Sila-Pummerer Rearrangements of Sulfoxides

Whereas conversion of sulfoxides to the corresponding a-acyloxysulfides by acid anhydrides, for example acetic anhydride, the Pummerer reaction [1], has been known for quite a time, the conversion of sulfoxides with silylating reagents via the unstable intermediate O-silyl compounds to a-silyloxysulfides, the Sila-Pummerer reaction is a relatively new reaction, which has recently been reviewed [1—4-]. 

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The synthesis of a-cyclocitral (161) in Scheme 39 illustrates further how trimethylsilyl sulfides can be synthesized and readily converted to aldehydes by the sila-Pununerer reaction. Compound (159) was prepared by a route involving a 2,3-sigmatropic rearrangement of the ylide derived from intermediate U )> The sila-Pummerer rearrangement of the sulfoxide derived from (158) occurred below room temperature. Unfortunately, however, the enhanced rate of the rearrangement to (160) conferred by the methyl substituent on the intermediate sulfoxide was accompanied by greater than normal difficulty in the (2,5-acetal hydrolysis step. 

Cyclization of the sulfoxide 1248 with TMSOTf 20/DlPEA affords a 4 1 mixture of the tetrahydroquinolines 1249 and 1250, in 97% yield, and HMDSO 7 [49]. On heating of the sulfoxide 1251 to 80 °C Brook rearrangement then Sila-Pummerer rearrangement-cyclization gives, via 1252, 17% 1253 [50] (Scheme 8.19). 

Formaldehyde anion synthon ( CHO). The anion of 1 (n-BuLi, THF, 0°) is readily alkylated, particularly by primary halides. The products can he converted into aldehydes under very mild conditions. Oxidation with m-chloroperbenzoic acid gives an unstable sulfoxide, which undergoes an sila-Pummerer rearrangement to an acetal. Addition of water liberates the free aldehyde. Epoxides can also be used as electrophiles.2 3 Example ... 

The first example of the sila-Pummerer rearrangement, which consists in the thermal conversion of sulfoxide 169 into O-silylated cyclic 0,S-acetal 170, has been described <1999TL185>. As shown in Scheme 29, a 1,3-migration of silicon atom to a sulfoxide oxygen resulted in ring expansion. 

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]. 

DMSO or other sulfoxides react with trimethylchlorosilanes (TCS) 14 or trimefhylsilyl bromide 16, via 789, to give the Sila-Pummerer product 1275. Rearrangement of 789 and further reaction with TCS 14 affords, with elimination of HMDSO 7 and via 1276 and 1277, methanesulfenyl chloride 1278, which is also accessible by chlorination of dimethyldisulfide, by treatment of DMSO with Me2SiCl2 48, with formation of silicon oil 56, or by reaction of DMSO with oxalyl chloride, whereupon CO and CO2 is evolved (cf also Section 8.2.2). On heating equimolar amounts of primary or secondary alcohols with DMSO and TCS 14 in benzene, formaldehyde acetals are formed in 76-96% yield [67]. Thus reaction of -butanol with DMSO and TCS 14 gives, via intermediate 1275 and the mixed acetal 1279, formaldehyde di-n-butyl acetal 1280 in 81% yield and methyl mercaptan (Scheme 8.26). Most importantly, use of DMSO-Dg furnishes acetals in which the 0,0 -methylene group is deuter-ated. Benzyl alcohol, however, affords, under these reaction conditions, 93% diben-zyl ether 1817 and no acetal [67]. 

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