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 1 H‐ and 13 C‐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 1 H‐ and 13 C‐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 CDCl 3 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.