Azodicarboxylic esters cycloaddition

Azobis(2,6-diphenylpyrylium-4-yl) and S, NR congeners, polarography, 60, 97 Azodicarboxylic esters cycloaddition to 2-azabuta-l,3-dienes,... 

The other main source of various pyridopyridazines from pyridines are the [4 + 2] cycloaddition reactions, already mentioned (Section 2.15.8.3), between vinylpyridines and azodicarboxylic esters (79T2027, 79KGS639) or triazolidinediones e.g. 78KGS651). 2-Vinyl-pyridines gave reduced pyrido[3,2-c]pyridazines (370), 4-vinylpyridines gave [3,4-c] analogues, whilst 2-methyl-5-vinylpyridine furnishes a mixture of the [2,3-c] and [4,3-c] compounds. Yields are low, however, and these remain curiosities for practical synthetic purposes. 

The [4 + 2]cycloaddition of 4,5-dicyano-l,3,2-diazaphospholes and acetylenes yields 1,2-azaphos-pholes (Chapter 3.15). Azodicarboxylic esters (R = Et, Pr ) give a 1,1-addition accompanied by a dimerization of the product (Scheme 32) [Pg.799]

The ene reaction of an azodicarboxylate ester was first observed in 196218-19 as a process competing with cycloaddition to dienes, but little additional work has been done on the ene reaction with simple alkenes. An elegant study in 1976 provided evidence for a concerted, suprafacial reaction between dimethyl azodicarboxylate and (S)-(Z)-l-deutero-4-methyl-l-phenyl-2-pentene (15) based on the direction and high level of chirality transfer observed20. 

Various azodicarboxylate esters have been used extensively as dienophiles.129 Several representative examples of this type of cycloaddition are listed in Table 6-III.33-40 As mentioned above, these reactions appear to be concerted processes that retain the 1,3-diene configuration in the adducts.303 The more bulky esters tend to react more slowly with dienes. Thus, dimethylazodicarboxylate reacts 5-6 times more rapidly with cyclopentadiene than does the ethyl ester,423 whereas the fert-butyl compound reacts sluggishly.42b... 

Pyridazine derivatives are also obtained by [4+2] cycloadditions. For instance, the tetrahydropyridazines 15 are produced by a Diels-Alder reaction from 1,3-dienes with azodicarboxylic ester. The pyridazines 17 are produced by addition of alkynes to 1,2,4,5-tetrazines followed by a retro-DiELS-Alder reaction of the adducts 16 with N2 elimination ... 

We were not able to obtain any cycloadduct from unactivated 2-azadienes 139 and esters of acetylenedicarboxylic acid. However, we found that 139 did cycloadd to typical electron-poor dienophiles such as esters of azodicarboxylic acid and tetracyanoethylene (Scheme 62). Thus, diethyl and diisopropyl azodicarboxylates underwent a concerted [4 + 2] cycloaddition with 139 to afford in a stereoselective manner triazines 278 in 85-90% yield (86CC1179). The minor reaction-rate variations observed with the solvent polarity excluded zwitterionic intermediates on the other hand, AS was calculated to be 48.1 cal K 1 mol-1 in CC14, a value which is in the range of a concerted [4 + 2] cycloaddition. Azadienes 139 again reacted at room temperature with the cyclic azo derivative 4-phenyl-1,2,4-triazoline-3,5-dione, leading stereoselectively to bicyclic derivatives 279... 

Mixed substituted orthoesters, e.g. (383 equation 180), can be obtained by addition of alcohols to ketene 0,0-acetals. With the aid of phosphoranes containing a ketene O,(7-acetal structure, various orthoesters, e.g. (384) and (385) (Scheme 70) were prepared. " Cyclic orthoesters (386)-(390) (Scheme 71) are formed in cycloaddition reactions of ketene 0,0-acetals with aldehydes, ketones, 7.848 gj.yj cyanides, oxiranes, a-keto esters, o-diketones or ketones (with irradiation), a,3-unsaturated aldehydes and ketones (under pressure or catalyzed by ZnCh), diazoaceto-phenone, 7 diazoacetone and azodicarboxylate. ... 

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