Aldoses branched-chain

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Several compounds of this type, formally derived from aldoses by oxidation of the terminal CH2OH group to -CHO, have been prepared. Dialdoses arise as intermediates in structural studies, but they are also valuable starting materials for synthetic conversions, in particular for natural-product synthesis. A branched-chain dialdose, streptose (167), occurs as a component of the antibiotic streptomycin. The structure of streptose was elucidated after extensive investigations of its derivatives and transformation products.371 The 2,5-dimethoxytetrahydrofuran fulvanol (168), an analogue of apiose, has been isolated from the plant Hemerocallis fulva 12... 

Synthesis of Branched-Chain Monosaccharides from Cs-Aldoses... 

SCHEME 13.72 Mukaiyama s synthesis of a branched-chain aldose. 

The Bilik reaction applied to 2-ketoses yield 2-hydroxymethyl aldoses in which the tertiary carbon originates from C2 of the ketose and the C2 hydroxyl is on the opposite side to the C3 hydroxyl of the ketose (in the Fischer projection). Thus, o-fructose yield o-hamamelose. The position of equilibrium, however, lies towards the straight-chain sugar, although it can be pulled over somewhat towards the branched-chain aldose by the addition of borate. The mechanism in Figure 6.9 again explains the main reaction, but not the formation of sorbose as a by-product, which probably arises from a metal ion-promoted hydride shift, as there is no isotope exchange with solvent. The Bilik reaction can be applied to the production of l-deoxy-o-xylulose from 2-C-methyl-D-erythrose the reaction is particularly clean and only the two... 

A considerable contribution to oin knowledge of the chemistry of dicarbonyl sugars, a group of sensitive and reactive compounds, has been made by Theander, who used, for the most part, acetals of D-glucose, D-mannose, and n-galactose. Carbonyl derivatives of aldoses and aldosides are of importance in the synthesis of branched-chain sugars. ... 

Branched-chain versus normal-chain selectivity is evidently a function of the aldose-ketose reaction equilibrium, and the driving force and degree of approach to that equilibrium this, in turn, is a function of the pH of the reaction and species concentrations, including that of the catalyst. Alditol-glycose selectivity is apparently controlled by the cross-Cannizzaro catalyst, as sodium hydroxide gives alditols, whereas calcium hydroxide affords glycoses. This, again, is probably more an effect of pH than of catalyst. 

Chain-extension reactions via aldehyde additions have been used to synthesize higher aldoses, enuloses, branched-chain enuloses and 1-deoxy-alduloses. Addition of 2-lithio-1,3-dithiane to partially blocked carbohydrate derivatives proceeds in certain cases with high stereoselectivity the authors suggest that the reaction is controlled by chelation of the oxygen functions at C-1, C-2, and C-4... 

If this reaction is appUed to a ketose it should result not in epimerization but in the formation of a branched-chain sugar, a 2-C-(hydroxymethyl)aldose, because the carbon atom which migrates carries a hydroxymethyl group. PreUmi-nary results show that this is indeed the outcome of the reaction [53]. 

---