1,3-dipolar cycloaddition 1-azadienes

Kobayashi et al. found that lanthanide triflates were excellent catalysts for activation of C-N double bonds —activation by other Lewis acids required more than stoichiometric amounts of the acids. Examples were aza Diels-Alder reactions, the Man-nich-type reaction of A-(a-aminoalkyl)benzotriazoles with silyl enol ethers, the 1,3-dipolar cycloaddition of nitrones to alkenes, the 1,2-cycloaddition of diazoesters to imines, and the nucleophilic addition reactions to imines [24], These reactions are efficiently catalyzed by Yb(OTf)3. The arylimines reacted with Danishefsky s diene to give the dihydropyridones (Eq. 14) [25,26], The arylimines acted as the azadienes when reacted with cyclopentadiene, vinyl ethers or vinyl thioethers, providing the tet-rahydroquinolines (Eq. 15). Silyl enol ethers derived from esters, ketones, and thio-esters reacted with N-(a-aminoalkyl)benzotriazoles to give the /5-amino carbonyl compounds (Eq. 16) [27]. The diastereoselectivity was independent of the geometry of the silyl enol ethers, and favored the anti products. Nitrones, prepared in situ from aldehydes and N-substituted hydroxylamines, added to alkenes to afford isoxazoli-dines (Eq. 17) [28]. Addition of diazoesters to imines afforded CK-aziridines as the major products (Eq. 18) [29]. In all the reactions the imines could be generated in situ and the three-component coupling reactions proceeded smoothly in one pot. 

There are two types of electrocyclic process which are of considerable value for heterocyclic ring synthesis one of these is 1,3-dipolar cycloaddition, and the second involves a Diels-Alder type addition using some type of azadiene the latter does not in general produce aromatic heterocycles and, important though it is, will not be dealt with here. 

However, its implementation in tandem intramolecular cycloadditions could expand the range of oxadiazoles that participate in the reaction cascade, extend their use to unsymmetrical dienophiles and oxadiazoles, control the cycloaddition regioselectivity, and increase the utility of tandem Diels Alder/l,3-dipolar cycloaddition reactions of such heterocyclic azadienes [196]. 

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