Tricyclic Hexahydrotetrazines

Bicyclic and tricyclic hexahydrotetrazines can also exist in sets four for the bicyclic and three for the tricyclic. The relationship of these sets to those for the monocyclic derivative is shown in Fig. 26. 

Conclusive evidence for the conformational equilibria of the hexahydrotetrazines was obtained from a study of compounds with fused rings, particularly the tricyclic derivatives. 

Whereas the low-temperature spectrum of 479 is too complex for interpretation even at 220 MHz (Fig. 27a), the specifically deuterated derivative 

481 shows clearly the presence of four AB quartets (Fig. 27b). The symmetrical conformers X and Y (see Fig. 26) each give rise to a single quartet (482 and 484), whereas the unsymmetrical conformer Z gives two equal quartets (483).373 This interpretation was confirmed by the l3C-NMR spectrum at 90°C, three singlets are observed for N—C—N, N—C—C and C—C=C. At — 30°C these singlets split into 4, 7, and 6 peaks, respectively (Fig. 28) which (given one overlap) is just what is expected for the three conformers.374 

In contrast, the saturated tricyclic compound 480 exists solely in the all-equatorial form Y.373 

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