Alkoxy enol silyl ether

Considerable use has also been made of allyl carbonates as substrates for the allylation of Pd enolates.9 The reaction of Pd° complexes with allyl enol carbonates119,120 proceeds by initial oxidative addition into the allylic C—O bond of the carbonate followed by decarboxylation, yielding an allylpalladium enolate, which subsequently produces Pd° and the allylated ketone (equation 22). In like fashion, except now in an intermolecular sense, allyl carbonates have been found to allylate enol silyl ethers (equation 23),121 enol acetates (with MeOSnBu3 as cocatalyst) (equation 24),122 ketene silyl acetals (equation 25)123 and anions a to nitro, cyano, sulfonyl and keto groups.115,124 In these cases, the alkoxy moiety liberated from the carbonate on decarboxylation serves as the key reagent in generating the Pd enolate. 

Stereoselective additions to chiral a- and -alkoxy aldehydes. Lewis-acid-catalyzed additions of enol silyl ethers to chiral ct-alkoxy or (3-alkoxy aldehydes can proceed with high 1,2- and 1,3-asymmetric induction. Moreover, the sense of induction can be controlled by the Lewis acid. Thus BF, which is nonchelating, can induce diastereo-... 

Recently, enol silyl ethers that bear chloro, alkoxy or amino groups on the silicon have been prepared (Scheme 18).4 ... 

Stereoselective reactions with acetals. Noyori et al. (10,438) have used this Lewis acid to promote an aldol-type reaction between enol silyl ethers and acetals and have noted high. syn-selectivity in this process. Molander and Haar report that reaction of acetals with cyanotrimethylsilane promoted by TMSOTf results in a-alkoxy cyanides and that this reaction can be diastereoselective when the acetal is substituted at the 4-position by an alkoxy group. The diastereoselectivity depends on the nature of the acetal and the 4-alkoxy group. Dimethoxy acetals show slight diastereoselectivity, but diisopropoxy and dibenzyl acetals can show diastereoselectivity of 5-10 1. The diastereoselectivity also depends on the type of 4-substituent. Acetoxy and t-butyldimethylsilyloxy groups have no effect on the diastereoselectivity, but methoxy, benzyloxy, and allyloxy groups promote anri-selectivity. Since a metal template is not involved, the diastereoselectivity... 

In each instance, the silyl enol ether approaches anti to the methyl substituent on the chelate. This results in a 3,4-syn relationship between the hydroxy and alkoxy groups for a-alkoxy aldehydes and a 3,5-anti relationship for (3-alkoxy aldehydes with the main chain in the extended conformation. 

For example, substituents in terminal olefins (43) in the resulting nitronates (35) are generally adjacent to the C-6 atom. The alkoxy and siloxy substituents from vinyl ethers and silyl enolates respectively, as well as the amino group from enamines, are oriented in the same positions. In the case of tris-substituted olefins, C-6 is the most crowded atom in the resulting nitronates. 

The dominance of an alkoxy group over a silyl group is manifested in the generation of amides from reaction of oc-silyl enol ethers with chlorosulfonyl isocyanate, followed by hydrolysis. Simple vinylsilanes and alkynylsilanes undergo carbamidation [34],... 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

The most versatile synthesis of w./i-unsaUi rated acyl silanes involves the use of allene methodology, developed by a number of groups14,22. Deprotonation and silylation of allenyl ethers followed by hydrolysis gives rise directly to ,/3-unsaturated acyl silanes via their enol ethers, 1-alkoxy-l-trimethylsilylallenes (Scheme 43). Indeed, the first example of an a,/l-unsaturated acyl silane was prepared by such a route223, as was the first example of an allenic acyl silane (from a l-trimethylsilyl-l-trimethylsilyloxy-l,2,3-alkatriene)22b. 

Silyl enol ethers of acyl silanes have been used in Lewis acid-mediated Mukaiyama reactions with acetals. Treatment of the resulting /1-alkoxy acyl silanes with tetrabutylammonium hydroxide or tetrabutylammonium fluoride gave the corresponding a,/J-unsaturated aldehydes (Scheme 99)210. 

Silyl enol ethers with stereogenic silicon atoms bearing chiral alkoxy groups on silicon, as in 193, induce modest stereoselectivity in peracid epoxidation of the enol double bond306. Aryl n participation has been observed in the epoxidation of the bicylooctene 194307. 

High 1,3- and 1,4-asymmetric induction is possible in addition reactions to chiral P-alkoxy aldehydes. The reaction with allylsilanes catalyzed by TiC shows high diastcr-eofacial selectivity (equation II). Essentially only one. vv -adduct of two possible dias-tereomers is obtained in reactions with silyl enol ethers (equation III). 

Aldol-type reactions. Trityl perchlorate catalyzes an aldol-type reaction between silyl enol ethers and acetals or ketals to give p-alkoxy ketones. The yields are comparable to those obtained with TiCU (6, 594). The iyn-aldol is formed predominantly ( 4 1). 

Alkoxy ketones.8 In the presence of ISi(CH3)3 or (CH3)3SiOTf, silyl enol ethers react with a-chloro ethers to give (3-alkoxy ketones selectively. Evidently ISi(CH3)3 activates only the C—Cl bond. Similarly, the reaction of silyl enol ethers with a-chloro sulfides gives rise only to P-alkylthio ketones. 

A more efficient route to 2-(benzoyloxy)vinyl ether (15) involves (0 °C, THF) conversion of the chiral alkoxy aldehyde 14 to its silyl enol followed by 0-acylation with benzoyl fluoride and a catalytic amount of TBAF (2 mol %) to form a separable mixture of the Z-vinyl ether (81%) and E-vinyl ethers (6%) (eq 8). 

Aldol-type condensation of silyl enol ethers with acetals under the influence of la is rather familiar. Unlike the Mukaiyama aldol reaction, 1-5 mol % loading of la is enough to complete the coupling reaction under mild conditions [20]. This transformation is applicable to the synthesis of a wide variety of / -alkoxy carbonyl substrates and has three characteristic features ... 

Asymmetric synthesis of 1,2-diol derivatives based on asymmetric aldol reactions of a-alkoxy silyl enol ethers with aldehydes has been developed. The reaction of (Z)-2-benzyloxy-l-(5)-ethyl-l-trimethylsiloxyethene with benzaldehyde was conducted in dichloromethane at -78 °C with a chiral promoter consisting of Sn(OTf)2, (5)-l-ethyl-2-[(piperidin-l-yl)methyl]pyrrolidine, and Bu2Sn(OAc)2, to afford the corresponding aldol adduct in 83 % yield with 99 % anti preference. The enantiomeric excess of anti aldol is 96 % [38a]. In the aldol reaction of several kinds of aldehydes, e.g. aromatic,... 

Again, much efficiency was gained by switching from alkoxy to siloxycyclopropanes . Dibromocarbene addition to silyl enol ethers generates cyclopropanes which open to a-bromo a,j5-unsaturated carbonyl compounds on thermolysis or treatment with acid in methanol (equation 137) . It has been shown that this homologation process also works for siloxycyclopropanes obtained by addition of other carbenoids (equation and that it is useful for terpene preparation . ... 

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