TES group

Et3SiCl, Pyr. Triethylsilyl chloride is by far the most common reagent for the introduction of the TES group. Silylation also occurs with imidazole and DMF arid with dimethylaminopyridine as a catalyst. Phenols, carboxylic acids, and amines have also been silylated with TESCl. 

By-product generation with TMS-alkynes and the sluggish coupling rate with TBDMS-alkynes rendered the triethylsilyl (TES)-alkyne 40a the best reactant for the coupling reaction. Indeed, C-protection with the TES group gave indole 41a in 80% yield and also provided sufficient hydrolytic stability and satisfactory reaction kinetics for use in large scale synthesis. 

Interestingly, we were intrigued by the ESI mass spectrum of the compound, as the observed base peak consisted of [M-S02+Na]+. This led us to explore a thermal retro-Diels-Alder reaction that could afford the desired enone 69. It is noteworthy that the chemistry of cyclic enol-sulfites would appear to be an under-explored area with a few references reporting their isolation being found [57]. At last, we were also able to prepare epoxy ketone 70 from 69 in three steps, albeit epoxidation did not take place unless the TES group was removed. Spartan models reaffirmed our initial conformational assessment of enone 69 and epoxy ketone 70, which contain sp3-hybridized C8a and s/r-hybridized C8b (p s e u d o-. v/r - h y b r i d i zed C8b for 70) at the AB-ring junction (Fig. 8.12) and displayed the desired twisted-boat conformation in A-ring. 

Three tactical approaches were surveyed in the evolution of our program. As outlined in Scheme 2.7, initially the aldol reaction (Path A) was performed direcdy between aldehyde 63 and the dianion derived from tricarbonyl 58. In this way, it was indeed possible to generate the Z-lithium enolate of 58 as shown in Scheme 2.7 which underwent successful aldol condensation. However, the resultant C7 P-hydroxyl functionality tended to cyclize to the C3 carbonyl group, thereby affording a rather unmanageable mixture of hydroxy ketone 59a and lactol 59b products. Lac-tol formation could be reversed following treatment of the crude aldol product under the conditions shown (Scheme 2.7) however, under these conditions an inseparable 4 1 mixture of diastereomeric products, 60 (a or b) 61 (a or b) [30], was obtained. This avenue was further impeded when it became apparent that neither the acetate nor TES groups were compatible with the remainder of the synthesis. 

The most representative example of the utility of (3-lactams as acylating agents is the coupling reaction of the (3-lactam 25 Scheme 10, with the sodium salt of vacatin III to give paclitaxel ([69-73] for a recent example involving kinetic resolution of racemic (3-lactams, see [74]), after mild deprotection of the TES group. 

Tellurium inserts into the P-group IV element bond in trialkylsilyl(-germyl, -stannyl)-diorganophosphanes in a benzene medium. The reaction proceeds via phosphane tellurides that rearrange to compounds with a P —Te-group IV element bond1-3. 

Exposure of Te-group IV element compounds to air resulted in replacement of tellurium by oxygen. 

The P = Te group is the most reactive part in phosphane tellurides. 

The Te — Te group is the most reactive part in most of the diorgano ditellurium compounds and reactions proceed with cleavage of the Te —Te bond, the elimination of one tellurium atom, or the elimination of two tellurium atoms,... 

Diphenyl ditellurium compounds with a stabilizing nitrogen functionality in the ortho-position to the tellurium atom insert one tellurium atom into the Te—Te group forming diaryl tritellurium compounds5,6. 

The ditellurium compounds, in which a Te —Te group joins two carbonyl groups, can be considered to be the tellurium analogs of peroxy compounds derived from carbonic acid or benzoic acids (e.g. benzoyl peroxides). Only a few of these compounds are known. During the reduction of aromatic nitro compounds with disodium telluride in dimethylfor-mamide, bis[dimethylaminocarbonyl] ditellurium was formed as a by-product in yields from 5 to 15%. The formation of this compound was attributed to the capture of the dimethylaminocarbonyl radical by the telluride anion and subsequent oxidation of the tellurocarbamoyl species4. 

The ratio of pleasure to pain is quite favourable with TES ethers. Like the TMS group, the TES group is easy to introduce and easy to remove in the presence of other silyl protecting groups but it is also sufficiently stable to endure column chromatography and it is resistant to many oxidation, reduction, and organometallic reactions,... 

The TES group is 10-100 times more stable to hydrolysis or nucleophilic attack than the TMS group but much more labile than the /m-butyldimethylsilyl (TBS) group. A quantitative measure of the relative stability of TES compared with the TMS and TBS groups can be gleaned from a synthesis of Rapamycin by Smith and co-workers.1 A salient problem was the choice of protecting group for the CIO hydroxyl [Scheme 4.15]. Three silyl ethers were evaluated. The TBS derivative of 15.1 was too stable and attempts to remove it under a... 

The TES group is the most labile of the common silyl protecting groups, apart from TMS, and can usually be removed in the presence of TBS, triisopropylsilyl (TIPS) and rerf-butyldiphenylsijyl (TBDPS) groups. Selective deprotection of a TES ether using aqueous trifluoroaoetic acid left two TBS ethers and two TIPS ethers intact in the Merck synthesis of the immunosuppressant FK-506 [Scheme 4.19].22 Weaker acids such as H2O-HOAC-THF (3 5 11) at room temperature,33 HF pyridine24 and pyridinium p-toluenesulfonate [Scheme 4.2Q]23 can accomplish similar transformations. 

Protection of the Cl 7 hydroxyl of the Discodermolide intermediate 92,1 [Scheme 4.92] was problematic,149 The TBS ether was stable to all deprotection conditions compatible with the molecule and the TES ether was too labile to survive subsequent steps, However, the DEIPS ether offered an added increment of stability over the TES group and was subsequently cleaved, along with the three TBS ethers using /Koluenesulfonic acid in aqueous THF. The diethyl-i sop ropy Isilyl triflate required for the conversion of 92,1 to 92.2 was prepared from diethylisopropylsilyl chloride75 and triflic acid. 

Three protecting group manipulations are performed. 3,4-Dimethoxybenzyl (DMPM) groups are cleaved oxidatively. TES groups are acid labile. 

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