Catalytic [3+3] Annulation of β‐Ketoethers and Cyclopropenones via C(sp³)—O/C—C Bond Cleavage under Transition‐Metal Free Conditions

Abstract

Main observation and conclusion The efficient cleavage of carbon‐oxygen (C—O) bond is highly important for the transformation of oxygen‐rich biomass and industry chemicals. Herein, an efficient [3+3] annulation of β‐ketoethers with cyclopropenones in the presence of catalytic base has been developed, which proceeds through the C(sp3)—O bonds cleavage in β‐ketoethers and C—C bond cleavage in cyclopropenones under transition‐metal free conditions. The cleavage of C(sp3)—O bonds in alkyl alkyl ethers and aryl alkyl ethers was realized. The reaction featured excellent functional group compatibility and chemoselectivity, affording various 2‐pyrones in good to excellent yields under mild conditions and simple operation.

DMAP‐Catalyzed [3+3] Annulation of Cyclopropenones with α‐Bromoketones for Synthesis of 2‐Pyrones

“…The overall process is exergonic and releases 58.4 kcal/mol of energy. Of note, an alternative pathway featuring the O ‐attack of the enolate 5 at the carbonyl of cyclopropenone followed by β‐cleavage of three‐membered ring and final ring closure might also be possible [9g–i] . For the verification, the O ‐attack process was computed which located a transition state TS‐3 associated with three‐membered ring opening (Figure 1a, path b).…”

“…The residue was purified via column chromatography (petroleum ether/ethyl acetate=20 : 1) to provide the desired products 3 . Among them, 3 a , [11] 3 b , [9f] 3 c , [9i] 3 d , [9i] 3 f , [9f] 3 g , [9i] 3 h – j , [9f] 3 l – n , [9f] 3 p , [9f] and 3 s [9i] are known compounds.…”

“…In particular, it has been reported that sulfoxonium ylides readily undergo [3+3] annulation with cyclopropenone to construct 2‐pyrones under Rh catalysis [9f] or catalyst free conditions [9g,h] (Scheme 2a). Bai and Chang [9i] very recently have disclosed that β‐ketoethers also undergo [3+3] annulation with cyclopropenones to form 2‐pyrones under the catalysis of t BuOK (Scheme 2b). Compared to β‐ketosulfoxonium ylides and β‐ketoethers, we surmised that the use of α‐bromoketones in the [3+3] annulation with cyclopropenones would also possible, which should significantly upgrade the synthetic method since α‐bromoketones are more readily available substances and are precursors of the two compounds (Scheme 2c).…”

Transition‐metal free synthesis of 2‐pyrones by [3 + 3] annulation of cyclopropenones and sulfur ylides

“…Among these methods, annulation reactions [23, 31–37] have been established as a versatile protocol to access 2‐pyrones, such as [4 + 2] [30, 38–45], [3 + 2 + 1] [46], and [3 + 3] [47–55] annulations. In 1971, Nozaki developed a direct annulation of sulphonium ylides and diphenylcyclopropenone for the formation of 2‐pyrones [56].…”

“…Recently, Jiang and co‐workers reported an Rh(III) catalyzed [3 + 3] annulation reaction between cyclopropenones and sulfoxonium ylides toward 2‐pyrone (Scheme 1, equation a) [53]. In 2021, an efficient [3 + 3] annulation of β‐ketoethers with cyclopropenones in the presence of a catalytic base was developed by Chang and coworkers (Scheme 1, equation b) [47]. Furthermore, a DMPA‐catalyzed [3 + 3] annulation of cyclopropenones with a‐bromoketones for the synthesis of 2‐pyrones was reported by Xu' group in 2022 (Scheme 1, equation c) [55].…”