engleski

(E)/(Z) Configurational Assignment of 4,7-Dihydro-4-hydroxyimino-6-nitro-1,3-dioxepins. NMR Problem Solving

NMR spectroscopy provides a set of experiments that allow the complete structure of organic molecules to be inferred in straightforward manner. The procedure of NMR structure elucidation problem solving is shown on the example of 4,7-dihiydro-4-hydroxyimino-6-nitro-1,3-dioxepins (1a, R = H; 1b, R = 4,7-dihydro-1,3-dioxepin-2-yl). In the course of syntheses and/or application of substituted 4,7-dihydro-5-nitro-1,3-dioxepins to the chemistry of pyridoxyne (B6 vitamin) the oxymes 1 were obtained as by-product in the Diels-Alder reaction (the step of B6 intermediates syntheses). The 1 was also obtained by direct nitrosation of nitrodioxepins with ethylnitrite. Irrespective of the route of their formation oxymes 1 have shown similar spectroscopic features, indicating the existence of only one configurational isomer in both cases. The investigation was aimed at the chemical and spectroscopic determination of their structures and configuration. Fig. Z- and E-configurations of 4,7-dihydro-4-hydroxyimino-6-nitro-1,3-dioxepins (1) The oxymes were underwent to Beckmann rearrangement, since it is a well known standard procedure for elucidation of oxyme stereochemistry. When oxymes are treated with PCl5 or a number of other reagents, they rearrange to substituted amides. The group that migrates is generally the one anti (E) to the hydroxyl group. Although this is not unequivocal, this chemical rearrangement is often used as a method of determining the configuration of the oxyme. Through elucidation the structure of obtained amide one can intermediary determine the stereochemistry of rearranged oxyme. Using different mild reagents (methane-, p-toluene- or p-acetylaminobenzensulfochlorides) in this case of Beckmann rearrangement, only O-sulfonates (2) were obtained, while under more drastic conditions, using either PCl5 or P2O5 in chloroform, both oxymes and their sulfonic esters furnished none of two possible dioxaazocines. In all studied cases, the only isolated product was 4-nitro-5H-furan-2-on (3) which was a result of hydrolisys and not rearrangement. Since the Beckmann rearrangement did not give an answer on the configuration of oxyme group in 1, to figure it out, these compounds were investigated by various one- and twodimensional homo- and heteronuclear correlation 1H and 13C NMR spectroscopy techniques (COSY, NOESY, HETCOR and HMBC). Assignment of 1H and 13C NMR spectra were performed using chemical and substituent shifts, H-H and C-H coupling constants and selected irradiation as well as connectivities in two-dimensional homo- and heteronuclear correlation spectra. The configuration of oxymes 1 could be assumed from 13C gated decoupled spectra as well. It is generally known that the Z-orientation of the lone electron pair to C-H bond gives raise to an increase in magnitude of the corresponding one-bond C-H coupling (c.a. 10-15 Hz) as compared to the E-orientation. However, in these reaction conditions, the only one isomer was isolated. The question was: in which one of two possible isomer forms the isolated oxymes 1 exist? The magnitude of 1JC-5,H-5 in 1a (163.7 Hz) and 1b (164.0 Hz) might correspond to E-arrangement of the lone electron pair to C5-H bond in parallelism with similar systems (e.g. acetaldoxyme, where 1JC,H is 163,0 Hz for E-isomer, while it is even 177,0 Hz for Z-isomer). This finding was supported with differential NOE measurements that showed spatial interaction between N-OH and H-5 and unambigously proved that both oxymes 1a and 1b have E-configuration. The NOE interaction between N-OH and H-5 was also in agrrement with ab initio calculated distance between these protons, amounting to 3.21 Ĺ. In conclusion, one can say that the analysis of 1H and 13C chemical shifts, magnitudes and patterns of couplings, substituent effects, as well as differential NOE and connectivities in COSY, NOESY, HETCOR and HMBC spectra enabled determination of the structure and configuration of these compounds and are certainly useful in the configurational determination of similar systems. 1. M. Jadrijeviă-Mladar Takač, I. Butula, D. Vikiă-Topiă and M. Dumiă, Chemistry of 1,3-Dioxepins. XIII. (E)/(Z) Configurational Assignment of 4,7-dihidro-4-hydroxyimino-6-nitro-1,3-dioxepins, Croat. Chem. Acta 71 (1998) 557-571.

nitrooxyiminodioxepin; configurational assignment; 1D and 2D NMR spectroscopy

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