ChemInform Abstract: Synthesis and Chemistry of Tetracyclo[8.2.2.2^2,5.2^6,9]-1,5,9-octadecatriene.

ChemInform Abstract

The key intermediate (VI) in the synthesis of the title compound (VII) is prepared from the monoacetal (I) by treatment with (II) and double Horner-Emmons reaction to give (IV), which is transformed to (VI) via the mixture of cis/trans isomers (V) [in the conversion (V)-(VI), deprotection has to precede the introduction of the double bonds to avoid isomerization]. The crucial titanium-induced cyclization of (VI) provides (VII) in reasonable yield, in spite of the considerable strain involved in its formation (e.g. boat conformation of all three rings). Contrary to expectations of possible trishomoaromaticity, the PE spectrum of (VII) (X-ray analysis: space group CZ/c) indicates that there is little interaction between the double bonds. - Many of the reactions of (VII) are governed by the fact that the cavity of the major ring system is too small to permit attack of a reagent from inside the tetracycle. On the other hand, syn addition to one of the double bonds increases the strain energy of the molecule. Thus, while (VII) can be cleanly converted to (VIII) and (X)-(XII), treatment of (VII) or (XII) with TFA and of (VII) with Pd/C in refluxing decalin results in double transannular cyclization giving rise to the products (XIII). Furthermore, it is remarkable that: 1) di- or hexahydro analogues of (VIII) could not be obtained; 2) "anti" opening of the epoxide rings being precluded, compound (XI) is stable towards acids (e.g. 12 M HCl); 3) the reduction of (XIIIc) to (XIIId) could not be achieved; and 4) the attempted dehydrogenation of (VII) yielded the hydrogenated compound (XIIId) instead, probably via disproportionation of (VII) with concomitant formation of an unidentified by-product.