Studies for a Variable Synthesis of Colchicinoids: Construction of Ring A on a Heptalene Moiety
作者:Markus Meyer、Khaled Abou-Hadeed、Hans-Jürgen Hansen
DOI:10.1002/1522-2675(20000906)83:9<2383::aid-hlca2383>3.0.co;2-0
日期:2000.9.6
It is shown that heptalene-4,5-dicarboxylates 2, which react with lithiated methyl sulfones mainly in a Michael fashion at C(3) (cf. Scheme-2), so that the formation of 3-sulfonylbenzo[a]heptalene-2,4-diols 5 is repressed or completely suppressed, can be transformed into corresponding pseudo-esters 15 (Scheme 4). These pseudo-esters, on treatment with lithiated methyl sulfones, followed by addition of BuLi, furnish the 3-sulfonylbenzo[a]heptalene-2,4-diols 5 in excellent-to-moderate yields without formation of Michael adducts or their follow-up products (cf. Scheme 5 and 6). The reaction of the pseudo-ester 15a with Li[C-13]H2SO2Ph, followed by treatment with non-labeled FiCH(2)SO(2)Ph and then BuLi, led to the exclusive formation of 3-(phenylsulfonyl)-[1-C-13]benzo[a]heptalene-2,4-diol 5a* (Scheme 9). This experiment demonstrates that the (phenylsulfonyl)acetyl groups at C(4) and C(5) of the heptalene core retain their individual positions in the course of the benzo[a]heptalene-2,4-diol formation. These findings are only compatible with an intramolecular rearrangement mechanism as depicted in Scheme 10.