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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.
An experimental and theoretical investigation on the iridium-catalyzed hydroacylation of C1-substituted oxabenzonorbornadienes with salicylaldehyde is reported. Utilizing commercially available [Ir(COD)Cl]2 in the presence of 5 M KOH in dioxane at 65 °C, provided a variety of hydroacylated bicyclic adducts in up to a 95% yield with complete stereo- and regioselectivity. The mechanism and origins of selectivity in the iridium-catalyzed hydroacylation reaction has been examined at the M06/Def2TZVP level of theory. The catalytic cycle consists of three key steps including oxidative addition into the aldehyde C–H bond, insertion of the olefin into the iridium hydride, and C–C bond-forming reductive elimination. Computational results indicate the origin of regioselectivity is involved in the reductive elimination step.
The relative stereo- and regiochemistry of the racemic title compound, C25H19NO7, were established from the crystal structure. The fused benzene ring forms dihedral angles of 77.3 (1) and 60.3 (1)° with the hydroxy-substituted benzene ring and the nitro-substituted benzene ring, respectively. The dihedral angle between the hydroxy-substituted benzene ring and the nitro-substituted benzene ring is 76.4 (1)°. An intramolecular O—H...O hydrogen bond closes an S(6) ring. In the crystal, weak C—H...O hydrogen bonds connect the molecules, forming layers parallel to (100). Within these layers, there are weak π–π stacking interactions with a ring centroid–ring centroid distance of 3.555 (1) Å.
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.
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