Photoinduced [2+2] cycloadditions (the Paterno–Büchi reaction) of 1-acetylisatin with enol ethers—regioselectivity, diastereoselectivity and acid catalysed transformations of the spirooxetane products

Abstract

Photoinduced [2+2] cycloadditions (the Paterno–Büchi reaction) of 1-acetylisatin 1 with cyclic enol ethers (furan 2, benzofuran 3, 2-phenylbenzofuran 4, 8-methoxypsoralen 5) and acyclic enol ethers (n-butyl vinyl ether 6 and vinyl acetate 7) afford the spiro(3H-indole-3,2′-oxetane)s 8–21 in benzene in high total yield (82–96%) and with high regio- and diastereoselectivity via the nπ* triplet state of 1 without single electron transfer (SET) involvement. The cycloaddition regioselectivity is determined by the formation of the most stable 1,4-diradical intermediate. As a result, head-to-head products with an acetal structure are formed for the cyclic 1,3-dienes 2–5, while for the acyclic monoalkenes 6 and 7, head-to-tail products are preferentially formed. The diastereoselectivity, which favors the thermodynamically less stable syn-spirooxetanes for all the alkenes 2–7, can be rationalized by considering the sterically more favorable 1,4-diradical conformers suitable for efficient intersystem crossing (ISC) and C–C bond formation according to the Salem–Rowland rules. Acid treatment of the oxetanes derived from furan and benzofuran results in oxetane ring opening and provides a convenient and high yielding access to the 3-(furan-3-yl)indole derivatives.