Saccharide recognition is one of the most attractive topics in host-guest and biomimicking chemistry. Herein we describe development of “meta-ethynylpyridine” poly-/oligomers as artificial hosts for saccharide recognition, which consist of 4-substituted pyridine rings linked 2,6-positions with acetylene bonds. When ethynylpyridine chains associate with a guest saccharide by multipoint hydrogen-bonds, a helical complex forms and its helicity is biassed by chirality of the guest saccharide. Substituents on the pyridine rings give additional characteristics such as solubility, amphiphilicity, basicity, and tautomeric ability. Amphiphilic polymer recognizes natural hexoses even in aqueous methanol. Basic and tautomeric ethynylpyridines switch their recognition ability by protonation and tautomerism, respectively. When a glycoside template is linked with ethynylpyridine oligomers covalently, the resulting helices are fixed by intramolecular association. Helical structure of azacrown-induced polymer is stabilized by collaboration with axial oligoammonium, accompanied with the improvement of glucoside recognition by pseudorotaxane formation between the crown and the oligoammonium.
Microfluidic conditions were applied to the acid-mediated reactions, namely, glycosylations, reductive opening of the benzylidene acetal groups, and dehydration, which are the keys to the practical synthesis of N-glycans and immunostimulating natural product, pristane. A distinctly different reactivity from that in the conventional batch stirring was found; the vigorous micromixing of the reactants with the concentrated acids is critical, especially for the “fast” reactions to be successful. Such a common feature might owe to the integration of all favorable aspects of microfluidic conditions, i.e., efficient mixing, precise temperature control, and the easy handling of the reactive intermediate by controlling the residence time. Distinctly different reactivity under microfluidic conditions also led to the success of the large-scale aldol condensation in aqueous biphasic system, as an example of the base-mediated reactions. The microfluidic reactions cited in this review, provoke the need to reinvestigate the traditional or imaginary reactions which have so far been performed and evaluated only in batch apparatus, and therefore, they could be recognized as a new strategy in synthesizing the natural products of the prominent biological activity in a “practical” and an “industrial” manner.
Polybenzoxazine synthesized by the ring-opening polymerization of cyclic benzoxazine monomer has been developed as a new type of phenolic resin. The monomers are easily prepared from phenols, amines and formaldehyde. We report here the performance improvement of polybenzoxazines by the molecular design of monomers, for instance, monomers having crosslinkable units, high molecular weight polymeric monomers, and liquid crystalline monomers. The wide variations of raw materials, phenols and amines, allow tremendous molecular-design flexibility for the cyclic monomers. Other approaches for performance enhancement of the polybenzoxazine, including polymer alloys and hybrids with inorganics, are also reviewed.
We have developed palladium(0)/monophosphine-catalyzed trans-selective alkylative cyclization reactions of alkyne- and allene-aldehydes with organoboron reagents leading to 3-substituted 2-cycloalken-1-ols and 3-cycloalken-1-ols, respectively. Three-component reaction involving secondary aliphatic amines as the third component affords the corresponding tertiary amines via in situ generated iminium ions. These cyclization reactions allow a combinatorial synthesis of biologically important indenes bearing three different substituents at 1,2,3-positions from available o-ethynylbenzaldehyde derivatives. 6-Endo-trig cyclizations of alkynyl- and allenyl-iminium ions, in situ prepared from 3-butynylamine and 2,3-butadienylamine with formaldehyde, afford 1,4-disubstituted 1,2,3,6-tetrahydropyridines. The remarkable trans-selectivity of these processes would result from the novel reaction mechanism involving “anti-Wacker”-type oxidative addition. Changes of the cyclization mode when enones were employed as electrophiles suggest that low tendency of palladium(0) catalyst to form π-complex with carbonyls and iminium ions cause the trans-selectivity.
In this account, polyacrylamide-based fluorescent sensors (thermometer, logic gate, and digital ion sensor) are reviewed. Generally, these polymeric sensors equilibrate in aqueous solution between an extended form with hydration by solvent water molecules and a globular form induced by hydrophobic interaction of side chains. Water-sensitive fluorophore 4-dimethylsulfamoyl-7-aminobenzofurazan incorporated into these polymeric sensors only emitted strong fluorescence at their globular forms where water molecules as quenchers could not get close to the fluorophore. Fluorescent thermometers were based on thermo-responsive poly (N-isopropylacrylamide), poly (N-n-propylacrylamide), and poly (N-isopropylmethacrylamide). Their modifications in solubility enabled intracellular temperature measurement for the first time. A fluorescent logic gate based on a copolymer of N-t-butylacrylamide and ionizable N, N-dimethylaminopropylacrylamide fluoresced only under limited conditions (i.e., high temperature and high pH). Digital fluorescent pH sensors were based on copolymers of N-isopropylacrylamide and ionizable acrylamide derivatives. Their fluorescence off-on switching was completed by changing pH by less than one unit.
The chemistry of CO2 has received much attention from the viewpoint of carbon resources and environmental problems, and the fixation of CO2 represents an attractive area in both organic and green chemistries. In this account, we describe a new synthetic methodology about “CO2-recycling transformation reactions”, in which the reaction would proceed via decarboxylation of the substrate followed by re-fixation of the resulting CO2. Phenoxy-substituted cyclic carbonates were produced when the palladium-catalyzed reactions of propargylic carbonates having a hydroxyl group with phenols were carried out. Applications to diastereo- and enantioselective reactions succeeded with high selectivities. Allylic and 1,3-dienylic carbonates were converted to the vinyl-substituted cyclic carbonates in the presence of palladium catalyst. Oxazolidinones were synthesized by the reaction of allylic carbonates having a amino group. DBU-promoted CO2-recycling reaction of propargylic carbonates also proceeded to afford the 5-vinylideneoxazolidin-2-ones.
Phosphoryl transfer reactions are one of the most important reactions in biological system and intense efforts have been devoted to understand the mechanism. Recently, some X-ray analyses of tri- and pentacoordinate phosphorus-enzyme complex as intermediates of phosphoryl transfer reactions were reported. In addition, highly reactive dinuclear Zn (II) complexes are recently reported to catalyze phosphoryl transfer reaction and used for the verification of the reaction mechanism.