Facile Construction of Furanoacenes by a Three-Step Sequence Going through Disilyl-exo-cyclic Dienes

Facile synthesis of various benzonaphthofurans was achieved by intramolecular hydroarylation of 1,4-disilyl-2-aryloxy-1,3-enynes followed by cycloaddition with arynes or alkenes and finally desilylaromatization. The three-step transformation can be operated sequentially in one-pot, providing with a range of furanoacenes easily and highly effectively.

The Inverse Demand Oxa-Diels-Alder Reaction of Resorcinarenes: An Experimental and Theoretical Analysis of Regioselectivity and Diastereoselectivity

The Diels-Alder reaction enables introduction of new functionalities onto the resorcinarene skeleton with simultaneous generation of new stereogenic centers and expansion of the internal cavity. We present highly regio- and diastereoselective inverse electron demand oxa-Diels-Alder reactions of resorcinarene ortho-quinone methide with benzofuran and indene, each generating 12 new stereogenic centers. The mechanism and reasons for regioselectivity and diastereoselectivity were analyzed using theoretical calculations (NBO charges, Fukui functions, transition state energies, and thermodynamic stability of the products). Enantiomers were separated, and their configurations were determined by comparison of experimental and theoretical electronic circular dichroism spectra.

Complete and Remarkable Reversal of Chemoselectivity in [4 + 2] Cycloadditions Involving Electron-Poor Indoles as Dienophiles. Diels−Alder versus Hetero-Diels−Alder Processes

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 from the participation of the indole 2,3-carbon-carbon double bond. The concomitant use of zinc chloride and high pressure (16 kbar) induces the primary cycloadduct to react further, and biscycloadducts 11 and 12 are now isolated in high yields, the result of two consecutive [4 + 2] processes on, first, the indole 2,3 C=C bond and, second, the 3-carbonyl unit. The possibility of using two different dienes in a tandem, sequential process is demonstrated by the preparation of tetracycle 13. Interactions between the carbonyl dienophile and Danishefsky diene yield exclusively yet another type of product, namely the gamma-dihydropyranones arising from the sequential [4 + 2] heterocycloaddition, hydrolysis of the silyl enol ether, and loss of methanol. Isolation of the Mukaiyama-type adduct 16 indicates that a stepwise mechanism may be involved, at least under zinc chloride catalysis. N,N-Disubstituted indole-3-glyoxamides undergo the expected, usual Diels-Alder process, with the 2,3 C=C bond acting as dienophile, and cycloadducts of the type 3 are obtained in high yields, regardless of the mode of activation. Remarkably, however, N-monosubstituted indole-3-glyoxamides react almost exclusively as heterodienophiles, the 3-carbonyl unit being now the preferred site of reactivity, and gamma-dihydropyranones of the type 6 are isolated in yields ranging from 72 to 92%. Conformational analysis of the Diels-Alder adducts based on both (1)H NMR spectrometry and X-ray diffraction data indicates that the newly created cyclohexene and cyclohexanone rings adopt a pseudoboat conformation.

Activation of the dienophilicity of indoles in normal electron demand [4 + 2] cycloadditions under high pressure

[reaction: see Text] Activation by either high pressure or a combination of Lewis acid catalysis and high pressure allows indole derivatives to behave as dienophiles in [4 + 2] cycloaddition reactions under mild conditions. The biactivation mode has the highest impact on the stereoselectivity of the reaction. The cycloadducts resulting from these reactions are characterized by boat-shape conformations that bear well-defined orthogonal planes.