Ketenes Pummerer reaction

A recent study demonstrates that trimethylsiloxy sulfides can also be prepared by the reaction of sulfoxides with 0-silylated ketene acetals (Scheme 40). ° Although by definition this transformation is not a sila-Pummerer reaction, it provides the first set of conditions by which compounds (153) are prepared using a silicon activating reagent. 

The Pummerer reaction, whose key step is a [2,3]-sigmatropic rearrangement, has never been observed to lead to efficient transfer of chirality starting from chiral sulfoxides in the presence of acetic anhydride [1632, 1633], A modification via silyloxysulfides, generated with O-methyl-OTBDMS ketene acetal at 65°C, allows asymmetric silicon-induced Pummerer reaction from chiral sulfoxides 10.29 with a high chirality transfer [1634] (Figure 10.11). The ( S)-sulfoxides generate the (5)-secondary ethers and vice-versa. 

Additive Pummerer reactions of the type described above using racemic a,P-unsaturated sulfoxides can be accomplished using the following electrophiles acyl chlorides [205,206], dithioacetic acid [207], acetic anhydride [208], mineral acids/alcohols [209], phosphorus pentachloride [210], silyl ketene acetals/zinc iodide [211], thionyl chloride [212], oxalyl chloride [213], trifluoroacetic acid and its anhydride [214-218], triflic anhydride/sodium acetate [219], and dichloroketene (see below). Selected recent examples of work in this area are presented here. 

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. 

Kita, Y., Shibata, N. Asymmetric Pummerer-type reactions induced by 0-silylated ketene acetals. Synlett 1996, 289-296. 

Kita, Y., Shibata, N., Yoshida, N., Fujita, S. First highly asymmetric Pummerer-type reaction in chiral, non-racemic acyclic sulfoxides induced by 0-silylated ketene acetal. J. Chem. Soc., Perkin Trans. 1 1994, 3335-3341. 

Practical and diversity-oriented synthesis of multi-substituted benzo[lr] furans was accomplished from simple phenols through a Pummerer annula-tion/cross-coupling sequence. Operationally simple and rapid reactions of phenols with ketene dithioacetal monoxides with the aid of trifluoroacetic anhydride provided the corresponding 2-methylsulfanylbenzo[ ]furans (14BCJ1349). 

Williams has demonstrated the use of 1-phenylsulfinyl-l-trimethylsilylethene (164) and phenyl vinyl sulfoxide as effective ketene equivalents for the Diels-Alder reaction [142,143], important as it is known that ketenes do not undergo satisfactory [4+2] cycloadditions. The initial cycloadduct (165), derived from the Diels-Alder reaction between 1-phenylsulfinyl-l-trimethylsilylethene (164) and cyclopentadiene, undergoes a facile sila-Pummerer rearrangement to give the thioacetal (166), which yields the product (167) upon hydrolysis (Scheme 5.55) [142]. 

Silyl ketene acetal adds photolytically to C50 give a-fullerene-substituted carboxylic esters and are used in L-quebrachitol and statine synthesis They induce the first highly asymmetric Pummerer-type reaction of acyclic sulphoxides, giving siloxy(arylthio)alkanes with >99% ee541. 

---