Laevo form

The melting-points of the dextto and laevo forms of any optically active compound may, as in this case, be virtually identical with that of the racemic fomi in many compounds however there is a marked difference in melting-point, and often in solubility, between the (-)-) and ( -) forms on one hand and the ( ) form on the other. 

A chiral molecule is one which exists in two forms, known as enantiomers. Each of the enantiomers is optically active, which means that they can rotate the plane of plane-polarized light. The enantiomer that rotates the plane to the right (clockwise) has been called the d (or dextro) form and the one that rotates it to the left (anticlockwise) the I (or laevo) form. Nowadays, it is more usual to refer to the d and I forms as the ( + ) and (—) forms, respectively. 

First there appears equal amounts of dextro and laevo forms in equilibrium with each other, but due to the presence of asymmetric reagent, the equilibrium gets displaced. This is actually asymmetric transformations. 

Some anomalies have been noted, the most interesting being the case of dexoxadrol (Relane, (+)- LXII) this isomer (but not the laevo form) increases... 

The sodium channel binding properties of racemic bupivacaine are stereospecific. The binding affinity of laevo- bupivacaine is less than dextro—hence the reduced cardiotoxic potential of the laevo form of the drug. 

Propranolol is chemically a naphthol derivative (dl-(isopropylamino)-3-(l-naphthyloxy)-2-propanol). It is a racemic mixture, and the laevo form is the active P-adrenergic blocking agent [10]. After oral administration, it is completely absorbed [11]. However, the systemic availability is relatively low with considerable variation in plasma levels [12,13]. The hepatic extraction of propranolol is about 80- 90%, and thus the main route of drug elimination is via hepatic metabolism [14]. One of the maj or metabolites of propranolol is 4-hydroxypropranolol, and the half-life has been reported to be 3 l-l/2h [15-18]. Since the drug is rapidly metabolized after oral administration, it necessitates a multiple dosage after oral strict patient compliance. 

Amfetamine has had multifarious uses. It is now obsolete for depression and as an appetite suppressant, and its use in sport is abuse (see before). There is concern that its illicit use as a psychostimulant is widespread. Amfetamine is a racemic compound the laevo-form is relatively inactive but dexamphetamine (the dextro- isomer) finds use in medicine. Amfetamine will be described, and structurally-related drugs only in the ways in which they differ. 

As an example of the co-existence of systematic, semi-systematic and trivial names, we could look at the monoterpenoid ketone, carvone. Carvone occurs in both enantiomeric forms in nature, the laevo-form in spearmint and the dextro-form in caraway. The trivial name carvone is derived from the Latin name for caraway, Carum carvi. The basic carbon skeleton is that of l-isopropyl-4-methylcyclohexane. This skeleton is very common in nature and is particularly important in the genus Mentha, which includes various types of mint, since it forms the backbone of most of the important components of mint oils. The skeleton has therefore been given the name p-menthane and the numbering system used for it is shown in Figure 1.3. Therefore, any of the following names may be used to describe the same molecule carvone, p-mentha-1,8-dien-6-one and 1 -methyl-4-(l-methylethenyl)cyclohex-l-ene-6-one. To classify it, we could say it was an unsaturated ketone of the /7-menthane family of monoterpenoids. 

The active laevo form, containing the two laevo rotating molecules. 

Dextro- and laevo-forms are now designated in all modern texts as (-h) and (-) respectively. The optically inactive racemate, a true double compound... 

The limonenes and dipentene occur in the leaf oils, the dextro-rotatory form reaching a maximum in Callitris arenosa, and the laevo-form in Callitris mtratropica. In these oils is seen a well-defined illustration of the formation in nature of the two active forms of limonene in the same plant, as well as the racemic modification. 

The ( (inversion of active tartaric acid into the inactive forms is known. s nwe mi Million, Aw< according to Winther is effected by the uiU uhangc ol the gi-oujis round each asymmetric carbon atom successively so that p.art of the active acid is fiisl con-wiled into luesotiut.nit acid, uliidi then passes into the laevo ariety,... 

It is obtained from American turpentine as dextro-o-pinene, or from French turpentine as laevo-a-pinene. It is also obtained in a very pure form as dextro-a-pinene from Greek oil of turpentine. Optically inactive o-pinene can be obtained by regeneration from the nitrosochloride. The purest specimens of a-pinene yet obtained have the following characters —... 

There appear-to be either several isomerides of pinonic acid, or such closely related oxidation products as to render the purification of the acid a matter of great difiiculty. The characters of the dextro- and laevo-rotatory forms of this acid have, ho t ever, been settled by the researches of Barbier and Grignard and himmel Co, ... 

By oxidation of d- and Z-pinene of high rotatory power, Barbier and Grignard obtained the optically active forms of pinonic acid. Z-pinene from French turpentine oil (boiling-point 155 to 157 , od - 37 2 157 to 160 , tto - 32 3°) was oxidised with permanganate. From the product of oxidation, which (after elimination of the volatile acids and of nopinic acid) boiled at 189 to 195 under 18 mm. pressure, Z-pinonic acid separated out in long crystalline needles, which, after recrystallisation from a mixture of ether and petroleum ether, melted at 67° to 69 . The acid was easily soluble in water and ether, fairly soluble in chloroform, and almost insoluble in petroleum ether. Its specific rotation is [a]o - 90-5 in chloroform solution. Oximation produced two oximes one, laevo-rotatory, melting-point 128 and the other, dextro-rotatory, melting-point 189° to 191°. 

Up till about three years ago, there appeared to be little reason to doubt that rhodinol was in fact an impure form of citronellol, the reduction product of citroneUal being dextro-citronellol, whilst the natural alcohol, which the French chemists had termed rhodinol was considered to be laevo-citronellol. 

Laevo-menthone is converted into its oxime by means of hydroxylamine.. This is treated with strong sulphuric acid, and so inverted to isomenthone oxime. This is treated with phosphorus trichloride in chloroform solution, when hydrochloric acid is given off, and menthonitrile is formed. The last named is reduced by sodium into menthonylamine, and the oxalic acid compound of this is warmed with sodium niteite solution when, menthonyl alcohol is formed. This body has the following characters —... 

According to Read and Smith i piperitone is, under natural conditions, optically inactive. By fractional distillation under reduced pressure, it is prepared, by means of its sodium bisulphite compound, in a laavo-rotatory form. The slight laevo-rotation is probably due to the presence of traces of cryptal. By fractional distillation alone, it is usually obtained in a laevo-rotatory form whether this is due to decomposition products or not is unknown. Piperitone has a considerable prospective economic value, as it forms thymol by treatment with formic chloride, inactive menthone by reduction when a nickel catalyst is employed, and inactive menthol by further reduction. Its char-Mters are as follows —... 

Carvone, CjjHj40, is the ketone characteristic of dill and caraway oils. It occurs in the dextro-rotatory form in these oils, and as laevo-carvone in kuromoji oil. 

Fenchone, Cj Hj O, is found in fennel oil and in the oil of Lavandula Stoechas, in its deirtro-rotatory form, and as laevo-fenchone in oil of thuja leaves. It can be extracted from these oils by treating the fraction boiling at 190° to 195° with nitric acid, or permar anate of potassium, and then steam distilling the unaltered fenchone. 

The laevo-rotatory form is the ordinary malic acid while the dextrorotatory form is an ethylene oxide resulting from the migration of the hydrogen of the hydroxyl on the asymmetric carbon to the carbonyl oxygen of the carboxyl group. 

In general, the maximum number of optically active isomers is given by 2n where n represents the number of asymmetric carbon atoms. Thus for a compound where n = 1, as in lactic acid, there would be two stereoisomers, one the dextro and the other the laevo. For a compound with two asymmetric carbon atoms, there would be 22 = 4 stereoisomers. But if the two asymmetric carbon atoms carry exactly identical groups, as in tartaric acid, the number would be fewer than four and we know that it exists in three forms, the d the 1 and the meso. 

---