Isomerism dextrorotatory

Levene and Walti also reduced phytochemically l-hydroxy-3-buta-none to the levorotatory 1,3-butanediol and l-hydroxy-2-heptanone to the dextrorotatory 1,2-heptanediol. It seems that the optically active glycols that are obtained by bioreduction of hydroxy ketones with fermenting yeast are configurationally related. But the 1,3-butanediol that is obtained by the reduction of the l-hydroxy-3-butanone has the opposite configuration from the product of bioreduction of the isomeric d,Z-acetaldol (see p. 81). 

When the above-mentioned ring expansion with diazomethane 74) of trimethyl-dioxo[2.2]metacyclophane 65 (methylation was necessary to increase the inversion barrier to > 130 kJ) was performed in the presence of optically active alcohols at —60 °C, asymmetric induction occurred to an extent of ca. 40% ee (enantiomeric excess as determined by nmr-spectroscopy in the presence of chiral shift reagents)85). (+)-DibutyI tartrate favoured the dextrorotatory diketone 66 ([a]D 160° for the optically pure product) — the isomeric 67 was formed only with 3% ee (—)-ethyl lactate on the other hand led to an excess of (+)-67 ([a]D +240°) but gave (+)-66 with only 10% ee85). 

Exercise 5-5 The work of the German chemist Wislicenus on hydroxypropanoic acids was influential in the development of van t Hoff s ideas on stereoisomerism. By 1869, Wislicenus had established that there are three isomeric hydroxypropanoic acids, let us call them A, B, and C, of partial structure CzH4(OH)(C02H). Isomer A was isolated from sour milk and Isomer B from a meat extract. Both A and B had the same physical properties, except for optical rotation, wherein A was levorotatory and B was dextrorotatory. Isomer C was not optically active and had considerably different physical and chemical properties from A or B. Work out structures A, B, and C in as much detail as you can from the information given. 

The beanlike seeds of the trees and shrubs of the genus Erythrina, a member of the legume family, contain substances that possess curare-like activity. The plants are widely distributed in the tropical and subtropical areas of the American continent, Asia, Africa, and Australia, but apparently they are not used by the natives in the preparation of arrow poisons. Of 105 known species, the seeds from more than 50 have been tested, and all were found to contain alkaloids with curariform properties. Erythroidine, from E. americana, was the first crystalline alkaloid of the group to be isolated. It consists of at least two isomeric alkaloids, a and P-erythroidine both are dextrorotatory. Most experimental and clinical study has centered on the b form because it is more readily obtainable in pure state. P-Erythroidine is a tertiary nitrogenous base. Several hydrogenated derivatives of p-erythroidine have been prepared of these, dihydro-P-erythroidine has been studied most carefully and subjected to clinical trial. Conversion of P-erythroidine into the quaternary metho salt (p-erythroidine methiodide) does not enhance, but rather almost entirely, abolishes its curariform activity this constitutes a notable exception to the rule that conversion of many alkaloids into quaternary metho salts results in the appearance of curare-like action. 

Optical isomerism occurs in molecules with an asymmetric carbon atom - that is one that is attached to four different atoms or functional groups. The two different optical isomers are called optical isomers or enantiomers and rotate polarized light in opposite directions. The cZ-isomer rotates it clockwise and is called dextrorotatory the Z-isomer rotates it anticlockwise and is called Zaevorotatory. Despite their structural similarity, optical isomers exhibit significantly different physiological properties. In the case of essential oils their smells can be very dissimilar. When a compound is made up of equal amounts of the d- and Z-isomers it will be optically inactive and is called a racemic mixture, or racemate. 

Early steroid chemists characterised cholesterol and cholest-5-ene as their 5,6-dibromides, but the work was complicated by the isolation of two isomeric dibromides in each case. It was recognised [i] that one isomer of each pair was stable and dextrorotatory, while the other was unstable andlaevorotatory. Solutions of each isomer in chloroform were found to exhibit mutarotation [2], corresponding to the establishment of equilibria between isomeric pairs of dibromides, and it is now recognised that the position of equilibrium is determined by conformational strains, mainly arising from the large syn diaxial interaction between a 6jS-bromo substituent and the Ca9)-methyl group (Fig. 43) of, p. 10). 

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,... 

Asymmetric Carbon.—Now van t Hoff and LeBel found that all optically active compounds contained at least one such carbon atom. They ascribed the existence of two optically active forms to the presence in the compound of this uns3anmetrically related or asymmetric carbon atom. The asymmetry of the compounds, in that one form is dextrorotatory the other levo-rotatory, is due to this asymmetric arrangement of the molecule in space. We emphasized the fact that our structural formulas as we have been using them are simply plane representations of relationships, and indicate nothing as to the arrangement in space of the atoms or groups in a molecule. The theory of van t Hoff and LeBel considers the molecule as it is arranged in space. The isomerism so explained is known as stereo-isomerism or space isomerism. 

This sugar is also an aldo-pentose and is stereo-isomeric with arabinose. It is known as wood sugar because it is obtained by the hydrolysis of wood gum, i.e.f of the pentosans present in this gum. It is crystalline and melts at 140°- 60°. It is optically active, being dextrorotatory. Its osazone melts at 160°. By reduction it yields a penta-hydroxy alcohol and by oxidation it yields tri-hydroxy glutaric acid. 

The diltiazem molecule ((3), R3 = OCOMe, R7 = H) has two chiral centres and it is also capable of cis-trans isomerism at these two carbon atoms. In general, the trans compounds do not cause vasodilation. Diltiazem is the dextrorotatory cis enantiomer. The laevorotatory cis enantiomer has a 10-fold longer duration of activity than diltiazem in increasing blood flow in the coronary sinus [75]. 

Talapatra and co-workers (200) also studied the Ravenia alkaloids and recorded the isolation of (—)-ravenoline mp 98-99° [a]D—5 1° (CHCIj) rather than the dextrorotatory enantiomer obtained by Paul and Bose. This discrepancy has not yet been resolved. Another alkaloid isomeric with ravenoline was first called spectabiline, but to avoid confusion with another compound of the same name this was later changed to lemobiline. (-)-Lemobiline (267) (mp 198° picrate, mp 198°) is identical with the alkaloid isolated from F. ifflaiana. Lemobiline and ifflaiamine retain water of crystallization tenaciously, and this probably accounts for the widely different melting points recorded for the alkaloids. 

In pyridine solutions, the statistically corrected relative catalytic coefficients of tertiary amines for 1-methylindene isomerization decreased in the order24 4. quinuclidine, 80 DABCO, 10 triethylamine, 1. The smaller catalytic effectiveness of DABCO than quinuclidine is attributable to its weaker basicity is —30eu for each of these bicyclic bases. On the other hand, triethylamine is about as basic as quinuclidine, but must lose considerable rotational freedom in the rate-limiting proton transfer. This is reflected in the more negative entropy of activation (—39eu) for the triethyl-amine-catalyzed reaction. In pyridine solution, there is a close correlation between pa s of the catalyzing base and A// for 1-methylindene isomerization. Asymmetric catalysis was demonstrated in the quinine-catalyzed isomerization of optically active 1-methylindene in pyridine at 25°C the dextrorotatory indene isomerized nearly twice as fast as its enantiometer247. 

Tryptoquivaline E (FTE), like tryptoquivalines C and D, is dextrorotatory and exhibits a positive Cotton effect in its o.r.d. spectrum. These observations, together with the similarity of the n.m.r. patterns owing to the three-proton systems at positions 12 and 13 in FTC, FTD, and FTE, strongly suggest that all three metabolites have the same stereochemistry. Hence the complete stereochemistry proposed for FTE is as shown in (48). FTH, the laevorotatory C-12 epimer of FTE, can be obtained by epimerization of FTE in the presence of dilute alkali, and thus has the stereochemistry shown in (49) this facile isomerization indicates that FTH may well be an artefact. "... 

Make a reaction flowchart (roadmap diagram), as in previous problems, to organize the information provided to solve this problem. An optically active compound A (assume that it is dextrorotatory) has the molecular formula CyHuBr. A reacts with hydrogen bromide, in the absence of peroxides, to yield isomeric products, B and C, with the molecular formula C7Hi2Br2. Compound B is optically active C is not. Treating B with 1 mol of potassium rerr-butoxide yields (+)-A. Treating C with 1 mol of potassium rerr-butoxide... 

Working in the mid-nineteenth century in a country famous for its wine industry, Pasteur was aware of two isomeric acids that deposit in wine casks during fermentation. One of these, called tartaric acid, was optically active and dextrorotatory. The other, at the time caiied racemic acid, was optically inactive. 

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