Azatrienes

The electiocyclic ring closure of 1-azatrienes to 1,2-dihydropyridines is a favorable process. The possibility of stereoisomerism at the nitrogen atom in certain derivatives (oximes and their benzoates) provides an important argument in favor of the steric and/or stereoelectronic factors discussed above for... 

SS20846A (183) as well as 184 and 185 were isolated from Streptomyces luteogriseus and their structures determined by EIMS, and two-dimensional NMR [456]. SS20846A had been isolated previously from Streptomyces sp. S20846, and was found to inhibit intestinal motility in mice [457]. A recent synthesis of 183 has been reported in which a LiC104 catalyzed stereoselective cycloaddition of a 1-azatriene iron tricarbonyl complex with Danishefsky s diene was used to form the piperidine ring [458]. 

Electrocyclization can also occur when heteroatoms are incorporated into diene, triene, or polyene systems. Most attention has focused on 1-azatriene and 1-oxatrienes, which lead to dihydropyridines and pyrans, respectively. 

Pyridinium ions 45 with iV-acceptor substituents also add O- and N-nucleophiles via C-2 to give 46. This is followed by ring opening, probably in an electrocyclic process, at N/C-2 resulting in the formation of 1-azatrienes 47 Zincke reaction, cf. the corresponding transformations of the pyrylium ion, see p 225) ... 

Sol 3. (a) Electrocyclic ring opening of 1,2-dihydropyridine derivative (I) gives 1-azatriene intermediate (II), which then undergoes [1,7] hydrogen shift to give III. Electrocyclization of III then affords IV. 

Reaction of enaminonitfiles with a, -unsaturated aldehydes catalysed by BF3 OEt2 gives 1-azatrienes which undergo 1,6-electrocyclizations to 1,2-dihydropyridines. A Br0nsted-acid-catalysed asymmetric 6 r-electrocyclization reaction has been reported for the synthesis of enantiomerically enriched 1,4-dihydropyridazines (Scheme 39). ... 

In 2014, it was shown that cross-conjugated 1-azatrienes ([3]-l-azadendralenes) 81 could also participate in the DTHDA cycloaddition (Scheme 2.13) [23]. The 1-azatrienes 81 prepared in situ from aldehydes 80 and amines underwent the initial HDA reaction with tosyl isocyanate to afford the mono-adduct 82 in high yields. The second DA reactions with TCNE and Ph-MI proceeded smoothly at rt or 113 C to produce bis-adducts 83 and 84, respectively, in good yields with high diastereo-ic-facial selectivity for both 83 and 84 and with endo selectivity for 84. 

Mechanistically, as illustrated in Scheme 12.66, this intermolecular process (279 -I- 280 282) involves a tandem Knoevenagel condensation/6jt-electron electrocyclic ring-closure of 1-azatrienes 281 [126,127] to give fused heterocyclic system 282 [128,129]. The regiochemical control [5], head-to-head versus head-to-tail (i.e., the Hickmott-Stille s aza-cycloaddition) [130], is the challenge in this and other related cycloadditions, which can be unpredictable and leads to complex mixtures [4-6,131]. However, in our cycloadditions, the use of a, 3-unsaturated iminium salts has led to head-... 

Meketa, M.L., Weinreb, S.M., Nakao, Y, and Fusetani, N. (2007) Appbcation of a 6-1-azatriene electrocydization strategy to the total synthesis of the marine sponge metabolite ageladine A and biological evaluation of synthetic analogues. J. Org. Chem., 72, 4892 899. 

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