STRUCTURAL REINVESTIGATION OF 2 DIASTEREOISOMERIC ASPIDOSPERMA EBURNEA TYPE BIS-INDOLES BY NMR METHODS - DETECTION OF A HIDDEN EXCHANGE PARTNER AND IMPLICATIONS IN NOE ANALYSIS

Full H-1 and C-13 assignments are reported for two aspidosperma-eburne a type diastereoisomers. Homonuclear NOE difference measurements revea led that the previously reported configurational assignment of these c ompounds is in error. More interestingly, rotation about the bond conn ecting the two indole units is hindered in both molecules and gives a strongly biased two-site chemical exchange system. At room temperature the exchange rates are slow on the chemical shift time-scale with reg ard to certain H-1 signals. In addition, the exchange rates are such t hat the signals due to the minor component are broadened to the extent that they escape detection in a conventional H-1 spectrum, whereas on ly a very small amount of broadening is observed on the main signals. The presence of this 'hidden' exchange partner can give rise to peculi ar effects in the NOE difference spectra. Such spurious effects can be the source of misinterpretation if the existence of the 'silent partn er' goes unnoticed. Thermodynamic parameters are evaluated based on ea rlier work addressing the problem of the 'hidden' exchange partner.