Asymmetric Synthesis

Asexuality should be treated as an anomaly of cerebral origin, in the sense of anesthesia of the psychosexual center. In such cases, various other disturbances of brain functions, symptoms of degeneration, and even anatomical signs of degeneration are usually noted. A decrease in libido as a result of sexual excesses rarely reaches the point of complete anesthesia, and therefore cannot be considered as true asexuality. Conversely, acquired sexual asexuality can in some cases be a consequence of organic lesions of the cerebral cortex or such mental states as, for example, melancholia. Thus, asexuality, in the sense of the absence or extinction of libido, can be of both peripherally incretory (from the side of the sex glands) and central origin. Physiologically, asexuality, in the sense of the absence of sexual desire, occurs in children and in old age, although Freud finds it possible to speak of manifestations of sexuality even in an infant.

G. Sakharov.

Asymmetric Synthesis, Resolution. During the artificial formation of asymmetric C atoms from molecules not containing them, there is an equal probability of the emergence of both optically opposite configurations, i.e., the production of a new substance in an optically inactive racemic configuration. If, however, a new C atom is created in the presence of already existing molecular asymmetry, then the relationships of the newly emerging asymmetric atom to its initial system are not identical for both antipode configurations, as a result of which the probability of the formation of one of the configurations is greater than the other. If one then removes the initial, optically active system from the molecule synthesized in this way, the newly emerged substance will be optically active. Such a synthesis is called asymmetric. Thus, for example, in the presence of quinine, the synthesis of benzaldehyde with hydrocyanic acid proceeds asymmetrically and leads to the formation of the optically active nitrile of mandelic acid. Quinine acts here as an optically active catalyst, just as the enzyme emulsin does, which causes the same synthesis. Asymmetric syntheses occur on a large scale in living cells. The very first organic synthesis, the synthesis of sugar in plants from CO₂ and H₂O, which proceeds with the participation of optically active chlorophyll grains, leads to the formation of optically active sugar. All numerous natural substances containing an asymmetric C atom turn out to be optically active, and not racemic, with very few exceptions, and moreover, almost all, with rare exceptions, are found in the form of only one of the optical antipodes. If their formation can be explained by asymmetric synthesis, the cause of the initial emergence of an optically active substance from an inactive one has not yet been determined. Syntheses can proceed asymmetrically even when introducing into the organism substances that are not its natural components; thus, for example, when introducing into the organism hydroxyphenylpyruvic acid HO.C₆H₄.CH₂.CO.COOH, which does not contain an asymmetric C atom, levorotatory tyrosine HO.C₆H₄CH₂.CH(NH₂).COOH is formed from it, i.e., the natural stereoisomer. In the laboratory, when obtaining optically active configurations, one can also start from racemic compounds, resolving them into optically active components. Pasteur provided three classical methods for the separation of such substances: selection of crystals by hand, resolution based on the different solubility of salts of optical antipodes with one and the same optically active substance, and the consumption of one of the antipodes by microorganisms; in most cases, the antipode that is found in nature is consumed. Asymmetric resolution of racemic compounds also takes place in the animal organism; thus, for example, the racemic dipeptide alanyl-leucine consists of two combinations: 1) d-alanyl-d-leucine + l-alanyl-l-leucine and 2) d-alanyl-l-leucine + l-alanyl-d-leucine; in nature, only d-alanine and l-leucine are found; accordingly, upon the action of pancreatic juice on the aforementioned racemic dipeptide, the first combination, as being formed by stereoisomers not found in nature, remains unchanged in racemic form, while from the second combination, its first part is decomposed into d-alanine and l-leucine, and the second remains untouched in the form of the now optically active l-alanyl-d-leucine.

S. Deyanovsky.