Synthesis of benzo- and naphthoquinonyl boronic acids: exploring the Diels-Alder reactivity.

Substituted 2-quinonyl boronic acids have been synthesised from 1,4-dimethoxy aromatic derivatives in two steps: regiocontrolled boronation and oxidative demethylation. The study of their dienophilic behaviour evidenced that the boron substituent significantly increases the reactivity and triggers an efficient domino process in which the Diels-Alder reaction was followed by a protodeboronation or dehydroboronation, depending on the substitution on both the quinone and diene partners. The boronic acid acts as a temporary controller, opening a direct access to trans-fused meta-regiosomeric adducts when 3-methyl-substituted 2-quinonyl boronic acids react with dienes with a substituent at C-1. A particularly valuable synthetic result was obtained in the reaction between 3,6-dimethyl-2-quinonyl boronic acid and piperylene under an oxygen atmosphere; trans-fused 8a-hydroxy-2,4a,8-trimethyl tetrahydronaphthoquinone was formed directly, in excellent yield and in a highly diastereoselective manner.