Mechanism of the inverse-electron demand Diels–Alder reaction of 2-aminopyrroles with 1,3,5-triazines: detection of an intermediate and effect of added base and acid

Aromatic Heterocycles as Productive Dienophiles in the Inverse Electron-Demand Diels-Alder Reactions of 1,3,5-Triazines

Heterocycles are often found as the structural nucleus in natural products and biological active compounds. Consequently, research toward the discovery and development of novel and efficient synthetic methodologies is of constant interest to organic chemists. Diels-Alder reactions are powerful at forming multiple bonds simultaneously, often with stereoselectivity, and thus are one of the most widely used synthetic methodologies in organic syntheses. Inverse electron-demand Diels-Alder (IEDDA) reactions, a subclass of Diels-Alder reactions, have been developed for the efficient synthesis of various heterocycles, with 1,3,5-triazines used as azadienes. The initial 1,3,5-triazine IEDDA reactions mostly included nonaromatic, electron-rich species such as vinyl ethers, enamines, ynamines, and amidines as dienophiles, producing in high yields pyrimidine derivatives with excellent regioselectivity. We hypothesized that some electron-rich aromatic heterocycles could be studied as potential dienophiles for 1,3,5-triazine IEDDA reactions; 5-aminopyrazoles proved to be productive dienophiles leading to high yields of pyrazolopyrimidines. Subsequently, many studies were reported to investigate the mechanism and scope of this new type of IEDDA reaction. Mechanistically, this new type of IEDDA reaction is quite interesting since it entails two aromatic compounds proceeding through a perceived high energy nonaromatic transition state, leading to a new aromatic compound, a counterintuitive process. Both theoretical and experimental studies provide key insights to this reaction mechanism, with learnings from these studies possibly stimulating unconventional thinking in other areas. Theoretical calculations of these cascade reactions of amino-substituted heterocycles with 1,3,5-triazines indicate that the reactions occur in a stepwise fashion via a highly polar zwitterionic intermediate; elimination of ammonia from IEDDA adducts and subsequent retro Diels-Alder reaction drive the reaction toward the fully aromatized IEDDA products. This amino substituent is critical in determining the regioselectivity and driving the cascade reactions to completion. With regard to reaction scope, many amino-heterocycles such as pyrroles, imidazoles, furans, thiophenes, and indoles all proved to be productive dienophiles for this new IEDDA reaction, leading to various fused-pyrimidines in a single step with moderate to high yields and high regioselectivity. Application of this new IEDDA reaction with 1,3,5-triazines was reported to lead to interesting heterocyclic compounds such as nucleoside natural product nebularine and analogues, as well as fluorine-containing fused pyrimidines with potential for biological activities.Herein, the scope of various aromatic heterocycles as dienophiles in the 1,3,5-triazine IEDDA reaction is reviewed. Moreover, both experimental and theoretical studies of the mechanisms for this interesting cascade IEDDA reaction are discussed. Finally, applications of this new type (aromatic heterocycles as dienophiles with 1,3,5-triazines as azadienes) of IEDDA reaction are also covered.

The Diels-Alder reaction of 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene: a case example of a stepwise cycloaddition

The reaction of 4,6-dinitrobenzofuroxan (DNBF) with 1-trimethylsilyloxybuta-1,3-diene (8) is shown to afford a mixture of [2+4] diastereomeric cycloadducts (10, 11) through stepwise addition-cyclization pathways. Zwitterionic intermediate sigma-adduct 9, which is involved in the processes, has been successfully characterized by (1)H and (13)C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three-parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile-electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, omega, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels-Alder reactions.