Abstract
Ascomycin ( 1a), a macrolactam antifungal antibiotic disclosed by Arai in 1962, 1 was found to display immunosuppressive activity more than 2 decades later by Okuhara and coworkers at Fujisawa. 2 Ascomycin ( 1a) and FK506 ( 1b) bind to a peptidyl-prolyl-isomerase, FKBP, a necessary but insufficient condition for drug activity. Both FK506 and ascomycin exist as a mixture of slowly interconverting cis and trans amide rotamers. It has also been shown that only the trans amide rotamer binds to FKBP. 24-epi-Ascomycin ( 3), 24-oxo-22-norascomyin ( 9a), and 22-norascomycin ( 9b), obtained by semisynthesis from ascomycin, exist as single rotamers on the NMR time scale. Their synthesis and the biological consequences of this observation are discussed.
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