Phenol condensation with /?-keto esters

The Pechmann synthesis of coumarins via condensation of phenols with / -keto esters also involves an intramolecular hydroxyalkylation, following initial... 

Coumarin synthesis. The Pschorr coumarin synthesis involves the reaction of phenols with -keto esters in the presence of a condensing agent (usually sulfuric acid). In a synthesis of the coumarin mold metabolite aflatoxin M, (3), Biichi and Weinreb found that the usual conditions could not be employed because the phenol (I) is exceptionally sensitive to acidic reagents. However, they effected the desired condensation of... 

The Pechmann synthesis of coumarins via condensation of phenols with keto esters also involves an intramolecular hydroxyalkylation, following initial transesterification, and subsequent dehydration. It was found that H-Beta could successfully replace the sulfuric acid conventionally used as catalyst. For example, reaction of resorcinol with ethyl acetoacetate afforded methylumbelliferone (Figure 11.9), a perfumery ingredient and insecticide... 

The Pechmann Condensation ( or Coumarin Synthesis) allows the synthesis of coumarins by reaction of phenols with p-keto esters. 

Finally, this tricyclic phenol (75) was utilized142 to synthesize the natural metabolite aflatoxin B2 (5) by the condensation with diethyl0-keto adipate, in von Pech-mann fashion, to afford the acyclic ester (79). Upon treatment of the derived acid chloride (80) with Lewis Acid (A1C13), there was obtained a single product which... 

Condensation-cycUzation. Woods developed a simple synthesis of coumarins by condensation of a phenol with a Q-keto ester in the presence of trifluoroacetic acid. However, in the example formulated the position of ring closure is activated by... 

Pechman condensation essentially comprise of the synthesis of coumarins by the interaction of phenols with (3-keto esters particularly in the presence of Lewis acid catalysts as given below ... 

Phosphorus oxychloride (POCI3) has proven useful as a promoter in a range of Pechmann condensations employing sterically encumbered 3-keto ester coupling partners and electronically deactivated phenols, reactions which fail using sulfuric acid as a promoter. Thus POCI3 has been found to successfully promote the condensation of a-benzylacetoacetate and 1-naphthol as well as the reaction of ethyl acetoacetate with 4-acyl-resorcinols. ... 

Steric effects likewise affect the regiochemical outcome of Pechmann condensations with poly-substituted phenols. Whereas resorcinol typically condenses with P-keto-esters to provide 7-hydroxycoumarin products (c/l... 

The Simonis chromone synthesis is the reaction of a phenol la-c with a P-keto ester 2 using an appropriate acid promoter to generate a chromone or benzo-y-pyrone 3 (also called a benzo-l,4-pyrone). While compound 3 is actually a chromenone, for this article, whether the double bond is present or not, the system will be characterized as a chromone. The condensation is related to the Pechmann-Duisberg reaction, which yields coumarins from the condensation of a phenol with a P-keto ester and like its relative, the reaction conditions require the loss of water from the ketone moiety and alcohol from the ester moiety. 

Pechmann Condensation of Various Phenols with jt-Keto Ester Using Zr-CAP-SG as the Catalyst... 

Gomberg, Cone and Kyriakides [105] modified the method by reacting chlorofumaric acid with hot sodium phenoxide, the excess of phenol having been previously removed under vacuum. In this way, a high yield (97 per cent) of chromone-2-carboxylic acid was obtained. More recently, an improved conversion of a thiophenol to a l-thiochromone-2-carboxylic ester was achieved by Bossert [106] by condensation with 3-oxoesters (/S-keto-esters) under the influence of polyphosphoric acid (PPA). 

Amides can also be obtained by AICI3 catalyzed ester amine exchange which proceeds primarily without racemization of chiral centers (eq 24). The reaction of phenols with /3-keto esters is known as the Pechmann condensation (eq 25). Aryl amines are used in the Riehm quinoline synthesis (eq 26) Aromatic systems may be coupled via the Scholl reaction (eq 27) and indole derivatives are prepared in the Stolle synthesis (eq 28). In the Zincke-Suhl reaction, phenols are converted to dienones (eq 29). 3 ... 

The term aromatic polyketide is used to describe phenolic compounds (or derivatives thereol) which are derived, wholly or in part, by the operation of the acyl-polymalonate biosynthetic route. Although fatty acids (and natural products derived therefromi) ate biosynthesized by condensation of an acyl-coenzyme A unit with a variable number of malonate or acetate units, the intermediate p-keto-ester is reduced after each condensation step. For this reason the biosynthesis of fatty acids etc. will not be considered in this Report. 

---