Tricarbonyl(3-methoxybenzocyclobutenedione)chromium: Regioselective Nucleophilic Addition Reactions, Dianionic Oxy-Cope Rearrangements, Regioselective Intramolecular Aldol Additions, and a Rare Case of an Anionic 1-Vinylcyclobutenol-Cyclohexadienol R

摘要

Tricarbonyl(3-methoxybenzocyclobutenedione)chromium (rac-3) has a lower symmetry than the corresponding unsubstituted benzocyclobutenedione complex. This has consequences for reaction sequences involving both ketone functionalities. The relative electrophilicity of the two ketone functins was evaluated by partial reduction of rac-3 and by a partial hydrolysis of the bis(ethylene acetal), the precursor of rac-3. In addition, monovinyl Grignard addition was performed. All three experiments indicated a higher electrophillicity of C-1 than of C-2, in agreement with the resonance formulas that can be drawn. Dianionic oxy-Cope rearrangements, followed by intramolecular aldol addition, occurred with remarkable regioselectivity: in five of six cases investigated only one of two expected reaction products was obtained. As an explanatin of this remarkable selectivity we considered a chelation effect of the intermediate bis(enolate) invovling the 3-methoxy substituent. This causes a rare if not the first, case of differentation of two enolate moieties present in the cyclooctane ring. Addition of 2-lithio-3,4-dihydro-2H-pyran did not give rise to a dianionic oxy-Cope rearrangement but instead to a rare example of an anionic 1-vinylcyclobutenol-cyclohexadienol rearrangement. Five compounds presented in this publication have been characterized by crystal structure analyses.