Cyclizations glycosylation methods

Skrydstrup, Beau and co-workers122 have adapted Stork s method to the SmI2-reduction of glycosyl pyridyl sulfones bearing a silicon-tethered unsaturated group at HO-C(2). An example is shown with the synthesis of methyl a-C-zso-maltoside 172 from alkyne 170 via the 5-exo-dig radical cyclization of 171 (Scheme 56).144... 

Intramolecular radical cyclization followed by trapping of the intermediate with an enone has been investigated by Fraser-Reid and co-workers [95JOC3871] as a method for the synthesis of highly functionalized C-glycosyl derivatives. The control of the anomeric stereochemistry as well as the facial selectivity in the addition to the chiral enone is impressive. 

In addition to the Hilbert-Johnson reaction, the so-called mercuri process,37 and, less frequently, the cyclization procedure of Shaw et a/.,53 64 have been used for the synthesis of nucleosides and their derivatives. l-Peracylglycosyl-4-alkoxy-2(l//)-pyrimidinones, the intermediates of the Hilbert-Johnson reaction, can be, in principle, prepared 56,56 also by the mercuri process, namely by reaction of 4-ethoxy-2(lZ/)-pyrimidinone chloromercuri salt with the corresponding halogenoses, but this method is of less importance because of the contamination of iV-l-glycosyl derivatives with the 0-2 isomers, namely, with 2-peracylglycosyloxy-4-alkoxypyrimidines. The advantageous features of the mercuri process in comparison with the Hilbert-Johnson reaction might be formulated as follows. 

Another method, based on cyclization of glycosyl fluorides,764,765 such as 143 (for a methyl glycoside as a side-product, see Ref. 766), or of 6-O-p-tolylsulfonyl derivatives of hexoses95,97 (145), in alkaline media, was used for the preparation of 2-amino-1,6-anhydro-2-deoxy-/3-D-glucopyranose and its N-substituted derivatives (144). 

Reaction of 2,3-isopropylidene-j3-D-ribofuranosylamine with iV-2,4-dinitro-phenyl-3-benzoyl-pyridinium chloride yielded the pyridinium nucleoside analogue (8). Glycosyl isocyanides yield formamidines with amines, which can be cyclized to nucleoside analogues in appropriate cases thus, methyl anthrani-late in presence of mercuric chloride yields the benz-pyrimidine derivative (9). Standard methods have been used to prepare l-(6-deoxy- 3-D-allo- and -ot-L-talo-furanosyl) uracil and 9-0[-L-rhamnopyranosyl-2-alkylthio-adenines. ... 

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