Molecular diversity through novel organosulfur synthons: versatile templates for heterocycle synthesis.

This review highlights some of our recent work on design and development of new synthetic methods for diverse classes of heterocycles employing novel organosulfur synthons i.e. polarized ketene dithioacetals, N, S-acetals, β-oxodithioesters and 2,3-(hetero)aryl-3-(methylthio)acrylonitriles, easily accessible from a wide range of active methylene compounds.

Efficient routes to pyrazolo[3,4-b]indoles and pyrazolo[1,5-a]benzimidazoles via palladium- and copper-catalyzed intramolecular C-C and C-N bond formation.

Efficient synthetic routes to pyrazolo[3,4-b]indoles and pyrazolo[1,5-a]benzimidazoles via intramolecular palladium- and copper-catalyzed cyclization of 1-aryl/1-unsubstituted 5-(2-bromoanilino)pyrazole precursors via intramolecular C-C and C-N bond formation have been reported.

A novel anionic domino process for the synthesis of o-cyanoaryl-methylthio/alkyl/aryl/heteroaryl acetylenes.

A novel unexpected anionic domino process involving n-BuLi-induced rearrangement of 3,3-bis(methylthio) or 3-methylthio-3-aryl/heteroaryl/alkyl-o-bromoarylacrylonitriles to o-cyanoarylacetylenes in synthetically useful yields has been reported. The scope and generality of the reaction has been examined, and a possible mechanism has been proposed.

[4 + 2] cycloaromatization of 4-bis(methylthio)-3-buten-2-one with active methylene ketones: a simple and facile phenol annulation.

A new method for beta-phenol annulation involving base-induced [4C + 2C] cycloaromatization of readily available 4-bis(methylthio)-3-buten-2-one with a variety of cyclic and acyclic active methylene ketones has been reported. Appropriate choice of ketones allows synthesis of diverse frameworks such as dihydroindan, tetrahydronaphthalenes, their higher homologues, dihydro/octahydrophenanthrenes, anthracene, and heteroannulated analogue in high yields with regiocontrol of phenolic functionality.

Domino carbocationic cyclization of functionalized cyclopropyl ketones: facile one-pot access to peri- and angularly fused polycyclic aromatic and heteroaromatic frameworks.

Conjugate adducts obtained by base-induced 1,4-addition-elimination of various aryl/heteroaryl acetonitriles with 1-(2-arylcyclopropyl)-3,3-(bismethylthio)-2-propen-1-ones have been shown to undergo facile acid-induced domino carbocationic rearrangement yielding a variety of substituted tricyclic aromatic and heteroaromatic frameworks in high yields in a one-pot operation. The methodology provides efficient, high-yield routes for synthesis of novel substituted dihydrophenalenes, dihydrobenzo[d,e]anthracene, cyclopenta[a]naphthalene, and fused heteroaromatics such as substituted 4,5-dihydrobenzo[c,d]indole, dihydronaphtho[1,8-b,c]thiophene, dihydroindeno[5,4-b]- and -[4,5-b]-thiophenes, cyclopenta[a]carbazole, and dihydrocyclopenta[e]indazol-3-one derivatives. The probable mechanism of this interesting domino process appears to involve stepwise or concomitant acid-induced ring opening and intramolecular cyclocondensation of cyclopropyl ketones to give benzo-fused arene (or heteroarene) intermediates bearing a reactive benzylic carbocation that is captured intramolecularly either by a preexisting aromatic (or heteroaromatic) ring or by a newly formed benzene ring to give either peri-fused or angularly fused products, respectively.