O-Silyl ketene acetals

In the presence of Znl2 in acetonitrile the saturated sulfoxide 1262 is converted by the O-trimefhylsilylketene acetal 663 into the sulfide 1263 in 55% yield and HMDSO 7 [54] whereas the unsaturated sulfoxide 1264 affords with excess O-silyl-ketene acetal 663 the bis-addition product 1265 in 45% yield [55, 56] (Scheme 8.22). 

Functionalized silacyclobutanes 16 result from photochemical decomposition of [azido-, isocya-nato- and isothiocyanato-bis(tert-butyl)silyl]diazoacetates 15. They undergo a remarkably facile ring-expansion reaction to cyclic O-silyl ketene acetals 17 even at 60°C. 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

The aldehyde (3.38 mmol) and the cinchonidinium catalyst (40 mg, 16.9 imol) in CH2C12 (0.8 ml) are added to the O-silyl ketene acetal (0.676 mmol), derived from... 

Arylacetate esters have been generated by coupling aryl bromides with enolates generated from O-silyl ketene acetals in the presence of tributylstannyl fluoride. 

Sulfonyl chlorides having an a-hydrogen are unstable under basic reaction conditions and can give variable results [96,97]. For base-labile sulfonyl chlorides, the use of O-silyl ketene acetals as scavengers for HC1 has been recommended [96]. Table 8.7 lists some illustrative procedures for the preparation of sulfonamides from primary amines on solid phase. Further examples have been reported [98-101]. 

As shown earlier (Figure 13.22), silyl ketene acetals can be prepared at -78 °C by the reaction of ester enolates with chlorosilanes. O-Allyl-O-silyl ketene acetals (A in Figure 14.48) are formed in this reaction if one employs allyl esters. Silyl ketene acetals of type A undergo [3,3]-rearrangements rapidly upon warming to room temperature. This variation of the Claisen rearrangement is referred to as the Ireland-Claisen rearrangement. 

Figure 14.51 shows four Ireland-Claisen rearrangements that exhibit simple diastereose-lectivity (see Section 11.1.3 for a definition of the term). The substrates are two cis, trans-iso-meric propionic acid esters. The propionic acid esters in Figure 14.51 are derived from achiral allyl alcohols. This is different from the situation in Figure 14.50. However, these esters contain a stereogenic C=C double bond. Both the esters in Figure 14.51 can be converted into their 7 "-enolates with LDA inpureTHF (cf. Figure 13.16). Silylation affords the two T -con-figured O-allyl-O-silyl ketene acetals A and D, respectively. Alternatively, the two esters of Figure 14.51 can be converted into their Z -enolates with LDA in a mixture of THF and DMPU (cf. Figure 13.17). Treatment with rert-BuMe,SiCl then leads to the Z-isomers B and C of the O-allyl-O-silyl ketene acetals A and D, respectively.

The observed rc-face differentiation of the electrophilic animation process was rationalized by the authors [14b]. NMR Nuclear Overhauser experiments agree with the ( -configuration of the O-silyl ketene acetals 35 and with a j-yn-periplanar disposition of the C -OSi and C2-Ha bonds. Electrophiles E+ , such as Lewis acids-co-ordinated DTBAD, attack 35 preferentially from the less hindered C(a)-Si (back) face (Scheme 17). 

Reactions of trimethylsilyl enol ethers with diazo ketones give cyclopropanes contaminated by ring opened compounds 60,61). Use of the more stable tert-BuMe2Si-derivatives or of Rh2 (0Ac)4 as a catalyst might eventually improve the situation. O-Silylated ketene acetals and O.S-ketene acetals, respectively, did not provide products with cyclopropane structure 61 ... 

Fig. 11.43. Claisen-Ireland rearrangement of two O-allyl-O-silyl ketene acetals. 7ran.v-sclective synthesis of disubstituted and E-selective synthesis of trisubstituted alkenes.

Treatment of chiral, nonracemic vinyl sulfoxides (214) with O-silylated ketene acetal (215) in the presence of a catalytic amount of zinc chloride resulted in an enantioselective additive Pummerer-type reaction, affording the corresponding enantiomerically enriched methyl-4-siloxy-4-sulfenylbuyrate (216) (Scheme 55).122 This is the overall addition of the enolate equivalent to the vinyl sulfoxide. 

A. Highly Enantioselective Pummerer-Type Rearrangement Induced by O-Silylated Ketene Acetals... 

Addition of S-Alkyl, O-Silyl Ketene Acetals to a,/ -Unsaturated Ketones (Scheme 43)... 

Conjugate addition of O-silyl ketene acetals to enones Addition of 1 -methoxy-1 -(r-butyldimcthylsilyloxy)ethylene to cyclohcxcnone proceeds in low yield when catalyzed by TiCI4 or TiCI4/Ti(0-/ -Pr)4, but is effected in 95% yield when catalyzed by 1.0 M LiCI04 in diethyl ether. 

Pummerer rearrangement. The O-silyl ketene acetal-induced rearrangement of sulfoxides also requires a catalyst such as Znl2. 

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