The 3,4-catechol derivative of propranolol, a minor dihydroxylated metabolite

The O,O-dibenzyl ether of the 3,4-catechol derivative of propranolol (11) was prepared to determine whether the catechol is a product of metabolic hydroxylation. 4-(Allyloxy)-1,2-naphthoquinone (5) was reduced with sodium dithionite and alkylated with benzyl chloride to produce ether 7. Osmium tetroxide oxidation of 7 afforded glycol 8. Subsequent monotosylation, oxirane formation with KOH, and opening with isopropylamine afforded benzyl ether 11. Although hydrogenolysis was successful, catechol 3 was rapidly oxidized to the corresponding o-quinone (12). Reduction of 12 with sodium bisulfite afforded 3, which was derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) to serve as a standard for the metabolic experiments. Gas chromatography-mass spectrometry of the Me3Si ethers of the products of metabolism of pseudoracemic propranolol (made up of equal molar (2R)-propranolol-d0/(2S)-propranolol-3',3'-d2) in the presence of the rat liver 9000g supernatant fraction showed four dihydroxylated metabolites, of which catechol 3 was in smallest amount, approximately 9% of the sum of dihydroxylated metabolites. Each of the four dihydroxylated propranolols arises stereoselectively from the 2R enantiomer of propranolol (by 1.15- to 2-fold), as determined by parent ion intensities at m/z 507 vs. 509. Quinone 12 was a nonselective competitive beta-adrenoceptor antagonist, being about 16-fold less potent than propranolol in both beta 1 and beta 2 assays.