Simple Aldoses

The simple aldoses are related to d- and L-glyceraldehyde in that structurally they may be considered to be derived from glyceraldehyde by the introduction of hydroxylated chiral carbon atoms between C-l and C-2 of the glyceraldehyde molecule. Thus, two tetroses result when CHOH is introduced into D-glyceraldehyde  

Question Two tetroses can be formed from L-glyceraldehyde. These tetroses are called l-erythrose and L-threose. Why is it unnecessary to invent new names for the tetroses derived from L-glyceraldehyde  

If the structures of the two tetroses are written alongside those of D-erythrose and D-threose using Fischer projection formulas, it is seen that two pairs of mirror images are given. That is, the four aldotetroses constitute two pairs of enantiomers  

Two simple aldopentoses can be derived structurally from each of the four aldotetroses described, making a total of eight aldopentoses. Therefore, there are 16 aldohexoses. 

Simplified structures for the eight aldopentoses and 16 aldohexoses are shown (o represents aldehyde — represents an OH group H atoms on carbons are omitted). 

---

FIGURE 7.1 Structure of a simple aldose (glyceraldehyde) aud a simple ketose (dihy-droxyacetoue). 

In addition to the common carbohydrates mentioned in previous sections, there are a variety of important carbohydrate-derived materials. Their structural resemblance to sugars is clear, but they aren t simple aldoses or ketoses. 

Note. For simple aldoses up to aldohexoses, and ketoses up to hept-2-uloses, the anomeric reference atom and the configurational atom are the same. 

Trivial names are common in carbohydrate nomenclature. Fifteen of them form the basis of the systematic nomenclature. They are assigned to the simple aldoses (polyhydroxyaldehydes), from triose to hexoses. 

In this Section, deoxy sugars, aminodeoxy sugars, and glycuronic acids are treated as modification products of simple aldoses that are classified here according to the structure of their formal, carbohydrate precursors. 

Decarbonylation of aldoses.2 Although this rhodium complex has been known since 1968 to effect decarbonylation of aldehydes, it has been used for decarbonylation of sugars only recently, probably for lack of a compatible solvent. Actually, this reaction when carried out in N-methyl-2-pyrrolidinone (NMP) at 110-130° is extremely useful in the case of simple aldoses, which are converted to the lower alditol with formation of carbonylchlorobis(triphenylphosphine)rhodium(I). The yields are 75-95%. This method of degradation has the further advantage that protecting groups are not necessary. Deoxyaldoses, particularly 2-deoxyaldoses, are decar-bonylated in 75-99% yield. A disadvantage of this reaction is that a full equivalent of the complex is required. 

There are two series of simple aldoses a d series and an l series. To determine to which series an aldose belongs, locate the chiral carbon atom most remote from the reducing group and determine its relationship to glyceraldehyde e.g., the sugar shown below, glucose, is called D-glucose ... 

The addition of cyanide to simple aldoses is essentially quantitative in solutions buffered at pH 9.1 increased acidity causes diminished reaction rates. The reaction can be conveniently effected using a solution of sodium cyanide and calcium chloride, but varied conditions may be required in order to obtain desired proportions of the epimeric products. The latter arise from the creation of a new asymmetric center, and are generally not produced in equal amoimts because of the asymmetric nature of syntheses using optically active starting materials. The epimeric preference may be so high as to give essentially quantitative yields of one product. For ex-... 

In a survey " of the behavior of various classes of carbohydrate derivatives in c.i.m.s., it was found that the principal ion is the MH (isobutane) or M -I- NH (ammonia) ion, or a simple elimination product from it. The ammonia-mediated spectra feature M -I- NH f as the major ion with simple aldoses, their acetylated methyl glycosides, 1,6-anhydro-aldohexopyranoses (and their triacetates), and acetylated 1-thioaldoses. Aldose diethyl dithioacetals show M -I- NHf — EtSH as the principal ion, whereas this ion for aldehydo-aldose peracetates is MH — AcOH. 

The O-methyloximes of several aldoses have been examined in solution by C-n.m.r. and by g.l.c. and the authors conclude that the 0-methyl oximes exist as acyclic modifications. Those derived from simple aldoses existed as the anfi-isomer, whereas those derived from 2-deoxyaldoses existed as anti- and jyn-mixtures.The oxime of D-glucose, however, exists as a mixture of the syn- and anft-forms and the two pyranose forms. All four isomers have been separated by t.l.c. and identified spectroscopically. ... 

Carbohydrate diacids (aldaric acids) are derived from oxidation of the terminal carbons of simple aldoses. Several aldaric acids are suggested as being reasonable candidates for commercial development, based on potential applications, and availability and cost of the individual simple sugar precursors. 

In comparison to the synthesis of analogous compounds from simple aldoses or ketoses, the situation for sialic acid is more complicated, due to the occurrence of several different functional groups. Synthetic studies on sialo-compounds have been reviewed by Box and Jeanloz (1969), Tuppy and Gottschalk (1972), Holmquist (1975), and Van der Vleugel (1981). 

Other simple aldoses that turn into meso-aldaric acids are D-erythrose, D-ribose, D-xylose, and D-galactose (see Figure 24-1). 

---