Aldehydes hemiacetals from

Base-catalyzed fragmentation also occurs on treatment of 5,6j9-epoxy-19-aldehyde (hemiacetal, hemiacetal-acetate or 3)5,6/ -acetal) accessible from nitrous acid-acetic acid treatment of 5a-bromo-6jS-hydroxy-19-oximes followed by mild base hydrolysis (yield not reported)... 

The structure of benulin (134), a lupene hemiacetal from Bursera arida, has been confirmed by X-ray analysis. An investigation of several Senecio species has resulted in the isolation of a series of lupane derivatives which include the acids (135), (137), and (139), the corresponding aldehydes (136), (138), and (140), the ketone (141), and betulonic acid. Three more triterpenoids (142)—(144) have... 

Hemiacetals are more readily formed from aldehydes than from ketones.77 4-Hydroxybutanal and 5-hydroxypentanal exist preponderantly as cyclic hemiacetals, containing only 11.4 and 6.1%, respectively, of the free aldehyde.79 These often-quoted data were, however, obtained for solutions in 3 1 1,4-dioxane-water, and, if experience gained with the hydroxyketones75 is also valid for the hydroxyaldehydes, the proportion of aldehyde would be much higher in aqueous solution (for which data are not yet available). 

RCHO - RCOOCH,. This conversion can be effected by oxidation of aldehydes with Br2 in CH3OH via a hemiacetal (equation I). The method is applicable o optically pure aldehydes obtained from sugars. Yields are moderate in the reaction with aromatic aldehydes.1... 

Hemiacetals from reaction of alcohols with aldehydes and ketones... 

Since a hemiacetal is formed so easily from a carbonyl compound and alcohol, it is not surprising to find that carbohydrates (polyhydroxy derivatives of aldehydes and ketones) frequently exist as cyclic structures in which a hemiacetal linkage is formed intramolccularly. Furthermore, since hemiacetal formation is a reversible process, many carbohydrates exhibit the phenomenon of mutarotation. The liberation of the free aldehyde (V) from the internal hemiacetal of the sugar (IV) destroys the optical activity of the hemiacetal carbon atom (in this case carbon 1), and reformation results in the formation of an equilibrium mixture of two diastereoisomers. 

In acidic media, polarized multiple bonds often undergo acid catalyzed addition, and a common mode of addition is the Ad 2. Deprotonation of the nucleophile by solvent gives the neutral compound. Common examples of this easily reversible Adg2 reaction are the formation of hydrates (NuH is H2O) and, if NuH is ROH, hemiacetals (from aldehydes) and hemiketals (from ketones). Usually this reaction favors reactants. 

ROH AdE2 Hemiacetal from aldehyde Hemiketal from ketone. OR C "OH... 

As mentioned in the introduction, reactions involving a protected sugar in the aldehydo tautomeric form do not differ basically from those of common aldehydes. For example, we can observe additions of organomagnesium, organolithium reagents, etc. These reactions can be very useful, especially in total chiral synthesis, but this type of chemistry should be, in principle, familiar to the reader. Here we will only consider sugars that are totally free or capable of aldehyde hemiacetal tautomerism. 

Ethers in which hydrogen atoms are replaced by halogen can be obtained by direct action of chlorine or bromine on the unhalogenated compounds. Other methods that start from aldehydes, hemiacetals, or acetals (see pages 232 and... 

HEMIACETALS FROM REACTION OF ALCOHOLS WITH ALDEHYDES AND KETONES... 

We have already described an important reaction of carbohydrates— the formation of glycosides under acid-catalyzed conditions (Section 23.14). Glycoside formation drew our attention to the fact that an OH group at the anomeric carbon of a furanose or pyranose form differs in reactivity from the other OH groups of a carbohydrate. It also demonstrated that what looks like a new reaction is one we saw before in a different guise. Mechanistically, glycoside formation is just a structural variation on the aldehyde hemiacetal acetal theme we saw when discussing the reactions of aldehydes and ketones. 

Silver oxide (AgO) is generally prepared in situ from silver nitrate (AgNOj) and base (NaOH). Reducing sugars derive their names from their ease of oxidation. These sugars possess aldehyde or masked aldehyde (hemiacetal but not acetal) functionality. 

The formation of side product ester 10 in oxidation of diol 5 (Table 3.1, entry 4) can be explained with the intermediacy of aldehyde as well. Upon generation of aldehyde 11 from 5, intermolecular nucleophilic attack of the carbonyl group by the free hydroxyl group produced hemiacetal 12, oxidation of which led to carboxylic ester 10 (Scheme 3.2). Formation of similar by-products during oxidation of primary hydroxyl groups in carbohydrates has been observed previously. ... 

A somewhat related two-step method for the synthesis of areno[a]quinolizines involves the preparation of a cyclic hemiacetal from a p-dicarbonyl compound and an a,p-unsaturated aldehyde. The subsequent reaction of this hemiacetal with tryptamine in the presence of a Brpnsted acid affords the target product, probably through a Pictet-Spengler reaction, and the use of chiral Brpnsted acids has been investigated and found to lead to good enantioselections in some cases [74]. The reaction between P-amidoesters derived from tryptamine and a,p-unsaturated aldehydes affords lactams derived from the indolo[2,3-fl]quinolizin-4-one system [75]. 

The condensation of aldehydes or ketones with secondary amines leads to "encunines via N-hemiacetals and immonium hydroxides, when the water is removed. In these conjugated systems electron density and nudeophilicity are largely transferred from the nitrogen to the a-carbon atom, and thus enamines are useful electroneutral d -reagents (G.A. Cook, 1969 S.F. Dyke, 1973). A bulky heterocyclic substituent supports regio- and stereoselective reactions. 

The synthesis of 11-oxaprostaglandlns from o-glucose uses the typical reactions of gl cofuranose diacetonide outlined on p. 267. Reduction of the hemiacetal group is achieved a thioacetal. The carbon chains are introduced by Wittig reactions on the aldehyde grou] which are liberated by periodate oxidation and laaone reduction (S. Hanessian, 1979 G Lourens, 1975). 

The methyl enol ether 37 is oxidized to the a,/3-unsaturated aldehyde 39 via hemiacetal 38. Unsaturated aldehyde 39, elongated one carbon from the aldehyde 36, is prepared by the Wittig reaction of 36 to give 37, and application of this reaction[ 88]. 

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hetni-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18], /-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19],... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

A variation involves the reaction of benzylamines with glyoxal hemiacetal (168). Cyclization of the intermediate (35) with sulfuric acid produces the same isoquinoline as that obtained from the Schiff base derived from an aromatic aldehyde and aminoacetal. This method has proved especially useful for the synthesis of 1-substituted isoquinolines. 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

Although 3y5-hydroxy-A -l9-aldehydes can be obtained from the corresponding oxime by acid treatment, the most efficient method for the production of A -19-aldehydes is nitrosation of the oximes followed by zinc reduction of the 5a-halo hemiacetal. ... 

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