[20533-29-3] · C10H20OSi · 3-Triethylsilyl-3-buten-2-one · (MW 184.39)
Physical Data: bp 83 °C/7 mmHg.
Analysis of Reagent Purity: 1H NMR spectrum (CDCl3): d 6.58 (d, J = 2 Hz, 1H, CH), 6.11 (d, J = 2 Hz, 1H, CH), 2.15 (s, 3H, CH3), 0.4-1.2 [m, 15H, Si(CH2CH3)3]; IR: 1669 cm-1; mass spectrum: m/e 184 (M+).
Preparative Methods: can be prepared by the reaction of 1-bromo-1-triethylsilylethylene with acetaldehyde.1 This substance was obtained pure only by preparative gas chromatography. The less pure distilled product can be used in annulations.2
Handling, Storage, and Precautions: this material showed no tendency to deteriorate when stored under an argon atmosphere at -20 °C.
One of the most useful synthetic applications of the Michael reaction is the Robinson annulation reaction,3 which is often utilized in the synthesis of steroids and terpenes. In general, the annulation of cyclohexanone with Methyl Vinyl Ketone (MVK) and its homologs, especially under aprotic, kinetic enolate conditions, produces rather poor yields of the desired products.4 This is mainly due to polymerization of the very reactive vinyl ketone under basic conditions, but other factors, including the fact that the intermediate Michael adduct and the original ketone have similar pK
In 1973, Stork and Ganem2 reported that under aprotic conditions, a-silylated vinyl ketones such as 3-triethylsilyl-3-buten-2-one participate in Michael additions with the lithium enolate of cyclohexanone to give, following cyclization of the product with Sodium Methoxide in methanol, the desired octalone in 80% overall yield (eq 1) (see also 3-Trimethylsilyl-3-buten-2-one). The silyl group is easily removed during the aldol cyclization step; removal occurs by nucleophilic attack on silicon, resulting in displacement of the ketone enolate. The corresponding reaction with methyl vinyl ketone itself gave less than 5% of the Michael addition product.2
The annulation of 2-alkylcyclohexanones with methyl vinyl ketone and its homologs has also been examined. While condensation at the less substituted a-carbon can be effected using the corresponding enamines,5 the basic conditions required for reaction at the more substituted a-carbon generally result in polymerization of the methyl vinyl ketone and low yields. This type of difficulty has been avoided with the use of alkyl halides instead of methyl vinyl ketone, but a number of steps are then required to transform the added alkyl group into the desired 3-ketoalkyl function.6 Application of the 3-triethylsilyl-3-buten-2-one technology, via reaction with the potassium enolate of 2-methylcyclohexanone in t-butanol, produced the desired functionalized octalone in 60% yield following cyclization using a
Bradley B. Brown
University of California, Berkeley, CA, USA