Coumarins phosphorus compounds

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-... 

The reaction of phenols with alicyclic 1,3-dicarbonyl compounds in phosphorus oxychloride or sulfuric acid also leads to 3,4-fused coumarins (40JA2405). Dehydrogenation to the dibenzopyranone occurs on heating with sulfur or with palladium-charcoal (73CB62). 

Since the phosphate anion is resonance stabilised, nucleophilic substitution of the bromine atom of the coumarin derivative can occur by either of the two free oxygen atoms. Thus two diastereomers will be produced, which may be distinguished between merely by considering the configuration at the phosphorus atom. In a publication it was shown that using the tetrabutyl-ammonium salt of cyclic AMP (cAMP) in acetonitrile the diastereoisomeric ratio was 85 15 in favour of the compound with an S-configured phosphorus atom [10]. 

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