Catalytic Oxidation of β‐Keto Esters by Manganese(III)/ Cobalt(II) and Consecutive Cyclization to Heterocycles

Abstract

The manganeseA C H T U N G T R E N N U N G (III)-mediated oxidation of b-keto esters becomes catalytic by the addition of a second cobalt(II) oxidant. This radical-initiated oxidation of b-keto esters is general under aerobic condition to produce a-oxo esters, which may consecutively undergo further heterocyclizations, either by intramolecular Diels-Alder reaction to dihydropyrans or by aromatic cyclization to furans depending upon the substitution pattern at the d-carbon. Since acetoacetates are very useful in carbon-carbon bond formation with allylic halides to give various b-keto esters, this one-pot catalytic oxidation and heterocy-A C H T U N G T R E N N U N G clization provides an efficient repertoire for the construction of diverse heterocyclic compounds.

4-Acetyl-2-hydroxy-2,5-dimethylfuran-3(2H)-one

“…The dimeric compound B should be more reactive than Hacac, thus compound B would be further oxidized and followed by capture of the dissolved molecular oxygen to produce the peroxy radical C. Radical C should be reduced by Mn(II), affording the peroxy anions D and the reproduced Mn(III) species [25,26]. The peroxy anion D must be converted to 4-acetylhexane-2,3,5-trione (H) via the 1,2-dioxetane formation [27]. Alternatively, the Mn(III)-enolate complex F would be formed by the reaction of the anion D with Mn(III) species, followed by the Criegee-type oxidation [2,28] accompanying the addition of water to also produce the trione H since the reaction was suppressed under dry conditions (Entries 5, 9).…”

Expeditious Synthesis of Fluorescent Bis(phenylfuryl)benzenes

“…Oxidative decarbonylation of β-ketoester in 4 was performed by the combined catalysts of Mn(OAc) 3 and CoCl 2 in AcOH at 60 °C under air. 5b The corresponding α-oxoesters 6a and 6d were obtained in 26% and 33% yield, respectively. Once again, Paal–Knorr hetero-aromatization was further proceeded after the double oxidative decarbonylation of 4b and 4c with an electron-rich methoxy substituent to directly provide meta -bis(phenylfuryl)benzenes 8b (30%) and 8c (17%), respectively.…”

“…Oxidative decarbonylation of β-ketoester in 3 was carried out by catalytic Mn(OAc) 3 with cocatalyst CoCl 2 in AcOH at 60 °C under air. 5b The peroxy radical, produced by trapping of molecular oxygen from the Mn(III)-promoted carbon radical at the activated α-methinyl carbon of β-ketoester 3 , reacted with the nearby acetyl group to form dioxetane intermediate, which was fragmented to give the corresponding α-oxoester 5a and 5d in 38% and 32% yield, respectively. 10 It was remarkable to note that Paal–Knorr hetero-aromatization had proceeded further after the double oxidative decarbonylation of 3b and 3c containing an electron-rich methoxy substituent to directly give para -bis(phenylfuryl)benzenes 7b (16%) and 7c (40%), respectively.…”

Synthesis of Phenyl-2,2′-bichalcophenes and Their Aza-Analogues by Catalytic Oxidative Deacetylation

“…13 Mn(OAc) 3 /CoCl 2cocatalyzed oxidative deacetylation progressed to give 1,4dicarbonyl compound 17a in 72% yield, where the reaction went through the -radical formation of -keto ester by Mn(III) and its capturing of atmospheric O 2 to give a peroxy radical, which promoted deacetylation via the formation of dioxetane intermediate A. 14 The reduced form of Mn(II) can be oxidized by Co(II), which can be regenerated from the oxidation by atmospheric O 2 .…”