Coumarins, Claisen rearrangements

For preparation of allyl coumarins and dihydrofuranocoumarins by tandem Claisen rearrangement-cyclization the usual procedures required vigorous reaction conditions, workup procedures were tedious, and long reaction times led to low yields. The rearrangement of allyloxycoumarins 48 to dihydrofuranocoumarins 49 has been optimized in good yields in a sealed Teflon containers with BF3-ether in N-methyl-... 

A review of Claisen rearrangements in aqueous solution has appeared. The synthesis of natural products utilizing tandem Diels-Alder additions with sigmatropic rearrangement processes has been reviewed, and a brief review of the regioselective synthesis of coumarins, quinolones and thiocoumarins with 3,4-fused pyran or furan ring systems by the Claisen rearrangement has been presented. ... 

The formation of the coumarin (388) from 7-allyloxy-3-methylf avone via a Claisen rearrangement and oxidation with DDQ is of a similar nature to the previous route, though yields are considerably lower (77TL473). 

The Claisen rearrangement has been widely used in the synthesis of natural heterocyclic products. Although much of this work has been in the coumarin series, there are now several alkaloid syntheses in which the Claisen rearrangement features as a key step. Indications are that the Claisen rearrangement will continue to hold an important role in organic chemistry. 

The Claisen rearrangement was also used for the preparation of coumarins and their derivatives. Thus, alkyl 3-acetoxy-2-methylenebutanoate 180 reacts with phenol to afford the ether 181, which rearranges into methylenecoumarin 182 (equation 83) . ... 

Photolysis of coumarin-3-carbonylazides gives nitrenes in the triplet state, and these have been trapped by hydrocarbons and by aniline. Prenyl ethers of 3-hydroxycoumarins undergo an abnormal Claisen rearrangement and give the 4-allyUc compound (193), but, in the presence of an 8-methoxy-group, the side-chain is further rearranged as in (192). ... 

Several naturally occurring 6-substituted and 3,6-disubstituted coumarins have been obtained by regioselective Claisen rearrangements and tandem Claisen-Cope rearrangements (94JCS(P1)3095,3101). 

Unfortunately the 4-hydroxycoumarins, and have not been demonstrated to occur naturally in Ruta. Nevertheless, the results suggest that 3-(l, 1 -dimethylallyl)-coumarins (e.g. and 34) arise by a Claisen rearrangement from the related 4-isopentenyloxycoumarin (33) (Fig. 7). 

Claisen Rearrangements — I. Synthesis of the Coumarin, Pinnarin. Tetrahedron... 

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