Selective O-methylations of 3-[(triphenylphosphoranylidene)acetyl]tetronic and -tetramic acids

Tetronic and tetramic acids react with the cumulated ylide Ph3PCCO to afford 3-[(triphenylphosphoranylidene)acetyl] derivatives 4 and 5. These can be selectively methylated with Meerwein's salt, Me3O+BF4−Me3O+BF4−, either at the 4-O atom of the heterocycle or, in the presence of 1,8-bis(dimethylamino)naphthalene (proton sponge), at the O-atom of the acyl ylide. While the 4-methoxy ylides 6 and 7 undergo Wittig reactions with aldehydes to give the corresponding 3-enoyltetronates and -tetramates as occurring in natural products, the 1′-methoxy isomers 8 and 9 are typically unreactive bisacyl ylides. This was rationalised by an X-ray structural analysis and ab initio calculations. However, the formal Wittig products of 8 and 9 are accessible in two steps via Wittig olefination of ylides 4/5 followed by 1′-O methylation of the resulting alkenes.

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