Rearrangement sila-Pummerer

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). 

Phenylthio-l-trimethylsilylalkanes are easily prepared by the alkylation of (phenylthioXtrimethylsilyl)mcthane as shown in Scheme 10 [40], The treatment of (phenylthio)(trimethylsilyl)methane with butyllithium/tetramethylethylene-diamine (TMEDA) in hexane followed by the addition of alkyl halides or epoxides produces alkylation products which can be oxidized electrochemically to yield the acetals. Since acetals are readily hydrolyzed to aldehydes, (phenylthioXtrimethylsilyl)methane provides a synthon of the formyl anion. This is an alternative to the oxidative transformation of a-thiosilanes to aldehydes via Sila-Pummerer rearrangement under application of MCPBA as oxidant [40, 41]. 

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. 

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. 

Ethoxyvinyllithium also adds stereoselectively to 1 to provide an adduct that is converted in several steps to the < n/(-carboxylic aeid 4. The, vvn-isomer 5 is available by addition of methyllithium, trapping with QH SeCI, and a sila-Pummerer rearrangement (Scheme 11). 

Ketone synthesis. The intermediate a in the synthesis of aldehydes can be alkylated in the presence of TMEDA to give the ketone equivalent 2. The silanes are converted into ketones (3) by a sila-Pummerer rearrangement (10, 314). Unfortunately, only primary alkyl halides give satisfactory yields. 

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. 

Sharpless oxidation, 8 Sila-Pummerer rearrangement, 314 Silica, 18, 185, 342, 346-347 Silicon(IV) chloride, 347 Silver carbonate, 350 Silver chloride, 347-348 Silvcr(II) dipicolinate, 348 Silver fluoride-Pyridine, 348 Silver imidazolate, 349-350 Silver(I) nitrate, 350 Silver(l) oxide, 350-351 Silver(II) oxide, 80, 352-354 Silver perchlorate, 354-355 Silver(I) trifluoracetate, 365 Silybin, 351,352 2-Silyl-l,3-dithianes, 380 Silyl nitronates, 381 SMEAH, 357... 

---