Synthesis of model morpholine derivatives with biological activities by laccase-catalysed reactions

The efficient enzyme-catalysed reaction of morpholines as model structures for bioactive compounds with para-dihydroxylated aromatic systems was carried out using the oxidoreductase laccase and atmospheric oxygen to produce eight novel morpholine-substituted aromatics. The laccase of Myceliophthora thermophila was used for cross-linking morpholines containing primary or secondary amino groups with para-dihydroxylated laccase substrates. We demonstrate that not only primary amino groups, but also secondary amino groups, are able to couple with para-dihydroxylated aromatic systems in laccase-catalysed reactions. The resulting model products (yields up to 80%) were isolated, structurally characterized and tested for their antibacterial, antifungal and cytotoxic activities. Four of the eight products showed low to moderate growth inhibition against several Gram-positive and -negative bacterial strains and against the yeasts Candida maltosa and Candida albicans. The antibacterial and antifungal activities were determined by an agar disc diffusion test and a modified method according to the EUCAST discussion document E.Dis 7.1 [Rodríguez-Tudela et al. (2003) Clin. Microbiol. Infect. 9, i-viii] for the evaluation of MIC (minimal inhibitory concentration). Differences in cytotoxicity against the human urinary bladder carcinoma cell line 5637 are discussed.