Thexyldimethylsilyl

To a solution of thexyldimethylsilyl chloride (11 mmol) and ImH (15 mmol) in DMF (5 ml) was added the alcohol (11 mmol) at ambient temperature. After being stirred at ambient temperature for 16 h, the mixture was diluted with hexane. The hexane phase was washed with water (2x), and then dried. Concentration followed by distillation (Kugelrohr) gave the silyl ether (86-93%). 

The tris(trimethylsilyl)silyl ligands can be easily modified by reactions with silyl chlorides, as shown with a series of phenylated species Mes Ph SiCl (n = 0-3). Furthermore, triisopropyl, thexyldimethylsilyl, or tert-butyldimethylsilyl substitution are all easily possible.190 The crystallographic characterization of some of the alkali metal derivatives indicates a direct correlation between ligand size and resulting structural parameters. 

The key intermediate 14, a stable tetrasubstituted tetrahydropyran derivative, was obtained from commercially available methyl o-glucopyranoside 12 via a three-step reaction sequence (Scheme 4). First the primary hydroxyl group was protected as thexyldimethylsilyl (TDS) ether, then the other OH groups were methylated under standard conditions to give 13. The latter glycoside was deoxy-genated at the anomeric position by treatment at room temperature with a Lewis acid (trimethylsilyl triflate) and a hydride donor (triethylsilane). This treatment also... 

Hereafter the anomeric center is protected. When the ceramide building block has to be connected to the oligosaccharide, normally selectively deprotection of the C-la position of the lactose is necessary. The thexyldimethylsilyl (TDS) group is a less common trialkylsilyl protecting group in carbohydrate synthesis. Advantageously it survives some rather harsh conditions. The liquid TDSC1 42 with imidazole 43 or DMAP as basic activator in a dipolar aprotic solvent like DMF forms the C-l protected compound 8 as a mixture of anomers.21... 

In order to prevent competing homoallylic asymmetric epoxidation (AE, which, it will be recalled, preferentially delivers the opposite enantiomer to that of the allylic alcohol AE), the primary alcohol in 12 was selectively blocked as a thexyldimethylsilyl ether. Conventional Sharpless AE7 with the oxidant derived from (—)-diethyl tartrate, titanium tetraisopropoxide, and f-butyl hydroperoxide next furnished the anticipated a, [3-epoxy alcohol 13 with excellent stereocontrol (for a more detailed discussion of the Sharpless AE see section 8.4). Selective O-desilylation was then effected with HF-triethylamine complex. The resulting diol was protected as a base-stable O-isopropylidene acetal using 2-methoxypropene and a catalytic quantity of p-toluenesulfonic acid in dimethylformamide (DMF). Note how this blocking protocol was fully compatible with the acid-labile epoxide. 

Thexyldimethylsilyl chloride (bp 55-56 01.3 kPa) and the corresponding tri-flate have been recommended152 as cheaper substitutes for rm-buty[dimethylsilyl chloride and /er/-butyldimethylsilyl triflate. Thexyldimethylsilyl ethers are formed at comparable rates and they are at least 2-3 times more stable than TBS ethers to acid and base hydrolysis but they introduce unwelcome clutter to NMR spectra, Thexyldimethylsilyl ethers are usually prepared by reaction of the substrate with thexyldimethylsilyl chloride in pyridine151 or in DMF in the presence of imidazole.153... 

Thexyldimethylsilyl feri-ButyIdIphenylsIlyl tert-ButyIdImethylsIlyl TDS TBDPS TBSorTBDMS... 

Thexyldimethylsilyl chloride is a liquid and is less expensive then TBSCL Furthermore, ROTDS ethers react more slowly under hydrolytic conditions than do RO-TBS ethers. 

Silyl protecting groups other than TMS and TBDMS are usually applied only if a modified acid stability is wanted. Thus, the triethylsilyl (TES) group shows a sensitivity to hydrolysis and nucleophilic attack intermediate between the TMS and the TBDMS groups. The triisopropylsilyl (TIPS), ° the r-bu-tyldiphenylsilyl (TBDPS) and the thexyldimethylsilyl (TDS) groups are considerably more stable... 

Morpholine, TBDMSCl, THF, 2 min, 20 C, >80% yield. In this case the ester was formed in the presence of a phenol. The functionally and sterically similar thexyldimethylsilyl ester is also formed under these conditions. ... 

Trialkylsilylacetylenes, 927 Trimethylsilyl, 928 (3-Cyanopropyl)dimethylsilyl, 930 Triethylsilyl, 930 f-Butyldimethylsilyl, 930 Thexyldimethylsilyl, 930 Benzyldimethylsilyl, 931... 

---