Isocoumarins and a-pyrones

The same reaction for isocoumarin preparation was performed using colloidal catalyst PS-PEO-PC-Pd in dimethylacetamide at 100 °C in the 

Coumarins, Isocoumarins, and a-Pyrones.—A two-step conversion of 2-hydroxyacetophenones into coumarins by a Wittig reaction with ethoxycarbo- 

The a-pyrone analogues (101) have been synthesized by a [24]cycloaddition reaction between cyclic a-keto-enamines (100) and di(thioethyl)ketene/ 5-Enamino- 8-keto-esters, derived from the corresponding acetylenic keto-esters, undergo smooth alkylation to give /35-diketo-esters, which can be cyclized to 4-hydroxy-a-pyrones with hot polyphosphoric acid/  

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Reaction Procedure (Scheme 3.48) Pd(OAc)2 (3 mg, 0.013 mmol), NazCOg (26.5 mg, 0.25 mmol), DMF (5 mL), LiCl (10.6 mg, 0.25 mmol), the allqme (0.5 mmol), and the ester (0.25 mmol) were placed in a 2-dram vial (the isocoumarins and hicyclic pyrones were prepared on twice this scale). The vial was heated in an oil hath at 100 °C for the necessary period of time. The reaction was monitored hy TLC to establish completion. The reaction mixture was cooled, diluted with ether, washed with saturated NH4CI, dried over anhydrous Na2S04, and filtered. The solvent was evaporated under reduced pressure, and the product was isolated by chromatography (EtOAc-hexanes) on a silica gel column. 

In 2007, the Mlura group reported a Rh(III)-catalyzed oxidative coupling of benzoic acids with internal alkynes to the synthesis of isocoumarins via aromatic C-H activation (Scheme 6.24a) [38]. Importantly, the reaction of benzoic acids with alkynes takes place efficiently even with a reduced amount (5mol%) of Cu(0Ac)2-H20 under air (Scheme 6.24b) [5b]. The same group also developed the rhodium-catalyzed coupling of acrylic acids with alkynes to provide corresponding a-pyrone via vinylic C-H bond cleavage (Scheme 6.24c) [5c]. In 2015, Wen and coworkers described a Rh(III)-catalyzed synthesis of... 

The cumulation reactions, which start from carboxylic acids and esters usually lead to the formation of pyrone derivatives. Methyl (Z)-3-iodoacrylate and 3-hexyne gave, for example, 5,6-diethyl-2-pyrone in acceptable yield (4.38.). Inclusion of the acrylate into a six membered ring starting from ethyl 2-bromocyclohexen-l-carboxylate, led to a condensed ring system, giving a partially reduced isocoumarin derivative.50... 

A special group of 8-lactones formally derived from the hydrox-ycinnamic acids are coumarins with a skeleton of 2//-benzopyran-2-one, which is also called chromen-2-one or 5,6-benzo-2-pyrone (8-99). More than 1000 coumarins are found in nature, but only the basic member of the homologous series of coumarins performs as an aromatic compound this is called coumarin. Plant materials also contain a number of non-volatile coumarins substituted with hydroxyl and methoxyl groups and their glycosides. These and other coumarins, generally taking on the role of phytoalexins, such as iso-coumarins, furanocoumarins and pyranocoumarins, are described in Section 10.3.2.5.2. Some isocoumarins are intensely sweet (such as phyllodulcin) or bitter substances (such as 6-methoxymellein),... 

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