Tetsutaro Igarashi

Complexation and proton dissociation behavior of 7-hydroxy-4-methylcoumarin and related compounds in the presence of β-cyclodextrin

Abstract The pH effects on the 1:1 complexation behavior of β-cyclodextrin (β-CDx) with the title compounds have been investigated by measuring ultraviolet (UV) absorption, circular dichroism (CD), fluorescence and 1 H nuclear magnetic resonance (NMR) spectra and fluorescence lifetimes of the β-CDx inclusion complexes formed. Steric effects of substituent attached at the four position of a guest molecule on the stability of the inclusion complex as well as on the induced CD spectral intensity demonstrated that the guest is included into the β-CDx cavity so as to locate this substituent around the rim of the cavity at low and even at high pH (where a tautomer anion is exclusively produced). The 1 H NMR data were consistent with the intracavity immersion of a given guest molecule from the secondary hydroxy group side and, additionally, substantiated the structure of the β-CDx inclusion complex generated at low pH. Through an analysis of the pH-dependent UV absorption and fluorescence spectra of 7-hydroxycoumarine derivatives, it was found that the proton dissociation abilities of these guests undergo negligible effects of the β-CDx inclusion in both the ground-state and the excited singlet-state. This finding strongly suggested that both the quinoid-type carbonyl and oxido groups in the tautomer anion guest are exposed to the bulk aqueous phase to interact with the surrounding water molecules.

Electron transfer-initiated photocyclization of substituted N-acetyl-α-dehydro(1-naphthyl)alanines to 1,2-dihydrobenzo[f]quinolinone derivatives: Scope and limitations

Abstract The irradiation of substituted N-acetyl-α-dehydro(1-naphthyl)alanines (1) in MeOH containing triethylamine with Pyrex-filtered light gave 1,2-dihydrobenzo[f]quinolinones (2) in preference to benzo[f]isoquinolines (3) and 1-azetines (4). Analysis of substituent effects on the product compositions revealed that the selectivity of 2 has a strong tendency to decrease with increasing steric bulkiness of the alkyl substituent introduced into the starting (Z)-1. From control and fluorescence quenching experiments it was shown that the photocyclization reaction of 1 affording 2 proceeds by an electron-transfer mechanism. The mechanism of this novel cyclization was discussed mainly based on substituent and solvent effects on both the reactivities of the excited singlet-state 1 and the selectivities of the products 2–4.

An efficient transformation of substituted N-acyl-α-dehydro(1-naphthyl)alanines into 1,2-dihydrobenzo[f]quinolinone derivatives via photoinduced intramolecular electron transfer

The irradiation of substituted (Z)-N-acyl-a-dehydro( 1-naphthyl)alanines (1) having the dialkylamino donor on the carboxamide side chain in methanol was found to give 1,2-dihydrobenzo[f]quinolinone derivatives (2) in goodyields, which were formed via the electron-transfer reaction in the excited-state (E)-isomers, while intramolecular photocyclization reactions in the (Z)- and (E)-isomers afforded minor amounts of benzo[f]isoquinolines (3) and 1-azetines (4), respectively. The replacement of the N-acetyl group by the benzoyl (having stronger electron-withdrawing ability and larger steric bulkiness than the former) increased the selectivity for 2 and 4, and this increased selectivity was mainly reflected in a great lowering of that for 3.

A new route to papaverine analogs via photocyclization of substituted N-acyl-α-dehydrophenylalaninamides

(Z)-N-Phenylacetyl-a-dehydro(3,4-dimethoxyphenyl)alaninamide derivatives [(Z)-1] were prepared in satisfactory yields, starting from 3,4-dimethoxybenzaldehyde. On irradiation in methanol, regioselective photocyclization of these derivatives proceeded to give papaverine analogs in good yields. Substituents introduced into (Z)-1 were found to exert only a minor effect on the conversion of the starting isomers (1) as well as on the selectivity of the analogs which were formed along with 1-azetines.

Photoinduced electron transfer-initiated enantioselective cyclization of N-benzoyl-α-dehydroarylalanine tert-butyl esters in the presence of chiral amine

The asymmetric photocyclization of the title compounds in 1,2-dichloroethane was found to proceed cleanly in the presence of primary. secondary, or tertiary chiral amine to give the corresponding cis- and trans-3,4-dihydrooxazole derivatives in enantiomeric excess (ee) of 11-38% and 6-33%, respectively, depending on the steric factor of the aryl substituent as well as on the hydrogen-bonding ability of the chiral amine.

Electron Transfer-initiated and Highly Selective Photocyclization of N-Acyl-α-dehydroarylalaninamides to 3,4-Dihydroquinolinone Derivatives

The irradiation of (Z)-N-acyl-α-dehydroarylalaninamides [(Z)-1] in methanol containing 1,8-diazabicyclo[5.4.0]undec-7-ene gave substituted 3,4-dihydro-2-quinolinones (2) in high yields along with minor amounts of 2-quinolinone (3) and 4,5-dihydrooxazole (4) derivatives. Analysis of substituent and tertiary amine base effects on the selectivity of each product showed that 2 and 4 are formed via electron transfer from the amine to the excited-state (E)-1, the cyclization of which proceeds in competition with this electron transfer to afford 3.

Oxidative cyclization of aryl-substituted (Z)-N-acetyl-α-dehydroalanines having a dialkylamino group, in the presence of dioxygen

It was found that the reactions of the title compounds (1) with dioxygen in methanol proceed according to the first-order kinetics to give (Z)-2-imidazolin-5-one derivatives and hydrogen peroxide in quantitative yields. Substituent and solvent effects on the rate constant for this oxidative cyclization reaction are consistent with the rate-determining electron transfer from the dialkylamino nitrogen in the starting 1 to dioxygen.

Cationic micellar effects on the proton transfer reactions of N-substituted 2-(7-hydroxycoumarin-4-yl)acetamides and related compound in the ground and excited singlet states

A spectroscopic investigation was undertaken to determine cationic micellar effects on the ground-state and excited singlet-state proton transfer reactions of the title compounds with and without triethylamine (TEA). It was found that the presence of hexadecyltrimethylammonium chloride (HTAC) micelles containing TEA apparently lowers the equilibrium constant for the formation of tautomer anions having hydrophobic alkyl or aryl side chains (owing to a decrease of the effective concentration of TEA in the micellar phase) but exerted only a minor effect on the tautomerization of 7-hydroxy-4-methylcoumarin. The latter finding was explained in terms of the adsorption of this guest on the cationic head group with the hydroxy group directed toward the bulk aqueous phase. Analysis of HTAC micellar effects on the fluorescence spectra and lifetimes of the guests and their tautomer anions in the absence of TEA revealed that the orientation of our bichromophoric guest molecules in the micellar phase is subject to dramatic electronic effects of aryl substituents attached at the 4-position of the hydroxycoumarin ring.

SUBSTITUENT EFFECTS ON THE ELECTRON TRANSFER-INITIATED PHOTOCHEMICAL TRANSFORMATION OF 1,2,4-TRIAZOLE-SUBSTITUTED α -DEHYDROARYLALANINAMIDES INTO 2(1H)-QUINOLINONE DERIVATIVES

An investigation was undertaken to elucidate substituent effects on the photoreactivity of 1,2,4-triazole-substituted α-dehydroarylalaninamides [(Z)-1] as well as on the selectivity of 2(1H)-quinolinone derivatives (2) from a synthetic point of view. It was found that photoinduced electron transfer-initiated cyclization of (Z)-1 bearing a meta-substituted phenyl or a 4-substituted naphthalen-1-yl group in methanol proceeds with a moderate to good efficiency affording the corresponding product 2 in a selectivity ranging from 33 to 100%.

Preferred Conformations of Diastereomeric N-Acyl-1-naphthylalanylprolinamides and Related Dipeptides and Their Fluorescence Quenching by Chiral Amines

Enantioselective fluorescence quenching was observed in the quenching processes of the l-naphthylalanyl-(S)-phenylglycinamide-derived diastereomers by enantiomeric pyrrolidine-2-methanols in 1,4-dioxane at room temperature, whereas the (S)-prolinamide-derived ones exhibited only a much weaker enantioselective emission quenching under the same conditions. These findings were explained on the basis of substituent and conformational effects on the stability of a singlet exciplex formed between given diastereomer and chiral amine.