Brønsted Acid Promoted Cyclization of Cross‐Conjugated Enynones into Dihydropyran‐4‐ones

Abstract

In triflic acid or sulfuric acid, diaryl‐substituted cross‐conjugated enynones undergo addition of the acid to the carbon–carbon triple bond to afford the corresponding vinyl triflates or sulfates. The vinyl triflates are stable under aqueous workup, whereas the vinyl sulfates are hydrolyzed to α,β‐unsaturated 1,3‐diketones (existing as conjugated enol forms). Extended reaction times lead to cyclization into dihydropyran‐4‐ones with yields of up to 95 %. The protonated forms of the vinyl triflates or sulfates in triflic and sulfuric acid, respectively, are studied as reactive intermediates by NMR spectroscopy. Plausible reaction mechanisms for the formation of dihydropyran‐4‐ones are discussed.

Triflic Acid Promoted Transformations of Linear‐Conjugated Enynones and their Reactions with Arenes. Synthesis of Dihydropyranones, Conjugated Dienones, and Indanes

“…Extending these reactions in NMR tubes till 3 days afforded O‐protonated forms of dihydropyranones Fa , b (Scheme 4, and see spectral data in SI). Similar protonated dihydropyranones, generated from cross‐conjugated enynones (1,5‐diarylpent‐1‐en‐4‐yn‐3‐ones), were observed by ourselves previously [11] . We also took NMR spectra of enynones 1 in neat TfOH, however these spectra had more complex character, they contained impurities of oligomeric products of transformations of these compounds in the superacid.…”

“…Longer reaction time has finally led to cyclization of species E (in its resonance form E′ ) into dihydropyranones 3 through an intermediate formation of species G and F . See also a detailed discussion on mechanism of such dihydropyranone formation in our previous studies [11,13] and in works [17,18] …”

“…Then, reactions of enynones 1 in the superacid TfOH were carried out. Analogously to transformations of other types of linear‐ [9] and cross‐ [11] conjugated enynones in TfOH, one would expect a formation of the corresponding vinyl triflates, as product of addition of TfOH to the acetylenic bond of enynones 1 . We run reactions of several enynones 1 with 1.5 equiv.…”

“…There are just a few examples of reactions of these enynones with nucleophiles, [6–7] and their intramolecular cyclization into various carbocycles [8] . Based on our recent studies on reactions of other types of linear‐ and cross‐conjugated enynones under the superelectrophilic activation conditions [9–12] and on our short preliminary communication on the cyclization of 1‐aryl‐5‐phenylpent‐4‐en‐2‐yn‐1‐ones into 6‐aryl‐2‐phenyl‐2,3‐dihydropyran‐4‐ones in triflic acid TfOH (CF 3 SO 3 H), [13] we undertook a special investigation on TfOH‐promoted transformations of the series of 1,5‐diarylpent‐4‐en‐2‐yn‐1‐ones.…”

Adsorption of neutral and protonated Lewis bases on $$\hbox {Na}_{8}$$ Na 8 -nanocluster: basicity enhancement of the Lewis bases

“…These compounds catalyze many important organic and inorganic reactions in chemical industries and laboratories. 1,2 Hence, design and synthesis of superacids and superbases and study of their structural and chemical properties are still an active research field in chemistry. [3][4][5][6] The main strategy to achieve a Brønsted superbase is to design a stable positively charged protonated molecule which can accommodate the positive charge.…”