Kei Maekawa

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.

A convenient synthesis of papaverine analogs via photocyclization of N-acyl-α-dehydroarylalanin- amide derivatives

Photochemical cyclizations of (Z)-N-(substituted phenylacetyl)-α-dehydro(3,4-dimethoxyphenyl)alaninamides in methanol were found to proceed regioselectively giving papaverine analogs in satisfactory yields, irrespective of the substituent introduced. MM2 and PM5 calculations revealed that steric hindrance of the methoxy group introduced at the meta-position on the styryl benzene ring is responsible for the regioselective photocyclization observed.