Studies on Tellurium-Containing Heterocycles. Part 20. Reactions of 2-Benzoselenopyrylium Salts and 2-Benzotelluropyrylium Salts with Nucleophiles: Formation of 1-Functionalized 1H-Isoselenochromenes and 1H-Isotellurochromenes

Abstract

Recently, the chemistry of the selenopyrylium [2][3][4] and telluropyrylium salts, [2][3][4][5] six-membered cationic heteroaromatics containing a selenium or tellurium atom, in both fields of organic synthesis and the chemical transformation of organotellurium and organoselenium compounds, has attracted considerable attention. Both of the monocyclic pyrylium 6,7) and 1-benzopyrylium salts [8][9][10] containing these atoms have been prepared, and the chemistry of these compounds has been reviewed. We have also previously reported the practical and facile preparation [11][12][13] and the reactions 11,14,15) of these 1-benzopyrylium salts. More recently, we succeeded in preparing 16) the stable 2-benzotelluropyrylium 17) and 2-benzoselenopyrylium salts, 18) structural isomers of them; the former is a previously unknown heterocyclic ring system. Therefore, this paper deals with the results of the examination of the reactivity of these novel 2-benzopyryliun salts with various nucleophiles using the stable compounds, 3-tert-butyl and (or) 3-unsubstituted derivatives as easily available pyrylium salts. The differences in the reactivity of these pyrylium salts containing a tellurium or selenium atom have received most of our attention thus far. Results and DiscussionReduction of Pyrylium Salts 23) were reported. In all cases, the major products are 4H-pyrans. The one-electron reduction of them when treated with zinc dust [24][25][26] in a degassed aprotic solvent gives the bichalcogenopyran derivatives via the pyranyl radical intermediates. We have previously described that the LiAlH 4 reduction of the 1-benzotelluropyrylium salts 11) and 1-benzoselenopyrylium salts, 14) the regioisomers of the title compounds, afforded a mixture of the corresponding 2H-and 4H-chromenes, respectively; the latter was the major product. In order to compare these behaviors, we first examined the hydride reduction of the 2-benzopyrylium salts (1). The reduction of 1A and 1B with LiAlH 4 in diethyl ether at 0°C afforded the 1H-isoselenochromenes (2A) and 1H-isotellurochoromenes (2B) in moderate yields as the sole products, respectively. The use of THF as a solvent almost gave similar results. The pyrylium salts (1) were also reduced by DIBAL-H to give the 1H-isochromenes (2) in quantitative yields. These compounds (2) were identical with the authentic samples, 16) which were the precursors for the synthesis of the corresponding pyrylium salts (1A) and (1B). The 3-tertbutyl-2-benzopyrylium salts (1Aa) and (1Ba) reacted with excess zinc dust in acetonitrile at 0°C under an argon atmosphere to give 1,1Ј-bis(tert-butylisochromenyl) (3Aa) and (3Ba) in 18 and 15% yield as the sole characterized products, respectively. The MS spectrum of compound 3Aa suggested the diselenide molecular formula of C 26 H 30 Se 2 as it measured a molecular ion at m/zϭ502 ( 80 Se) and the expected isotope pattern for Se 2 . The base peak was m/zϭ251, which was a half of the molecular ion. In addition the HR-MS of 3Aa showed the exact molecular formula. There was no c...

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

“…These pyrylium salts (1) easily reacted with various nucleophiles 9) such as the alkoxide ion, amine, Grignard reagent, organocopper reagent, 10) and an active methylene compound to afford the corresponding 1-substituted 1H-isoselenochromenes (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.…”

“…The starting 2-benzoselenopyrylium salts (9a-f) were prepared by the reported method, 6,9) 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 (Ph 3 C ϩ BF 4 Ϫ ) to afford the corresponding selenopyrylium tetrafluoroborates (9g, h) in good yields.…”

“…9) 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-ing with selenium element extrusion.…”

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

“…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.…”

“…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.…”