The formation of trimethylsilyl ethers of ecdysones. A reappraisal

The hydroxyl groups of 20-hydroxyecdysone react with trimethylsilylimidazole with varying ease, in the positional order 2,3,22,25 greater than 20 greater than 14. The 14alpha-hydroxyl group can only be silylated under forcing conditions. Confusion in silylation procedures has been caused by failure to recognize incomplete reaction. The conclusions are supported by mass spectra. In the presence of a catalyst, and absence of a 14alpha-oxy substituent, enol ethers are readily formed, but the rate is considerably reduced with a C-14 substituent present.