Cyclic acetals, nomenclature

Cyclic acetals formed by the reaction of saccharides or saccharide derivatives with aldehydes or ketones are named in accordance with 2-Carb-24.1, bivalent substituent names (formed by general organic nomenclature principles) being used as prefixes. In indicating more than one cyclic acetal grouping of the same kind, the appropriate pairs of locants are separated typographically when the exact placement of the acetal groups is known. 

Acetals, nomenclature, 123-124 cyclic, nomenclature, 121-122 Acid degradation, monosaccharides, 457-459 Acid hydrolysis... 

The cyclic acetals are written by extending the rules applicable to alcohol derivatives with, as a prefix, the name of the divalent radical as from general nomenclature. Two such examples are methyl 4,6-0-benzylidene-a-D-glucopyranoside 4.31 and 1,2 5,6-di-O-isopropylidene-a-D-glucofuranoside 4.32. 

Rigorous application of nomenclature to the hnal example, 7-6, would treat the cyclic acetal as a fused dioxole. The alternative that names the compound as a derivative of the 16,17-diol is more common. The name thus becomes 21-chloro-9a-fluoro-l l)8,16a,17a-trihydroxypregn-l,4-dien-3,20-dione 16,17-acetonide. 

The Chemical Abstracts nomenclature is used for most of the acetals described in this chapter. The compounds are named either as dialkoxy derivatives or as derivatives of acetals or ketals. Confusion exists in the earlier literature on naming cyclic acetals. For example, the acetals prepared from glycerol and an aldehyde were at one time referred to as 1,2- or 1,3-alkylidene (or arylidene) glycerol however, today they are named as shown below ... 

The cyclic hemiacetal and hemiketal forms of monosaccharides are capable of reacting with an alcohol to form acetals and ketals (see Section 7.2). The acetal or ketal product is termed a glycoside, and the non-carbohydrate portion is referred to as an aglycone. In the nomenclature of glycosides we replace the suffix -ose in the sugar with -oside. Simple glycosides may be synthesized by treating an alcoholic solution of the monosaccharide with an acidic catalyst, but the reaction mixture usually then contains a mixture of products. This is an accepted problem with many carbohydrate reactions it is often difficult to carry out selective transformations because of their multifunctional nature. 

The common method of naming aldehydes corresponds very closely to that of the related acids (see Carboxylic acids), in that the term aldehyde is added to the base name of the acid. For example, formaldehyde (qv) comes from formic acid, acetaldehyde (qv) from acetic acid, and butyraldehyde (qv) from butyric acid. If the compound contains more than two aldehyde groups, or is cyclic, the name is formed using carbaldehyde to indicate the functionality. The IUPAC system of aldehyde nomenclature drops the final e from the name of the parent acyclic hydrocarbon and adds al. If two aldehyde functional groups are present, the suffix - dial is used The prefix formyl is used with polyfunctional compounds. Examples of nomenclature types are shown in Table 1. 

Anomeric carbon (Section 22.2C) The hemiacetal or acetal carbon in the cyclic form of a carbohydrate. The anomeric carbon can have either the a or /3 stereochemical configuration (using carbohydrate nomenclature), resulting in diastereomeric forms of the carbohydrate called anomers (a-anomers and /3-anomers). Anomers differ on/y in the stereochemistry at the anomeric carbon. 

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