Dedicated to Professor Brindaban Chandra Ranu on the occasion of his 75th birthday.
Abstract
The present article provides a personalized account of our recent work on the synthesis
of substituted and fused five-membered heterocycles using various organosulfur building
blocks, derived primarily through base-mediated condensation of active methylene compounds
with (het)aryl/alkyl dithioesters, which have not been previously explored. We initially
describe the ring-opening transformations of 4-[(methylthio)-(het)aryl-methylene]-2-phenyl-5-oxazolones,
leading to the synthesis of functionalized oxazoles, thiazoles, and bisoxazoles. We
then go on to focus on the synthesis of substituted benzothiophenes, indoles, and
benzofurans, as well as their hetero-fused analogs. These compounds are synthesized
via transition-metal-catalyzed intramolecular C–heteroatom (C–S, C–N, C–O) bond formation
(via cross-coupling or C–H bond functionalization) of various reactive organosulfur
intermediates, derived from base-mediated condensation of 2-bromo(het)arylacetonitriles,
acetates, or desoxybenzoins or the corresponding 2-unsubstituted precursors. Finally,
we highlight the synthetic applications of a new class of previously unexplored organosulfur
building blocks, namely, unsymmetrically substituted 1,3-bis(het)aryl-1,3-monothioketones,
derived via base-mediated condensation of ketones with (het)aryl/alkyl dithioesters,
for the regioselective synthesis of substituted pyrazoles, isoxazoles, thiophenes,
imidazoles, and benzothiophenes.
1 Introduction
2 4-[(Methylthio)-het(aryl)-methylene]-2-phenyl/2-(2-thienyl)-5-oxazolones: Versatile
Templates for the Synthesis of Oxazoles, Thiazoles, and Bisoxazoles
3 Synthesis of Benzothiophenes, Indoles, and Benzofurans via Transition-Metal-Catalyzed
Intramolecular C–Heteroatom Bond Formation
4 1,3-Bis(het)arylmonothio-1,3-diketones and 1,3-Bis(Het)aryl-3-(methylthio)-2-propenones:
Versatile Intermediates for the Regioselective Synthesis of Five-Membered Heterocycles
5 Conclusion
Key words organosulfur building blocks - (het)aryl/alkyl dithioesters - 4-[(methylthio)-het(aryl)-methylene]-2-phenyl-5-oxazolones
- transition metal catalyzed intramolecular C-heteroatom bond formation - 1,3-bis(het)aryl-1,3-monothioketones