Conversion of Weinreb Amides into Benzene Rings Incorporating the Amide Carbonyl Carbon

Esters, acids and acid chlorides can be converted via the intermediacy of their Corresponding Weinreb amides into benzene derivatives that incorporate the original carbonyl carbon as part of the benzene ring. The process involves treatment of the derived Weinreb amides with 3-butenylmagnesium bromide and an allylic Grignard reagent, followed by ring-closing metathesis, dehydration and dehydrogenation. The dehydration-dehydrogenation can be done under acidic conditions with a mixture of TsOH center dot H2O and DDQ or in two steps with SOCl2/Pyridine, followed by treatment with DDQ. Application of the method to carbohydrates provides a convenient route to C-5 aryl pyranosides.