Chem. Pharm. Bull. 53(1) 60—63 (2005)

construction of various heterocyclic rings. The reactions of pyrylium or thiopyrylium salts, six-membered cationic heteroaromatics containing an oxygen or sulfur atom, with nitrogen-containing nucleophiles to afford a variety of nitrogen-containing heterocyclic compounds provide one of the most effective examples of the ring transformation. Sixmembered heterocycles such as pyridines, quinolines, isoquinolines, acridines and pyridine N-oxides are obtained by the reactions of the pyrylium salts with ammonia derivatives. The reaction of the pyrylium salts with hydrazine affords the pyridinium betaines. The thiopyrylium salts are also converted into these heterocycles with less reactivity. In 1974, Snieckus et al. described the preparation of the monocyclic 4H-1,2-diazepines by the reaction of 2,4,6-triphenylthiopyrylium salts with hydrazine via the ring transformation of the thiopyrylium salts. While the chemistry of the selenopyrylium salts including the ring transformation has not yet been sufficiently examined, we previously succeeded in the practical and facile preparations of the 2-benzoselenopyrylium (1A) and 2benzotelluropyrylium salts (1B), which are seleniumor tellurium-containing six-membered heteroaromatic cation rings. These pyrylium salts (1) easily reacted with various nucleophiles such as the alkoxide ion, amine, Grignard reagent, organocopper reagent, and an active methylene compound to afford the corresponding 1-substituted 1Hisoselenochromenes (2A) or isotellurochromenes (2B) (Chart 1). More recently, we reported the interconversion of the 1-benzoselenopyrylium salts into the 1,3-benzoselenazepines through thermal ring expansion of the 2-azidoselenochromenes. In our knowledge, there are two reports on the syntheses of the 2,3-benzodiazepines, fully unsaturated nitrogencontaining seven-membered heterocycles, as shown in Chart 2. Sharp et al. reported the preparation of 1H-2,3-benzodiazepines (5) from the tosylhydrazones (3) via the diazo intermediates (4) by thermal intramolecular cyclization in 1973. The 5H-2,3-benzodiazepines (8) were obtained by photochemical ring expansion of the isoquinoline N-imides (7) under basic conditions via the presumed tautomerized 1H derivatives. In this paper, we report the one-pot ring conversion of the 2-benzoselenopyrylium salts into the 5H-2,3-benzodiazepines. The starting 2-benzoselenopyrylium salts (9a—f) were prepared by the reported method, while other pyrylium salts (9g, h) were synthesized from 9f as shown in Chart 3. The salt (9f) was reacted with Grignard reagents to give the 1-substituted isoselenochromenes (10g, h), which were treated with triphenylcarbenium tetrafluoroborate (Ph3C BF4 ) to afford the corresponding selenopyrylium tetrafluoroborates (9g, h) in good yields. The reaction of the 3-tert-butyl-2-benzoselenopyrylium salt (9a) with methylhydrazine under a solvent-free condition at 20 °C resulted in the selective nucleophilic attack at the C-1 position on the heterocyclic cation ring to give 1-methylhydrazino-1H-isoselenochromene (11) in very high yield as the sole product in analogy with the amines as described in a previous paper. Phenylhydrazine also reacted with the salt (9a) to afford the 1-phenylhydrazinoisochromene (12). In contrast, when the 2-benzoselenopyrylium salt (9a) was treated with anhydrous hydrazine, the transformation reaction of the selenium-containing six-membered ring skeleton into the 5H-2,3-benzodiazepine (13a) occurred accompany-