Unusual fragmentation of trimethylsilylated enols derived from m‐ and p‐hydroxyacetophenones

When 4‐hydroxyacetophenone is treated with MSTFA the corresponding bis‐trimethylsilylated enol ether (1a) is obtained. The mass spectrum of la is characterized by a [M – 1]+ base peak. Extensive deuteration experiments revealed that the hydrogen is mainly removed from a ring position, but originates also to some extent from the side chain (methylidene group) and even to a very small amount from the hydrogens of the methyl groups of the enolic trimethylsilyl group. A mechanism for this fragmentation behaviour is formulated.