BASE-CATALYZED REACTIONS OF ANTHRONES WITH DIENOPHILES

Anthrone in the presence of an amine functions as a reactive diene in [4 + 2] cycloadditions. Dienophiles which differ in reactivity over a range of ca. 105give products at 25°C. Thus N-methylmaleimide (NMM) gives cycloadduct within minutes in various solvents containing catalytic triethylamine. The reaction of dimethyl fumarate is slower, and a convenient rate with methyl acrylate requires the use of the dienophile as solvent or a primary amine catalyst. The reactions take a different course in methanol solvent, ultimately leading to Michael adduct. Cycloadducts are detected at short reaction times and are shown to be viable precursors to Michael adducts. A 5-alkoxy group on naphthacene directs Diels-Alder reaction mainly to the unsubstituted central ring. The analogous reaction with naphthacen-5-one and amine catalyst affords the bridgehead hydroxyl product. Dithranol (1,8-dihydroxyanthrone) reacts more rapidly than anthrone and exhibits a strong tendency to give Michael adduct. Possible intramolecular protonation of a carbanion (enolate) intermediate was ruled out by the use of deuterated substrate. The product with NMM is deuterated stereoselectively trans to the anthracenyl ring, implying sterically controlled intermolecular reaction. Similar stereoselectivity was observed for the conversion of anthrone-NMM cycloadduct to Michael adduct in MeOD cosolvent. Cycloadducts may be formed either by stepwise (Michael + aldol) reactions, or by oxyanion accelerated concerted Diels-Alder reactions. Arguments favoring the latter mechanism are presented. The reaction is discussed in the context of other base-induced and base-catalyzed cycloadditions of 1-oxido and 2-oxido 1,3-dienes. © 1990, American Chemical Society. All rights reserved.