Ring oxygen replacement carbon

Carbon, ring oxygen replacement by, 141-143 2-Carboxy-5-(2-hydroxymethyl)-4-methylthiazole, synthesis and transformations, 284-286 Carrageenans, 366-368,418-419 Cellotetraose hemihydrate, 331 Cellulose, 326, 329-332 alternate unit cells, 329-330 derivatives, 332... 

Replacement of the ring oxygen atom of the cyclic form of an aldose or ketose by sulfur is indicated in the same way, the number of the non-anomeric adjacent carbon atom of the ring being used as locant. 

The (non-detachable) prefix carba- signifies replacement of a heteroatom by carbon in general natural product nomenclature [26], and may be applied to replacement of the hemiacetal ring oxygen in carbohydrates if there is a desire to stress homomorphic relationships. If the original heteroatom is unnumbered, the new carbon atom is assigned the locant of the non-anomeric adjacent skeletal atom, with suffix a . 

Other class of polyhydroxylated derivatives acting as sugar mimics consists of carbasugars i.e. sugars in which the ring-oxygen atom is replaced by the carbon atom. Their synthesis can be initiated either from achiral (or racemic) derivatives (this approach will not be dealt with) or more conveniently from simple sugars (chirons). Syntheses of these important compounds were comprehensively reviewed recently.4... 

Students will be familiar with carbocyclic compounds, such as cyclo-hexane and benzene, that are built from rings of carbon atoms. If one or more of the carbon atoms is replaced by another element, the product is a heterocycle. Multiple replacements are commonplace, and the elements involved need not be the same. The most common are oxygen. 

Strictly speaking, the basic motif of a carbasugar possesses a carbohydrate counterpart, be it a real molecule or only hypothetically obtainable. To be precise, the carba counterpart of a monosaccharide arises from the replacement of the hemiacetal ring oxygen in carbohydrates by a carbon atom (usually a CH2), leaving the rest of the structure and chirality intact. In literature, however, this distinction does not appear to be so clear and often polyhydroxylated carbocyclic... 

Replacement of secondary sulphonate esters which are adjacent to the anomeric carbon atom is not normally possible with azide ion owing to a combination of the electron-withdrawing effect of the acetal and unfavourable polar effects in the transition state . The latter effect is best visualized by considering the Newman projections (128) and (129) along the C(ij-C(2) bond for the 5 12 transition states of both a- and j8-anomers. In the case of the a-anomer the developing dipoles of the transition state are almost parallel and opposed to the C(i)— methoxyl and C j-ring oxygen dipoles. The situation is similar,... 

It has already been remarked that the CO group, when it does not occur as a member of a closed carbon ring, cannot act as a chromophor, and only becomes one when the oxygen is replaced by sulphur or by a nitrogenous group. So far as the present state of knowledge reaches, all benzophenone derivatives are colourless. On the other hand, thiobenzophenone yields coloured derivatives. Tetramethyldiamidothiobenzophenone [3] —... 

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