Abstract
The ^1^H and ^13^C NMR spectra of methyl‐8‐(2‐furyl)‐5‐methyl‐1,3‐dioxo‐3,3a,4,6,8,9,9a,9b‐octahydrofuro[3,4‐f]isoquinoline‐7(1__H__)‐carboxylate (1) and trimethyl 8‐methyl‐3‐phenyl‐3,4,4a,7‐tetrahydro‐2,5,6(1__H__)‐isoquinolinetricarboxylate (2) at room temperature displayed doubling of the majority of signals, suggesting the presence of two rotational conformers (rotamers) in a ratio ∼1:1.2 (in a mixture of CDCl~3~ and C~6~D~6~), ∼1:1 (in CD~2~Cl~2~) and ∼1:1.4 (in C~6~D~6~). On the basis of the temperature‐dependent ^1^H NMR spectra of 1 and 2, the barrier to interconversion of the rotamers was calculated to be ∼16 kcal mol^−1^. The average chemical shifts and spin–spin coupling constants were analyzed for the resolution‐enhanced 300 MHz ^1^H NMR spectrum of 1 at 333 K in CDCl~3~ solution. From analysis of the spin–spin coupling constants, it is proposed that the nitrogen‐containing ring is in a chair conformation with C‐2—H equatorial. Low‐ and room‐temperature NOESY experiments in conjunction with theoretical ab initio calculations supported the hypothesis that the two rotamers interchange via rotation about the C—N bond. Spectral assignments of all proton and carbon resonances were made on the basis of one‐ and two‐dimensional NMR experiments (DEPT, DQCOSY, NOESY, HETCOR and gHMBC). Copyright © 2003 John Wiley & Sons, Ltd.