Alkylation reactions at the benzo moiety of 2,4-Dimethoxy-3-(phenylsulfonyl)benzo[a]heptalenes – model compounds of colchicinoids

Abstract

Alkylation reactions of 3-(X-sulfonyl)benzo[a]heptalene-2,4-diols (X¼Ph, morpholin-4-yl) and their dimethyl ethers were studied. The diols form with K2CO3/MeI in aqueous media the 1-methylated benzoheptalenes, but in yields not surpassing 20% (Table 1). On the other hand, 2,4-dimethoxybenzo[a]heptalenes can easily be lithiated at C(3) with BuLi and then treated with alkyl iodides to give the 3-alkylated forms in good yield (Table 2). Surprising is the reaction with two equiv. or more of t-BuLi since the alkylation at C(4) is accompanied by the reductive elimination of the X-sulfonyl group at C(3) (Table 3). Most exciting is also the course of 2,4-dimethoxy-3-(phenylsulfonyl) benzo[a]heptalenes in the presence of an excess of MeLi. After the expected exchange of MeO against Me at C(4) (Scheme 6), rearrangement takes place under formation of 4-benzyl-2-methoxybenzo[a]heptalenes and concomitant loss of the sulfonyl group at C(3) (Table 4). In the case of X=morpholin-4-yl, rearrangement cannot occur. However, the intermediate benzyl anions of Type E (Scheme 8) react easily with O2 of the air to build up corresponding benzo[a]heptalene-4-methanols (Table 6).