1.2.4.5- Tetrazine-3-thiones, tetrahydro

Synthetic approaches to representatives of this ring system have been discussed in CHEC-II(1996) <1996CHEC-II(8)496>. Research activity in this area has been considerably extended during the past years. Thus, the basic starting material is a 6,6-disubstituted tetrahydro[l,2,4,5]tetrazin-3-thione 52, which has been converted in three different ways reaction with phenacyl bromides led to 3,3-disubstituted 3,4-dihydro-6-phenyl-2//-thiazolo[3,2-4]-[l,2,4,5]tetrazines 53, reaction of 52 with 1,2-dibromoethane gave 3,4,6,7-tetrahydro-2//-thiazolo[3,2-7][l,2,4,5]tetra-zines 54, whereas transformation of 52 with chloroacetic acid in the presence of sodium acetate yields substituted 3,4-di hydro-1-2//-thiazolo[3,2- 1 [ 1,2,4,5]tctrazin-6(7//)-oncs 55 <2001IJB584> (Scheme 17). Details are shown in Table 2. 

Arylidene-33-diethyl-3,4,6,7-tetrahydro-2//-thiazolo[3,2-/j][l, 4 ]-tetrazin-6-ones 53 have been synthesized in a single step by the condensation of tetrahydro-l, 4 -tetrazin-3-thione 52 with ethyl chloioacetate and aldehydes in the presence of pyridine and piperidine. Condensation of 53 with hydrazine afforded fused derivatives 54 <98IJC(B)819>. 

Treatment of verdazyls (48) with mineral acids resulted in disproportionation to 1,2,3,4-tetrahydro-1,2,4,5-tetrazines (75) and l,6-dihydro-l,2,4,5-tetrazinium salts (76). Here one molecule of the verdazyl is reduced to (75) and the other is oxidized to (76). The mechanism of this reaction has been studied by Polumbrik and his group (72ZOR1925). Heating 3-phenyl-1,2-dihydro-1,2,4,5-tetrazine-6(5/f)-thione (77) in 2N hydrochloric acid led to the isolation of 3-phenyl-l,2,4-triazole-5-thione (78) (77KGS1564). 

Verdazyls of the type 27 are very stable. They are produced by dehydrogenation of 1,4,5,6-tetrahydro-l,2,4,5-tetrazin-3(2/ /)-ones or -thiones 26 [178] ... 

Spiro-l,2,4,5-tetrazines. 4-(p-Methoxyphenyl)-2,2-pentamethyleneimidazoline-5-thione refluxed 72 hrs. in ethanol with hydrazine hydrate in the presence of triethylamine 6-(a-hydrazono-p-methoxybenzyl)-l,2,3,4-tetrahydro-s-tetrazine-3-spirocyclohexane. Y 83%. F. e. s. F. Asinger and W. Leuchtenberger, A. 1974, 157. 

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