Carbohydrates acetal formation

Structurally, 0-glycosides are mixed acetals that involve the anomeric position of furanose and pyranose forms of carbohydrates. Recall the sequence of intermediates in acetal formation (Section 17.8) ... 

D. M. Clode, Carbohydrate cyclic acetal formation and migration, Chem. Rev. 79 491 (1979). 

Recently, a cyclization-elimination route to carbohydrate-based oxepines is proposed (Scheme 11). After silyl protection and hydroboration/oxidation, starting hept-l-enitols 31 give the protected heptan-l-itols 32. Swern oxidation of the latter followed by sequential acetal formation/cyclization provide methyl 2-deoxyseptanosides 33 that undergo elimination reactions to give the carbohydrate-based oxepines 34 <2005JOC3312>. 

S. J. Angyal and R. J. Beveridge, Intramolecular acetal formation by primary versus secondary hydroxyl groups, Carbohydr. Res., 65 (1978) 229-234. 

Carbohydrate chemistry has provided a rich mine of information on the stability, selective formation, and selective cleavage of cyclic acetals and reviews devoted wholly to the synthesis and reactivity of carbohydrate acetals should be consulted for a more detailed coverage of the subject. The Royal Society of Chemistry publishes annual reviews in its Specialist Periodical Reports series entitled Carbohydrates that gives extensive accounts of recent protecting group developments. 

Carbohydrate Cyclic Acetals Formation and Migration. Clode, D. M. Chem. Rev. 1979, 79, 491. 

Terminally fluorinated carbohydrates may be obtained by treating the tosyl esters, or the more reactive mesyl esters, with potassium fluoride in methanol. Because of the alkaline reaction of potassium fluoride, the remaining hydroxyl groups must be protected, as for example, by acetal formation,... 

Methane fermentation in natural ecosystems is usually described as a two-stage system in which nonmethanogenic bacteria ferment organic matter such as carbohydrate yielding such products as acetate, formate, hydrogen, and carbon dioxide. The methanogenic bacteria are restricted... 

TEA, CH2CI2, rt, 5-30 min, 84-99% yield. An adamantyl glycoside was stable to these conditions. Secondary carbohydrate PMB ethers are cleaved faster than the primary PMB. The reaction has also been performed in the presence of anisole to scavenge the liberated benzyl carbenium ion. This method is probably preferred for the cleavage of two adjacent PMB ethers since competing benzylidine acetal formation is not a problem. ... 

Homolysis of the S-acyl bond is responsible for formation of phenylthio-cyclohexane and 3-phenylthiocyclohexene from. SiS -diphenyl dithiocarbonate by irradiation in cyclohexene.117 A cyclic mechanism, however, has been proposed to account for the photodecomposition of certain carbohydrate dithiobisthio-formates.118 On irradiation, 0-ethyl thioacetate is converted into cis- and / 5-2,3-diethoxybut-2-ene, together with small amounts of 2,3-diethoxybut-l-ene, 2,3-diethoxybuta-1,3-diene, and 1-ethoxyethane-l-thiol.119 121 Free sulphur was trapped by reaction with 1,2-dimethyIenecyclohexane. Details of the photoelimination of sulphur dioxide from D-glucofuranosyl phenyl sulphone acetates have been published.180 An efficient synthesis of another 1,8-bridged naphthalene... 

D. M. Oode, Carbdiydrate cyclic acetal formation and migration, Chem. Rev. 79 491 (1979). A. H. Haines, The selective tnoioval of protecting groups in carix>hydrate chemistry, Adv. Carbohydr. Chem. Blochem. 39 13 (1981). 

The applications of mass spectrometry to carbohydrate chemistry have been re vie wed.In general, stereoisomeric acetates or methyl ethers of carbohydrates are not well differentiated by mass spectrometry, but when the mode of derivatization depends on the stereochemistry of the parent sugar and leads to new ring-systems, as in acetal formation, structural isomers are produced that are more apt to show significant mass-spectral differences. 

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. 

The reactions of carbohydrates, with few exceptions, are the reactions of functional groups that we have studied in earlier chapters, especially those of aldehydes, ketones, and alcohols. The most central reactions of carbohydrates are those of hemiacetal and acetal formation and hydrolysis. Hemiacetal groups form the pyranose and furanose rings in carbohydrates, and acetal groups form glycoside derivatives and join monosaccharides together to form di-, tri-, oligo-, and polysaccharides. 

OH groups of the D-glucoside. Figure 22.43 shows the reactions one can use to obtain each of the alcohol groups of a-D-glucopyranoside independently available for further modification. The reactions used for protecting are typically a combination of acetal formation, alkylation, acylation, silylation, and/or reduction. Keep in mind that every carbohydrate is different. So, for example, the method that works for D-glucose may not work for D-mannose. 

In Section 17.9 we explored the use of diols to protect carbonyl groups. Entry 5 shows how carbohydrates can function as the diol component in forming a cyclic acetal from benzaldehyde, thereby protecting two of the carbohydrate s hydroxyl groups. This is yet another example of acetal formation, in this case involving the aldehyde group of benzaldehyde and the C-4 and C-6 hydroxyl groups of the carbohydrate. 

Acetal formation Glycoside synthesis, from carbohydrates and alcohol Hydrolysis of enamines Epoxidation of fats by H2O2, followed by hydrolysis to transglycol... 

---