DMTST

Frejd and co-workers utilized a different tactic for aniline cyclization by first employing a Heck-Jeffery protocol under solvent-free conditions to prepare o-amino dehydrophenylalanine derivatives from o-aminoaryl iodides with the former undergoing a spontaneous la cyclization-elimination sequence to afford 2-methoxycarbonyl indoles <06S1183>. Dimethyl(methylthio)sulfonium trifluoromethanesulfonate (DMTST) was used by the Okuma group to promote the cyclization of o-vinyl-A-p-toluenesulfonylanilide to N-tosylindole <06CL1122>. 

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).

Scheme 5.3 Stepwise assembly of the solid-phase-bound heptasaccharide thioglycosides as glycosyl donors and DMTST as activator.29...

These final model experiments indicate that thioglycosides may be used in the block synthesis of 1.2- cis- linked glycosides with DMTST as promoter. With unreactive acc.eptors it may be advisable to refrain from transforming the reaction into a halide-assisted one by adding halide ion to the mixture. When the acceptor reactivity is sufficiently high, however, the presence of halide ion may significantly improve the stereospecificity and the yield of the desired product. 

Scheme 8 Thioglycoside-based glycosidation reactions, (a) thiophile = JV-btvmosuccwimide (NBS) [35], methyl tnflate [38], dimethyl(methyIthto)sulfonium triflate (DMTST) [39], or AModo-succmiraide (NlS)-triflic acid [40]. (b) TBPA+- = tris (4-bromophenyl)ammoniumyl hexachloroanti-monate [42]. (c) mCPHA = m-chloroptrbenmic acid [43],...

In the initial studies, as briefly described earlier, the methyl a-2-thioglycoside of Neu5Ac 5 [22,24], or the -1 1 anomeric mixture 11 [26,27], which can be almost quantitatively prepared from 9 in one step, was coupled with lightly protected sugar acceptors such as 14,16, and 18 in the presence of DMTST in acetonitrile to give exclusively the sialyl a(2—>3)- or sialyl ot(2—>6)-D-galactose or lactose derivatives (20, 22, and 24) in 50-70% yields even in large-scale reactions [26],... 

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