ABSTRACT
The combination of two prostereogenic substrate molecules, the carbonyl and the alkene component, in the course of a Paternò-Büchi reaction leads to a photoadduct with three new stereogenic centers. Control of the relative and absolute configuration of these stereogenic centers is a challenge for synthetic chemistry in that many interesting products could in principle be derived from oxetane precursors. A detailed knowledge of the photophysical properties of the electronically excited compound (which in most cases is the carbonyl addend) is necessary to understand (and predict) the stereochemical result of such a [2+2]-cycloaddition reaction. Therefore, the configuration of the excited state, its lifetime, and IC as well as intersystem crossing (ISC) properties should be known. For clean transformations, the carbonyl group should be the only absorbing chromophore in the reaction mixture (i.e., the product should not absorb at the wavelength used), the solvent should not interfere with the cycloaddition step by competing reactions (e.g., hydrogen abstraction), and the polarity influence of the reaction medium on biradical or photoinduced electron transfer (PET) steps ought to be carefully investigated.
