: Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate which can be readily activated by transition metal complexes with great face selectivity due to its dual-faced nature and intrinsic angle strain on the alkene. To date, the understanding of transition-metal catalyzed reactions of OBD itself has burgeoned; however, this has not been the case for unsymmetrical OBDs. Throughout the development of these reactions, the nature of C1-substituent has proven to have a profound effect on both the reactivity and selectivity of the outcome of the reaction. Upon substitution, different modes of reactivity arise, contributing to the possibility of multiple stereo-, regio-, and in extreme cases, constitutional isomers which can provide unique means of constructing a variety of synthetically useful cyclic frameworks. To maximize selectivity, an understanding of bridgehead substituent effects is crucial. To that end, this review outlines hitherto reported examples of bridgehead substituent effects on the chemistry of unsymmetrical C1-substituted OBDs.

The present work demonstrates the ability of carboxylic acid-tethered cyclopropanated oxabenzonorbornadienes (CPOBDs) to undergo ring-opening reactions in mild acidic conditions. The optimized reaction conditions involve the use of pTsOH in DCE at 90 °C. Two regioisomers are formed but the reactions are highly regioselective towards type 3 ring-opened products. It was observed that substitution at the C-5 and aryl positions of CPOBD significantly hinders the ring opening reactions leading to decreased yields of ring-opened product, although high regioselectivity for the Type 3 ring-opened product is still maintained. Herein, we report the first examples of acid-catalyzed intramolecular ring-opening reactions of CPOBD with carboxylic acid nucleophiles.