Phenylselenyl triflate

Ionic Inflate derivatives of nonmetallic elements such as selenium, sulfur, phosphorus, and iodine form an important class of reagents lor organic chemistry. Highly electrophilic phenylselenyl triflate can be used in the cyclization of 5- and 6-hydroxyalkenes, affording the corresponding tetrahydrofurans and pyrans [132] (equation 68). 

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).

A mixture of 0.55 g (1.8 mmol) of phenylselenyl triflate and 0.18 g (1.8 mmol) of 3-methyl-4-penten-l-ol (5) in 32 mL of CH2C12 is stirred at 0°C for 15 min. After dilution with 50 mL of Et20, the organic layer is washed with sat. brine. Drying over MgS04 and removal of the solvent leaves a residue which is purified by column chromatography (silica gel, petroleum cther/EtOAc 9 1) yield 0.369 g (79%) d.r. (trans lets) 80 20. The structures of the trans- and cw-isomers are confirmed by differential NOE 1H NMR at 500 MHz. 

Few examples are reported for the cyclization of 5-alkenols to 2,3-disubstituted tetrahydro-2//-pyrans where the stereoselectivity depends on the nature of the allylic substituent. The cyclization of 4-methyl-5-hexen-l-ol (1), performed with phenylselenyl triflate in dichloromethane at — 78 "C, proceeds with low 1,2-asymmetric induction and the corresponding tetrahydro-2//-pyran 2 is recovered in 85% yield as a 66 34 (Ircms/cis) diastereomeric mixture19. 

A 2-thio-neuraminic acid glycosyl xanthate gives good yields of a-linked disaccharides when used in conjunction with phenylselenyl triflate. ... 

Another cyclomaltohexaose isomer was synthesized in which the D-glucose residues were replaced by D-mannose residues [63,64]. The key intermediate was the formation of a mannohexaosyl methylthioglycoside that was cyclized by an intramolecular glycosylation reaction using phenylselenyl triflate as a catalyst. 

Another unique cyclodextrin has been synthesized to give the first cyclodextrin with L-monosaccharide residues [66]. L-Rhamnose was converted into an a-1 ->4-linked L-rhamnohexasaccharide derivative (2,3-dodeca-o-benzyl-l-methyl-a-thio-L-rhamnohexaoside) that was cyclized using phenylselenyl triflate to form a-1 4-linked cyclo-L-rhamnohexaose (see Fig. 8.9 for the structure). 

Trimethylsilyl triflate was used to bring about the cyclopentannulation shown in equation 4754. It was used to simultaneously catalyze the phenylselenylation of the double bond and cyclization of vinylsilane 2055 (equation 48). 

The electrophilic addition of arylbis(arylthio)sulfonium salts to glycals in the presence of alcohols, directly provide 2-thio-(5-glycosides with good stereoselectivity [162]. Stereoselective syntheses of [5-glycosides were also obtained from the reaction of 1,2-trans acetoxysele-nides, prepared by treatment of glycals with phenylselenyl chloride and silver acetate, in the presence of trimethylsilyl triflate [163]. Combined application of this method and the trichlo-roacetimidate method (see section 3.3) was suggested for the synthesis of functionalized precursors of the olivomycin C-D-E trisaccharide [164] (Scheme 34). 

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