Oxygen, anomeric

Male palaearctic bees (Andrena wilkella) in Sweden produce 2,8-dimethyl-l,7-dioxaspiro[5.5]undecane (88, Figure 4.34) as their pheromone. Its (25, 6R,83 )-isomer is pheromonally active, while the opposite enantiomer (2R,6S,8R)-88 is inactive.63 There are six stereoisomers of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecane (88), three as shown in Figure 4.34 and their opposite enantiomers. Due to the oxygen-anomeric effect, (2S,6R,8S)-88 and its opposite enantiomer are the stable isomers. 

Based upon the results of the studies of VaseUa et al. [85] and Franck et al. [95], it can be concluded that the reverse anomeric effect does not exist. The electropositive substituent at the anomeric carbon produces the enhanced endo-snometic effect but not a reverse anomeric effect [97]. Non-solvated electropositive axial anomeric substituents such as nitro and sulfone groups show the ent/o-anomeric effect to be much larger than the oxygen anomeric effect, namely, 2.35 kcal/mol for the nitro group and 3.05 kcal/mol for the sulfone group, as opposed to the endo-anomeric effect for the oxygen atom of 0.85-1.54 kcal/mol. 

Orienf fhe Haworth formula of the carbohydrate with the ring oxygen at the back and the anomeric carbon at the right... 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

The anomeric carbon of a furanose or pyranose form of a ketose bears both a hydroxyl group and a carbon substituent In the case of 2 ketoses this substituent is a CH2OH group As with aldoses the anomeric carbon of a cyclic hemiacetal is readily identifi able because it is bonded to two oxygens... 

An electronegative substituent adjacent to a ring oxygen atom also shows a preference for an axial orientation. This is known as the anomeric effect , and is particularly significant to the conformations of carbohydrates (B-71MI20100, B-83MI20100). 

The preference for the gauche arrangement is an example of the anomeric effect. An oxygen lone pair is anti to fluorine in the stable conformation but not in the unstable conformation. 

Even molecules as simple as dimethoxymethane give evidence of anomeric effects. The preferred conformation of dimethoxymethane aligns each C—O bond with a lone-pair orbital of the adjacent oxygen. ... 

In cyclic systems such as 1, the dominant conformation is the one with the maximum anomeric effect. In the case of 1, only conformation lA provides the preferred antiperiplanar geometry for both oxygens. Antiperiplanar relationships are indicated by including lone pairs in the oxygen orbitals. Other effects, such as torsional strain and nonbonded repulsion, contribute to the conformational equilibrium, of course. Normally, a value of about 1.5 kcal/mol is assigned to the stabilization due to an optimum anomeric interaction in an acetal. 

Gluconolactone shows no exchange. The reason is that the tetrahedral intermediate is formed and breaks down stereoselectively. Even though proton exchange can occur in the tetrahedral intermediate, the anomeric effect leads to preferential loss of the axial oxygen. 

A. J. Kirby, The Anomeric Effect and Related Stereoelectronic Effects at Oxygen, Springer-Verlag, New %rk, 1983. 

A point to be emphasized about glycoside fonnation is that, despite the presence of a number of other hydroxyl groups in the carbohydrate, only the anomeric hydroxyl group is replaced. This is because a carbocation at the anomeric position is stabilized by the ring oxygen and is the only one capable of being fomned under the reaction conditions. 

Electron pair on ring oxygen can stabilize carbocation at anomeric position only. 

The anomeric carbon in a cyclic acetal is the one attached to two oxygens. It is the carbon that conesponds to the car bonyl carbon in the open-chain form. The symbols a and p refer to the configuration at the anomeric carbon. 

Anomeric carbon (Section 25.6) The carbon atom in a fura-nose orpyranose form that is derived from the carbonyl carbon of the open-chain form. It is the ring carbon that is bonded to two oxygens. 

If a cyclic form of a sugar is to be represented in the Fischer projection, a long bond can be drawn between the oxygen involved in ring formation and the (anomeric)... 

If a cyclic form is to be named, the locants of the anomeric centre and of the carbon atom bearing the ring oxygen atom must be given (in that order) (cf. 2-Carb-6.4). If there is more than one ring size designator, they are placed in alphabetical order (e.g. furanose before pyranose). 

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 . 

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