Diastereomers, crystal structures

Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers. ... 

Even acetophenone reacts with the magnesium compound 17 (R1 = R2 = H) to yield the w-diastereomer 18 with 90 % de 22 24. The structure of the metal-organic precursor and, as well, of the major product was determined by an X-ray crystal structure analysis23. 

Oxidative addition of H2 and Ph3SiH to IrBr(CO)(chiraphos), where chiraphos 2,S, 3,S -bis (diphenylphosphino)butane, leads to stable kinetic (115a,115b), followed by thermodynamic (116a,116b) diastereomers as shown in reaction Scheme 15.2 3 The crystal structure of cis-[IrH2(chiraphos)2]BF4 has been reported.204... 

Crystal structures are available for many (N)4Co-amino acid complexes (Table I). Many of the diastereomers (AS, AS) in the bis-en series have been resolved using classic crystallization (usually via bromocamphor sulfonate, arsenyl-, or antimonyl-tartrate salts) or ion exchange methods (Table II). Reversed-phase ion-pair HPLC, using aryl phosphate or aryl/alkyl sulfonate ion pairing reagents in MeOH/ H20 eluent, has allowed diastereomer separations to be carried out on analytical amounts (28) (Table II). 

The methylenecyclopropane derivative 3-SPh with its capto-dative substitution pattern has demonstrated essentially the same reaction mode and underwent dimerization to afford a mixture of E) and (Z)-17 (ratio 1.3 1) upon attempted cycloaddition of 3-SPh onto bicyclopropylidene [7h, 291 (Scheme 5). The assignment of these diastereomers was secured by an X-ray crystal structure analysis of E) and (Z)-16 [11c, 30] as well as E)-17 [29]. 

The strongest influence of configuration has been observed for Pi -substituted diastereomers of Z-Phe-(aTfm)Ala-Ala-NH2. The crystal structures of both dia-stereomers have been solved, which enables a better interpretation of this rather interesting effect. While the (S,S,S)-diastereomer has been shown to be almost as stable as the Aib-substituted peptide, the (S,R,S)-diastereomer was hydrolyzed very quickly within the same time range. Molecular modeling studies readily support the formation of hydrogen bonds as a possible explanation for this effect [18,54]. With the known crystal structure of the a-chymotrypsin/phenyl boronic... 

Monosubstituted methylenccyclopropanes 22 or unsymmetrically 1,1-disubstituted methylene-cyclopropane 24 give mixtures of cisftrans isomeric cyclobutanes 23 and 25, respectively.12-13 The [2 + 2]dimerization of 24 proceeded particularly cleanly under high pressure so that crystals of both diastereomers could be obtained and stereochemical assignment be made by X-ray crystal structure analysis.64... 

Many diasteromeric peptides of type (R,S)-27 and (S,S)-28 (Scheme 1) could be crystallized and their absolute configuration determined based on the known ( -configuration of L-phenylalanine. Table 1 shows a compilation of the relevant torsional angles (4>1/ P1) derived from crystal structures of the peptides shown.108 It is interesting to note that the (S,S)-diastereomers show a high prevalence for P-tum type I conformations in the crystal state. 

Meso- and D,L-2,3-dimethoxy-2,3-diphenylsuccinonitrile 39 were obtained in a 1 1 ratio by a preparative dimerization procedure. Their configurations were assigned by crystal structure analysis 74). According to nmr both diastereomers have a configuration with the phenyl groups in the anti position. From a conformational analysis by the force field method it was concluded that these conformations remain the preferred ones also for the isolated molecules meso-39 was calculated to be 1.2 kcal mol-1... 

Our group has also encountered success in cycloadding an isomiinchnone dipole across an indole double bond [28]. Cycloadduct 91 was generated in high yield as a single diastereomer from the Rh2(OAc)4-catalyzed reaction of diazoimide 90. The assignment was unequivocally established by an X-ray crystal structure. The ready construction of these poly-heterocycles in one step and in... 

This review will illustrate examples of computer projected models of inclusion complexes of structural isomers (ortho, meta, para nitrophenol), enantiomers (d- and 1- propranolol) and diastereomers [cis and trans. l(p-B-dimethylaminoethoxy-phenyl-butene), tamoxifen] in either a- or B-cyclodextrin. The use of these computer projections of the crystal structures of these complexes allows for the demonstration and prediction of the chromatographic behavior of these agents on immobilized cyclodextrin. 

Compound 86 exists in two diastereomeric forms due to the chiral silicon centers, which are evident in the various NMR spectra (Table XXV). The crystal structure of 86 shows a molecular inversion center, and hence belongs to the meso diastereomer. The geometry around silicon in the crystal is a distorted TBP and the chloride is more than 7 A away from silicon, in accord with a pentacoordinate siliconium chloride structure. The dative Si bonds are exceptionally short in 86 (1.802 and 1.807 A) relative to those in other pentacoordinate complexes.2,3,1115... 

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