Thermal Ring Expansion of 2-Azidoselenochromenes: First Synthetic Examples of 1,3-Benzoselenazepines

Abstract

Benzothiazepines, [2][3][4][5][6] fully conjugated seven-membered heterocycles containing both the heteroatoms of sulfur and nitrogen, have received much attention from the synthetic, theoretical, and reactive points of view. In contrast, there are only a limited number of reports on the synthesis of their selenium and tellurium analogues, the selenazepine and tellurazepine ring systems; dibenzo [b,f][1,4]-selenazepines 7,8) and -tellurazepines 9) have been prepared by the thermal or photochemical rearrangements of the 9-azidoselenoxanthenes and 9-azidotelluroxanthenes, respectively. However, no monocyclic and benzene ring-fused selenazepines have yet been reported to the best of our knowledge. Recently, we have succeeded in the construction of the first benzotellurazepine ring system, the 1,5-benzotellurazepinines.10,11) Furthermore, we have focused attention on the syntheses and reactions of the telluro- [12][13][14][15][16][17] and selenopyrylium salts, [18][19][20][21] six-membered heterocyclic cations containing a tellurium or selunium element.Since Le Roux and coworkers 22,23) discovered the conversion of pyrylium salts into 1,3-oxazepines through azidopyran intermediates 25 years ago, several thermal and photo rearrangements of the azide compounds, prepared from pyrylium salts and relatives with sodium azide, have been reported for the purpose of synthesizing seven-membered heterocycles. [24][25][26][27] Among them, neither the monocyclic selenazepines nor benzoselenazepines were prepared by any method including the above ring expansion access from the azide compounds. In this paper, we describe the first isolation of stable selenazepines, 2-tert-butyl-1,3-benzoselenazepines, using the thermal ring expansion of 2-azidoselenochromenes, which are readily obtained from 1-benzoselenopyrylium salts. 19,21) The reaction of 1-benzoselenopyrylium salts (1) with NaN 3 is shown in Chart 1. 2-tert-Butyl-1-benzoselenopyrylium tetrafluoroborate (1a) 19) and 2-tert-butyl-4-phenyl-1-benzoselenopyrylium tetrafluoroborate (1b) 21) were treated with 4.0 eq of sodium azide in anhydrous CH 3 CN at room temperature to give the 2-azido-2-tert-butyl-2H-selenochromene (2a) and 2-azido-2-tert-butyl-4-phenyl-2H-selenochromene (2b) in 95% and 85% yields, respectively, in spite of the steric hindrance of the tert-butyl group at the C-2 position. Similarly, 2-azido-2H-selenochromene (2c) was obtained from the parent selenopyryrylium salt (1c) 19) in 90% yield. During this azidation of selenopyrylium salts (1), the preparation of 2-azido derivatives with any other substituents at the C-2 or -4 position failed. For example, the treatment of the 4-ethyl-1-benzoselenopyrylium salts (1d, 21) e 21)) with sodium azide resulted in d-hydrogen elimination of the methylene carbon of the ethyl group to afford the (Z)-ethylideneselenochromenes (3d, e) in 91% and 90% yield, respectively. The stereochemistry of the olefin moiety of 3 was determined by nuclear Overhauser enhancement (NOE) measurement. The NOE was observed between the 4Ј-H and the ar...

Reactions of 2-Benzoselenopyrylium Salts with Hydrazine: A New Easy Access to 5H-2,3-Benzodiazepines

“…More recently, we reported the interconversion of the 1-benzoselenopyrylium salts into the 1,3-benzoselenazepines through thermal ring expansion of the 2-azidoselenochromenes. 11) In our knowledge, there are two reports 12,13) on the syntheses of the 2,3-benzodiazepines, fully unsaturated nitrogencontaining seven-membered heterocycles, as shown in Chart 2. Sharp et al 12) reported the preparation of 1H-2,3-benzodiazepines (5) from the tosylhydrazones (3) via the diazo intermediates (4) by thermal intramolecular cyclization in 1973.…”