Chirality occurrence

Folic acid, 4-amino-4-deoxy-10-methyl-, 1, 164 3, 325 as anticancer drug, 1, 263 biological activity, 3, 325 Folic acid, 4-amino-10-methyl-toxicity, 1, 141 Folic acid, 7,8-dihydro-biosynthesis, 3, 320 synthesis, 1, 161, 3, 307 Folic acid, 4-dimethylamino-hydrolysis, 3, 294 Folic acid, 5-formiminotetrahydro-biological activity, 3, 325 Folic acid, 5-formyl-5,6,7,8-tetrahydro-biological activity, 3, 325 chirality, 3, 281 occurrence, 3, 325 Folic acid, 10-forfnyltetrahydro-biological activity, 3, 325 Folic acid, 5,10-methenyl-5,6,7,8-tetrahydro-biological activity, 3, 325 chirality, 3, 281 Folic acid, 5-methyl-chirality, 3, 281 Folic acid, 9-methyl-toxicity, 1, 141... 

The introduction of synthetic materials into natural products, often described as adulteration , is a common occurrence in food processing. The types of compounds introduced, however, are often chiral in nature, e.g. the addition of terpenes into fruit juices. The degree to which a synthetic terpene has been added to a natural product may be subsequently determined if chiral quantitation of the target species is enabled, since synthetic terpenes are manufactured as racemates. Two-dimensional GC has a long history as the methodology of choice for this particular aspect of organic analysis (38). 

For cations 74-75 (Fig. 27), low temperature NMR experiments were necessary to reveal stereodynamical behaviors and allow the observation of split signals for the enantiomers [38,144]. Stereoselective recognition between the chiral cations and anions was observed in essentially all cases as integration of the split signals revealed the preferential occurrence of one diastereomeric salt over the other. 

The Rosetta mission with its planned landing on a comet, with analysis of cometary material (see Sect. 3.2), should provide more information on the occurrence of chiral molecular species in the cosmos (Adam, 2002). The GC-MS apparatus installed in the robotic lander RoLand is also able to separate and analyse chiral organic molecules (Thiemann and Meierhenrich, 2001). 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

Keywords Chirality Hydrolysis Metabolism Mobility Occurrence ... 

Buser, H.-R., Poiger, T. and Muller, M. D. (1999). Occurrence and environmental behaviour of the chiral pharmaceutical ibuprofen in surface waters and in waste-water, Environ. Sci. Technol., 33, 2529-2535. 

The configurational stability of chiral allenylmetal reagents depends to a large extent on the nature of the metal substituent. The mechanism of the racemization process has not been studied in detail, but two reasonable pathways can be proposed, based on known reactivity characteristics of these compounds. The first entails reversible intermolecular SE- rearrangement to the propargylic isomer. This process could proceed by a pure syn or anti pathway, in which case no racemization would take place. However, the occurrence of both pathways would result in racemization (Scheme 9.5). 

Traditional models for diastereoface selectivity were first advanced by Cram and later by Felkin for predicting the stereochemical outcome of aldol reactions occurring between an enolate and a chiral aldehyde. [37] During our investigations directed toward a practical synthesis of dEpoB, we were pleased to discover an unanticipated bias in the relative diastereoface selectivity observed in the aldol condensation between the Z-lithium enolate B and aldehyde C, Scheme 2.6. The aldol reaction proceeds with the expected simple diastereoselectivity with the major product displaying the C6-C7 syn relationship shown in Scheme 2.7 (by ul addition) however, the C7-C8 relationship of the principal product was anti (by Ik addition). [38] Thus, the observed symanti relationship between C6-C7 C7-C8 in the aldol reaction between the Z-lithium enolate of 62 and aldehyde 63 was wholly unanticipated. These fortuitous results prompted us to investigate the cause for this unanticipated but fortunate occurrence. 

About new emerging pesticides such as chiral pesticides most of the works are focused on environmental occurrence [221, 222]. 

S,2S,AS) —)-trans-V vpcntonc oxide (dihydro rotundifolone) has been isolated for the first time from Mentha longifolia by Reitsema and Varnis. The presence of the (15,25,4i )(—l-ar-isomer was reported in aAf. spicata mutant. Only enantiomerically pure levorotatory piperitone oxides, (15,25, AS)-trans piperitone oxide and (15,25, AR)-cis piperitone oxide, were detected by chiral GC analysis of Micromeria fruticosa and Mentha longifolia The occurrence of the cis- and trans- piperitone oxides was dependent on the population of the species. In most cases, the enantiomeric composition of trans- and cis- piperitone oxides detected in various essential oils is unknown. 

Cyclizable, chiral and dehalogenation probe compounds have been successfully used to detect the occurrence of ET during the addition reactions of Grignard and other reagents. However, no such probe experiment has been examined for lithium reagents. 

Stereoselective polymerization may proceed by ionic or coordination mechanisms. In many cases one admits that in the counterion or in the catalytic complex enantiomeric active centers exist, which give rise to predominantly (R) or (S) chains, respectively. Such centers may exist prior to polymerization or may be formed by reaction of a nonchiral precursor with the enantiomeric mixture of the monomers. Alternatively, one can think that the stereoselectivity depends mainly on the interaction between the entering monomer molecule (which is chiral) and the last unit in the chain (also chiral) according to this hypothesis, the enantiomeric excess inside each chain is generally low, because the occurrence of an accidental error brings about an inversion of the sense of stereoselection. 

Whereas the simple bidentate nitrogen ligands proved to be rather limited, the frequent occurrence of a set of four P-phenyl or alkyl substituents, e. g., in coordinated Binap, MeO-Biphep, Josiphos or Duphos (shown, from left to right in Scheme 1.4), offered many more reporters . In this way, one can develop a more detailed NOE picture of how the complexed substrate interacts with the chiral pocket offered by these auxiliaries. From these NOE studies [97, 98] it can be shown that the atropisomeric bidentate ligands Binap and MeO-Biphep tend to have fairly classical axial and equatorial P-phenyl substituents. 

Thirty different esterases and lipases were tested. The rate of release of 14 in the wells of standard microtiter plates was monitored by fluorescence (70,86). Control experiments ensured that the apparent rate of release of 14 is directly proportional to the rate of acetate hydrolysis. The predicted and observed E and ee values (as checked by standard HPLC assay on a chiral phase) were found to match within + 20%. Only in one case was a larger discrepancy observed, a result that was believed to be caused by the occurrence of an unusually low value of Km for one of the enantiomers. 

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