Enantiotopic Faces Research Articles

Molecular asymmetry of an N-alkylporphyrin with enantiotopic faces. Resolution and spectroscopic characterizations of optical antipodes of N-methyletioporphyrin I

The optical antipodes of N-methyletioporphyrin I were resolved and characterized as the zinc complex with a chiral axial ligand by n.m.r. spectroscopy.

Transfer of structural information from Langmuir monolayers to three-dimensional growing crystals

One enantiomer of an initially racemic mixture of α-amino acids can be preferentially removed from solution by selective incorporation into the opposite enantiotopic faces of growing centrosymmetric crystals of the α-form of glycine1,16. We report here that oriented growth of crystals of the α form of glycine has been achieved under chiral Langmuir monolayers comprising amphiphilic α-amino acids, by virtue of a structural match between the monolayer and the ac surface layer of the attached growing glycine crystals. Such monolayers of α-amino acid of R configuration, containing long hydrocarbon chains, induce glycine to crystallize with its (010) face attached to the monolayer, and by symmetry the corresponding S amino-acid monolayers induce attachment of the (0&1macr;0) face of glycine. Replacement of the hydrocarbon by a fluorocarbon chain induces analogous crystallizations, albeit, with only a partial degree of orientation, whereas monolayers of a resolved amino acid bearing a cholestanoyl moiety do not promote crystallization of glycine. Monolayers of the R, S-amino acids induce attachment of both (010) and (0&1macr;0) faces of glycine. These results on oriented crystal growth provide a new route for efficient amplification of optical activity of amino acids present in solution, through the enantioselective occlusion into the growing crystals of glycine1 at water–air interfaces covered by a monolayer.

ChemInform Abstract: STEREOSELECTIVITY IN THE REACTIONS BETWEEN THE ANION OF 4‐PHENYL‐5,6‐DIHYDRO‐2H‐THIOPYRAN 1‐OXIDE AND ELECTROPHILES

AbstractDas Sulfoxid (V) bildet beim Behandeln mit LDA ein instabiles Anion, das sich nur schlecht mit Elektrophilen (außer mit Mel) abfangen läßt. Bei der Reaktion mit MeI wird die Methylgruppe trans in bezug auf das Sul?nyl‐Oaddiert: Bildung von (X).

Stereoselectivity in the Reactions between the Anion of 4-Phenyl-5,6-dihydro-2H-thiopyran 1-Oxide and Electrophiles

Treatment of the anion(17), prepared from the six- membered ring allylic sulfoxide 4-phenyl-5,6-dihydro-2H-thiopyran 1-oxide (8) with primary alkyl halides at -78°, affords trans-2-alkyl-4-phenyl-5,6-dihydro-2H-thiopyran 1-oxides exclusively. The stereospecificity of these reactions thus corresponds to that obtained from the related saturated cyclic sulfoxides . However, when the anion (17) is treated with 2-iodopropane (-78° → 20°) a mixture of 2-cis and 2-trans products results. Independent experiments indicate that the lack of stereoselectivity is due to anion exchange reactions at the higher temperature required to effect alkylation. 2-cis and 2-trans Adducts also are obtained in reactions of the anion (17) with benzaldehydes (-78° → 0°), and from X-ray crystallographic analyses it has been established that the cis adducts predominate, with little selectivity in attack on the two enantiotopic faces of the aldehyde.

Spontaneous generation and amplification of optical activity in α-amino acids by enantioselective occlusion into centrosymmetric crystals of glycine

Recent studies of crystal growth in the presence of specific additives have suggested a new approach to the generation and amplification of optical activity in initially racemic mixtures1–4. This approach is based on the observation that particular enantiomers can be selectively incorporated not only into chiral crystals5–7 but also into the opposite enantiotopic faces of centrosymmetric crystals3,4,8. If one such face of a centrosymmetric crystal is blocked, therefore, enantioselective occlusion into the opposite face will remove one enantiomer preferentially from the racemic mixture. We report here that this process can take place with centrosymmetric crystals of glycine, in a way that may be of relevance to the generation of optical activity during prebiotic evolution. Glycine crystals float in solution, such that only one face is available for growth. The addition of a particular enantiomer of a hydro-phobic amino acid orientates the floating crystals in such a way that the face that occludes amino acids of the opposite chirality is exposed to the solution. Thus, a statistical fluctuation in the orientation of crystals floating in an initially racemic solution could lead to the generation and amplification of optical activity.

ChemInform Abstract: COB(I)ALAMIN AS CATALYST. 8TH COMMUNICATION. COB(I)ALAMIN AND HEPTAMETHYL COB(I)YRINATE DURING THE REDUCTION OF α,β‐UNSATURATED CARBONYL DERIVATIVES

AbstractUntersuchungen zeigen, daß die Reduktion der ungesättigten Carbonyl‐Verbindungen (I) mit Z‐Konfiguration, katalysiert durch die aus ihren Co(III)‐Analogen (V) und (VII) mit Zn/ AcOH gebildeten Co(I)‐Komplexe (VI) bzw. (VIII) enantioselektiv verläuft und zu den gesättigten Produkten (II) in der S‐Konfiguration mit vom verwendeten Katalysator abhängigen Enantiomerenüberschuß führt.

Cob(I)alamin as Catalyst. 8th Communication. Cob(I)alamin and Heptamethyl Cob(I)yrinate During the Reduction of α,β‐Unsaturated Carbonyl Derivatives

AbstractHydrogen bonds as presented in Figure 2 cannot account for the enantioselective attack of cob(I)alamin (4(I)) or heptamethyl cob(I)yrinate (5(I)) on one of the two enantiotopic faces of the substrates. The attack of the strongly nucleophilic 3d orbital is preferentially directed to the re‐side of the starting materials with (Z)‐configuration and leads, after the highly stereoselective reductive cleavage of the Co, C bond, to saturated products with (S)‐configuration in varying enantiomeric excesses (see Schemes 1, 3 and Table 1).

Enantioselektive 1,2‐Additionen von Li‐, Mg‐, Zn‐ und Cu‐organischen Verbindungen und von Li‐Enolaten an Carbonylverbindungen im chiralen Medium DDB

Enantioselective 1,2‐addition of Li‐, Mg‐, Zn‐, and Cu‐organic compounds and of Li‐enolates to carbonyl derivatives in the chiral medium DDBDDB = 1,4‐Dimethylamino‐2,3‐dimethoxybutan (= 2,3‐Dimethoxy‐N,N,N′,N′‐tetramethyl‐1,4‐butandiamin). The (+)‐enantiomer of the methoxyamine DDB is used as a chiral cosolvent in the title reactions. As evident from the results listed in the table, enantiotopic faces are generally differentiated with preferences ranging from 55:45 to 60:40.

ChemInform Abstract: ASYMMETRIC REDUCTION OF α‐KETOESTERS WITH HANTZSCH ESTERS (DIALKOXYCARBONYLDIHYDROPYRIDINES)

Single and double asymmetric inductions were achieved by the reduction of pyruvates and benzoylformates with 2,6-dimethyl-3,5-dialkoxycarbonyl-1,4-dihydropyridine in the presence of a mono-ionised zinc species; differentiation is made between enantiotopic faces of the substrate carbonyl by diastereotopic hydrogen, and between diastereotopic faces by equivalent and diastereotopic hydrogen.

Asymmetric reduction of α-ketoesters with Hantzsch esters (dialkoxycarbonyldihydropyridines)

Single and double asymmetric inductions were achieved by the reduction of pyruvates and benzoylformates with 2,6-dimethyl-3,5-dialkoxycarbonyl-1,4-dihydropyridine in the presence of a mono-ionised zinc species; differentiation is made between enantiotopic faces of the substrate carbonyl by diastereotopic hydrogen, and between diastereotopic faces by equivalent and diastereotopic hydrogen.

Stereoselective attack at on enantiotopic face of a carbonium ion by chiral nucleophile

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStereoselective attack at on enantiotopic face of a carbonium ion by chiral nucleophileJames L. FryCite this: J. Am. Chem. Soc. 1971, 93, 14, 3558–3559Publication Date (Print):July 1, 1971Publication History Published online1 May 2002Published inissue 1 July 1971https://doi.org/10.1021/ja00743a061RIGHTS & PERMISSIONSArticle Views214Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (280 KB) Get e-Alerts Get e-Alerts