Diels-Alder reaction of β-fluoro-β-nitrostyrenes with cyclic dienes

Cycloadditions of 4-Alkenyl-2-aminothiazoles with Nitroalkenes in the Formal Synthesis of Pramipexole: An Experimental and Computational Study

4-Alkenyl-2-dialkylaminothiazoles act as in-out dienes in [4 + 2] cycloaddition reactions with nitroalkenes, furnishing 2-amino-6-nitro-4,5,6,7-tetrahydrobenzothiazoles in moderate to good yields, accompanied by a subsequent 1,3-H migration. These transformations proceed with exquisite site-, regio-, and diastereoselectivity. This strategy is further enriched by revealing a novel route for pramipexole synthesis. The examination of the potential energy surfaces associated with the four possible reaction pathways for the Diels-Alder cycloaddition (relative approach of the diene-dienophile and endo/exo approach of the nitro group) not only aligns with experimental observations but also unveils key mechanistic insights. Specifically, computational analyses uncover the favored pathway yielding 6-nitro-4,5,6,7-tetrahydrobenzothiazoles, with some instances proceeding through a two-step mechanism involving a tandem sequence of chemical processes, and the influence of various factors such as dienophile structure and the approach mode of the nitro group. Additionally, the stabilization of the exo-transition states, particularly facilitated by phenyl substitution in the dienophile, is highlighted. Asynchronicity, dipole moment, and other parameters indicative of polar character further characterize these Diels-Alder reactions. Conceptual DFT calculations underscore the pivotal role of the 1,3-thiazole ring in enhancing dienic activation and dictating regioselectivity, emphasizing interactions between the C5 of the thiazole nucleus and the Cβ atom of the nitroalkenes.

gem-Heteroatom-Substituted Fluoroalkenes as Mimics of Amide Derivatives or Phosphates: A Comprehensive Review

gem-Heteroatom-substituted fluoroalkenes have received little attention despite their great potential in medicinal chemistry or in fine chemistry. Indeed, due to the electronic and steric similarity between the fluoroalkene moiety and the amide bond as well as the high strength of the carbon-fluorine bond, these gem-heteroatom-substituted fluoroalkenes could be envisioned as stable mimics of various important organic functions, such as phosphates, carbamates, S-thiocarbamates and ureas. We present herein an overview describing the syntheses over the last decade of heteroatom-substituted fluoroalkenes in geminal position. This review will be divided into several sections covering each the common following heteroatom: oxygen-, nitrogen-, sulfur-, phosphorus-, boron- and silicon-substituted fluoroalkenes.

An Easy Synthesis of Monofluorinated Derivatives of Pyrroles from β-Fluoro-β-Nitrostyrenes

The catalyst-free conjugate addition of pyrroles to β-Fluoro-β-nitrostyrenes was investigated. The reaction was found to proceed under solvent-free conditions to form 2-(2-Fluoro-2-nitro-1-arylethyl)-1H-pyrroles. The effectiveness of this approach was demonstrated through the preparation of a series of the target products in a quantitative yield. The kinetics of a conjugate addition of pyrrole was studied in detail to reveal the substituent effect and activation parameters of the reaction. The subsequent base-induced elimination of nitrous acid afforded a series of novel 2-(2-Fluoro-1-arylvinyl)-1H-pyrroles prepared in up to an 85% isolated yield. The two-step sequence herein proposed is an indispensable alternative to a direct reaction with elusive and unstable 1-Fluoroacetylenes.