Aldol condensation, enolsilanes, with

NMR study, 42 162 oxidation of, 25 327, 328 reaction with ethylene oxide, 35 295-297 Acetals, aldol condensation of enolsilanes, 38 265-273 Acetic acid... 

Circulation flow system, measurement of reaction rate, 28 175-178 Clausius-Clapeyron equation, 38 171 Clay see also specific types color tests, 27 101 compensation behavior, 26 304-307 minerals, ship-in-bottle synthesis, metal clusters, 38 368-379 organic syntheses on, 38 264-279 active sites on montmorillonite for aldol reaction, 38 268-269 aldol condensation of enolsilanes with aldehydes and acetals, 38 265-273 Al-Mont acid strength, 38 270-271, 273 comparison of catalysis between Al-Mont and trifluorometfaanesulfonic acid, 38 269-270... 

In large measure, the problem associated with the execution of a stereoselective aldol condensation has been reduced to the generation of a specific enolate geometry. The recent results of Kuwajima (66a), which demonstrate that enolsilanes may be transformed into boryl enolates without apparent loss of stereochemistry (eq. [53]), should enhance the utility of vinyloxyboranes in stereoselective synthesis. The only current drawback to this procedure is associated with the presence of trimethylsilyl triflate (69), which must be removed from the reaction medium before the aldol condensation. It has recently been established that 69 is an effective catalyst for the aldol process (4). 

A. Aldol Condensation of Enolsilanes with Aldehydes and Acetals... 

In recent organic synthesis, stereoselective aldol condensations has been performed under two different conditions. Under the influence of acid, stabilized enol derivatives, enolsilanes (M = SiMe3), can condense with aldehydes or acetals in a stereoselective fashion [Eq. (12)]. In this reaction the role of the acid is to activate aldehydes or acetals. Alternatively, under basic conditions, the same process can be carried out directly with aldehydes and reactive, preformed metal enolates (M = Li, MgL, ZnL, AIL2, BL2, etc.) of defined geometry. 

A number of methods that utilize enolsilanes directly in the aldol process with either aldehydes or acetals have been developed recently. These reactions are usually performed either in the presence of Lewis acids such as titanium tetrachloride (67) or with fluoride ion (68). Recently trimethylsilyl triflate (CF3S03SiMe3) was found to be an efficient acid catalyst for condensation... 

The remainder of the right-hand fragment was prepared (Scheme 5) in accord with our efforts in the theopederin D synthesis. Keto-aldehyde 21, available through the condensation of the enamine of isobutyraldehyde with acetyl chloride, was subjected to a Krische allylation to provide secondary alcohol 22 in 93% ee. Protection as a triethylsilyl ether and exposure to TMSOTf yielded enolsilane 23. The enolsilane was selected as the nucleo-phile for the fragment-coupling aldol reaction based on Evans studies on the influence of various substituents on complex aldol reactions. While the silyloxy group in aldehyde 20 was expected to direct nucleophiles toward the undesired face of the aldehyde, consistent with the extended Felkin model,the methyl group adjacent to the aldehyde was expected to direct the nucleophile to the desired face of the aldehyde, in accord with the... 

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