The CH/π-interaction is a weak hydrogen bond occurring between CH groups and π-electron systems. Evidence for such a weak attractive molecular force has been presented. These include data from IR, NMR, CD spectroscopies, and X-ray crystallography. The conclusion has been supported by ab initio MO calculations. An important point is that CH- and π-groups are arranged generally in chemical structures and have many chance to interact each other. Unlike ordinary hydrogen bondings, the CH/π interaction may occur in protic media as well as in nonpolar atmosphere. Consequences in supramolecular chemistry, selectivities in chemical reactions, as well as the substrate specificities of proteins were discussed in the light of the CH/π-interaction hypothesis.
Elicitor-active hexa-β-D-glucopyranosyl-D-glucitol (1) (oligosaccharin), isolated from mycelial wall of Phytophthora megasperma f. sp. glycinea, induces antibiotic phytoalexin accumulation in soybeans. Our interests associated with the elicitor-active oligosaccharide are to develop a new synthetic method for the oligosaccharin and to elucidate three dimensional structure of the oligosaccharin in water. Synthesis of the oligosaccharides has made considerable progress as a result of the development of new glycosyl donors and glycosylation reagents. However, only a few methodologies directed to the synthesis of oligosaccharides has been developed. In the first part of this report we describe a development of a new one-pot glycosylation methodology that allows us to construct two glycosidic linkages sequentially in one-pot operations, and its application to the synthesis of an elicitor-active hexaglucoside 2. In the second part, we describe the development of a protocol for revealing the stable conformation of oligosaccharide in aqueous solution based on AMBER (united atom), GB/SA solvation model and NMR experiments. Its application to the conformational analyses of trisaccharide 41 and hexaglucoside 2 in water is also presented. In the last part we focused our attention on a remote glycosylation that allows us to form a branched trisaccharide in intramolecular fashion. The molecular design of the spacer molecule for the remote glycosylation was examined based on molecular mechanics calculations.
A lithium amide conjugate addition approach to the synthesis of β-amino acid derivatives is described. Lithium amides derived from α-methylbenzylamine, such as lithium (α-methylbenzyl) benzylamide undergo highly diastereoselective 1, 4-conjugate addition to a variety of α, β-unsaturated carbonyl compounds. The benzyl substituents on the amino group can be readily removed by hydrogenolysis to afford a wide range of β-amino acid derivatives. The enolate intermediate can be trapped by electrophiles such as alkylhalides and (camphorsulphonyl) oxaziridine to give α-alkyl and α-hydroxy-β-amino acids in a highly stereocontroled fashion. The synthetic utility of the methodology is demonstrated by the syntheses of numbers of natural products and other important synthetic intermediates such as taxol C-13 side chain, cispentacin, and (+) -negamycin. The origin of the stereoselectivity is briefly discussed.
Some optically active tricoordinate selenium and tellurium compounds, such as selenonium ylide, telluronium ylide, selenonium imide, selenonium salt, telluronium salt, and telluroxide, were isolated by means of optical resolution of corresponding diastereomeric mixtures or asymmetric syntheses. Absolute configurations of the optically active isomers obtained were determined by their X-ray crystallographic analyses, specific rotations, and circular dichroism spectra. Racemization of the isomers by pyramidal inversion or via achiral hydrate were also examined, and activation energies for the pyramidal inversion were estimated based on ab initio MO calculations.
Synthetic strategy and concept of multi-step synthesis of biologically important compounds are discussed with special reference under stereochemical control. A-strain in cyclic exo-olefin was expanded to acyclic cases to allow high stereoselectivity in conjugate addition, epoxidation, etc. A special synthetic methodology named heteroconjugate addition has been improved in various aspects (i) preparation of heteroolefin, (ii) diastereo-switching of syn or anti product, (iii) enantio-switching of D or L from D-sugar analog and (iv) coupling with other fragments in the total syntheses of maytansinoids, okadaic acid, tautomycin, etc. Recent advances on sugar acetylene chemistry are also introduced to use the corresponding biscobalthexacarbonyl complexes and Nicholas type reaction for medium-size ether ring formation. These syntheses request multiple organic reactions such as carbanions, carbenium cations, organometallic reagents, heteroatom chemistry for bond formation in good harmony with functional group preparation of the target natural products.
Non-recognition for dienes in ketene reactions has long been an important problem in organic chemistry since the diphenylketene-cyclopentadiene reaction was found by Staudinger in 1920. Recently, we have discovered that the ketene recognizes conjugated dienes. The ketene is a dienophile not for well-known [2+2] cycloadditions but for [4+2] (Diels-Alder) reactions across its C=O bond. This paper outlines the long history of ketene chemistry and describes the way of how the problem has been tackled. The frontier-orbital theory and ab initio calculations have predicted that ketene should react with cyclopentadiene via the [4+2] cycloaddition and a subsequent Claisen rearrangement. Careful low-temperature experiments and NMR spectroscopy of the reaction have demonstrated the first detection of the [4+2] -type cycloadduct and the conversion to the final product, cyclobutanone, by the rearrangement.