Dextro-rotatory compounds

Faolini and Divizia have succeeded in partially resolving inactive linalol into its optically active isomers hut only to the extent of optical rotations of + 1° 70 and — 1° 60 respectively. Linalol was converted into its acid phthalate, and an alcoholic solution of this compound was treated with the equivalent quantity of strychnine. By fractional crystallisation the laevo-rotatory salt, yielding dextro-rotatory linalol, separates first, leaving the more soluble dextro-rotatory strychnine salt, which yielded laevo-rotatory linalol in the mother liquor. 

By hydrolysis of a dextro-rotatory 2-octyl nitrate in a neutral medium, this compound has been decomposed to yield a laevo-rotatory product containing 13% of 2-octanone and 87% of 2-octanol, the latter comprising 71% of (-)2-octanol and 29% racemate, i.e. 85.5 parts of laevo-rotatory and 14.5 parts of dextro-rotatory alcohol. 

In solution, enantiomers rotate the plane of oscillation of linearly polarized light in opposite directions hence they are referred to as "dextro-rotatory" or levo-rotatory designated by the prefixes d- or (+)- and /-or (-)-, respectively. The direction of rotation gives no clue concerning the spatial structure of enantiomers. The absolute configuration, as determined by certain rules, is described by the prefixes (S)- and (R)-. In some compounds, designation as the D-and L-forms is possible by reference to the structure of d- and L-glyceraldehyde. 

Dextro, Levo and Inactive Compoimds.— All optically active substances rotate the plane of polarized light either in one direction to the right or in the contrary direction to the left. On this account they are known as right-handed or dextro-rotatory and left-handed or levo-rotatory. The phenomenon of polarization being purely a physical one will not be discussed here. An explanation of it may be found in text books of physics or in chemical books which consider in detail such subjects as the sugars. All that need be added is that optically active compounds are readily examined by means of an instrument known as a polariscope and the direction of rotation (right or left) and the exact amount of rotation in degrees may be accurately determined. 

What we are concerned with at this time is an explanation on chemical grounds of the important fact that three amyl alcohols or pentanolSf are known all of which possess the same structural formula viz., 2-methyl butanol-i and that one of these compounds is dextrorotatory another is levo-rotatory and the third one is inactive. These three are different individual compounds with practically the same physical properties other than optical. The inactive variety of 2-methyl butanol-1 differs, however, from the other seven structurally isomeric pentanols which are likewise inactiye not only in its structure but also in the fact that by means of certain reactions there may be obtained from it both the dextro-rotatory and the levo-rotatory compounds. In it, and in other inactive compounds of the same kind, there are present equivalent amounts of the two oppositely active compounds,... 

The malic acid obtained from fruits is optically active. Its aqueous solutions exhibit a behavior toward polarized light which is unusual. In concentrated solutions the acid is dextro-rotatory as the liquid is diluted the extent of rotation decreases until it becomes zero, when, upon further dilution, the solution becomes levorotatory. A number of theories have been put forward to explain these and similar facts, but none is satisfactory. There are many unsolved problems in stereo-chemistry. Optically active substances are involved in the building up of living matter, and nature in selecting the units for this purpose differentiates between dextro and levo compounds. The importance of this field of investigation is evident, and valuable work is being done in it. 

Racemic acid is of considerable historical interest as it was the first inactive substance to be resolved into optically active compounds. The remarkable discovery was made by Pasteur in 1848 in an investigation of the crystalline structure of the salts of racemic acid. It was found that two kinds of crystals, which differed slightly in the relative position of the faces they contained, were formed when a solution of the sodium ammonium salt of racemic acid was allowed to crystallize spontaneously. The relation in form which the two kinds of crystals bear to each other, is that of an object and its reflection in a mirror. Pasteur separated the two kinds of crystals and examined the solutions of each in polarized light. He found that one solution was dextro-rotatory and the other was levo-rotatory. From the two salts two acids were isolated one was ordinary d-tartaric acid, the other a new acid which was levo-rotatory. When equal weights of the two acids were mixed and recrystallized, inactive racemic acid was obtained. 

The third method devised by Pasteur depends on the action of certain moulds and bacteria on racemic compounds. One of the active substances is more rapidly destroyed than the other. When the mould penicillium glaucum develops in a dilute solution of racemic acid, the dextro-rotatory acid is destroyed and Z-tartaric acid is obtained. 

For every optically active dextro- or laevo-rotatory compound, a corresponding stereo-isomer, or epimer, exists, having the same formula but exactly opposite optical properties. This can only be explained by assmning that the groups attached to each asymmetric carbon atom can be arranged in either of two ways, and that the fact that chemical compounds occupy tri-dimensional space must be recognised in constructing formulae. 

The authors found that on elimination of the halogen acid from this compound, rhodinol, and not citronellol, is regenerated, dextro-rhodinol from dextro-citroneUol, and laevo-rhodinol from the laevo-rotatory alcohol from oil of rosesior geranium, the two bodies, in the latter case being identical. 

Pasteur.— The discovery that optically active substances exist in two forms dextro-rotatoryj and levo-rotatory and that the corresponding inactive compound is composed of equal amounts of the dextro and levo and may be split into these two forms, was made by Pasteur, who, because of his later remarkable work in the field of pathology, is not generally known as a chemist. Pasteur made this discovery during a study of tartaric acid, and it will be spoken of again when we come to that compound. 

Asymmetric Photodecarboxylations - Acridine and diphenylacetic acid are both achiral molecules, but despite this fact the two compounds self-assemble to form dextro- and /aevo-rotatory chiral mixed crystals from acetonitrile solution. Irradiation of either the dextro- or /aevo-rotating crystals causes a stereospecific decarboxylating condensation, which yields excess of (+)- or (—)-(68), respectively, with about 35% ee. ... 

During the elucidation of the structure of mannosidostreptomycin by methylation. Fried and Stavely isolated two derivatives of a dimethyl ether of iV-methyl-L-glucosamine. In order to compare their physical constants with those of a known compound in the n-series. Fried and Walz prepared derivatives of 2-deoxy-3,6-di-0-methyl-2-methylamino-n-glucose. The addition of methylamine and hydrogen cyanide to 2,5-di-0-methyl-D-arabinose (prepared by the periodate oxidation of 3,6-di-O-methyl-D-glucose) was followed by hydrolysis, and jdelded a mixture of a dextro- and a levo-rotatory acid. The dextrorotatory product was con-... 

The term racemic is used as a general term in describing stereo-isomers to indicate that an inactive substance consists either of a mixture of the dextro- and levo-rotatory forms or is produced as the result of the formation of a molecular compound from these forms (315). Thus the, acid which has been called inactive malic acid is said to be the racemic form of malic acid. 

Much confusion has arisen from the indiscriminate use of the small prefix d or Z to denote chemical relationship as well as optical rotation, and to avoid mistakes it is now usual to employ the prefix (+) or (—) to indicate if a compound is dextro- or laevo-rotatory, the large capital D or L being used to denote lineal descent from the (+) or (—) chemical ancestor. 

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