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

The irradiation of substituted N-acetyl-alpha -dehydro(1-naphthyl)alanines (1) in MeOH containing triethylamine with Pyrex-filtered light gave 1,2-dih ydrobenzo[f]quinolinones (2) in preference to benzo[f]isoquinolines (3) and 1-azetines (4). Analysis of substituent effects on the product composition s revealed that the selectivity of 2 has a strong tendency to decrease with increasing steric bulkiness of the alkyl substituent introduced into the s tarting (Z)-1. From control and fluorescence quenching experiments it was s hown that the photocyclization reaction of 1 affording 2 proceeds by an ele ctron-transfer mechanism. The mechanism of this novel cyclization was discu ssed mainly based on substituent and solvent effects on both the reactiviti es of the excited singlet-state 1 and the selectivities of the products 2-4 . (C) 2001 Elsevier Science Ltd. All rights reserved.