions

When the substituent in 10 at C2 is a carboxaldehyde group, i.e. 10b, this cationic cyclopropana-tion reaction was accompanied by transannular cyclization to form a mixture of regiomeric olefins 12b. However, a selective reaction occurred with 10a on silylation with trimethylsilyl triflate in toluene at — 20 °C and subsequent desilylation with potassium fluoride in methanol a mixture of olefins 12a was obtained in 80% yield. Applying these conditions to 10b did not lead to 12 b. Instead, efficient transannular cyclization took place to form the 3,6-secoprotoil-ludane system. These reactions are examples of a regio- and stereoselective transformation of the humulene skeleton into cyclohumulanoids. 

One early example was reported by Chatteijee in 1979 for stereospecific synthesis of the sesquiterpene isocomene by transannular cationic cyclization. The acid-catalyzed cyclization of epoxide 152 through transannular participation of the remote double bond led to the formation of 153 (Scheme 20.37), which was further transformed to isocomene. The reaction was stereoselective since formation of any other diastereomers will lead to the formation of much strained polycyclic systems with trans-fased cyclopentanes. 

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. ... 

The radical cations of diene systems in cyclic molecules are also capable of reaction as demonstrated by Demnth, Roth and their coworkers. They have studied the influence of phase on the photochemical reactivity of some naturally occurring dienes. Thns the irradiation of the diene 10 in homogeneous solution (acetonitrile/water) in the presence of an electron-accepting sensitizer such as cyanonaphthalene (CN) or DCB brings about fraws,cw-isomerization only. However, when the electron transfer reaction is carried out in the presence of sodinm dodecyl sulphate, transannular hydrogen abstraction reactions yield the two prodncts 11 and 12. Similar reactivity is observed with frans-geranyl acetate 13 and all-trawi-famesyl acetate 14. The authors report that these cyclizations are the flrst examples of biomimetic processes brought about under SET conditions. 

A transannular 6-endo reaction takes place in the boron trifluoride-diethyl ether complex induced cyclization of a bicyclic aldehyde. After the Markovnikov-orientated ring-closure step, a tertiary cation is generated, which is stabilized by //-proton loss and tetracycle 18, bearing a cyclopropyl unit, is formed in 67% yield24. In this case an ene reaction is impossible. 

Stereoselective transannular cyclizations to hydroazulene systems are also described. These reactions are initiated by ionization of cyclodecenol to form an allylic cation or by oxirane ring opening in cyclodecadiene monoepoxides. 

Other examples include the free radical polymerization of vinyl isocyanate with subsequent anionic polymerization of the isocyanate side groups, as well as the corresponding polymerization of acrylonitrile or poly(acrylonitrile). Free radical and anionic polymerizations are to be preferred over those initiated by cations, since cationic polymerization frequently tends to give rise to transfer reactions which interrupt the transannulation. For example, in the cyclization of natural rubber with the aid of concentrated acids or Lewis acids, an average of only three transannular rings is obtained [see Equation (25-9)]. The intermolecular elimination of water from poly(vinyl methyl ketone) only produces single, double, and triple rings ... 

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