Generation and trapping of α-oxa and oxo o-quinodimethanes for natural product synthesis
Photolysis of 1-o—toluyl-3,4-dihydroisoquinolines gives protoberberine alkaloids through the intermediacy of x-hydroxy quinodimethanes and spirobenzyl isoquinol ines. Ortho trimethylsilylmethyl benzoyl chlorides on treatment with fluoride ions afford ketene quinodimethanes which can be trapped with reactive dienophiles. The reactivity pattern of these intermediates is considered in terms of the frontier molecular orbital theory. The synthetic utility of some ketene-quinodimethane equivalents is discussed. Ortho-quinodimethanes have attracted considerable attention ever since the pioneering work of Cave (ref. 1). Their imense synthetic potential was demonstrated by Oppolzer, Kametani, Vollhardt, Magnus, Ito and others (ref. 2). For some time we have been interested in a-oxa and x-oxo o-quinodimethanes which on trapping can produce target molecules with additional inbuilt structural features, especially the anthracyclines, the lignans and the alkaloids. Some novel aspects of purely chemical interest are also associated with these reactive intermediates e.g., the unique molecular orbital array of ketenequinodimethane can have an intriguing bearing on the reactivity and the regioselectivity in their 41T + 27r cycloadditions. The a-hydroxy quinodimethanes which are readily generated through photoenolisation (ref. 3) can undergo, besides the widely explored cycloaddition, interesting reactions with electrophiles. We have investigated the competition between these two processes in the substrates of the type shown in fig. 2. Here the hydroxyquinodimethane, formed on irradiation, can yield a protoberberine (path A) or a spirobenzylisoquinoline (path B) alkaloid.