Sugar aldehydes cyclization

Another stereoselective synthesis (Scheme 11) is based on sugar aldehyde 35 and intermediate 36 subsequent 1,2-O-isopropylidine deprotection, N,0-debenzylation/olefin reduction/reductive cyclization in a single pot, and O-acetylation result in the formation of bicyclic aza sugar 37 (09TA1217). 

The three-component reaction of indole (2) with sugar hydroxyaldehyde 281 and Meldrum s acid 282, with a catalytic amount of D,L-proline, afforded the 3-substitution product 283 as a single isomer [203]. The substituent possesses the czs-fused furo [ 3,2- b ] pyranonc skeleton. The proline catalyzes the Knoevenagel condensation of the sugar aldehyde 281 and Meldrum s acid 282 to provide the alkylidene derivative 284 of Meldrum s acid. Then a diastereo-selective Michael addition of indole and an intramolecular cyclization of this adduct 285 with evolution of carbon dioxide and elimination of acetone furnish the furopyranone in one-pot (Scheme 62). 

Oxidative processes (route B) represent another common route to triazolopyridines (compounds described in Schemes 54 and 55). These preparations all start from aldehyde hydrazones and use different oxidative reagents for the cyclization (Table 2). Generally, those conditions are milder than condensation methods. Moreover, the oxidizing reagents are compatible with other moieties, even the sugar-derived polyol 209. In the case of compound 208, the hydrazone (major diastereomer) was obtained by tautomerization of the corresponding enhydrazine, the... 

An interesting approach to carbobicyclic polyhydroxylated compounds was proposed by Jarosz.43 It was based on a Lewis acid transformation of sugar allyltins 82 into dieno-aldehydes 83 with the fi-geometry across the internal double bond exclusively. The Wittig-type reaction of 83 afforded a triene, which underwent cyclization under high pressure, providing derivatives of bicyclo[4.3.0]nonane 84. Alternatively, dienoaldehyde 83 was converted into the phosphonate 85, which afforded the bicyclo[4.4.0]-decene derivatives (86) upon reaction with an aldehyde and simultaneous cyclization of the resulting triene.43 (Fig. 29). 

The samarium diiodide-mediated reductive cyclization of a sugar-derived oxime ether provides a trace amount of an unexpected cyclized product <1997JOC7397>. It is thought that its formation probably arises from an intermediate (methylthio)methyl ether formed with the excess of Swern reagent used to generate the aldehyde in situ after cyclization (Equation 59). 

Monosaccharides Cyclize to Form Hemiacetals Aldehydes can add hydroxyl compounds to the carbonyl group. If a molecule of water is added, the product is an aldehyde hydrate, as shown in figure 12.4. If a molecule of alcohol is added, the product is a hemiacetal the addition of a second alcohol results in an acetal. Sugars readily form intramolecular hemiacetals in cases in which the resulting compound has a five- or six-member ring. 

Hemiacetals that can be formed by intramolecular cyclization of an alcohol on to an aldehyde are also often stable, especially if a five- or six-membered ring is formed. You met this in Chapter 6— many sugars (for example, glucose) are cyclic hemiacetals, and exist in solution as a mixture of open-chain and cyclic forms. 

Hemiacetals are formed from hydroxy aldehydes " or ketones which are usually first in a protected form and are deblocked prior to cyclization. Ozonolysis of cyclohexene derivative 5 and reductive workup, followed by hydrolysis of the acid chloride formed in situ and borane reduction of the carboxylic acid, leads to the six-membered hemiacetal 6. a precursor for fluo-rinated sugars that are potentially versatile as molecular probes in the elucidation of biochemical processes. ... 

The synthesis of oxazole C-nucleosides in moderate yields by Tosmic addition to sugar derived aldehydes and concomitant cyclization has been reported. In particular, aldehyde 130 gave 131 in 48% yield <03SL1619>. 

Pyrrole and N-substituted pyrroles are formed by a reaction analogous to the conversion of sugars to furan aldehydes. Ammonium and substituted ammonium salts of mucic acid, H02C(CH0H)4C0jH, are cyclized and de-carboxylated by pyrolysis. The yields of pyrrole and its N-phenyl and N-methyl derivatives are about 40%. Tetrahydropyrroles (pyrrolidines) are formed from various 4-substituted amines by elimination of water, ammonia, or hydrogen halide. ... 

Hemiacetals in sugars are formed in the same way that other hemiacetals are formed—that is. by cyclization of hydroxy aldehydes. Thus, the hemiacetal of glucose is formed by cyclization of an acyclic po/yhydroxy aldehyde (A), as shown in the accompanying equation. This process illustrates two important features. 

The predominant forms of ribose, glucose, fructose, and many other sugars in solution are not open chains. Rather, the open-chain forms of these sugars cyclize into rings. In general, an aldehyde can react with an alcohol to form a... 

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