Sugar condensation

Sugars condense with anhydrous 2-propanone in the presence of an acid catalyst to form cyclic ketals known as isopropylidene derivatives ... 

It must be concluded that these experiments did not demonstrate unequivocally that a nitromethane-aldose sugar condensation had been achieved. 

If the medium becomes more dilute, the rate of Amadori compoimd formation is in general drastically reduced. However, this does not apply to the cysteine reaction. As a consequence, cysteine becomes clearly the most reactive amino acid in dilute neutral media (Table II). A possible explanation for the relatively high reactivity of cysteine in dilute aqueous systems is the high concentration of the cysteine-sugar condensation products thanks to the assistance by the thiol group. Therefore, in dilute systems the concentration of the Schiff s base of cysteine 6 will be much higher than the concentrations of the corresponding Schiff s bases of the other amino acids. 

Although aldol condensation is another type of ionic reaction, it deserves special consideration. It can lead to two classes of sugars. Condensation of an aldehyde in an a-position of a free carbonyl group under mildly akaline conditions still seems possible. This very simple reaction can also be quite efficient. Thus diacetal 7.59, quantitatively prepared by treatment of mannose with acetone in an acidic medium, is at equilibrium with the carbonyl tautomer. Condensation with formaldehyde, used in aqueous solution in the presence of potassium carbonate, gives the hydroxymethylated sugar 7.60 in 86% yield (Ho 1979). 

Aquallzer EJ. [Kolmar Labs] Polyamino sugar condensate, urea. 

PEG-240 PEG-75 cocoa butter glycerides PEG-50 lanolin PEG-85 lanolin PEG-25M PEG-45M PEG-10 propylene glycol PEG-3 stearate PEG-8 stearate PEG-20 stearate Phytantriol Placental protein Polyamino sugar condensate Polydextrose Polyglucuronic acid... 

Polyamino sugar condensate 120144-90-3 Methylaluminoxane 120200-78-4 Mannitan laurate 120359-07-1... 

Intramolecular glycosylation using a thioglycoside as donra (see VoL 27, p. 245) has been applied ro the synthesis of AZT.73 Base-sugar condensation was used to prepare some AZT analogues of type 69.10 The 3-amino-2, 3 -dideoxy-p-D-ribofuranosides of adoiine and guanine... 

Tetrazolyl)-2, 3 -dideoxythymidine has been prepared from a 2,3 -anhydronucleoside, whilst the hydantoin nucleoside 72, and the 5 -amino-2, 5 -dideoxy-structure, were made by heterocycle-sugar condensation. 

In the area of nucleoside phosphonates, the C-phosphonosugar unit 202 has been prepared by addition of dimethyl phosphite to the 3-ketosugar followed by radical deoxygenation, both reactions occurring with -selectivity. The 3 -deoxy-3 -C-phosphononucleosides 203 (R H or OH) were then made fairly conventionally from 202.2 The mimics 204 of nucleoside S -monophosphates were produced from a known sugar unit O etrahedron Lett., 1992,33, 1839) by base-sugar condensation.261... 

Although the sugars condense with nitromethane in aqueous alkali (148a), the reaction usually is carried out in methanol solution (or suspension) with sodium methoxide acting as the basic catalyst. In the most favorable examples of the condensation reaction, the sodium salts of the C-nitroalco-hols precipitate from this solution and can be isolated simply by filtration. As in the cyanohydrin condensation, two epimeric products are formed, usually in unequal amounts. After removal of the sodium by ion exchange, the epimeric (7-nitroalcohols frequently can be separated by fractional crystallization. For some syntheses, partially substituted sugars, such as the benzylidene derivatives, may be used to advantage. 

The mesylated cyanohydrins 135 (B = pyrimidine) were the major epimers obtained from cyanide attack on the 3 -ketonucIeosides these compounds could be cyclized in base and deprotected to give the spiro-systems 136. The 3 -epimers could also be obtained as minor products from this chemistry, 168,169 but were better obtained by making nitrile-mesylate 137 stereo-specifically, followed by base-sugar condensation and cyclization.1 0... 

Conventional heterocycle-sugar condensation methods have been used to prepare p-D-ribofuranosyl derivatives of various mono- and bicyclic imldes,4 3-substituted pyrroles, 5-alkylcjrtoslnes, M -acylcytosines,7 pyridazlnones of type (3), and 5-benzyl- and 5-tm-benzylo3y)benzyl uracil, acyclonucleoside derivatives of which are inhibitors of uridine phosphorylase in this case P-D-arabinofuranosyl and 2 -deo3qr systems were also prepared via the 2,2 -anhydronucleoside.9... 

Base-sugar condensations have been used to prepare 2 -deoxyribosides of pyrazin-2-ones,30.31 3,4,6-trisubstituted pyrazolo[3,4-d]pyrimidines,32 and indazoles, where about equal amounts of 1- and 2-glycosylated products were obtained, although with P-selectivity.33 Reaction of 2-deoxy 3,5-di-O-toluoyl-a-D-cryt/iro-pentofuranosyl chloride with 2-amino-6-chloropurine under Seela s phase-transfer conditions gives the p-A. giy osylated product as the major isomer.34... 

A number of -acyl- and thioureido-derivatives of 3 -amino-3 -deoxythymidine have been prepared. 1 0 Michael addition of phthalimide to enal (81) was used in the synthesis of the 3 -amino-2 ,3 -dideoxyhexo-furanose nucleosides (82) (all four isomers, R=H, Me, halogen),191 and a similar enal prepared by mercuric-ion catalysed hydrolysis of L-rhamnal was used in the same way to make the L-acosaminyl nucleoside (83) and its -anomer, and the L-ristosaminyl systems (epimers at C-3 ). 2 Michael addition of 1,2,4-triazole to (81) led, after base-sugar condensation, to the triazolyl-substituted nucleosides (84, B=T, U), together with the analogous hexopyranose systems. 93 xhe imidazolyl nucleosides (85) can be made by reaction of 3 -amino-3 -deoxythymidine with l,4-dinitroimidazoles, 9 whilst the unsaturated imidazole nucleoside (87) was made by treating the phenylselenone (86) (see Section 6) with imidazole. 195... 

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