Racemates, crystal structure

Yokota, M., Doki, N., Shimizu, K. (2006). Chiral separation of a racemic compound induced by transformation of racemic crystal structures DL-Glutamic acid. Crystal Growth Design, Vol. 6, No. 7,1588-1590... 

Gonnade RG, Iwama S, Sugiwake R, Manoj K, Takahashi H, Tsue H, Tamura R. Occurrence of spontaneous resolution of ketoprofen with a racemic crystal structure by simple crystallization under nonequiUbrium preferential enrichment conditions. Chem. Commun. 2012 48(22) 2791-2793. 

Trivalent Co complexes of the mixed donor amino arsine emda (2-aminoethyl)dimethylarsine) have been synthesized.936 Examples include trans(X,X), cA(As,As)-[CoX2(edma)2]+ (X = C1, Br, and I), trans( As,As)- and trara(As,N)-[Co(acac)(edma)2]2+, trara(As,N)-[Co(C03)(edma)2]+, [Co(acac)2(edma)]+, and /ao[Co(edma)3]3+. The /ao[Co(edma)3]3+ complex was resolved and the absolute configuration of the (+)49o CD isomer was assigned as A on the basis of the CD spectrum. Racemization of this complex was first order in both complex and hydroxide ions. The crystal structure of [Co(acac)2(emda)]C104 has been reported.937... 

A derived crystal packing model proved to be useful in resolving the crystal structure of a metastable polymorph of racemic modafinil, where details of the solved crystal structure of one polymorph was used as a basis for developing the structure of the other [12]. It was found that the calculated XRPD pattern matched well with the experimental data, indicating the correctness of the analysis. The powder diffraction of two polymorphs of chlorothalonil were solved to obtain... 

The late stages of the synthesis (Scheme 1.17) proceeded with Wittig methylenation of ketone 144 with Ph3P=CH2 at 70 °C to furnish exocyclic alkene 145 in 77 % yield. Finally, the alcohol was installed via a Se02-mediated allylic hydroxylation [57] of the exocyclic alkene 145 to afford ( )-nominine (1) in 66 % and 7 1 dr. The structure of nominine (1) was verified via an X-ray crystal structure determination, thereby completing the racemic total synthesis of ( )-nominine (1). 

In order to understand the rates of racemization of biphenyls and bihetero-cyclics, an accurate knowledge of their geometric structure is essential. Such knowledge makes it possible to estimate the amount of interference caused by substituents in the vicinity of the pivot bond in an assumed coplanar transition state for rotation. A study of the crystal structure of 1 (X = Se) found that the selenophene rings have a small but significant deviation from planarity and are nearly perpendicular to each other.17 The deviation from 9(T is such that the carboxyl groups are in transoid positions. 

An X-ray crystal structure of 28 bound in the thumb-region of the NS5B polymerase showed little interaction of the acetamide moiety with the protein. Alterations at this position were explored in order to improve the physical properties of the compound. Incorporation of basic amines as part of this side-chain, leading to zwitterionic compounds, reduces plasma binding and has a beneficial effect on cell activity and pharmacokinetic profiles. In the cell-based replicon assay, racemic 29 has an EC50 of 152 nM in the presence of 10% fetal calf serum and 376 nM in the presence of 50% normal human serum [71],... 

Crystals composed of the R and S enantiomers of the same racemic mixture must be related by mirror symmetry in terms of both their internal structure and external shape. Enantiomorphous crystals may be sorted visually only if the crystals develop recognizable hemihedral faces. [Opposite (hid) and (hkl) crystal faces are hemihedral if their surface structures are not related to each other by symmetry other than translation, in which case the crystal structure is polar along a vector joining the two faces. Under such circumstances the hemihedral (hkl) and (hkl) faces may not be morphologically equivalent.] It is well known that Pasteur s discovery of enantiomorphism through die asymmetric shape of die crystals of racemic sodium ammonium tartrate was due in part to a confluence of favorable circumstances. In the cold climate of Paris, Pasteur obtained crystals in the form of conglomerates. These crystals were large and exhibited easily seen hemihedral faces. In contrast, at temperatures above 27°C sodium ammonium tartrate forms a racemic compound. 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

Kim s group in Seoul report the application of another of the E. coli ATs, the aromatic l-AAT encoded by the tyrB gene, to enrich the D-component of racemic preparations of alanine substituted at the /3-position with pyrazole, triazole, and imidazole. They also carried out an in silico investigation based on the crystal structure (PDB 3TAT) providing a reasonable rationalization (and therefore also potentially prediction) of substrate specificities. 

The absolute configurations of amino acids 198a (the racemic counterpart was designated 186c) and 198i on the basis of X-ray crystal structure analysis data of the a-azido esters 197a-Me and 197i-Bn were (R) and (S,S,S), respectively. 

Starting from the Michael adducts of (4R,5S)-4,5-diphenyloxazolidine-2-one (100) onto chlorospiropentylideneacetates 2 c (102c-Bn and 102c-Me) and juggling with the set of transformation discussed above, the spiropentane amino-carboxylic acids 207,208 and 213 were also prepared either in racemic (208) or enantiomerically pure (207, 213) forms, the absolute configurations of which were determined on the basis of X-ray crystal structure analysis of the precursor... 

The crystal structure of five members of the ferrioxamine family has been determined ferrioxamine Di , ferrioxamine E °, desferrioxamine E , the retro-isomer of fer-richrome E and ferrioxamine B . While all of the Fe(III)-ferrioxamine structures (Table 2) crystallize as racemic mixtures of A- and A-cis isomers , the configuration of the binary ferrioxamine-B in EhuD complex (2.0 A resolution) is A-C-trans,cis, indicating that the interaction between ferrioxamine-B and FhuD is enantioselective, and also exhibils geomelric seleclivily. 

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