Diastereoselektion in der Paternò-Büchi-Reaktion von 1,2-Dicarbonylverbindungen mit a-Cyanenaminen
Reactions of photoexcited 1,2-dicarbonyl compounds with chirally labelled and unlabelled captodative alkenes have been investigated, with special emphasis on their diastereoselection in cycloadditions. Irradiation of methyl phenylglyoxylate in presence of unlabelled alkenes yields to the corresponding rel-(2R, 3R)-head-to-head-oxetanes. Using the chirally labelled (S)-(methoxymethylpiperidinyl)propenenitrile gives rise to two oxetanes, one of which was isolated and identified as (2R, 3R)-head-to-head-oxetane by X-ray cristallography. Additionally, the photoreduction product meso-dimethyl-2,3-diphenyltartrate of the ester was formed in all reactions. The photoreactions of benziles with chirally labelled c,d-alkenes give rise to oxetanes in low yields, in these cases the products of Norrish type I reactions become competitive.