1

N NH2

OH H0 OH

To Ac OAc

1. Br2, Ph2PCI

3. TMSBr

coupling

POs*

Photolysis of the anomeric cobaloxime 245 in the presence of maleic anhydride and PhSSPh gave 246 in 46% yield. The reaction proceeds via radical intermediates and the formed a-carbonyl radical is trapped with PhSSPh to give the formed ad-ducts. Then syn-^-H elimination led to 247 in quantitative yield (Scheme 46) [68].

D. Use of Rhodium

McDonald et al. used a rhodium-catalyzed cyclotrimerization reaction to gain access to aryl C-glycosides (Scheme 47). Their work began with the conversion of lactone 248 to alkynyl glycal 249 by standard methods. Exposure of 249 or 251 to 254 in the presence of ClRh(Ph3P)3 in protic solvent furnished the product glycals 252 and 253, respectively, in the indicated yields [69]. Dialkyne 255 was also subjected to similar conditions and gave C-aryl spiroglycoside 257, a compound structurally related to the papulacandin natural products.

E. Chromium, Tungsten, and Molybdenum

Dotz and coworkers prepared several interesting and novel glycosylidene carbenes (259-261) by reaction of lithiated glycal 258 with the appropriate metal carbonyl derivative. The synthetic utility of the formed carbenes was demonstrated by reaction with 3-hexyne to give a mixture of complexed and uncomplexed adducts 262 and 263, respectively. The anomeric chromium carbene 264 was converted to 265 by exposure to ethoxy ethyne (Scheme 48) [70]. The same workers have carried out a similar chromium-mediated benzannulation (266 ^ 268), this time with the chromium on the aromatic fragment [71]. Other sugar-based carbenes have also been prepared [72].

Postema and coworkers have used ring-closing metathesis to prepare C1-gly-cals, compounds convertible to C-glycosides (Scheme 49). The sequence is both general and convergent. Ester formation (269 ^ 270) is followed by methylenation to give 271, and ring-closing metathesis is then carried out with the Schrock catalyst 272 to deliver the generic Cl-glycal 273. The reaction tolerates alkyl and aryl groups at the anomeric center. Very hindered groups (R = f-Bu) were not good substrates because the methylenation step did not proceed in good yield. This was presumably due to the steric hindrance [73].

The chemistry has been extended to the preparation of a number 1,6-linked C-disaccharides. The conversion of 274 to 277 exemplifies the strength and versatility of this approach to C-glycoside synthesis. Compound 277 was converted to the 2-deoxy ^-C-disaccharide and also to the ^-C-disaccharide by standard methods (not shown). A small library of glycal C-disaccharides was prepared by use of this novel methodology [74].

Scheme 47

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