Isomers chiral

Technetium-99m coordination compounds are used very widely as noniavasive imaging tools (35) (see Imaging technology Radioactive tracers). Different coordination species concentrate ia different organs. Several of the [Tc O(chelate)2] types have been used. In fact, the large majority of nuclear medicine scans ia the United States are of technetium-99m complexes. Moreover, chiral transition-metal complexes have been used to probe nucleic acid stmcture (see Nucleic acids). For example, the two chiral isomers of tris(1,10-phenanthroline)mthenium (IT) [24162-09-2] (14) iateract differentiy with DNA. These compounds are enantioselective and provide an addition tool for DNA stmctural iaterpretation (36). 

It appears as if an axiom of stereochemistry, the absolute identity of the most important chemical and physical properties of chiral isomers, is no longer valid. Experiments using the amino acid tyrosine (Tyr) showed unexpected differences in the solubility of D-and L-Tyr in water a supersaturated solution of 10 mM L-Tyr crystallised much more slowly than that of D-Tyr under the same conditions. The saturated solution of L-Tyr was more concentrated than that of D-Tyr. Supersaturated solutions of DL-Tyr in water formed precipitates containing mainly D-Tyr and DL-Tyr, so that there was an excess of L-Tyr in the saturated solution. The experiments were carried out with extremely great care in order to exclude the possibility of contamination. Further experiments will show whether this is a particular property of tyrosine, or whether other amino acids will show similar behaviour. Possible... 

Kaneko, H., M. Matsuo, and J. Miyamoto. 1986. Differential metabolism of fenvalerate and granuloma formation. I. Identification of a cholesterol ester derived from a specific chiral isomer of fenvalerate. Toxicol. Appl. Pharmacol. 83 148-156. 

In addition, three types of lipophilic conjugates have been found in pyrethroid metabolism studies (Fig. 4). They are cholesterol ester (fenvalerate) [15], glyceride (3-PBacid, a common metabolite of several pyrethroids) [16], and bile acid conjugates (fluvalinate) [17]. It is noteworthy that one isomer out of the four chiral isomers of fenvalerate yields a cholesterol ester conjugate from its acid moiety [15]. This chiral-specific formation of the cholesterol ester has been demonstrated to be mediated by transesterification reactions of carboxylesterase(s) in microsomes, not by any of the three known biosynthetic pathways of endogenous cholesterol esters... 

As mentioned in Sect. IV-B, enantiotopic groups even in achiral substances may show nonequivalence. For example, this property sometimes can be used to distinguish meso compounds from their chiral isomers. Thus in the presence of TFPE, the meso isomer of dimethyl 2,3-diaminosuccinate (46) shows two equally intense methoxy singlets and an AB quartet for the now diastereotopic methine hydrogens (88). This coupling clearly shows 46 to be the... 

Another ring-opening substitution reaction was observed for tricyclane 55 the attack occurred exclusively at the tatiary carbon, not at the quaternary one. The chiral isomer 56, of the symmetrical 55, has two 3 -4 bonds either of which may be the site of spin and charge, possibly in an equilibrium. Regardless of the actual structure of the radical cation, it appears that the attack of the nucleophile is less hindered at the carbon further removed from the dimethyl-substituted bridge (approach a). The isolated product 57 is optically active, and formed by backside attack on the less hindered carbon. ... 

The nucleophilic capture of tricyclane radical cations 115 " and 117 " supports the role of conventional steric hindrance 115 reacts at the 3° carbon ( 116 ), whereas the chiral isomer 117 + is captured by backside attack at the less hindered 3° carbon ( 118 ). " Both reactions are regio- and stereospecific and avoid attack at the neopentyl-type carbon (denoted by an asterisk). 

A variety of chiral [m.n]cyclophanes has been described, including [2.2](2,6> naphthalenophane (27) 49-50> and the corresponding diene 49), or [2.2](2,5)pyridino-phanes (28)51). In both cases (27, 28) achiral and chiral isomers (a, b) were isolated and their structures assigned mainly by H-nmr spectroscopy. The chiral structure... 

Labetalol is a reversible adrenoceptor antagonist available as a racemic mixture of two pairs of chiral isomers (the molecule has two centers of asymmetry). The (S,S)- and (R,S)-isomers are nearly inactive, (S,R)- is a potent blocker, and the (R,R)-isomer is a potent 13 blocker. Labetalol s affinity for receptors is less than that of phentolamine, but labetalol is -selective. Its 13-blocking potency is somewhat lower than that of propranolol. Hypotension induced by labetalol is accompanied by less tachycardia than occurs with phentolamine and similar a blockers. 

A particularly interesting case is the partial hydrolysis of the racemic N-tri-fluoroacetyl derivative 59 with hog kidney aminoacylase (HKA) (EC 3.5.1.14) to prepare chiral isomers of 2-trifluoromethylalanine 60 [53]. The stereochemical preference of hog kidney aminoacylase is to hydrolyze amino acid amides bearing the larger C-2 substituent in the pro-S position [54]. The enzymatic hydrolysis of compound 60 follows this trend. 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

FIGURE 5. Schematic diagram for stereomutation paths interrelating 1,2-d,-cyclo-propanes and a particular EE intermediate. The four C,(ts) structures link EE with chiral isomers the two C,(ts) structures separate EE and cu-1,2-d,-cyclopropane... 

Methods of designating geometrical (ZE nomenclature) and chiral isomers (RS nomenclature) are given in the IUPAC Rules for Nomenclature of Organic Chemistry, Section E,18 and will not be discussed here. 

It is clear, then, that the number of chiral isomers that may exist for a given structural isomer is 2n, where n = the number of different chiral elements (centers, axes, or planes). When identical chiral elements are present, the 2n formula does nut hold. 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

The steric requirements of the tetramesityl porphyrin ligand are likewise determining the optimal geometry of the transition states in these reactions. Therefore, it was consequent to look for a chiral porphyrin ligand which could add enantioselectivity to the processes already described. Simonneaux et al. [370] have synthesized a set of chiral porphyrins derived from the four atrop-isomers of tetrakis(o-aminophenyl) porphyrin, H2(ToAPP), by acylation with (R)-( + )-a-methoxy-a-trifluormethylphenylacetyl chloride. This mixture of chiral porphyrins H2(P ) was metallated with Ru3(CO)i2 in o-dichlorobenzene and the resulting chiral isomers of RuCO(P )THF separated by thin layer... 

The products of principal moments of inertia IA /B / calculated from the B3LYP/6-31G geometries, symmetry numbers a, numbers of chiral isomers for the C6QH2n hydrofullerenes are presented in Table 4.2. 

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