Ketoses trimethylsilylation

Reaction of ribose 5-phosphate 116 with dihydroxyacetone phosphate, catalyzed by fructose 1,6-diphosphate aldolase from rabbit muscle (RAMA) affords the ketose diphosphate 117. Dihydroxyacetone phosphate was formed in situ from fructose 1,6-diphosphate by action of RAMA and triose phosphate isome-rase (TPI). The diphosphate 117 was dephosphorylated enzymatically using acid phosphatase, and the ketose 118 was reduced directly into the a-C-manno-side 119 by treatment with bistrimethylsilyltrifluoroacetamide, trimethylsilyl-triflate and triethylsilane (Scheme 28) [45]. 

Ketoses appear to react slowly, and several authors have commented on the time required for complete trimethylsilylation of D-fructose. Thus five peaks were observed after trimethylsilylation for one hour, compared to three peaks when the reaction was allowed to continue for 24 hours.119-123 Sorbose reacts in a similar, slow man-... 

Although many analyses are performed on alditol acetates (see Section VII, p. 56), in order to avoid the formation of multiple peaks, such a reduction is not practical when the mixture contains ketoses, notably fructose. Such analyses are mainly encountered with medical samples and in the examination of sugars occurring free in Nature. Furthermore, the peak-area ratios may be used as a means of identification, to check on the completeness of trimethylsilylation,67,89 and, despite the complex chromatograms obtained from trimethyl-silyl derivatives, they have the merit of being rapidly formed.89 For all of these reasons, improvements in the separation of monosaccharides as their trimethylsilyl derivatives continue to be of considerable importance. 

Recent improvements in the standard synthetic procedure using haloacetates have featured the use of trialkyi phosphites and activated acetates in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) as activating reagent. " This reaction has proved to be of great value in constructing complex sugar phosphonates. For example, in the presence of TMSOTf, the Michaelis-Arbuzov reaction between protected ketose acetates and triethyl phosphite takes place in ( 1 FC F at room temperature to produce, after deprotection, the expected diethyl l-(methoxycarbonyl)alkyl-phosphonates in high yields. -" ... 

The ring size of ketoses and the structure of gan osides have been examined by using g.l.c.-m.s., and the mass spectra of per-O-(trimethylsilyl)alditols have been reported. ... 

In recent years, the Lewis acid-catalyzed azidation of protected sugar derivatives has been extended to more-complex starting materials. For instance, isopropyli-dene-protected ketose derivatives were treated with trimethylsilyl azide and a catalyst to give various ketosyl azides.Thus, the reaction of 6-O-tert-huX.-yldimethylsilyl-l,2 3,4-di-(9-isopropylidene-p-D-psicofuranose (90) gave a mixture of partially deprotected anomeric azides 91-94 in 54% overall yield. The corresponding transformation of 3-deoxy-l,2 4,5-di-(9-isopropylidene-p-D-erjr/zro-hex-2-ulopyranose (95) led to the azide mixture 96 in 75% yield.2,3,4,6-Tetra-... 

Peracetates of aldoses, disaccharides, and methyl glycosides have been obtained in good yields by reaction of the trimethylsilylated sugars with iron(III) chloride-acetic anhydride without change in their stereochemistry. The same reaction on ketoses produced dehydration products only. 

---