Bis(1,3,4-thiadiazolo)-1,3,5-triazinium Halides. 2. † Intramolecular Ring Transformation and Synthesis of Novel Highly Substituted Guanidines

Bis(1,3,4-thiadiazolo)-1,3,5-triazinium halides 6 can be easily attacked by nucleophiles at either the C(3a) or the C(4a) position of the central six-membered (cationic) ring. Nucleophilic attack leads to at least two reaction channels, one of which has been previously detected (pathway a) and leads to novel aminals 19. In this paper we report on a second channel (pathway b). Attack of primary or secondary amines 8 at C(3a) or C(4a) in 6 (and their analogues 7) leads to the weakly stabilized intermediates 14. A cascade of several proton shifts, ring openings, rearrangements, and ring closure processes is initiated which finally leads via 17 and 18 to novel highly substituted guanidines 9, 10, 12, and 13. Pathway b seems to be the result of well-balanced negative-hyperconjugative effects in 14 and/or 17 which control the highly selective opening of a relatively stable central 1,3,5-triazinium ring to yield the crucial intermediate 18. Some representatives of the guanidines have been characterized by X-ray analyses. Since some of the guanidines contain one or two chirality centers, an effort was made to investigate the stereochemistry of these compounds.