Triflate, trimethylsilyl alcohols

O-Tritylation.2 A trimethylsilylated alcohol, phenol, or carboxylic acid can be converted into the corresponding trityl derivative by reaction with 1 catalyzed by trimethylsilyl triflate (equation I). t-Butoxytrimethylsilane can be used, but it is less effective. ... 

In the area of long-chain alkyl glycosides, 2,3,4,5,6-penta-O-acetyl-D-glucose treated with trimethylsilylated alcohols in the presence of trimethylsilyl triflate has been used to give the dialkylacetals following base catalysed deacetylation. Mild acid hydrolysis then gives the... 

Thus, D-ribofuranosides with high a-stereoselectivity were obtained with the combined use of a catalytic amount of tin tetrachloride and tin(II) triflate and a stoichiometric amount of lithium perchlorate using l-6)-acetyl-2,3,5-tri-0-benzyl-3-D-ribofuranose as a glycosyl donor and O-trimethylsilylated alcohols or monosaccharides as acceptors [92],... 

The use of the enolsilyl ether of 1-menthone [16, 19, 21-23] and of some free triflic acid favors the formation of the thermodynamically controlled products as with free 2,2 -dihydroxydiphenyl [22] and only subsequently added HMDS 2 [22]. On reacting silylated alcohols and carbonyl compounds with pure trimethylsilyl triflate 20 under strictly anhydrous conditions no conversion to acetals is observed [24]. Apparently, only addition of minor amounts of humidity to hydrolyze TMSOTf 20 to the much stronger free triflic acid and hexamethyldisiloxane 7 or addition of traces of free triflic acid [18-21, 24, 26] or HCIO4 [25] leads to formation of acetals. 

Trimethylsilyl triflate is also a powerful catalyst for acylation by anhydrides. Reactions of alcohols with a modest excess (1.5 equival) of anhydride proceed in inert solvents at 0°C. Even tertiary alcohols react rapidly.114 The active acylation reagent is presumably generated by O-silylation of the anhydride. 

With boron trifluoride etherate instead of trimethylsilyl triflate the yield was 51%, without catalyst 40% and without catalyst and HMPA 25%.t88],[89] With CH3MgI instead of CH3MgBr the ketone is further transformed into a tertiary alcohol.[893 b) Reaction in the presence of FeCl3.[90J... 

The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[f>]thiophene 101 mediated by boron trifluoride-etherate . 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

The introduction of the allylic silane moiety required for the intermolec-ular Hosomi-Sakurai reaction is depicted in Scheme 16. Following the formation of the enol triflate 97, a Stille coupling provided excess to the allylic alcohol 98 [51]. The allylic alcohol (98) was endowed with a phosphate leaving group for the subsequent allylic substitution. Utilizing a trimethylsilyl cuprate as nucleophile for the 5 2 reaction, the allylic phosphate was converted into the allylic silane 89. A useful substrate-induced diastereoselectivity in favour of (14i )-89 was encountered at small scale but decreased significantly upon up-scaling. 

O-isopropylideneglycerol in the presence of a Lewis acid (pre-racemization of the glycerol was always observed) [49], condensation of the alcohol 72 [49] with 65 in the presence of the usual trimethylsilyl triflate gave the glycoside 73. This glycoside could then be manipulated to form the epoxides 66 (n = l) and 67... 

The Lewis acid trimethylsilyl triflate brings about the rearrangement of 2,3-epoxyamines to the corresponding 2-trimethylsilyloxymethylaziridinium ions.38 Such intermediates react regiospecifically with nitrogen nucleophiles to form 1-substituted 2,3-amino alcohols with full stereochemical control. 

It has been demonstrated89 that the reaction of acetoxybenziodazole (73) with alcohols in the presence of trimethylsilyl triflate produces 3-iminobenziodoxoles (74). A plausible mechanism for the process is shown in Scheme 18. 

In their prominent synthesis of brevetoxin B, Nicolaou et al. have investigated, among some other methods for construction of oxepine ring system, the reductive cyclization of keto alcohol 37 with triethylsilane and trimethylsilyl triflate giving oxepane 38 (Equation 12) <1995JA1173>. 

Yamashiro et al. 1972), boron trifluoride etherate in acetic acid (Schnabel et al. 1971), trimethylsylil triflate (Schmidt et al. 1987), trimethylsilyl perchlorate (Vorbrueggen Krolikiewicz 1975), and, most frequently, trifluoroacetic acid (Farowicki Kocienski 1995 and references therein). Deprotection of the /-HOC group under neutral conditions was not described until recently, yet it is highly desirable. Now it has been found that the tert-butoxycarbonyl protecting group for amines, alcohols, or thiols is removed efficiently (90-99% yield) with use of 0.2 equivalent of cerium ammonium nitrate in acetonitrile at 80°C (Hwu et al. 1996) ... 

Acetals and ethers can also be used to produce benzylic car-bocations by reacting with a Lewis acid. Treatment of acetals derived from chiral 2-substituted benzaldehyde chromium tricarbonyl complexes with trimethylsilyl triflate followed by reaction with a silyl enol ether give diastereomerically pure alkylation products (Scheme 98). Benzylic alcohols can be... 

A definite improvement in the synthesis of A -methoxy aziridines was achieved by substituting boron trifluoride with trimethylsilyl triflate and diethyl ether with dichloromethane"9. In this way, the A -methoxy aziridines were obtained in good yields from a variety of linear and cyclic alkenes, e.g., 6-8. For comparison, the aziridine 8 was obtained in 50% yield by using boron trifluoride- diethyl ether complex in dichloromethane. Complex product mixtures were obtained with allyl and crotyl alcohols and with cyclohexenone. Further transformation of the A -methoxy aziridines into the N-H aziridines was possible using sodium/ ammonia reduction, e.g., 9. 

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