Acetylene-C-glycosides

A further review of compounds bearing CF2X substituents includes references to relevant C-glycosides, and one on acetylenic coupling covers Vasella s work on couplings involving acetylenic C-glycosides. ... 

The synthetic plan in Figure 6 has been realized as full steps shown in Figure 7. C-Glycosidation of 18 with phenylthiotrimethylsilyl-acetylene and boron trifluoride etherate followed by treatment with biscobaltoctacarbonyl gave the biscobalthexacarbonyl complex 25 in 92% yield. Epimerization of the cobalt complex 25 was achieved with trifluoromethane sulfonic acid in... 

With the exception of photochemical reactions, the chemistry of allyltin compounds is analogous to that of allylsilanes and translates to the use of acetylenic tin reagents. As a final illustration of the utility of tin reagents to the formation of C-glycosides, the use of acetylenic tin reagents, shown in Scheme 7.20, is addressed. In the cited report, Zhai et al. [98] observed the formation of a-C-acetylenic glycosides in good yields. The reactions were catalyzed by zinc chloride and run in carbon tetrachloride. 

The configuration of the acetylene group was inverted through an acetylene-cobalt complex prepared by treatment of the acetylene with Co2(CO)g (O Scheme 31) [52]. Upon treatment of the complex with acid, epimerization occurred via a propargylic cation intermediate stabilized by the cobalt complex to afford the thermodynamically more stable -C-glycoside as the... 

Sugar acetylene reacted with a Fisher chromium carbene complex to give a phenol derivative, a possible intermediate for naturally occurring aryl-C-glycoside antibiotics (O Scheme 34) [55]. 

Further examples of the utility of 2-acetoxyglycals as C-glycosidation substrates were reported by Tsukiyama, et al.,68 and are illustrated in Scheme 2.3.42. This study demonstrated the highly stereospecific high yielding nature of these reactions. Furthermore, the generality of these reactions was demonstrated in their application to a variety of silylated acetylenes. 

The methods described thus far represent only a small subset of the available methodologies for the formation of C-furanosides and C-pyranosides. Other methods shown to be useful include cyclizations of halo olefins and ene-ynes. As shown in Scheme 7.6.1, Lee, et al.,29 prepared halo olefins from suitable alcohols and acetylenic esters. Subsequent application of free radical conditions thus effected cyclizations. All reactions proceeded in yields exceeding 95%. Furthermore, where the formation of diastereomers was an issue, selective cis formation was observed. Thus the ease of preparation of the vinyl ether substrates required for these cyclization reactions makes this methodology an extremely useful addition to the technology surrounding the preparation of C-glycosides. 

---