Sila-Pummerer reaction

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-]. 

Nucleophilic Substitutions and Cyclizations via Sila-Pummerer Reactions... 

Sila-Pummerer reaction of the /1-ketosulfoxide 1257 with the enol silyl ether of acetophenone 653 in the presence of BSA 22 a and stannous triflate affords the C-substituted sulfide 1258 in 82% yield and HMDSO 7 [52]. The allylic sulfoxide 1259 reacts with 653 in the presence of TMSOTf 20/DIPEA to give the unsaturated sulfide 1260 in 62% yield or, with the enol silyl ether of cyclohexanone 107a , the unsaturated sulfide 1261 in 63% yield and HMDSO 7 [53] (Scheme 8.21). 

The Sila-Pummerer reaction of a-alkoxy sulfoxides such as 1269 with excess tri-mefhylsilyl cyanide 18 and Znl2 affords, in quantitative yield, the a-cyanoether 1270 and trimethylsilyl methylsulfenate 1271 [58] (Scheme 8.24). 

Salicylate annelation, 279-280 Salutaridines, 593 Samarium(ll) iodide, 464-465 Sarkomycin, 457, 458 Schweizer s reagent, 597 Selenium, 465-466 Selenones, 123 Selenuranes, 223 Shapiro reaction, 563-565 Sharpless epoxidation, 263, 600 Shikimic acid, 3, 4, 548 Sibirinone, 596 Sila-Pummerer reaction, 576 Silica, 466... 

Additive and Vinylogous Additive Pummerer Reactions Sila-Pummerer Reactions Preparation ofTrimethylsiloxy Sulfides The Abnormal Pummerer Reaction... 

Sila-Pummerer Reactions Preparation of Trimethylsiloxy Sulfides... 

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. 

Kocienski has reported that the ylides derived from the reaction of allylic silylmethylsulphonium salts [e.g. (49)] with n-butyl-lithium undergo a [2,3]-sigmatropic rearrangement to homoallylic a-methylthiosilanes these are precursors to /8,y-unsaturated aldehydes via the sila-Pummerer reaction e.g. Scheme 64). " ... 

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]. 

Acetalization or ketalization with silylated glycols or 1,3-propanediols and the formation of thioketals by use of silylated 1,2-ethylenedithiols and silylated 2-mer-captoethylamines have already been discussed in Sections 5.1.1 and 5.1.5. For cyclizations of ketones such as cyclohexanone or of benzaldehyde dimethyl acetal 121 with co-silyl oxyallyltrimethylsilanes 640 to form unsaturated spiro ethers 642 and substituted tetrahydrofurans such as 647, see also Section 5.1.4. (cf. also the reaction of 654 to give 655 in Section 5.2) Likewise, Sila-Pummerer cyclizations have been discussed in Chapter 8 (Schemes 8.17-8.20). 

Tetrahydrothiophene-fused Cgg can be generated by its reaction with the thio-carbonyl ylide precursor bis(trimethylsilylmefhyl) sulfoxide 262 [314, 315]. Thermal sila-Pummerer rearrangement leads in situ to the ylide 263, which is readily added to CgQ (Scheme 4.45). 

The formation of 29, the product of a sila-Pummerer rearrangement, as a minor reaction product points to the intermediacy of the ion pair 28. 

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. 

---