Conformational Analysis of the cis‐ and trans‐Isomers of FK506 by NMR and Molecular Dynamics

The potent immunosuppressant drug FK506 (2) has been examined by ¹H‐ and ¹³C‐NMR spectroscopy and NOE‐restrained molecular dynamics to elucidate the conformation in solution. A combination of two‐ and three‐dimensional NMR techniques was used to completely assign the ¹H‐ and ¹³C‐NMR chemical shifts of the two configurational isomers resulting from the cis‐trans isomerization about the single amide bond. Hetero‐ and homonuclear coupling constants were measured to assign the diastereotopic methylene protons at C(16), C(18), and C(23). Intramolecular HH distances were defined from NOESY spectra recorded at −30° in CDCl₃ and used as constraints in molecular‐dynamics simulations. The conformational preferences of 2 in solution are discussed in light of the constitutional features recently proposed to be necessary for binding and activity.