As an illustration it may be pointed out that in the case of the two known types of lactones - the y-lactones, which contain four carbon atoms and one oxygen atom in the ring, are more readily formed and more stable (less readily hydrolysed) than the S-lactones, which contain one oxygen and five carbon atoms in the ring.
As a useful preliminary it is convenient to divide heterocyclic ring systems into two leading groups: (I) systems resulting from simple internal dehydration (or similar condensations) of saturated aliphatic compounds - such compounds are: the internal anhydrides or cyclic ethers of the glycols and thioglycols (ethylene oxide, &c.); the cyclic alkyleneimides resulting from the splitting off of ammonia between the amino groups of diaminoparaffins (pyrrolidine, piperazine, &c.); the cyclic esters of oxycarboxylic acids (lactones, lactides); the internal anhydrides of aminocarboxylic acids (lactams, betaines); cyclic derivatives of dicarboxylic acids (anhydrides, imides, alkylen-esters, alkylenamides, &c.).
- The cyanhydrins on hydrolysis give monocarboxylic acids, which yield lactones; these compounds when reduced by sodium amalgam in sulphuric acid solution yield a sugar containing one more carbon atom.
Other forms are: d- and l-gulose, prepared from the lactones of the corresponding gulonic acids, which are obtained from d- and /-glucose by oxidation and inversion; d- and l-idose, obtained by inverting with pyridine d- and l-gulonic acids, and reducing the resulting idionic acids; d- and l-galactose, the first being obtained by hydrolysing milk sugar with dilute sulphuric acid, and the second by fermenting inactive galactose (from the reduction of the lactone of d, l-galactonic acid) with yeast; and d- and l-talose obtained by inverting the galactonic acids by pyridine into d- and l-talonic acids and reduction.
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