Intramolecular site selectivity in cation radical Diels-Alder cycloadditions of difunctional and trifunctional dienophiles

The cation radical Diels-Alder reactions of several difunctional and trifun ctional dienophiles with 1,3-cyclopentadiene, catalyzed by tris(4-bromophen yl)aminium hexachloroantimonate, were determined. When both of the reactive sites are of the 4-methoxystyrene type, reaction occurs smoothly at both s tyrene moieties to give the bis-adducts. However, when one reactive moiety is of the styrene type and one is of the enol ether type, reaction occurs e xclusively at the styrene moiety to give a mono-adduct, even in the presenc e of a large excess of cyclopentadiene. When three potentially reactive moi eties are included, two of them being of the styrene type and one of the 1, 2-diaryloxyethene type, the reaction is again specific for the styrene moie ties, giving only the bis-adduct. The high site selectivity observed in the se reactions is especially noteworthy in view of the observation that monof unctional molecules containing all three of these moieties are found to be reactive toward Diels-Alder adduct formation under the same reaction condit ions and that these adducts are relatively stable under the reaction condit ions. Copyright (C) 2000 John Wiley & Sons, Ltd.