Leah Schleifer

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.

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.

1,4,5,8-Tetraacyl-1,4,5,8-tetraazadecalins: Configurational and conformational analysis in the crystal and in solution: Structure and conformation of heterocycles-12☆

Abstract A series of 1,4,5,8-tetraacyl-1,4,5,8-tetraazadecalins was studied and found to exist in trans form in analogy with the parent system 2 . Only tetraacetyltetraazadecalin ( TATAD ) could be obtained in both trans and cis forms, which were investigated in detail by X-ray diffraction analysis and NMR techniques, trans - TATAD ( 8 ) occurs in the crystal in a centrosymmetric double twist-boat conformation whereas cis - TATAD ( 9 ) has one ring as a flattened chair and the other as a twist boat with all the amide COs syn-oriented. Three out of seven possible rotameric forms of 9 occur at low temperature in solution. The free energy of activation of amide rotation in trans - TATAD ( 8 ) is 14.5 kcal/mol, in contrast to ca 12kcal/mol in the unique cis isomer ( 9 ).

Structure, conformation and stereoelectronics of 9,10-annelated-1,4,5,8-tetraoxadecalins: A multidisciplinary probe☆☆☆★

Abstract The structure and conformation of the 9,10-annelated-1,4,5,8-tetraoxadecalins (3,4 and 5) were investigated in solution using NMR spectroscopy, in the crystal using X-ray diffraction analysis and by computation, using the MM2 force field, suitably parameterized to include structural manifestations of the anomeric effect. The study focused on the geometry, viz., the structural parameters of the anomeric moieties and the ring conformation and degree of puckering, as affected by changes in the dihedral angle of the 9,10-annelating system. Very good agreement between the three methods was obtained, showing increased chair deformation, as the 9,10-substituents approach coplanarity.

Photochemistry of homoconjugated ketones: 2-(carbomethoxy)spiro[5.5]undeca-1, 3-dien-7-one.

Abstract The direct and sensitized irradiation of the title compound ( 2 c) was studied with particular attention to its oxa-di-π-methane rearrangement, the product of which is of notable structural and synthetic interest. X-ray diffraction data of the final product (8) are given to support structural assignments.

Conformational energy and entropy differences of the t-butoxy group and implications in stereochemistry and stereoelectronics

Abstract The conformational energy and entropy differences of the t-butoxy group were determined using 1H- and 13C-NMR variable temperature techniques and ΔEconf was calculated by molecular mechanics (MM2). Strikingly small values were observed and discussed in conjunction with the alleviated anomeric effect in 2-t-butoxytetrahydropyran systems.

Unusual photorearrangements of homoconjugated diacylcyclohexa-2,4-dienes

The unprecedented triplet sensitized electrocyclic ring opening of various 3a,7a-diacyl-3a,7a-dihydroindans (3)–(5) to the corresponding cyclononatriene derivatives (7)–(11) is reported, direct irradiation at all absorbing wavelengths of the keto-ester (5) resulting in a diastereoselective oxa-di-π-methane (ODPM) rearrangement to the tricyclic product (6); dimethyl 1,2-dimethyl-1,2-dihydrophthalate (15) reacted similarly to give dimethyl E,E,E-2,7-dimethyl-2,4,6-octatrienoate (16), whose structure has been determined by single crystal X-ray diffraction analysis.