PHOTOCYCLOADDITIONS WITH ALPHA-NAPHTHALDEHYDE AND BETA-NAPHTHALDEHYDE - COMPLETE INVERSION OF DIASTEREOSELECTIVITY AS A CONSEQUENCE OF DIFFERENTLY CONFIGURATED ELECTRONIC STATES

The diastereoselectivity of the [2 + 2] photocycloaddition reaction between aromatic aldehydes and 2,3-dihydrofuran (3) is completely inverted when changing the electronic configuration of the excited triplet from 3(n-pi*) to 3(pi-pi*). Mesitylenaldehyde, which corresponds to the first case, reacts endoselectively, whereas alpha- and beta-naphthaldehyde (with low-lying pi-pi* triplets and reactive n-pi* singlets) react exo-selectively. The configuration of the alpha-naphthaldehyde adduct is proven by means of X-ray structure analysis. This type of spin-correlation effect can be rationalized by comparing the conformation of the reactive biradical intermediates for rapid intersystem crossing and/or bond closure. In the furan (1) case, this effect cannot be observed. All carbonyl compounds investigated here react exo-selectively. A possible explanation for this pronounced selectivity is the assumption of a secondary orbital effect.