Ethers, enol, addition hydrolysis mechanism

The concept of silyl enol ether synthesis via / -elimination from a Brook rearrangement-derived carbanion also appeared in Wicha s studies on additions of 1-phenyl-l//-tetrazol-5-yl (PT) sulfonyl anions to acyl silanes. When PT sulfone 34 was deprotonated in the presence of acyl(triphenyl)silane, ketone 36 was isolated in good yield after hydrolysis of the silyl enol ether intermediate 35. The mechanism involved addition of the... 

The reaction mechanism proposed for the addition of organostannanes [29] is similar to that for organoboronic acids. An example of the reaction of methyl vinyl ketone 42 is outlined in Scheme 3.15. The catalytic cycle involves a cationic rhodium complex G, phenylrhodium H, and oxa-n -allylrhodium I. Stannyl enol ether 44 is formed by the reaction of oxa-n -allylrhodium I with Me3SnBF4, which upon hydrolysis gives the ketone 43. The lower yields in the absence of water were explained by the further reaction of 44 with methyl vinyl ketone 42. The rapid hydrolysis with water may prevent such oligomerization. 

Following the mechanism given in Figure 12.23, the addition of an acetal to a simple enol ether (in contrast to the dienol ether B shown above) leads to a /3-alkoxy acetal. This reaction is known as the Mukaiyama aldol addition. If this is followed by a hydrolysis (of the... 

Allylic azides, e.g., 1, were produced by treatment of the triisopropylsilyl enol ethers of cyclic ketones with azidotrimethylsilane and iodosobenzene78, but by lowering the temperature and in the presence of the stable radical 2,2,6,6-tetramethylpiperidine-/V-oxyl (TEMPO), 1-triso-propylsilyloxy-l,2-diazides, e.g., 2, became the predominant product79. The radical mechanism of the reaction was demonstrated. A number of 1,2-diazides (Table 4) were produced in the determined optimum conditions (Method B 16h). The simple diastereoselectivity (trans addition) was complete only with the enol ethers of unsubstituted cycloalkanones or 4-tert-butylcy-clohexanone. This 1,2-bis-azidonation procedure has not been exploited to prepare a-azide ketones, which should be available by simple hydrolysis of the adducts. Instead, the cis-l-triiso-propylsilyloxy-1,2-diazides were applied to the preparation of cw-2-azido tertiary cyclohexanols by selective substitution of the C-l azide group by nucleophiles in the presence of Lewis acids. 

Sodium Azide/Ammonium Cerium(IV) Nitrate. Silyl enol ethers give a-azido ketones on treament with sodium azide and anhydrous ammonium cerium(IV) nitrate in anhydrous acetonitrile (see Eq. 97).297 325 33i With a glycal, the 2-azido-1-hydroxy nitrate derivative is formed.332 Low yields due to hydrolysis of the silyl enol ether may be improved by use of the triisopropylsilyl (TIPS) derivatives,331 although with a sterically encumbered taxane-derived enol ether the TMS derivative gives higher yields than the TIPS derivative.325 The mechanism is believed to involve addition of an azide radical to the double bond. 

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