Benzotriazole-mediated 4-position derivatization of 2,6-diarylpyrylium cations by electrophiles

Abstract

Recently, we introduced new methodology for the easy indirect electrophilic derivatization of π-electron-deficient heteronium salts [1], utilizing the properties of benzotriazole as an efficient synthetic auxiliary [2]. Benzo [b]pyrylium, xanthylium, thioxanthylium, acridinium and 2',3-bridged pyrylium salts in this two pot, three step procedure yield the corresponding heterocycles, substituted at position 4 to the heteroatom (Scheme 1). Abstract. 2,6-Diaryl-4H-(benzotriazolyl)pyrans (7a-c) on treatment with n-butyllithium undergo smooth lithiation at the position α to the benzotriazolyl moiety. In contrast to fused and bridged pyranyl derivatives, these pyranyl anions react with electrophiles by two routes: i) the expected elec- Benzotriazole-Mediated 4-PositionThe success of this strategy was demonstrated [1] by the preparation of a series of long chain (up to 22 carbons) alkyl-, ω-arylalkyl-, ω-halogenalkyl-, (α,ω-alkylidene)bisheteronium salts, which are of interest for synthetic, physico-organic and pharmaceutical applications [3]. Significantly, only a few short-chain 4-alkyl derivatives of simple, non-bridged pyrylium salts are known; they were synthesized in low yield by condensations of either acetaldehyde or alkyl acetates with aryl methylene ketones [4, p. 884]. Isolated examples of 4-(arylmethyl)-and 4-(hetarylmethyl)-2,6-diarylpyrylium salts were obtained by reaction of pyranylphosphonate carbanions with corresponding aldehydes [5]. 4-Alkylpyrylium salts with a long-chain alkyl group were not previously described. During our expansion of the new methodology to the synthesis of 4-alkyl-2,6-diarylpyrylium cations (6) we found that in some cases the intermediate 2,6-diaryl-4-benzotriazolylpyran-4-yl anions undergo rearrangement by ring contraction, and we now report on this alternative reaction pathway. Results and DiscussionFollowing our normal procedure we prepared 2,6-diaryl-4-benzotriazolylpyran analogues of compound 2 and investigated the behavior of 2,6-diaryl-4-benzotriazolylpyranyl anions in reaction with electrophiles. Easily accessible 2,6-diarylpyrylium tetrafluoroborates (6a-c) [6] underwent facile regiospecific addition of the bentrophilic substitution resulting in various 4-alkyl-and 4-(ω-alkylfunctionalized)-pyrylium salts (12)(13)(14)(15)(16)(17)(18)(19) or ii) pyrylium ring rearrangement of 2,6-diarylpyrylium anions (8a-c), leading to 1,2-diaryl-2,4-cyclopentadien-1-ols (10a-c).