Coumarin heteroaromatic

In the Diels-Alder reaction with inverse electron demand, the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group at the oxabutadiene at the 3-position lowers its LUMO dramatically, the cycloaddition as well as the condensation usually take place at room or slightly elevated temperature. There is actually no restriction for the aldehydes. Thus, aromatic, heteroaromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. For example, a-oxocarbocylic esters or 1,2-dike-tones for instance have been employed as ketones. Furthermore, 1,3-dicarbonyl compounds cyclic and acyclic substances such as Meldmm s acid, barbituric acid and derivates, coumarins, any type of cycloalkane-1,3-dione, (1-ketoesters, and 1,3-diones as well as their phosphorus, nitrogen and sulfur analogues, can also be ap-... 

Diketene is of course a masked form of acetoacetic ester, and as such reacts in much the expected manner with a variety of mono- and di-nucleophiles. Aromatic and heteroaromatic amines, and phenols, for example, give acetoacetanilides and aryl acetoacetates the latter can be cyclized in excellent yield to coumarins while reaction of the former with excess of diketene followed by cyclization gives dioxopyridines (e.g. equation 164). Amidines, ureas, thioureas, S- alkylthioureas and carbodiimides also react with diketene to give pyrimidines e.g. equation 165), although in the case of amidines, S- alkylthioureas and carbodiimides the initially formed products are 1,3-oxazines which are converted into pyrimidines on subsequent treatment with acid or base. 

The pyrylium salts (76) are the six-membered oxygen heteroaromatic compounds. The 2- and 4-pyrones [(77) and (78) respectively] and the corresponding benzo analogues [coumarins (79) and chromones (80)] are represented by examples which further illustrate the usefulness of retrosynthetic analysis in the derivation of preparative methods. 

An elegant new synthesis of polycyclic heteroaromatic compounds via quinone-methide intermediates is illustrated in Scheme 60. The required quinone-methides have also been generated from hydroxy-pyridones and -coumarins, and these have been allowed to react with a wide selection of aromatic amines. ... 

Microwave-assisted High-temperature Metalation of Weakly Activated Aromatics and Heteroaromatics. Over the last few decades, microwave irradiation has been used to accelerate numerous organic reactions, including metalations. Organozinc reagents of the type RZnX possess a good thermal stability and tolerate functional groups, even at elevated temperatures. The direct zincation of coumarin requires 7 days at room temperature to achieve 85% conversion. However, under microwave irradiation, the zincation was achieved in over 90% yield (80 °C, 1 h) (eq 7). ... 

Orientation effects in the formation of coumarins via a Pechmaim-type reaction using CNMR have been investigated by A. G. Osborne et al. A mixture of alkylcoumarins 124 was produced from the condensation of 4-acetyl resorcinole 123 with mahc acid. CNMR study showed the production of 10% of 4-hydroxycoumarins as a minor product, whereas the reaction with EAA gives 7-hydroxycoumarins as sole products. The O-proximity effects on the CNMR of DMCs have been studied to reach this conclusion. It can be correlated with similar effects in other aromatic and heteroaromatic systems (Scheme 53) (81T2021). 

In addition to the examples described above, many other heteroaromatic substrates have been employed in A -arylation reactions, including triazines [175], thiophenes and benzothiophenes [176-180], pyridines and pyridones [181-183], quinolines and quinolones [184, 185], coumarins [186, 187], chromenes and chromones [188, 189], pyridazines [190], azaindoles [191], pyrroles andpyrazolo-heteroarenes [192, 193], indazoles [194], indolizinones [195], triazines [196], and... 

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