Acyclic 2-azadienes

Unusual Diels-Alder Reactivity of Acyclic 2-Azadienes... 

Heterocyclic and acyclic azadiene Diels-Alder reaction in total synthesis of nothapodyine B, alkaloid with antiviral activity 98JHC1003. 

Boger D. L. Heterocyclic and Acyclic Azadiene Diels-Alder Reactions Total Synthesis of Nothapodytine B. J. Heterocycl. Chem. 1998 35 1003-1011 Keywords inverse electron-demand Diels-Alder reactions, acyclic azadienes, synthesis of natural products... 

Primary enamines 115 have been obtained by flash thermolysis of adducts 116 at 600 °C in vacuo. A retro-Diels-Alder reaction occurs to give vinylamine (R = R1 = R2 = H). These primary enamines have been identified by their spectroscopic properties at — 80 °C. At temperatures > 80 °C isomerization to the imine occurs as well as self-condensation to various nitrogen heterocycles or acyclic azadienes. 

The pyridine ring system has also been obtained via the [4 + 2] cycloaddition reaction between enamines and acyclic azadienes like a,/ -unsaturated carbodiimides165,166. In this way, compounds like 297 (obtained from the vinyliminotriphenylphosphorane 296... 

The use of a range of modified reaction conditions including the use of protic acids or conventional and nonconventional Lewis acid catalysts, pressure-promoted reaction conditions, - cation-radical catalysts, and dry-state adsorption reaction conditions has been employed to accelerate the 4it participation of sensitive heterodienes in thermally slow or problematic Diels-Alder reactions. The former two techniques have proven useful for promoting the typically poor reactions of simple, unactivated 1-oxa-1,3-butadienes or acyclic azadienes. 

Bis-l,2,4-triazole-3,5-diones such as 110 have also been used in Diels-Alder reactions, and give bispyridazines.171 The pyridazine derivative 111 is formed in quantitative yield from PTAD and 2,7-dimethyl-2,3,5,6-octatetraene,172 and the azadiene, 4-aza-l,3,5-triphenylpenta-2,4-diene, also reacts readily with PTAD to give 112.173 There are many other examples of Diels-Alder additions of ADC compounds to simple acyclic dienes which proceed entirely as expected the above selection has been limited to reactions of synthetic potential and with novel features. 

Acyclic C-acyl imines have recently been studied as dienophiles.32-34 p j example, Prato and coworkers examined the reaction of imines (41) (equation 13) with several cyclic and acyclic 1,3-dienes. Under neutral conditions, (41) is unreactive as a dienophile. However, under Lewis acid catalysis these imines react to afford mixtures of adducts. With 1,3-cyclohexadiene, bicyclic adducts (42) and (43) are produced along with (44) in which the imine has acted as an azadiene. The ratios of these sorts of products are dependent upon the particular imine and diene used. The formation of adducts of type (43) proved to be both regio- and stereo-selective. Product formation in these cases can be rationalized lx>th by concerted and by stepwise ionic mechanisms. ... 

Synthesis. - Cycloadditions. The azadiene (26) behaves as a dienophile with cyclopentadiene but as a diene with acyclic 1,3-dienes and with cyclohexa-... 

The first compound selected for this study was the azadiene 1. Previous studies demonstrated that tetraphenyl-substitution in positions 3 and 5 of the P,y-unsaturated s) tem promotes efficient DPM, ODPM, and 1-ADPM reactions in acyclic 1,4-dienes," P,y-unsaturated aldehydes, and l-aza-1,4-dienes, respectively. Based on these precedents, the 2-azadiene 1 was considered to be a suitable candidate to undergo rearrangement of the di-7l-methane type. Triplet-sensitized irradiation of azadiene 1, using acetophenone, for 25 min, afforded two new products that were identified as the cyclopropyl imine 2 (11%) and the vinylaziridine 3 (3%) (Scheme 1). Compound 2 hydrolyzes to the corresponding cyclopropyl amine during isolation. ... 

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