Diazadienes

Conjugated 1,3-diazadienes are oxidized by m-chloroperoxybenzoic acid to I-oxa-2,4-diazoles [SS] (equation 80)... 

Diazadienes have been used in organic synthesis for the preparation of various heterocyclic compounds. Alkylation of 1,3-diazadienes 207 and the benz-fused analog 210 at the nitrogen atom by aryl acyl bromides provided the iV-alkyl amidinium bromides 208 and 211, which underwent annulation to the 2,3-dihydro-imidazo[2,l-A]thiazole 209 and imidazo[2,l- ]benzothiazoles 212, respectively (Equations 92 and 93) <2001S741, 2002J(P1)741>. 

In a similar manner, aldazines afford pyridazinones 317 (Scheme 7.104). 1,3-Diazadienes such as l-aryl-4-(dimethylamino)-2-phenyl-l,3-diaza-l,3-butadienes or l-aryl-4-(dimethylamino)-2-methylthio-l,3-diaza-l,3-butadienes react with oxazolones and give rise to pyrimidin-6-ones 319 and 320 as single diastereo-... 

TABLE 7.29. PYRIMIDIN-6-ONES EROM CYCLOADDITION REACTIONS OF SATURATED 5(4i7)-OXAZOLONES WITH 1,3-DIAZADIENES ... 

Diazadienes have been shown by various groups to be suitable precursors of imidazoline derivatives by means of [4 + 1 ] cycloaddition reactions. In 1976 Matsuda and co-workers were able to cycloadd heterodienes 303, available from AMrimethylsilyl benzophenone imine and phenyl isocyanate, with cyclohexyl isocyanide to obtain 305 in 91% yield, after methanolysis of the initial adduct 304 [76JCS(P1)1523] (Scheme 67). 

Burger et al. have largely studied the synthetic utility of 1,3-diazadienes 314 as useful edducts for the construction of five-membered heterocycles... 

Some attention has been paid in our group to 1,3-diazadienes while working with trimethylsilyl imines, since these reagents allowed us to readily prepare a variety of 2-trimethylsilyloxy- and 2-trimethylsilylthio-1,3-dienes 357 (89S228) (Scheme 79). We first tested their potential as heterodienes toward aromatic isocyanates and found that treatment of a dichloromethane solution of 357 and isocyanate at room temperature resulted in a clean conversion into triazinedione and 4-thioxotriazinone derivatives 358 in 84-96% yield. 

Since no cycloaddition reactions between 1,3-diazadienes and electron-rich dienophiles, e.g., enamines, were previously recorded in the literature, we studied the inverse-electron demand [4 + 2] cycloaddition of 357 with acyclic and cyclic enamines derived from pyrrolidine or morpholine (89TL4573 90TH1) (Scheme 80). Thus, pyrimidinones and -thiones 359... 

An example dealing with the intramolecular [4 + 2] cycloaddition of 1,3-diazadienes with a nitrile was provided in 1985 by Foucaud et al. (85JOC771) (Scheme 82). They reacted ester 364 with isocyanides to give intermediates 365, which underwent at room temperature intramolecular cycloaddition producing l,3,5-triazabicyclo[4,3,0]nona-2,5,8-trienes 366 in 60-70% yield. 

Volume 57 of our series consists of five chapters. In the first, J. Bar-luenga and M. Tomas of the University of Oviedo, Spain, summarize the varied methods for the synthesis of heterocycles from azadienes with sections covering 1-azadienes, 2-azadienes, and 1,3-diazadienes, all classes of compounds whose importance has increased enormously in the past few years and to whose chemistry the Oviedo group has contributed most significantly. 

Conjugated 1,3-diazadienes 5 as azomethines with two imino groups are cyclo-oxidized by 3-chloroperoxybenzoic acid to 1,2,4-oxadiazoles 6.237... 

To 1,3-diazadiene 5 (10 mmol) was slowly added dropwise a solution of MC l BA (12 mmol) in anhyd CHClj (20 111L) at rt. An exothermic reaction ensued and decolorization occurred as an indication of the end of the oxidation. The solution was washed with coned itq NaIIC03 (lOmL), H20 (10 ml.,), and then dried (Na2S04). After removing the solvent, the residue was recrystallized (hexane) to give the product 6. 

Diazadienes 370 and 371, formed in situ from the corresponding silyl imine and phenylisocyanate, readily undergo [4 + 2] cycloadditions with enamines derived from butyraldehyde and cyclohexanone leading, after deamination, to pyrimidone 372 and pyrimidothione 373, respectively224. 

Recently, we reported that reactive trifluoroalkyl substituted 1.3-diazadienes in a formal [4+1] cycloaddition form five-membered heterocycles, eg., /BuN-C(Ph)=N-C(CF3)2-SiCl2 [2]. New investigations have shown that this strategy can be extended, with some decisive modifications, to other heterobutadienes, e g., l,4-diaza-l,3-butadienes I... 

In contrast to the 1,3,5-oxadiazines discussed in the previous section, the formation of 1,3,5-thiadiazines by cycloadditions involving a 1,3-diazadiene system is uncommon. 

However, 1,2,4-triazines react as 1,3-diazadienes across the ring positions N-2 and C-5 with alkynylamines, also in a [4+2] cycloaddition ... 

The synthesis of various types of quinazoline derivatives 329-330 by thermal six-electron electrocyclic ring closure of 1-aryl-1,3-diazadienes 328 has been achieved by Rossi et al. (Scheme 72). Thus, N-imidoylimino triphenylphosphorane 327 (X = PPh,) reacted with aliphatic and aromatic aldehydes in boiling xylene to give, through intermediate 328, 3,4-dihydroquinazolines 330 and/or quinazolines 329 in 46-90% yield [90TL(31)903 91T5819]. Similarly, 5,5-dimethyl-N-(A/-arylbenzimidoyl)... 

The addition of ketene to amidines (1,3-diazadienes) leads to [4 - - 2] cycloaddirion to form 5,6-dihydro-4-pyrimidones. When diazabutadienes 360 are employed as the diene constituent, either [2 + 2] 361 or [4 + 2]... 

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