Azomethine imines intramolecular cycloadditions

Azomethine imine cycloadditions provide access to pyrazolidines, pyrazolines and pyrazoles. Intramolecular cyclizations were first reported in 1970.78 The main method for generation of azomethine im-ines involves reaction of a 1,2-disubstituted hydrazine with an aldehyde or an aldehyde precursor. 

The products derived from intermolecular cycloaddition of sydnones readily lose carbon dioxide. If the original cycloaddition is carried out on a diene, the resulting azomethine imine intermediate can be trapped intramolecularly. Thus, 1,5-cyclooctadiene cycloadded to a sydnone to afford the cycloadduct (168).78b It is theorized that the intermolecular monocycloadduct (169) was formed, lost carbon dioxide, and cycloadded as a cyclic azomethine imine to the remaining C—C double bond. Conjugated dienes have been shown to undergo a similar sequence.87... 

The concept of intramolecular 1,3-dipolar cycloaddition reaction has been extended to include azides, azomethine imines, nitrile oxides, nitrile imines and azomethine ylides. Such reactions are summarized in Table 1. 

This report covers two topics (1) The generation of 2-thioxo-2,4-dihydro-3fT-imidazol-l-ium-l-imides as intermediates in the course of [3+2] cycloaddition reactions of azoalkenes and thiocyanic acid resulting in the formation of l-aminoimidazole-2-thione derivatives some further reactions of these heterocycles are presented as well. (2) The rhodium-catalyzed intramolecular interaction of co-diazenyl a -diazo ketones giving rise to the formation of mostly two cyclic azomethine imine isomers with an exocyclic terminal nitrogen atom and with all three... 

Not long after Kishi s synthesis was completed, Jacobi published an equally impressive campaign featuring the development of an intramolecular 1,3-dipolar cycloaddition of an azomethine imine to a 2-imidazolone to... 

Intramolecular cycloaddition of azomethine imines and olefins has been used to synthesize various diazabicyclic systems (Scheme 29). A double Diels-Alder addition of coumalic acid with butadienes has been used to prepare tricyclo[3,2,l,0 ]oct-3-enes (475). ... 

A cornucopia of synthetically useful 1,3-dipoles have been exploited for the preparation of azacycles and amines. One elegant example is the intramolecular cycloaddition of azomethine imines [48] reported by Jacobi in a total synthesis of the shellfish toxin saxitoxin (20, Scheme 18.5) [49]. The acid-catalyzed condensation of hydrazines and acetals is a mild method for the generation of azomethine imines [50]. In the key cyclization event, the inter-... 

Synthetic work commenced with evaluation of an azomethine ylide dipole for the proposed intramolecular dipolar cycloaddition. A number of methods exist for the preparation of azomethine ylides, including, inter alia, transformations based on fluoride-mediated desilylation of a-silyliminium species, electrocyclic ring opening of aziridines, and tautomerization of a-amino acid ester imines [37]. In particular, the fluoride-mediated desilylation of a-silyliminium species, first reported by Vedejs in 1979 [38], is among the most widely used methods for the generation of non-stabilized azomethine ylides (Scheme 1.6). 

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