A conformational NMR analysis of methymycin aglycones: Complete and unambiguous assignments of stereochemically diverse glycosylated methymycin analogs by 1D and 2D NMR techniques and molecular modeling

The 1H and 13C NMR spectra of 10-deoxymethynolide (1), 8.9-dihydro-10-deoxymethynolide (2) and its glycosylated derivatives (3-9) were analyzed using gradient-selected NMR techniques, including 1D TOCSY, gCOSY, 1D NOESY (DPFGSENOE), NOESY, gHMBC, gHSQC and gHSQC-TOCSY. The NMR spectral parameters (chemical shifts and coupling constants) of 1-9 were determined by iterative analysis. For the first time, complete and unambiguous assignment of the 1H NMR spectrum of 10-deoxymethynolide (1) has been achieved in CDCl3, CD3OD and C6D6 solvents. The 1H NMR spectrum of 8,9-dihydro-10-deoxymethynolide (2) was recorded in CDCl3, (CD3)2CO and CD3OD solutions to determine the conformation. NMR-based conformational analysis of 1 and 2 in conjugation with molecular modeling concluded that the 12-membered ring of the macrolactones may predominantly exist in a single stable conformation in all solvents examined. In all cases, a change in solvent caused only small changes in chemical shifts and coupling constants, suggesting that all glycosylated methymycin analogs exist with similar conformations of the aglycone ring in solution.