Specific rotations synthesis

Optically Active PO. The synthesis of optically pure PO has been accompHshed by microbial asymmetric reduction of chloroacetone [78-95-5] (90). (3)-2-Meth5loxirane [16088-62-3] (PO) can be prepared in 90% optical purity from ethyl (3)-lactate in 44% overall yield (91). This method gives good optical purity from inexpensive reagents without the need for chromatography or a fermentation step. (3)-PO is available from Aldrich Chemical Company, having a specific rotation [0 ] ° 7.2 (c = 1, CHCl ). 

Chondrodendron polyanthum, 371 Chondrodendron tomentosum, 363, 371, 373, 377, 391 alkaloids, 376 Chondrodine, 363, 364 Chondrofoline, 364, 365 Chrycentrine, 172, 313 Chiysanthemine, 773 Chrysanthemum cineraricefoHum, 773 Chuchuara, 781 Chuehuhuasha, 781 Cicuta virosa, 13 Cinchamidine, 419, 429 Cinchene, 439 Cinchenine, 438, 439, 440 apoCinchenine, 440, 441 Cincholoipon, 438 Cincholoiponic acid, 438, 443 Cinchomeronic acid, 183 Cinchona alkaloid structure, synthesis, 457 Cinchona alkaloids, bactericidal action of some derivatives, 478 centres of asymmetry, 445 constitution, 435 formulae and characters of transformation products, 449, 451 general formula, 443 hydroxydihydro-bases, 448, 452-4 melting-points and specific rotations, 446... 

Coal, structure of, 517 Coal tar, compounds from, 517 Cocaine, specific rotation of, 296 structure of. 64, 916 structure proof of. 875 synthesis of, 915... 

SN1 reaction and, 379-380 Sjvj2 reaction and, 370-371 Sorbitol, structure of, 992 Spandex, synthesis of, 1214 Specific rotation, 295 table of, 296... 

The structures of these bases have been established mainly on the grounds of their physicochemical data and have been confirmed by synthesis. In Table II the melting points, specific rotations, absolute configurations, and IR and UV spectral features are collected. 

As this synthesis started from an achiral starting material, compound 199 must be resolved to secure enantiomerically pure intermediates for the synthesis of taxol. Treatment of (+ )-diol 199 with excess ( lA)-( )-camphanic chloride in methylene chloride in the presence of Et3N forms two diastereomeric monoesters for chromatographic separation. Enantiomerically pure diol 199 can be regenerated from the ester in 90% yield with a specific rotation of +187 (c = 0.5, CHC13). 

Aminoallenes constitute an important class of functionalized allenes with interesting chemical properties. They are known as attractive substrates for constructing three- to six-membered azacycles [78]. In 1999, Ohno and co-workers reported the stereoselective synthesis of chiral a-aminoallenes 179 and 181 by RCu(CN)M-medi-ated anti-SN2 substitution of chiral 2-ethynylaziridines 178 and 180 (Scheme 4.47) [79]. The X-ray data and specific rotations of the allenes were consistent with a net anti-S- 2 substitution reaction. 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

An interesting method for the estimation of optical purity of sulfoxides, which consists of the combination of chemical methods with NMR spectroscopy, was elaborated by Mislow and Raban (241). The optical purity is usually determined by the conversion of a mixture of enantiomers into a mixture of diastereomers, the ratio of which may be easily determined by NMR spectroscopy. In contrast to this, Mislow and Raban used as starting material for the synthesis of enantiomeric sulfoxides a diastereomeric mixture of pinacolyl p-toluenesulfinates 210. The ratio of the starting sulfinates 210 was 60.5 39.5, as evidenced by the H NMR spectrum. Since the Grignard reaction occurs with full stereospecificity, the ratio of enantiomers of the sulfoxide formed is expected to be almost identical to that of 210. This corresponds to a calculated optical purity of the sulfoxide of 20%. In this way the specific rotations of other alkyl or aryl p-tolyl sulfoxides can conveniently be determined. 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

Recently, studies of the conformation of oligomers were extended to peptides derived from /3-methyl-L-aspartates. Their synthesis (n = 2 up to 14) was described by Goodman and Boardman (82), and later the specific rotations of their solutions in dimethyl formamide, dichloro-acetic acid and in chloroform were determined (83). The oligomers exist in a random-coil form in the first two solvents, but helices become stable in chloroform for n — 11 and 14. These peptides are unusual since their L-amino-acid residues produce a left-hand helix (84, 85) whereas most of the investigated polyamino acids crystallise as a right-hand helix (86). 

It is essential that the specific rotation of the product and the pure enantiomer be measured in the same solvent, at the same wavelength and temperature, and at a similar concentration if possible both measurements should be made at the same time. Optical yields in enzymic reactions carried out under laboratory conditions approach 100 per cent an asymmetric chemical synthesis may be regarded as promising if the optical yield ranges upwards from 20 per cent. 

The structure of cycleahomine (33) has been unequivocally confirmed by partial synthesis as the N (rather than N -) monoquatemary derivative of tetrandrine (see Section IV,A, 1). The specific rotation of the chloride, [a]j=>5 (CHC13, c 0.2), is +228°, rather than +103° as earlier reported (20). 

There are two different ways of making 2-ethoxyoctane from octan-2-ol using the Williamson ether synthesis. When pure (— )-octan-2-ol of specific rotation —8.24° is treated with sodium metal and then ethyl iodide, the product is 2-ethoxyoctane with a specific rotation of —15.6°. When pure (— )-octan-2-ol is treated with tosyl chloride and pyridine and then with sodium ethoxide, the product is also 2-ethoxyoctane. Predict the rotation of the 2-ethoxyoctane made using the tosylation/ sodium ethoxide procedure, and propose a detailed mechanism to support your prediction. 

The essence of asymmetric synthesis is the creation of asymmetric centers under the influence of other asymmetric centers in such a way that the resulting enantiomers or diastereoisomers are formed in unequal proportions. Most reactions in asymmetric synthesis that have been described involve the conversion of trigonal carbon atoms into asymmetric, quadrivalent carbon atoms, and this article will be principally concerned with such reactions, although, in many instances, the principles involved may also be applied to asymmetric reactions in which, for example, chiral phosphorus or sulfur atoms are formed. In all reactions in which are formed mixtures of enantiomers having one enantiomer in preponderance, it is possible to describe the stereoselectivity of the reaction in terms of optical yield (optical purity, or enantiomeric yield). The precise significance of these terms has been described in detail elsewhere,1 but, practically, where at a selected wavelength, [a] is the specific rotation of the reaction product and [A] is the specific rotation of a pure enantiomer, the optical yield = [a]/[A]. Thus, the value of the optical yield is a measure of the excess of one enantiomer over the other. 

All of these graft polymers retained some optical activity but the method of synthesis results in racemization thus no specific rotations are recorded b By elemental analysis... 

The most definitive stereochemical studies have concerned configurational changes at the a carbon. Alexander et al. (2, 138) showed that (-I-)540-CpFe(CO)2CH(Me)Ph reacts with neat SOg at —60° or — 10°C, or with SO2 in saturated pentane at 27°C, to give the corresponding iS-sulfinate. The specific rotation of the product, [a]546, varied somewhat with the method of synthesis (—186°, —176°, and —158°, respectively). It was concluded that the insertion is a substantially stereospecific process, but whether it involves retention or inversion could not be determined. 

Corossolin was isolated by a French group in 1991, and the absolute configuration of its C-10 hydroxyl group remained unknown until its total synthesis was achieved by us in 1999 (Scheme 10-8). The first total synthesis of corossolin was achieved in 1996 by Makabe et al. in Japan. However, the specific rotations and nuclear magnetic resonances (NMRs) of their two synthetic C-10 epimers are... 

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