Triflic acid promoter

In earher work, the Hall group found that catalytic amounts of triflic acid promoted the addition of aUylboronates to aldehydes [132], While aUylboration reactions catalyzed by chiral Lewis acids in general led to only low levels of enan-tioselection [133], Hall found that chiral LBA 1 catalyzes the asymmetric addition of allyl- and crotylboronates to various aldehydes to provide products in excellent yields and moderate to high ee s (Scheme 5.71) [134]. Further, double diastereose-lective crotylboration could also be achieved with high selectivities using the... 

The triflic acid-promoted hydrolysis of bis-triphenylphosphonio-isophos-phindolide cation (38) has been followed using NMR spectroscopy. No evidence was found for direct phosphorus protonation or alkylation which had previously been postulated. 

DMB) analogue of 24, namely donor 25 (see Scheme 11). Glycosylation with this donor led only to glycoside, 26, and no acyl transfer. Further, if less triflic acid promoter was used, a mixture of neutral orthoester (27) and glycoside 26 could be formed. If this mixture was treated with more triflic acid then only glycoside 26 and a small amount of hydrolysis product were isolated. This example clearly decouples neutral orthoester formation from acyl transfer, at least for such donors as 24. 

In this process (Fig. 22), ketone 61 underwent reduction with an over-stoichiometric amount of borane in THF in the presence of 5 mol % of the proline-derived Corey s oxa-zaborolidine 62 to afford alcohol 63 in 100% yield and 92% e.e. SN2 displacement carried out on the alcohol mesylate with a higher-order cuprate gave adduct 64, that was transformed first into tetralone 65 by a triflic acid-promoted Friedel-Crafts-type reaction, and then into sertraline 66 (86% e.e.) by imine formation and borohydride reduction. 

The stmctural architecture of siUcone polymers, such as the number of D, T, and Q sites and the number and type of cross-link sites, can be deterrnined by a degradative analysis technique in which the polymer is allowed to react with a large excess of a capping agent, such as hexamethyidisiloxane, in the presence of a suitable equiUbration catalyst (eq. 38). Triflic acid is often used as a catalyst because it promotes the depolymerization process at ambient temperature (444). A related process employs the KOH- or KOC2H -catalyzed reaction of siUcones with excess Si(OC2H )4 (eq. 39) to produce ethoxylated methylsiUcon species, which are quantitatively deterrnined by gc (445). 

Scheme 5.1 Mechanism of thioglycoside activation (a) by thiophiles X1 such as /V-bromosuccinimicle (NBS), 11,12 methyl triflate,13 dimethyl(methylthio)sulfonium triflate (DMTST),14 phenylselenyl triflate (PhSeOTf),17,18 iV-iodosuccinimide/triflic acid (MS/TfOH),19 20 and iodonium di-sym-collidine perchlorate (IDCP)21 (b) by tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA,+)25 and (c) via anomeric sulfoxides.26 The stereochemical outcome of these glycosylations follows the same general trends as with many other glycosyl donor/promoter combinations (m-CPBA = mcta-chloroperbenzoic acid).

Photoinduced oxidation of Cjq has been achieved by electron transfer from excited to a strong electron acceptor such as p-chloranil [72, 73], p-benzoquinone [73], tetracyano-p-quinodimethane (TCNQ) or tetracyanoethylene (TONE). This electron transfer proceeds efficiently only by addition of promoters such as Sc(OTf)3 or triflic acid, both of which strongly enhance the electron-transfer process [72, 73]. Another possibility to produce the cation is the electron transfer from to the singlet excited state of a strong electron acceptor such as N-methylacridinium hexafluorophosphate (NMA ) [74, 75], triphenylpyriliumtetrafluoroborate (TPP" )... 

Laali et al.234 have developed a method to the highly selective pura-adamantylation of arenes (toluene, ethylbenzene, anisole) with haloadamantanes (1-chloro- and 1-bromoadamantane, l-bromo-3,5,7-trimethyladamantane) and 1-adamantanol promoted by triflic acid in butylmethylimidazolium triflate [BMIM][OTf] ionic liquid. In contrast to reactions mn in 1,2-dichloroethane, little or no adamantane byproduct was detected in [BMIM][OTf. Furthermore, no isomerization of para-tolyladamantane was observed supporting the intramolecular nature of the formation of meta isomers. In competitive experiments with benzene-toluene mixture (1 1 molar ratio), high substrate selectivities were found (kT/kB = 16-17) irrespective of the alkylating agent. This is in sharp contrast to values about unity measured in 1,2-dichloroethane. 

Triflic acid effectively promotes the phenylamination of aromatics with phenylazide in a fast, convenient, high-yield process598 (Table 5.34). The high ortho/para selectivity with only a small amount of meta product and high substrate selectivity (kT/kB = 11) indicate the involvement of a substantially electron-deficient species, the phenylami-nodiazonium ion intermediate with the possible protosolvation by triflic acid. 

Wong and co-workers753 and Hashimoto and co-workers754 have demonstrated that triflic acid is an ideal promoter in chemo- and stereoselective glycosylation with phosphites [Eq. (5.285)]. 

In another application of catalytic glycosylation with triflic acid, two disaccharides were synthesized with donors bearing trichloroacetimidate function and then the disaccharides thus prepared were condensed in a similar manner to a tetrasaccharide757 (1,2-dichloroethane, 55°C, 1.5 h, 50-85% yields). The same protocol was used in the synthesis of a decasaccharide from mono-, tetra-, and pentasaccharide building blocks.758 The trichloroacetimidate procedure has been successfully used in the glycosylation of /3-cyclodextrins promoted by triflic acid.759 760... 

Triflic acid has proved to be an efficient catalyst to promote direct C-alkylation of 1,3-dicarbonyl compounds with benzyl alcohols979 [Eq. (5.353)]. Alkylation of... 

The inner disaccharide unit of the trisaccharide hapten of the M. avium serovar 8 GPL148 was assembled in a manner similar to that of the serovar 20, but with reaction of the rhamnosyl trichloroacetimidate (80b) with the benzylidene acetal (81). O-Deacetylation of the product yielded the disaccharide acceptor (84) for the next glycosylation. Incorporation of the pyruvate acetal moiety into the terminal 3-O-methyl-D-glucose residue of 85 was achieved by transacetalation with methyl pyruvate diethyl dithioacetal, with sulfuryl chloride-triflic acid as catalyst. From the mixture of products the desired diastereomer was separated and converted by successive O-debenzylation, acetylation, selective 1-O-deacetylation, and reaction with trichloroacetonitrile into the trichloroacetimidate 86. Reaction of glycosyl donor 86 with acceptor 84, with trimethylsilyl triflate as promoter, afforded fully... 

As previously described, the 3//-pyrrolium adduct 50 is obtained from the TBSOTf-promoted aldol reaction between the 1-methylpyrrole complex 21 and acetaldehyde diethylacetal (Figure 11). Deprotonation of 50 occurs at C-3 with i-Pr2EtN to give the corresponding -substituted lH-pyrrole complex. Addition of triflic acid results in the elimination of ethanol to give the azafulvenium complex 141 as a 3 2 mixture of diastereomers (Figure 25). Deprotonation of 141 results in formation of the unstable unsubstituted P-vinylpyrrole complex 142, which can be trapped in situ with N-phenyl maleimide (vide infra). 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

Thio-sugars [60,61] are excellent glycosyl donors for oligosaccharide synthesis. Excellent reactivity of activated thioglycosides [62,63] by common promoters such as methyltriflate, dimethyl(methylthio)sulfonium triflate (DMSTS), iodonium dicollidine perchlorate (IDCP), and A-iodosuccininude (NIS) alone or with a combination of triflic acid or silver triflate makes them versatile glycosylation donors. 

Chiral oxazaborolidine salts are useful catalysts not only for enantioselective reductions (i.e., CBS reduction), but also for promoting enantioselective D-A reactions. The cationic Lewis acids formed by reaction of chiral oxazaborolidines with triflic acid [TfOH] or trifluoromethanesulfonimide [(Tf)2NH] coordinate to dienophiles to direct subsequent cycloadditions in a highly controlled fashion. The D-A reactions using these chiral salts proceed in CH2CI2 under very mild conditions at temperatures as low as -95 °C (Table 9.4). ... 

The synthesis of 4 -thioxy loadcnosine 126, described in the pioneering work by Reist et has been improved" by direct glycosylation of V-benzoyladenine with the 1,4-dithio-D-xylofuranoside 123 in the presence of V-iodosuccinimide and catalytic triflic acid, followed by deprotection. Based on the ratio of isomers obtained (a -.ft 20 34) an iodium ion-promoted elimination-addition reaction mechanism has been proposed, as outlined in Scheme 35. 

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