Optical activity, origin

Note 2 The optical activity originates from the presence of chiral elements in a polymer such as chiral centers or chiral axes due to long-range conformational order in a polymer (helicity) (see [2], p. 182 for helicity). 

Hydrolysis of Enol Esters. Enzyme-mediated enantioface-differentiating hydrolysis of enol esters is an original method for generating optically active a-substituted ketones (84—86). If the protonation of a double bond occurs from one side with the simultaneous elimination of the acyl group (Fig. 3), then the optically active ketone should be produced. Indeed, the incubation of l-acetoxy-2-methylcyclohexene [1196-73-2] (68) with Pichia... 

Camphor was originally obtained from the camphor tree Lauras eamphora in which it appeared in the optically active dextro-rotary form. Since about 1920 the racemic ( ) mixture derived from oil of turpentine has been more generally used. By fractional distillation of oil of turpentine the product pinene is obtained. By treating this with hydrochloric acid, pinene hydrochloride (also known as bomyl chloride) may be produced. This is then boiled with acetic acid to hydrolyse the material to the racemic bomeol, which on oxidation yields camphor. Camphor is a white crystalline solid (m.p. 175°C) with the structure shown in Figure 22.3. 

In the meantime, Spath and Gohring had effected another synthesis of both inactive bases, and resolved each of the latter into the two possible optically active forms. This synthesis was effected by the stages represented by the following five equations, which need no descriptive explanation the original paper should be consulted for a full discussion of the reactions involved. 

Citronellal which was originally termed citronellone, is one of the characteristic constituents of citronella oil, in which it occurs to a considerable extent. It can be prepared by shaking the essential oil with a hot solution of sodium bisulphite, and decomposing the resulting bisulphite compound by means of sodium carbonate and distilling the citronellal in a current of steam. Citronellal is optically active, and it is probable that the specimens isolated from natural sources are mixtures of the two optically active isomers, so that the actual theoretical rotation is not known with certainty. 

Several features of the rearrangement have been elucidated. Although in the treatment of the ester 26 with acetic acid the products were isolated in only 64% yield, evidence was obtained (22) that finally no 1,2-unsaturated compounds remained, since the noncrystalline portion on hydrogenation and deacetylation afforded only 3-deoxy-D-ribo and -d-arabino-hexose and no 1,5-anhydrohexitols. That the components of the final mixture were in equilibrium was indicated by the observation that the main component 27 underwent reaction in boiling acetic acid to give a solution with the same optical activity as that of the original reaction mixture. Thus the 2,3-unsaturated compounds are more stable than the hydroxyglycal derivatives and the a isomer 27 is more stable than its anomer 28. 

Working carefully with tweezers, Pasteur was able to separate the crystal into two piles, one of "right-handed" crystals and one of "left-handed" crys tals like those shown in Figure 9.6. Although the original sample, a 50 50 mix lure of right and left, was optically inactive, solutions of the crystals from eacl of the sorted piles were optically active, and their specific rotations were equa in amount but opposite in sign. 

Fortunately, the optical activity of the polymer freed from the optically active substituent was of the opposite direction than that of the original polymer. Hence, the observed effect cannot be due to a trace of alcohol which was not removed in the hydrolysis. 

The stereogenic sulfur atom in sulfoxides is usually configurationally stable at room temperature thus, sulfoxides may be chiral based on this property alone1. In fact, there are many examples of optically active sulfoxides of both synthetic and natural origin. This chapter reviews the important methods for obtaining optically active sulfoxides, and discusses some reactions at sulfur which either leave the coordination number at three or increase it to four, generally with preservation of optical activity. It also describes briefly some recent studies on the conformational analysis and chiroptical properties of sulfoxides. 

Optically pure (S)-benzyl methyl sulfoxide 139 can be converted to the corresponding a-lithio-derivative, which upon reaction with acetone gave a diastereomeric mixture (15 1) of the /S-hydroxysulfoxide 140. This addition reaction gave preferentially the product in which the configuration of the original carbanion is maintained. By this reaction, an optically active epoxy compound 142 was prepared from the cyclohexanone adduct 141181. Johnson and Schroeck188,189 succeeded in obtaining optically active styrene oxide by recrystallization of the condensation product of (+ )-(S)-n-butyl methyl sulfoxide 143 with benzaldehyde. 

Ogasawara and coworkers reported a concise route to (—)-kainic acid (Figure 6.51), an excitatory neurotransmitter of marine origin, via a lipase-mediated kinetic resolution of an N-Cbz aminocyclopentenol [139]. More recently, (—)-kainic acid was synthesized from an optically active butenolide prepared by enzymatic DKR [140]. 

We have previously discussed the possibilities of racemization or inversion of the product RS of a solvolysis reaction. However, the formation of an ion pair followed by internal return can also affect the stereochemistry of the substrate molecule RX. Cases have been found where internal return racemizes an original optically active RX, an example being solvolysis in aqueous acetone of a-p-anisylethyl p-nitrobenzoate, while in other cases partial or complete retention is found, for example, solvolysis in aqueous acetone of p-chloro benzhydryl p-nitrobenzoate. the pathway RX R+X some cases where internal return involves racemization, it has been shown that such racemization is faster than solvolysis. For example, optically active p-chlorobenzhydryl chloride racemizes 30 times faster than it solvolyzes in acetic acid. ... 

The Au SG clusters were optically active in the visible and UV spectral range, with the anisotropy factors in the order of several tens to hundred parts per million. The origin of the observed optical activity in core-based electronic transitions has been previously discussed [24]. The key question is whether the gold cluster is intrinsically chiral or whether the optical activity is induced by the chiral ligation environment. The first possibility has been supported by theoretical studies performed by Garzon and... 

Application of this technique to a study of the photoelimination of azo compounds has been reported by Porter, Landis, and Marrett.aw Photolysis of the unsymmetrically substituted azo compound (1) in solvents of varying viscosity revealed a dependence of on solvent viscosity as shown in Table 11.2. Photolysis of optically active (1) (40% completion) and examination of the remaining azo compound indicated that 26% of the original optical activity had been lost. This is explained by the following mechanism involving stepwise homolysis ... 

Addadi L, Lahav M (1979) in Walker DC (ed) Origins of optical activity in nature. Elsevier, New York, chapt 14... 

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

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