Mechanism and Scope of the Mn^III‐Initiated Oxidation of β‐Ketocarbonyl Compounds: Furan Synthesis

Abstract

Unless the MnIII‐produced carbon‐radical from β‐ketocarbonyl compounds undergoes smooth intramolecular addition to alkenes, it traps molecular oxygen in the reaction medium to produce a peroxy radical, which reacts with the neighbouring carbonyl group to form 1,2‐dioxetane. Thermal decomposition of 1,2‐dioxetane completes the oxidation to produce α‐oxo ester. This oxidation seems to be general at 50 °C under aerobic conditions, and can be catalytic for MnIII in AcOH with ultrasonic irradiation. Thus, the development of a new synthetic method for diversely substituted furans has been accomplished based on a couple of the MnIII‐initiated domino oxidation of β‐ketocarbonyl compounds with a suitable α‐allylic substitution.

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

“…Five β‐keto esters 18 , 21 , 23 , 25 , and 27 were subjected to the catalytic Mn(III)/Co(II) oxidation/cyclization condition in AcOH (0.2 M) at 60 °C for 3 h under air (Table 3). The sequence of Mn(III)‐mediated oxidations, and aromatic cyclization worked well under these catalytic conditions to give rise to various furan derivatives 19 , 22 , 24 , 26 , and 28 in reasonable yields, even though these yields were lower by ∼10% in some cases compared to those under the stoichiometric conditions 1b…”

“…As is described in Scheme , the oxidation is initiated by Mn(III) to produce an α‐carbon radical, which traps molecular oxygen to produce the peroxy radical A 3. We proposed the 1,2‐dioxetane intermediate B in order to explain the formation of α‐oxo ester C 1b,4. Mn(III) can be regenerated by the formation and fragmentation of the intermediate B .…”

“…It was interesting to note that the fate of the double oxidation products, δ‐hydroxy‐β,γ‐unsaturated‐α‐oxo esters was decided by the degree of substitution at the δ‐carbon. Unlike the previous case containing a tert ‐hydroxy group at the δ‐carbon, it is possible to form furans by the aromatic cyclization at a δ‐carbon containing a sec ‐hydroxy group in the series of β‐keto esters in Table 3 1b…”

“…To a stirred solution of 18 1b (0.12 g, 0.49 mmol) in AcOH (2.5 mL) were added Mn(OAc) 3 ⋅2 H 2 O (7 mg, 0.025 mmol) and CoCl 2 (2 mg, 0.010 mmol). The reaction mixture was heated to 60 °C for 3 h under air, and cooled to room temperature.…”

“…Following the standard procedure for 19 , 21 1b (0.115 g, 0.33 mmol) was reacted with Mn(OAc) 3 ⋅2 H 2 O (5 mg, 0.017 mmol) and CoCl 2 (1 mg, 0.0066 mmol) in AcOH (2.5 mL) at 60 °C for 3 h under air to give 22 as a colorless oil; yield: 51 mg (0.16 mmol. 49%).…”