Benzion Fuchs

Diagnostic structural criteria for the anomeric effect in carbohydrates and inferences of general significance on their scope and limitations

Abstract A systematic analysis of 529 carbohydrate structures that contain OCO units, retrieved from the Cambridge Structural Database, was performed with regard to the bond lengths (CO) and bond (COC and OCO) angles as they depend on the dihedral angles in the sequence COCOC. This dependence was then interpreted in terms of the anomeric effect. Known and new concepts that concern the manifestations of the anomeric and exo -anomeric effects were thus reassessed. A set of structural criteria of diagnostic value was defined which, together with qualifying arguments, allow evaluation of the scope and limitations of these stereoelectronic effects in carbohydrate systems.

New supramolecular host systems. 2. 1,3,5,7-tetraoxadecalin, 1,2-dimethoxyethane and the Gauche effect reappraised. Theory vs. experiment.

Abstract The three 1,3,5,7-tetraoxadecalin diastereomers (1, 2 & 3), core moieties of new host and macromolecular systems, were calculated “

Computation of O-C-F and N-C-F systems: AB-initio calculations and a MM2 parameterization study. Theory vs. experiment☆

Abstract A parameterization scheme of Allingers MM2 force field for the anomeric effect in O-C-F and N-C-F systems is presented. The scarcity of experimental data, in particular for the N-C-F case, dictated the use of ab-initio calculations to account for the energetic and structural manifestations of the effect. The resulting modified force field was tested against available X-ray, microwave and NMR results leading to a very good agreement between calculations and experiment. In addition, ab-initio results were used to demonstrate the role of the anomeric effect in lowering barriers to N-inversion and elevating barriers for rotation around single CN bonds. The results for the fluoro compounds, when juxtaposed to other systems, provide a complete treatment of the anomeric effect for the first row elements.

Computation of N-C-N systems: Theory vs. experiment☆

Abstract In sequel to a previous recent study4, netgtkebeduanube (MDA) was calculated by higher level ab initio methods namely 6-31G* abd NO3//6-31G* levels. The influence of polarization functions on the calculated pyramidality of nitrogen and on the magnitude of the anomeric effect is discussed. At the same time, a structural-statistical study was performed on molecules containing the N-C-N moiety retrieved from the Cambridge Structural Database (CSD). Both methods complement and improve results from the previous study. Special attention was given to the tertiary amine case and the N-C-N bond angle for which new MM2 parameters are put forward. These are mainly based on ab initio calculations of N,N-dimethylmethylenediamine and tetramethylmethylenediamine and on a comparative study of CN bond lengths in primary, secondary and tertiary amines.

Structure, conformation, and stereoelectronics of 1,4,5,8-tetraazadecalins. Chemical, multinuclear NMR and molecular mechanics studies

Abstract We present a multidisciplinary investigation of some known and new 1,4,5,8-tetraazadecalin (TAD) derivatives. Their structure, static and dynamic conformational analysis, isomerization pathways and relative stabilities were studied using 1H-, 13C-, and 15N-NMR techniques. Molecular mechanics calculations were carried out using the MM2 force field and an earlier modified version for NCN containing systems. The peculiar stereoelectronic features of the CNCNC moieties in the TAD systems are emphasized.

Modified MM2 scheme for computation of O-C-N-containing systems

A modification of Allingers MM2 force field for the anomeric effect in OCN systems is presented. For optimal consistency, it was parameterized by alternate use of ab initio (3‐21G level) and X‐ray results to account for the energetic and structural manifestations of the effect in the gas or condensed phase, respectively. The results obtained with the modified force field are in good agreement with those from both theoretical and experimental methods. The parameterization scheme explicitly treats all structural parameters of the COCNC moiety as well as CN bond lengths in tertiary amines contained within an anomeric unit. In addition, it includes directional H‐bond type interactions. A limited number of parameters is put forward, in accord with the general MM2 force‐field approach.

Diazadioxadecalin and salen podands and macrocycles within dynamic combinatorial virtual libraries: structure, prototropy, complexation and enantioselective catalysis

The reactions of l-1,4-diaminobutanediol (3) and d-2,3-diaminobutanediol (4) with salicyl aldehyde provide the tautomeric manifolds of l-1,4-bis(salicylideneamino)-2,3-butanediol (5) and d-2,3-bis(salicylideneamino)-1,4-butanediol (6), respectively. O-alkylation of the salicyl moiety stabilizes the closed dioxadiazadecalin (DODAD) and diazadioxadecalin (DADOD) isomers (7″, 8″) and accordingly, the dialdehyde 1,2-bis(o-formylphenoxy)-ethane (9) led to the respective macrocyclic manifolds (10–10″ and 11–11″). These tautomeric manifolds are typical target-driven dynamic combinatorial virtual libraries, which can be biased by complex formation with metal ions of different ionic radius. A rare instance of simultaneous occurrence of keto–enamine and phenol–imine tautomers in the solid state of 6 was unravelled (X-ray at two temperatures) and the strength of the intramolecular hydrogen bonding (and hence, the extent of ring closure) in 6 is temperature dependent. Compounds 6, 11 and 12–14 constitute a new class of salens, which form heavy and transition metal complexes. Some such Mn(III) complexes are good chirality inducing catalysts, as found in asymmetric indene epoxidation reactions.

Structure and conformation of heterocycles. 13. conformational analysis of 2,3-DI(R)OXY-1,4-dioxanes: anomeric and gauche effects

Abstract A series of trans - and cis -2,3-di R)oxy-1,4-dIoxanes (R = Me,Ph, Ac) and some 2,5-disubstituted analogis were prepared and analysed by various NMR techniques. The trans isomers occur in more than 96% as diaxial conformers. Molecular mechanics calculations largely confirm these findings, which are interpreted -in terms of combined anomeric and gauche effects.

New supramolecular host systems. 41. novel diacetal podands, diazacrowns and cryptands

Abstract New and important key podands bearing the cis -1,3,5,7-tetraoxadecalin (TOD) core are reported, viz., 2,6-di(hydroxymethyl)- ( 2 ) and 2,6-di(aminomethyl)- cis -1,3,5,7-TOD ( 6 ), which lead to the first diazacrown-TOD compounds ( 9a ) and ( 9b ) and a corresponding cryptand ( 11 ) in the series. 9a&b exhibit very good alkali and earth-alkali ion inclusion ability with selectivity for K + and Ba +2 . Molecular dynamics (Insight/Discover/AMBER) on the crowns and cryptand were performed, including reparametrized MM3-GE calculations. A convincing case for the TOD based hosts was thus established.

New supramolecular host systems. 1. A structural and conformational study of the 1,3,5,7-tetraoxadecalin core system

Abstract This is the first in a series of studies of new host systems based on the 1,3,5,7-tetraoxadecalin (TOD) core moiety. The parent molecule ( 2 ) an