Primary triflates

Indeed, metallation of diethyl difluoromethyl phosphonate affords an anion. This species can displace a primary triflate in position or add onto an aldehyde (in... 

Spontaneous ionization requires both good leaving groups and that the resulting carbenium ions are sufficiently stable. For example, although primary triflates are very stable covalent species which do not self-ionize, secondary triflates with phenyl substitents are very reactive and spontaneously ionize. The ionization equilibrium of styryl triflate could not be established because of side reactions such as Friedel-Crafts alkylation [56], On the other hand, methoxymethylium triflate is partially ionized with equilibrium constants Kj = 5-10 4 at 10° C and Kt = 210 4 at -70° C in S02 [57]. In this system, ionization is endothermic. Secondary triflates with alkoxy substituents, such as those in polymerizations of vinyl ethers, are apparently more strongly ionized than their primary counterparts [58,59],... 

It should be noted that direct substitution of sulfonate or iodide derivatives at C6 is sensitive to steric hindrance. Kishi reported a failure in the substitution of a primary tosylate by vinyl cuprate in the synthesis of mycalamides where C4 had two geminal methyl substituents. An alternate five step route was developed to introduce the vinyl substituent [119]. Direct substitution of a primary triflate by an aryl Grignard reagent is a key step in the formal synthesis of (+)-apicularen A from D-glucal [194]. Displacement of a triflate with an alkynyllithium has also been used in the total synthesis of (+)-panaxacol [195]. 

Stereochemical control in 0-alkylative /3-glycosylation relies on the kinetic anomeric effect [110] namely, that the oxyanion derived from a reducing sugar is more reactive in its 8-ori-ented form 71B than the corresponding a-anion 71A which should be more abundant. This tactic has met with partial success in cases of reactive substrate [111] (O Scheme 38). For example, reaction of 72 with a primary triflate in THF in the presence of NaH afforded the 8-glycoside in 62% yield. 

One carbon elongation has also been achieved by displacement of primary triflate groups vrith potassium cyanide/ and by opening of S,6-epoxide 40 wth 2-lithio-2-trimethy]silyl-l,3-dithiane to 2-... 

Anomeric O-Alkylation of Ribofuranose witii Primary Triflates Efliect of the Protecting Group at 0-5 of Ribofuranose... 

Schmidt and Reichrath performed the anomeric 0-alkylation of ribofuranose with primary triflates with complete stereocontrol as shown in Scheme 3 [4]. Ribofur-... 

Anomeric O-Alkylation of Mannofuranose with Primary Triflates The Crown Ether Effect... 

Schmidt et al. reported an effect of a crown ether on the glycosylation of mannofuranose. Scheme 4 shows an example of stereoselective coupling of mannofuranose 2,3 5,6-di-O-isopropylidene acetal (6) with primary triflate (7) in the presence of NaH [5, 6]. The coupling reaction in the absence of the crown ether afforded only P-glycosides (9). Mannofuranose is different from ribofuranose in having two endo hydroxyl groups at the 2,3-positions. Thus mannofuranose could be a better ligand for complex formation. In particular, the P anion preferentially forms a complex... 

Anomeric O-AIkylation of Gluco- and Galactopyranoses with Primary Triflates High -Selectivity as a Result of the Reactive Anomeric P-Anion... 

Scheme 5. Anomeric O-alkylation of glucopyranoses and primary triflates. MTr = p-methoxytrityl...

Anomeric O-Alkylation of Acyl-Protected Nucleophiles with Primary Triflates... 

Anomeric 0 Alkylation of Mannopyranose wUli Primary Triflates Possibility of Intramolecular Complexation of the Nudeophile... 

Anomeric O-Alkylation of Partially Protected Aldoses with Primary Triflate... 

Isomerization of the stannylene acetal from 1,2-0 to 2,3-0 and the resultant equatorial 3-0 activation rationalize the formation of the /t.vctA/o-disaccharide as shown in Scheme 17. The epimerization of the stannylene acetal seems much faster than alkylation. The use of the analogous 3-0-benzyl ether effectively prevented the unfavorable coupling. Thus, generation of the etheric by-product was suppressed in the reaction with the same primary triflate (57). 

Whereas nucleophilic attack of an anomeric alkoxide on primary triflates is extremely efficient (50-80%), the attack on secondary triflates is somewhat lower... 

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