Silyl enol ethers hexamethyldisilazane

In another example, addition of phenyl vinyl selenoxide into a mixture of an indanedione, hexamethyldisilazane, and TMSCl in dichloromethane affords the a-trimethylsiloxyselenide in 70% yield (eq 67). The reaction proceeds through a sequence of in situ formation of TMS silyl enol ether, Michael addition onto the phenyl vinyl selenoxide, and seleno Pummerer rearrangement of the resulting selenoxide. Trifluoroacetic anhydride and various other trialkylchlorosilanes give the same product for this reaction, but in much lower yields. 

Grieco and coworkers19 employed the trimethyliodosilane/hexamethyldisilazane (HMDS) combination, first introduced by Miller and McKean20, to prepare dienol silyl ether 2 (equation 13). This enol silyl ether was converted via the Rubottom reaction to the sensitive a-hydroxy enone A ring of Quassimarin, 3. 

Partial (3, 97) or complete (98) silylation may be obtained. Hydroxyl groups at C-3, 6a, Ip, and 12 are transformed into silyl ethers when treated with hexamethyldisilazane, 0.03 ml, in dry dimethylformamide, 0.06 ml, at 50 °C for 3hr (77). Complete silylation, both of equatorial and axial hydroyxl groups, is obtained in pyridine(dry)-hexamethyldisilazane-trimethyl-chlorosilane, 10 5 2 (v/v), 15 min, room temperature (98). Prolonged reaction times may give enol silyl ethers with keto bile acids. In an analogous reaction VandenHeuvel and Brady have prepared chloromethyldimethylsilyl ethers (99). Silyl ethers are very readily hydrolyzed. 

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