Study of spontaneous E/Z isomerization of bis[(Z)-cyanomethylidene]-diazapentacyclodienedicarboxylates by 1 H, 13 C, and 15 N NMR spectroscopy, X-ray, and quantum chemical calculation data.

X-ray data show that the diethyl 6,13-bis[(Z)-cyanomethylidene]-5,5,14,14-tetramethyl-4,15-dioxa-7,12-diazapentacyclo[9.5.2.02,10 .03,7 .012,16 ]octadeca-8,17-diene-10,17-dicarboxylate is formed as the ZZ isomer and diastereomer with the (1R*,2R*,3R*,10S*,11R*,12R*,16R*) configuration. The 1 H, 13 C, and 15 N NMR data exhibit that on standing in chloroform-d solution, there is a spontaneous isomerization of this compound resulting in a thermodynamically stable mixture of the ZZ, ZE, EE, and EZ isomers with the same backbone. Using the 2D [1 H-1 H] COSY, [1 H-13 C] HSQC, and [1 H-13 C, 1 H-15 N] HMBC NMR techniques and quantum chemical calculations makes it possible a complete assignment of signals in the 1 H, 13 C, and 15 N NMR spectra of each of the isomers. Such isomerization does not occur for similar compounds with the more bulky substituents at the 1,3-oxazolidine rings. Copyright © 2016 John Wiley & Sons, Ltd.