Asymmetric [2 + 2] Photocycloaddition Utilizing Thiourea Derivatives as H-bonding Hosts via Host-Guest Chemistry

Photochemical transformations have been demonstrated extensively in natural product synthesis, and are particularly advantageous when mild and efficient C–C and C–O bond formations are employed. Utilizing [2+2] cycloaddition reactions to prepare cyclobutane derivatives, up to four carbon stereocenters can be generated in one synthetic step. Recent reports suggest progress in the development of enantioselective photochemical reactions through intermolecular host-guest pairing in solution through hydrogen bonding with a rigid, chiral host. In order to expand upon this methodology, chinchona-based ureas and thioureas are prepared and investigated as potential chiral hosts. Quinolone-derived substrates are known to undergo facile [2+2] with various substituted alkenes. Their behavior under irradiation, and effect of prepared hosts in the stereoselectivity of the cycloaddition is reported