Pyrones, Benzopyrans, etc

On treatment with IBD, tetraketones 213 are transformed to pyrones 215. This reaction probably occurs via intermediate cyclic ene-hemiacetal 214. In contrast, lead tetra-acetate (LTA), which generally behaves similarly to IBD, shows a different reactivity pattern in the oxidation of 213, thereby yielding the isomeric pyrone 216 (80TL1575 83JHC1389). 

6-Bromo-2//-chromene (223) is conveniently prepared by intramolecular cyclization of iodonium salt 222. The latter is obtained by the oxidation of alkenyltrimethylsilane 221 with iodosobenzene [86JCS(CC)1382]. 

N-Acyl-3-(3,4-dihydroxyphenyl)-L-alanines (224) on oxidative cycliza-tion provide a one-step synthesis of 3,4-dihydrocouumarins 225 (70HCA1708). This example, in fact, is based on oxidative cyclization of phenols. 

Although conversion 226 to 228 is quite general, the presence of a methoxyl group at the para position of ring A of the chalcone 226 changes the course of this process. The chalcone 226a, for example, on treatment 

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