| Abstract
| - The reaction between indole-3-carboxaldehyde 1a or indole-3-glyoxalate 1b and 2,3-dimethylbutadiene under thermal activation leads exclusively to the Diels−Alder cycloadducts resulting fromthe participation of the indole 2,3-carbon−carbon double bond. The concomitant use of zinc chlorideand high pressure (16 kbar) induces the primary cycloadduct to react further, and biscycloadducts11 and 12 are now isolated in high yields, the result of two consecutive [4 + 2] processes on, first,the indole 2,3 CC bond and, second, the 3-carbonyl unit. The possibility of using two differentdienes in a tandem, sequential process is demonstrated by the preparation of tetracycle 13.Interactions between the carbonyl dienophile and Danishefsky diene yield exclusively yet anothertype of product, namely the γ-dihydropyranones arising from the sequential [4 + 2] heterocycloaddition, hydrolysis of the silyl enol ether, and loss of methanol. Isolation of the Mukaiyama-typeadduct 16 indicates that a stepwise mechanism may be involved, at least under zinc chloridecatalysis. N,N-Disubstituted indole-3-glyoxamides undergo the expected, usual Diels−Alder process,with the 2,3 CC bond acting as dienophile, and cycloadducts of the type 3 are obtained in highyields, regardless of the mode of activation. Remarkably, however, N-monosubstituted indole-3-glyoxamides react almost exclusively as heterodienophiles, the 3-carbonyl unit being now thepreferred site of reactivity, and γ-dihydropyranones of the type 6 are isolated in yields rangingfrom 72 to 92%. Conformational analysis of the Diels−Alder adducts based on both 1H NMRspectrometry and X-ray diffraction data indicates that the newly created cyclohexene andcyclohexanone rings adopt a pseudoboat conformation.
|
