Ethers, triethylsilyl

The triethylsilyl ether is approximately 10-100 times more stable than the TMS ether and thus shows a greater stability to many reagents. Although TMS ethers can be cleaved in the presence of TES ethers, steric factors will play an important role in determining selectivity. The TES ether can be cleaved in the presence of a /-butyldimethylsilyl ether using 2% HE in acetonitrile. In general, methods used to cleave the TBDMS ether are effective for cleavage of the TES ether. 

From intermediate 43, the path to monensin would seemingly be straightforward. A significant task which would remain would be the construction of the l,6-dioxaspiro[4.5]decane substructure of monensin. You will note that the oxygen atoms affixed to carbons 5 and 12 in 43 reside in proximity to the ketone carbonyl at C-9. In such a favorable setting, it is conceivable that the action of acid on 43 could induce cleavage of both triethylsilyl ethers to give a keto triol which could then participate in a spontaneous, thermodynamically controlled spiroketalization reaction. Saponification of the C-l methyl ester would then complete the synthesis of monensin. 

The completion of the total synthesis only requires a few deprotection steps. It was gratifying to find that the final deprotections could be conducted smoothly and without compromising the newly introduced and potentially labile trisulfide residue. In particular, exposure of intermediate 101 to the action of HF pyridine results in the cleavage of all five triethylsilyl ethers, providing 102 in 90% yield (Scheme 23). Finally, hydrolytic cleavage of the ethylene ketal with aqueous para-toluenesulfonic acid in THF, followed by removal of the FMOC protecting group with diethylamine furnishes calicheamicin y (1) (see Scheme 24). Synthetic calicheami-cin y, produced in this manner, exhibited physical and spectroscopic properties identical to those of an authentic sample. 

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

To a solution of 4-t-butylcyclohexanone (lmmol), tris(triphenylphos-phine)ruthenium(n) chloride (0.05 mmol) and silver trifluoroacetate (0.05 mmol) in toluene (5 ml) was added triethylsilane (1.5 mmol). The mixture was heated under reflux for 20 h, and concentrated under reduced pressure. The residue was diluted with hexane (3 ml), filtered and distilled to give a mixture of triethylsilyl ethers (0.96mmol, 96%), b.p. 70°CI 0.1 mmHg. G.l.c. analysis shows an axial (cis) equatorial (trans) ratio of 5 95—a result comparable to the best LAH results. 

QHijOjP 122-52-1) see Foscamet sodium Gesirinone 7-O-triethylsilylbaccatin III (C,7H520 Si 115437-21-3) see Paclitaxel 4-triethylsilyl 3-butyn-l-ol triethylsilyl ether (C 4Hj40Si2 160194-28-5) see Rizatriptan benzoate triethylsilyl chloride... 

Benzil is reductively triethylsilylated to the bis(silyl) ether in 83% yield.411 The combination of Et3SiH/ZnCl2 reductively triethylsilylates ketones in good yield.382 Excellent yields of triethylsilyl ethers from ketones are accomplished with the use of triethylsilane and catalyst 73 or 74.412 tert-liutyIditnelliyIsily 1 ethers can be synthesized by the reaction of TBSH/TBSOTf with a ketone (Eq. 240).392... 

This reaction was also used in a synthesis of 13-cis-retinoic acid.2 Thus reduction of 3 under the same conditions gives the triethylsilyl ether (4) of 13-cis-retinol, with retention of the geometry of the terminal double bond. This product can be converted to 13-cis-retinoic acid by deprotection and oxidation (60% yield). 

Doyle and co-workers have employed Rh2(pfb)4 as a highly selective catalyst for the room temperature synthesis of silyl ethers from alcohols and triethylsilane.159 The selectivity of the catalyst is demonstrated by reactions of olefinic alcohols, in which hydrosilylation is not competitive with silane alcoholysis when equimolar amounts of silane and alcohol are employed. High yields (>85%) of triethylsilyl ethers are obtained from reactions of alcohols such as benzyl alcohol, 1-octanol, 3-buten-l-ol, cholesterol, and phenol. Tertiary alcohols are not active in this system. 

In addition, trimethylsilyl ether 197a and triethylsilyl ether 197b have conformations different from those of compounds 198a and 198b247, which bear a bulky PhsSi or a (z -Pr) vSi group, respectively. The change of conformation is due to the steric hindrance from these two bulky silyl groups. 

Oxidation of alkyl trimethyl- and triethylsilyl ethers, ROSi(CH3)3 or ROSi-(C2H5)3.1 Silyl ethers of this type can be oxidized directly to carbonyl compounds by the Swern reagent. This oxidation provides an efficient route to the Corey aldehyde (2). 

The final step in a recent synthesis of the antifungal agent Papulacandin D involved deprotection of five O-silyl groups including a di-ferr-butylsilylene group, a triethylsilyl ether and two phenolic tri-isopropylsilyl ethers [Scheme 3.115],215 Acid conditions were precluded by the add lability of the side chain. Use of TBAF was complicated by problems in separating the product from tetrabuty-Lammonium salts. The desired global deprotection was accomplished with tris-(dime thy iamino)sulfoni urn difluorotrimethylsilicate (TAS-F). 

Three of the most common procedures for the formation TES ethers have been selected for their large scale preparative value. Triethylsilyl ethers are prepared by the reaction of the alcohol with chlorotriethylsilane in the presence of a catalytic amount of imidazole [Scheme 4.23]30 or DMAP31 Triethylsilyl triflate in the presence of pyridine [Scheme 4.24]32 or 2,6-lutidine [Scheme 4.25]23 can be... 

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