COVALENTLY LINKED TYROSINE PYROPHEOPHORBIDE-A AND TRYPTOPHAN PYROPHEOPHORBIDE-A COMPOUNDS - SYNTHESIS AND PHOTOINDUCED CROSS COUPLING WITH 1,4-BENZOQUINONE

Tyrosine-linked pyropheophorbide a (2) and tryptophan-linked pyropheophorbide a (3) were synthesized. Neither the phenol group of the tyrosine end nor the indole group of the tryptophan end showed upfield shifts as observed for the tyrosine-linked mesoporphyrin II (17) and the tryptophan-linked mesoporphyrin II (18). When the keto group at the C-9 of 2 and 3 was reduced to methylene group [tyrosine-linked 9-deoxopyropheophorbide a (5) and tryptophan-linked 9-deoxopyropheophorbide a (6)], the phenol group of the tyrosine and the indole group of the tryptophan showed upfield shifts, indicating the intramolecular hydrogen bonds between amino acid moieties and the keto group at the C-9 in 2 and 3. Irradiation of 2 and 1,4-benzoquinone (BQ) in benzene led to a clean formation of a quinone-linked pyropheophorbide a (8) in 56% yield. Triplet radical pair composed of tyrosine radical and semiquinone radical was assigned as the reaction intermediate for the formation of 8 by means of CIDNP technique. Irradiation of 5 under the identical conditions led to the formation of benzoquinone-linked 9-deoxo-mesopyropheophorbide a (10) and benzoquinone-linked 9-deoxophylloerythrin a (11) in 45 and 22%, respectively. Magnesium and zinc complexes of 2 underwent the similar photoaddition reaction much less efficiently. However, irradiation of the tryptophan-linked model compounds 3 and 6 in the presence of BQ did not give the coupling product.