Tetroses, racemic

Eschenmoser and co-workers [35] studied the aldomerization of glycoaldehyde phosphate which led to mixtures containing mostly racemates of the two diastereomeric tetrose 2,4-di-phosphate and eight hexose 2,4,6-triphosphates (O Scheme 2, route A). At 20 °C in the absence of air, a 0.08-molar solution of glycolaldehyde phosphate 2 in 2-M NaOH gave 80% yield of a 1 10 mixture of tetrose 3 and hexose 4 derivatives with DL-allose 2,4,6-triphosphate comprising up to 50% of the mixture of sugar phosphate [36]. 

Butlerov found out that in alkaline medium (calcium hydroxide), formaldehyde HCHO polymerizes to form about 20 different sugars as racemic mixtures, Butlerov 1861. The reaction requires a divalent metal ion. Breslow found a detailed mechanism of reaction that explains the reaction products, (Breslow 1959). He found that glycol-aldehyde is the first product that is subsequently converted into glyceral-dehyde (a triose), di-hydroxy-acetone, and then into various other sugars, tetrose, pentose, and hexose. The formose reaction advances in an autocatalytic way in which the reaction product is itself the catalyst for that reaction with a long induction period. The intermediary steps proceed via aldol and retro-aldol condensations and, in addition, keto-enol tautomerizations. It remains unexplained how the phosphorylation of 3-glyceraldehyde leads to glycral-3-phosphate (Fig. 3.6). Future work should study whether or not ribozymes exist that can carry out this reaction in a stereo-specific way. 

The two racemic tetroses have been prepared from 43 by a sequence of reactions involving the extension of the carbon atom chain by a Grignard reaction with ethyl orthoformate to four atoms, acetylation, and half-reduction of the triple bond to a cis olefin (48). cw-Hyroxylation of 48 with potassium permanganate, peracetylation (for the purpose of isolation), and final hydrolytic deprotection gave DL-erythrose. franj-Hydroxylation of 48 with peroxyacids failed. Therefore acetal 48 was hydrolyzed with 70% aq. formic acid to 4-acetoxycro-tonaldehyde (49). Reacetalization of 49 followed by cii-hydroxylation led to dl-threose in about 14% yield. 

Aldol condensation of 2,2-diethyl-l,3-dioxolan-4-one lithium or zirconium enolates with aldehydo sugars has afforded higher carbon aldonic acid derivatives, e.g. 1. The synthesis of L-ribono-1,4-lactone has been achieved from d-isoascorbic acid by way of the tetrose and pentitol derivatives 2 and 3 and the d-ribonolactone derivative 4 has been efficiently epimerized to the L-lyxonolactone 5 (Scheme 1). A selective i yn-epoxidation of racemic 2-0-benzyl-4-alkenamides followed by hydrolysis has afforded 3-deoxy-pentono-1,4-lactones. 

The peptide-catalysed stereospecific synthesis of tetroses (threose and erythrose) was reported by Weber and Pizzarello using homochiral L-configured dipeptides. " In the course of their investigations several dipeptides containing a- or (3-branched amino acids were employed revealing peptide 46 as the most useful catalyst and yielding the u-erythrose in enantiomeric excess of 82%, whereas the u-threose was obtained nearly racemic (Scheme 13.26). Generally, most of the tested peptides followed this trend and had an asymmetric effect on erythrose, whereas threose remained unaffected. 

Other indirect support for the pyranose half-chair structure of the transition state of the secondary process is provided by the behavior of alditols [21] and furanoid tetroses or 5-deoxypentoses [5, 6] under the conditions of the Bilik reaction. The former, which although able to form planar bidentate complexes are without the deformation necessary for the hydrogen exchange, do not undergo any structural change while the latter, sterically much more demanding furanoses (aparently unable to form bidentate molybdate complexes via their trans-oriented 2,3-hydroxyls), do not undergo racemization. 

Trioses to Hexoses.- A kinetic analysis of the formose reaction has led to the conclusion that the carbonyl rearrangement and retro-aldol reaction steps play a substantial role in the autocatalytic process. The condensation of D-glyceraldehyde acetonide with racemic alkyl (alkylthiomethyl) sulfoxides proceeded with high selectivity to produce diastereomeric l-alkylsulfinyl-l-alkylthio-3,4-0-isopropylidene-D-tetroses with a predominance of e/yrhro-products. An example is given in... 

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