Aldonic acids rules

The discovery of empirical rules of rotation of derivatives of the aldonic acids which indicated the configuration of the y-, and later the a-carbon atoms, made possible an advance in the solution of the problem of establishing the configurations of the synthetic higher-carbon sugars and the naturally-occurring rhamnose and fucose. The lactone rule was the earliest of these generalizations it was inferred from very extensive data. 

Hudson lactone rule. The value of the rotation of aldonic acid lactones is decisively affected by the configuration of that carbon atom whose hydroxyl group is engaged in the cyclization. If, in the normal Fischer projection formula, the lactone ring is written on the right, the lactone is dextrorotatory if it is written on the left, the lactone is levorotatory. 

Methods for lengthening the carbon chains of sugars may involve the formation of aldonic acids as intermediates. The Kiliani cyanohydrin synthesis (see p. 106) creates two new aldonic acids with one more carbon atom than in the original aldose. The configuration of the new asymmetric atom can be assigned by use of the lactone rule discussed below. 

Normally the alkali salts of the aldonic acids are slightly more dextrorotatory than the free acids, when the hydroxyl of carbon 2 lies on the right 23). This correlation might be considered to be an extension of the hydrazide rule. The lead salts present an exception, apparently because a complex is formed between the lead ion and the hydroxyl on carbon 2 24)- The lead salts are acidic in contrast to the normal type. The rotatory displacement in relation to the calcium salts is levorotatory when the hydroxyl of carbon 2 is on the right. 

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