Double Condensation of 3-Coumaranone with Aromatic Carbonyl Compounds Catalyzed by Brønsted Hyperacids ^†

Abstract

Condensation of two molecule of 3-coumaranone with aromatic carbonyl compounds is catalyzed by PFAT under microwave irradiation, new dicoumararan-3-ones are obtained.

1. Introduction

The acidic condensation of 3-coumaranone (benzofuran-2(3H)-one) with aromatic carbonyl compounds (aldehydes, diones) conduct to different products depending on the acidity of the catalyst. With a poorly Bronsted acid such as clay, aurons (3-benzylidenebenzofuran-2(3H)-one and (or) 3-(hydroxyl(phenyl) methyl)benzofuran-2(3H)-on were formed with microwave activation [1,2].

2. Results and Discussion

We reported herein that double condensation of 3-coumaranone with aromatic carbonyl compounds take place with Brønsted hyperacids. The condensation of 3-coumaranone and 8-hydroxy-3-coumaranone with diones (2a–2d) and aldehydes (2e,2f) were studied in dimethylsulfoxide (DMSO) with pentafluorophenylammonium trifluoromethanesulfonate (PFAT) as a catalyst [3] (Figure 1).

New 3,3′-(phenylmethylene)bis(benzofuran-2(3H)-ones (bicoumaranones) were obtained according to a condensation reaction (Figure 2), and the results are reported

Table 1. Hyperacidic condensation of 3-coumaranones with carbonyl compounds into bicoumaranones.

Entry	Coumaranone	Carbonyl Comp.	Bicoumaranone	Yield (%)
1	1a	2a	3a	70
2	1a	2b	3b	80
3	1a	2c	3c	78
4	1a	2d	3d	62
5	1a	2e	3e	78
6	1a	2f	3f	75
7	1b	2a	4a	60
8	1b	2d	4d	75

. Bicoumaranones were identified by NMR and mass spectrometry (HRMS).

Similar results can be obtained with trifluoromethanesulfonic acid (triflic acid) [4], without a solvent as and a catalyst, under microwave irradiation with a slightly less than good yield.

To our knowledge, bicoumaran-3-one is only cited in one paper in the literature as a by-product [5].

3. Reaction Mechanism

A reaction mechanism (Figure 3) involving the addition of a carbocation to an enolic form is proposed, and this mechanism is very close to that known for the condensation of aldehyde acids with two molecules of 4-hydroxycoumarone to form dicoumarols [6].

4. Experimental

4.1. General Information

The ¹H NMR and ¹³C NMR spectra were recorded on a Bruker AC 400 spectrometer at 400 MHz (Bruker corporation, Billerica, MA, USA).

The samples were recorded in CDCl₃ solutions using TMS as an internal standard. The chemical shifts are expressed in δ units (ppm) and quoted downfield from TMS.

The mass spectra were recorded on Xevo G2-XS QTof Waters (Waters, Milford, MA, USA).

Microwave irradiations were performed at 2450 MHz with an Anton–Paar Monowave 300 (Anton-Paar GmbH, Graz, Austria).

4.2. Starting Reactants

The 6-hydroxybenzofuran-3(2H)-one (1b) was prepared according to the literature [7]. Benzofuran-3(2H)-one (1a), benzil (2a), phenanthraquinone (2b), acenaphthoquinone (2c), isatin (2d), piperonal (2e) and benzaldehyde (2f) are commercially obtained (Alfa).

4.3. Typical Experiment

In a typical experiment, 1a (2 mmol) and 2b (1 mmol) were dissolved in DMSO (1 mL) with PFAT (0.01 mmol). The mixture was irradiated at 160 °C for 10 min. The mixture then become dark red.

Based on the analysis with Xevo G2-XS Qtof Waters: ESI + (M + 23(Na⁺) = 481.1047, corresponding to C₃₀H₁₈O₅; ESI⁻ (M + 23-H) = 457.1071.

5. Conclusions

Aurones are known for their numerous biological properties, and it is likely that 3,3′-(phenylmethylene) bis(benzofuran-2(3H)-ones have interesting properties which will be studied in the near future.