Reaktionen von Azinen und Iminen (Azomethinen, Schiff'schen Basen) mit Dienophilen

The reactions of dienophilic compounds with azines and imines represent a valuable enlargement to synthetic heterocyclic chemistry. Azines derived from aromatic aldehydes or from hexafluoroacetone undergo the so-called criss-cross addition with 2 mol of a dienophilic compound to yield 1,5-diazabicyclo[3.3.0]octane derivatives (derivatives of pyrazolo[1,2-a]pyrazoles). The addition reaction of equimolar amounts of the same reagents leads to the formation of azomethinimines and pyrazolines. Schiff bases of aromatic aldehydes react with acetylenic dienophiles to give partially hydrogenated pyridine derivatives, and with olefinic dienophiles to give spiro-{pyrrolidine-3,1′(2′H)-(pyrrolo[3,4-c]pyrroles}. Spiro compounds are also formed from pyrazine or phthalazine and maleic anhydride or N-alkylmaleimides. The manifold types of dienophilic addition reactions with the C=N double bond of other heterocyclic compounds are reviewed. Azines and Schiff bases of cyclic ketones, as well as free ketones and enamines, add dienophiles in a Michael- or ene synthesis, with formation of the corresponding ring-substituted products. Reactions of dienophiles with imines substituted on the N-atom by N- or O-containing groups are discussed. Cycloadditions of heterocumulenes are only mentioned for comparison in a few cases. 1. Addition of Dienophiles to Azines 1.1. Aromatic Aldazines 1.1.1. criss-cross Reactions 1.1.2. Higher-molecular criss-cross Adducts 1.1.3. criss-cross Addition of Epoxides 1.2. Aliphatic Aldazines and Ketazines 1.2.1. criss-cross- and Other Additions to Hexafluoroacetone Azine 1.2.2. Reaction of Further Aliphatic Azines with Dienophiles 1.2.3. Addition of Dienophiles to Cycloaliphatic Ketazines and to Cyclohexanecarboxaldazine 2. Addition of Dienophiles to Imines (Schiff Bases, Azomethines) 2.1. Aliphatic Imine Systems 2.1.1. Reaction of Dienophiles with Enamines derived from Ketimines 2.1.2. 1,4-Diene Additions of Dienophiles to Schiff Bases derived from Aromatic Amines 2.1.3. Addition of Maleic Acid Derivatives to produce Spiro-linked Five-membered Ring Systems (Dipolar Spiro-Bisaddition) 2.1.4. Formation of Six-membered Ring Systems by Addition of Acetylenedicarboxylic Esters and Other Dienophiles to Imines (1,4-Dipolar Cycloadditions, Co-cyclotrimerizations) 2.2. Aromatic Imine Systems 2.2.1. Addition of Dienophiles to Polyazabenzenes 2.2.2. Addition of Dienophiles to Other N-Heterocyclic Compounds 2.2.3. Reaction of Dienophiles with 2-Methylquinolines and 2-Methylquinoxalines 3. Reaction of Dienophiles with Imines having an O- or N-Substituent on Nitrogen 3.1. Azomethinimines, Azomethine Ylids, Sydnones, Nitrones, N-Acylimines 3.2. Heterocyclic Imine Oxides, Nitrile Oxides 3.3. Hydrazones 3.4. Ketoximes