Lithiated heterocycles as substrates for dianionic oxy-Cope rearrangements- Chelation accounts for regioselective enolate hydrolysis

Lithiated heterocycles and phenyllithium were tested as alkenyllithium comp onents in the dianionic oxy-Cope rearrangement taking place upon their addi tion to (eta (6)-benzo-cyclobutenedione) tricarbonylchromium (1). It was fo und that, with 5-lithio-2,3-dihydrofuran, the rearrangement and subsequent intramolecular aldol addition takes place diastereoselectively. Treatment o f 1 with phenyllithium, 2-lithio-Nmethylpyrrole, 2-lithiofuran, or 2-lithio thiophene, however, resulted only in the formation of single adducts and pr oximally or distally ring-opened diadducts, presumably because of the activ ation energy required to negate the aromatic character of two added anions. When, therefore, the monovinyl adduct rac-19 was used as starting material , addition both of 2-lithiofuran and of 2-lithiothiophene resulted in the d esired dianionic oxy-Cope rearrangements, followed by intramolecular aldol addition. Remarkably, the intramolecular aldol addition takes place with co mplete regioselectivity, meaning that the bis(enolate) hydrolysis is regios elective. This is explained by a chelate stabilization of one enolate moiet y in preference to the other one, hydrolysis of which is thus highly favore d.