This paper deals with resonance and strain energies in non-planar cyclic conjugate molecules, such as C60 and corannulene 1, including quantitative discussions based on combustion calorimetry. Before the evaluation of those energies, energetics of planar aromatic molecules is reasoned in order to provide reference energies. The energetics is extended to the non-planar sp2 carbon networks. The formation enthalpies of graphite fragments, hollow fullerenes and fullerene onions are predicted as functions of the number of carbon atoms. Fullerenes are predicted to be less stable than infinite size of graphite, but to be more stable than finite size of graphite fragments. Among aggregates of around 1000 carbon atoms, fullerene onions are found to be more stable than hollow fullerened and graphite fragments.
Synthetic studies of nitrogen-containing terpenes isolated from marine organism are described. Agelasidine A was synthesized by using hetero-Claisen rearrangement of allyl xanthate. Biomimetic synthesis of agelasidine A employing sigmatropic rearrangement of allyl sulfinate realized three-step synthesis starting from farnesol. Biomimetic synthesis of aminobisabolene and theonellin using the Ritter reaction was achieved. Allyl cyanate-to-isocyanate rearrangement was established as a new synthetic method for the construction of allyl amine moiety at sterically congested positions. Application of this new method completed the synthesis of geranyllinaloisocyanide.
Cyclopolymerization of 1, 2 : 5, 6-dianhydrohexitol has been studied in terms of regio- and stereoselectivity. The anionic cyclopolymerizations of 1, 2 : 5, 6-dianhydro-D-mannitol and 1, 2 : 5, 6-dianhydro-L-iditol produced (1→6) - and (6→1) -2, 5-anhydro-D-glucitol, respectively. On the other hand, the cyclopolymerizations using a cationic initiator gave the polymer consisting of 2, 5-anhydro-D-glucitol as the cyclic constitutional unit along with 1, 6 : 2, 5-dianhydro-D-glucitol as a by-product. For the cationic cyclopolymerization of 1, 2 : 5, 6-dianhydro-D-glucitol, the resulting polymer was confirmed to have the constitutional unit mainly of 2, 5-anhydro-D-mannitol, while the polymer obtained using anionic initiator consisted of two cyclic repeating units, 2, 5-anhydro-D-mannitol unit and 2, 5-anhydro -L-iditol unit. The cyclopolymerization of 1, 2 : 5, 6-dianhydrohexitol was regio- and stereoselective, i.e., the intramolecular cyclization proceeded through the α-scission with inversion of the configuration at the α-carbon and the intermolecular propagation through the β-scission with retention. The polymer consisting of 2, 5-anhydrohexitol, a novel polymeric carbohydrate, showed a metal cationbinding ability and a chiral recognition property toward the racemic amino acid derivatives.
Development of efficient methods for highly selective partial fluorination of organic compounds is becoming increasingly important in various fields such as medicinal chemistry and material science. In contrast to chemical methods using flouorinating reagents, which are hazardous, difficult to handle, or very costly, electrochemical partial fluorination is an ideal method for direct fluorination since fluorine atoms can be introduced into organic molecules in one step under safe conditions. However, elctrochemical partial fluorination has not been developed owing to low selectivity for the fluorination, low nucleophilicity of fluoride ions, and passivation of electrodes. From these viewpoints, we have developed highly selective electrochemical mono- and difluorination of organic compounds using Et3N·3HF and Et4NF·4HF as the supporting electrolyte and fluorine source. This account deals with highly selective anodic fluorination of various heteroatom compounds such as sulfides, selenides, and tellurides together with sulfur and/or nitrogen and oxygen containig heterocycles including β-lactams. Furthermore, indirect anodic mono- and gem-difluorodesulfurizations using mediators such as halogen, hypervalent fluoroiodoarenes, and triarylamines are also described. This electrochemical mehtod is much superior to conventional chemical methods because any hazardous reagents are not required and the yields are much higher compaired to the chemical methods.
Reaction of N-sulfonyl amino acid with 1 equiv. of BH3 THF easily gave the oxazaborolidinone which promoted the aldol reaction of a variety of aldehydes with silyl ketene acetals with very high enantioselectivity. For example, isobutyrate aldols were obtained with >96% ee and acetate aldols were obtained with >98% ee by using a silyl nucleophile derived from ethyl 1, 3-dithiolane-2-carboxylate. The level of the selectivity is surely acceptable for practical syntheses although the promoter is used as stoichiometric amounts. The scope and limitation of the oxazaborolidinone-promoted aldol reaction will be discussed by surveying the stereochemical outcome of the reaction inducing the high enantioselectivity.
In order to create specific inhibitors against glycosyltransferases, tethering glycosyl donor to acceptor was planned. Mono-O-methylated UDP-Gal and related analogues were synthesized, and their behaviors toward β-1, 4-galactosyltransferase were examined. The allowance for the O-methylation of Gal moiety was decreased in the following order : 2-, 3-, 4- and 6-positions and it was suggested that the modification at the 2-position does not affect the affinity but retards the reaction rate by preventing the conformational change to the transition state, which develops so-called dynamic binding. Modification of the glycosyl acceptor (G1cNAc) moiety showed the tethering probability in the 3' - and 6' - positions. Bisubstrate tricomponent analogues linked through 2, 6' -methylene and 2, 6' -ethylene groups were synthesized and found to be potent inhibitors against bovine G1cNAc : β-1, 4-galactosyltransferase, with Ki values of 1.35 and 1.95 μM, respectively, for the acceptor.