Photochemical [2+2] cycloadditions of naphthalene acrylic acids: templated and untemplated photoreactivity, selective homo/heterodimerizations, and conformational analysis

Selective homo- and heterodimerization reactions of naphthalene acrylic acids have been achieved in a diastereocontrolled manner with the use of 1,8-dihydroxynaphthalene as a covalent template in photochemical [2 + 2] cycloadditions. When the reactions were run in solution, cycloaddition products were obtained in good yields (64-88%), which were subsequently detached from the template via a transesterification reaction affording the symmetrical and unsymmetrical cyclobutane products in 75-91% yields. A careful examination of the crystal structures of diester 6a and cycloadduct 15a along with detailed powder XRD and ATR-IR spectroscopic studies revealed an intriguing case of conformational isomerism in 15a arising from the different possible orientations of its two ester carbonyls. The rotational barriers between the different conformers of 15a were calculated to be within the range of 4.19-7.10 kcal mol-1, indicative of rapid interconversion between these conformers in solution at room temperature.