Subject inverse-electron-demand

The initial report of an imine acting as a heterodienophile was briefly mentioned by Alder almost five decades ago. Since that time an ever increasing number of examples of this type of cycloaddition have appeared. The large preponderance of imino Diels-Alder reactions have utilized electron-deficient im-ines, although a few cases of inverse electron demand cyclizations exist. Detailed reviews of this subject have previously been published. ... 

In addition to the [4 + 2] cycloadditions of oxazoles (Section 1) and substituted 1,2,4,5-tetrazines (Section 14), the Diels-Alder cycloadditions of substituted 1,2,4-triazines constitute one of the most thoroughly investigated heteroaromatic azadiene systems capable of 4tt diene participation.3,89 In contrast to the oxazole or sym-tetrazine series, two potential and observed modes of cycloaddition are open to 1,2,4-triazines cycloaddition across C-3/C-6 or C-5/N-2 of the 1,2,4-triazine nucleus, and the former is subject to 1,2,4-triazine substituent control of the observed regioselectivity.90 The complementary addition of electron-withdrawing substituents to the 1,2,4-triazine nucleus generally increases its rate of participation in inverse electron demand Diels-Alder reactions, influences the mode of [4 + 2] cycloaddition (C-3/C-6 versus C-5/N-2 cycloaddition), and controls the observed regioselectivity. In addition, the reactivity of the electron-rich dienophile as well as the reaction conditions, polar versus nonpolar solvent, have a pronounced effect on the observed course of the [4 -I- 2] cycloadditions.89... 

Diels-Alder reactions. The version with inverse electron demand involving a-pyrones and vinyl ethers is subjected to asymmetric induction by a BINOL-ytterbium complex. 

In addition to metal-based catalysts, organocatalysts are also selective promoters of asymmetric Diels-Alder reactions. Several groups reported the use of cinchona alkaloid catalysts in standard Diels-Alder reactions. Deng combined 2-pyrones with a,P unsaturated ketones, while Bernard and Ricci focused on the reactions of vinylindoles with quinones and maleimides. Lectka reported enantioselective inverse electron demand hetero Diels-Alder reactions of ketene enolates and o-benzoquininone diimides catalyzed by a combination of benzoylquinidine and zinc triflate. For example, subjecting diimide 51 to the standard reaction conditions yields cycloadduct 52 as a single stereoisomer, which can be easily converted to... 

Recently pyrimidine substituted alkynes 476 were subjected to intramolecular inverse-electron-demand hetero-Diels-Alder reaction with extrusion of HCN affording fused fluorinated pyridines 478. The reaction proceeds at high temperatures in sealed tubes for small amount of the starting materials [256] or as scalable flow process [257] (Scheme 89)... 

---