Shigeyoshi Kanoh

Isomerization of cyclic ethers having a carbonyl functional group: new entries into different heterocyclic compounds

Oxiranes (epoxides) and oxetanes having a carbonyl functional group are chemoselectively isomerized to different heterocyclic compounds via Lewis acid-promoted 1,6- and 1,7-intramolecular nucleophilic attacks of the carbonyl oxygen on the electron-deficient carbon neighboring the oxonium oxygen: for example, cyclic imides to bicyclic acetals, esters to bicyclic orthoesters, sec-amides to 4,5-dihydrooxazole or 5,6-dihydro-4H-1,3-oxazines, and tert-amides to bicyclic acetals or azetidines. The intramolecular attack of a 1,5-positioned carbonyl oxygen predominantly results in a propagating-end isomerization polymerization. On the other hand, cyclic ethers having a 1,8- or farther positioned carbonyl group undergo conventional ring-opening polymerization. A tetrahydrofuran (oxolane) ring does not open, even with a 1,6-positioned carbonyl group.

Asymmetric reduction of aromatic ketones with borane-amine complexes modified with optically pure 2,2′-dihydroxy-6,6′-dimethylbiphenyl

Highly enantioselective reduction of prochiral aromatic ketones was achieved with a chiral reagent (2) prepared from BH3·THF, optically pure 2,2′-dihydroxy-6,6′-dimethylbiphenyl (1), and an achiral amine.

Side Chain Liquid Crystalline Polyoxetanes with a Spacer-Separated Azobenzene Moiety. I. Preparation and Characterization.

Several oxetane derivatives carrying the azobenzene moiety at the C-3 position of the oxetane ring through the spacer arms of differing lengths were prepared by a substitution reaction of the corresponding bromide or p-toluenesulfonate residue of the oxetane with 4-hydroxyazobenzenes. The polymers of these oxetane derivatives were readily obtained by cationic ring-opening polymerization using the adequately increased amount of a THF· BF3 complex as an initiator at 20—30 °C. The liquid crystalline property of the polymers thus obtained were examined by differential scanning calorimetry and by optical polarized microscopy. From these measurements the influences of the p′-substituted azobenzene and of the spacer arm on the liquid crystalline property were found.

Chemoselective isomerization of amide-substituted oxetanes with Lewis acid to give oxazine derivatives or bicyclic amide acetals

The Lewis-acid catalyzed isomerization of secondary and tertiary amide-substituted oxetanes takes place chemoselectively, giving 5-hydroxymethyl-5,6-dihydro-4H-1,3-oxazines and reactive amide acetals consisting of a bicyclo[2.2.2]octane skeleton, respectively.

Substituent Effect on the Cationic Monomer-Isomerization Polymerization of (Cyclic Imide)-Substituted Oxetanes with Two Different Ring-Opening Routes

This study reports the effect of substituent on the cationic monomer-isomerization ring-opening polymerization of 3-(R 1 -methyl)-substituted 3-R 2 -oxetanes (1), in which R 1 is phthalimide, maleimide, succinimide, or glutarimide and R 2 is ethyl, benzyl, phenyl, or isopropyl. The acid-catalyzed polymerization of 1 gave polyacetal (3) or polyether (4), together with an isomeric bicyclic acetal (2). The isomerization of 1 to 2 took place prior to polymerization. Subsequently the polymerization of 2 occured by either single or double ring opening depending on temperature. The polymerization mechanism is discussed in detail based on the coordination of 1 to Lewis acid and the substituent effect on the polymerization manner. In the double ring-opening polymerization of 2 at 130°c, a carbon-carbon double bond of the lactam ring was indispensable for stabilizing the carboxonium-propagating end. Therefore, 2 carrying a saturated lactam ring did not polymerize in such a manner. Phenyl-substitued oxetane phthalimide was unique in undergoing an unusual cyclodimerization at 130°C, primarily because of the high susceptibility of the neophyl-type carbon skeleton to a cation transfer. On the other hand, most 2 brought about the single ring-opening polymerization below room temperature, regardless of the lactam substituent and the R 2 group. This polymerization was an equilibrium polymerization through a bicyclic oxonium-propragating end, and the thermodynamic parameters of polymerization were determined. Thus, 3 was transformed into 4 in one pot, by a combination of the depolymerization of 3 and the repolymerization of 2 above the ceiling temperature. From the structure analysis of 2 it was inferred that the single ring-opening polymerizability arises from dipole-dipole repulsion between the parallel standing lone pairs of two acetalic oxygen atoms in a nearly symmetric bicycle. There fore, 2 having a somewhat twisted bicyle showed no single ring-opening polymerizability.

Enantioselective reduction of ketones with optically active 2,2′-diamino-6,6′-dimethylbiphenyl-lithium aluminum hydride complex

Abstract Enantioselective reductions of prociral ketones with chiral hydride reagent prepared from optically active 2,2′-diamino-6,6′-dimethylbiphenyl and lithium aluminum hydride were accomplished in O.Y. more than 50%.

Chiral fluorescent sensors based on cellulose derivatives bearing terthienyl pendants

Chiral fluorescent sensors were designed and synthesized from naturally occurring optically active cellulose, in which a terthienyl moiety was used as a fluorescent signaling unit, and their chiral recognition abilities were investigated on the basis of an enantioselective fluorescence response to aromatic nitro compounds as quenchers. A cellulose phenylcarbamate derivative (Ce-1b) exhibited an apparent enantioselectivity for various types of aromatic nitro compounds containing either central or axial chirality. Considering the fact that a corresponding monosaccharide derivative showed almost no enantioselectivity, the chiral recognition ability of the cellulose-based fluorescence sensor is attributed to its regular higher-order structures, probably a one-handed helical conformation. The resolution ability of Ce-1b as a chiral stationary phase for high-performance liquid chromatography was basically higher than that of the previously reported cellulose tris(phenylcarbamate), revealing that the terthienyl-based pendant provides attractive chiral recognition sites along the helical backbone as well as the fluorescent functionality.

Synthesis and chiroptical properties of a π-conjugated polymer containing glucose-linked biphenyl units in the main chain capable of folding into a helical conformation

An optically active π-conjugated polymer (poly-9) containing glucose-linked biphenyl units in the main chain was synthesized through the copolymerization of 5,5′-diiodo-2,2′-bithiophene and a new diethynyl compound, whose molecular design has taken inspiration from naturally occurring ellagitannins. The chiroptical properties of poly-9 were investigated in solution and the solid state. The absorption and circular dichroism spectra of poly-9 were clearly changed depending on the solvents due to the conformational alteration within a single polymer chain. Because a corresponding model molecule and polymer did not show marked solvent-dependent spectral changes, the backbone conformation of poly-9 is considered to be capable of interconverting between random-coil and preferred-handed helical states in response to the exterior environment. We also found that poly-9 exhibited efficient circularly polarized luminescence with a green color, whose dissymmetry factor reached 1.9 × 10−2 when the backbone was folded into the helical conformation in the film state.

Unusual cyclodimerization of small cyclic ethers via neighboring carbonyl-group participation and cation transfer

Oxetanes having both a carbonyl functional group and a phenyl group at the 3-position bring about unusual cyclodimerization under the influence of trifluoromethanesulfonic acid, to give a cis/trans-isomer mixture of substituted 1,3-dioxanes via neighboring carbonyl-group participation, cation transfer including phenonium rearrangement, and cyclodimerization. Oxiranes having a carbonyl functional group also undergo a similar cyclodimerization to form a 1,3-dioxolane ring. No phenyl substituent is required in the cases of the oxiranes, because the cation transfer process involves a 1,2-hydride shift alone. These five- and six-membered cyclic acetals are quite different from ring-expanded cyclic ether dimers expected simply from the back-biting reaction known in the cationic ring-opening polymerization of oxiranes and oxetanes.

Chiral stationary phases consisting of π-conjugated polymers bearing glucose-linked biphenyl units: reversible switching of resolution abilities based on a coil-to-helix transition

A series of optically active π-conjugated polymers (poly-2–poly-5) consisting of alternating thieno[3,4-b]thiophene and glucose-linked biphenyl units were synthesized through copolymerizations by Sonogashira–Hagihara cross-coupling. Absorption and circular dichroism spectroscopy and theoretical calculations revealed that poly-2 undergoes a conformational transition between the random-coil and helix in both solution and the solid state in response to the external solvent environment (chloroform and acetonitrile). Coated-type chiral stationary phases (CSPs) for high-performance liquid chromatography were prepared from both the random-coil and helical poly-2, and the influence of the polymer backbone structure on the chiral recognition ability was evaluated. The two CSPs showed somewhat complementary resolution abilities and the kinds of resolved racemates were dependent on the backbone conformation. In addition, an immobilized-type CSP with universal solvent durability was also prepared through copper-catalyzed click cyclization between an alkyne-appended poly-2 analogue and an azide-functionalized silica gel. The resulting CSP displayed repeatable switching of the chiral recognition ability based on a coil-to-helix transition of the polymer backbone by alternate column treatment with common organic solvents, such as chloroform and acetonitrile.