Reassignment of the 13 C NMR spectrum of minomycin

1 H and 13 C NMR spectra of minomycin were determined and a full assignment was carried out with the aid of HMQC, HMBC and other techniques. It was shown that the previous assignment for 13 C-NMR signals required some correction


Introduction
As an extension of our synthetic and structural studies of organogermanium compounds 1 , we were interested to study the changes of physiological activity induced in antibiotics by the introduction of a germanium-containing moiety.
The first antibiotic we chose for this purpose was minomycin (minocycline) hydrochloride (1).It is essential to have a full assignment of the 1 H and 13 C NMR spectra of 1 in order to elucidate the structure of the reaction products of germylation.Two papers have so far been published on the NMR spectra of 1, one by Nilges et al. 2 on 1 H NMR, and another by Mazzola et al. 3 on 13 C NMR spectra.In the paper by Nilges et al. 2 , as well as considering the conventional chemical shifts, coupling constants were also used for the spectral assignment.Mazzola et al. 3 employed chemical shift considerations and coupled splitting patterns in their study, although no modern techniques, such as 2D NMR, were used.It occurred to us that it was worthwhile to attempt a re-examination of the NMR spectra of 1 with the aid of 2D and other techniques.In this paper we report the reassignment of the 13 C NMR spectrum of 1.

H NMR spectra
Nilges et al. determined 1 H NMR spectra of 1 in CD 3 OD and DMSO-d 6 in order to differentiate between exchangeable and non-exchangeable protons.For protons 4, 4a, 5a, 5', 5", 6'and 6", homonuclear spin decoupling was extensively employed to facilitate the assignment.Signals due to H4a and H5a were obscured by the strong peak of one of the N(CH 3 ) 2 moieties at δ 2.99.They identified these signals by comparing the spectrum of a related compound that does not contain this N(CH 3 ) 2 moiety .The assignment of the remaining aliphatic ring protons was essentially based on the Karplus-type relation.From these results, the authors concluded that 1 has the same basic conformation as other tetracyclines.
We confirmed their assignment using a systematic proton homonuclear decoupling experiment.Thus, we irradiated the CD 3 OD solution of 1 at δ 1.63 (H5'), 2.19 (H6'), 2.9 (H4a and H5a), 3.41 (H6") and 4.26 (H4).For instance, by irradiating H5', a doublet for H4a collapsed into a singlet, and a multiplet at δ 2.9 became a singlet, which indicated that the low field multiplet centered at δ 2.21 is due to H5" rather than H6' thus confirming the assignment made by Nilges et al. 2 We further confirmed the assignment by an H-H COSY experiment.The starting point of choice is the signal for H4 at δ 4.26, which is a doublet with very small splitting due to coupling with H4a.There are no other couplings involved, and hence it is easy to identify from the splitting pattern.This signal is correlated with the signal for H4a at δ 2.9, and the correlation can be extended to the signals for H5'(δ 1.48) and H5"(δ 2.21) to confirm the reported assignment.
Chemical shifts and coupling constants are summarized in Table 1.

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C NMR spectra Mazzola et al 3 reported the assignment of the 13 C NMR spectrum of 1 based on a comparison of chemical shifts against other tetracyclones, the splitting pattern under a proton coupled condition and the multiplicity observed in SFORD (single frequency off resonance decoupling) experiments.In one case they also used spin-lattice relaxation times (T 1 ).
Although the multiplicity in SFORD experiments and the splitting pattern in proton coupled spectra will give useful information, comparisons of chemical shifts between similar compounds are not always unambiguous, particularly among compounds with many substituents.
For this reason, we undertook our own assignment.Altogether twenty-one signals were observed for twenty-three carbon nuclei in 1, with two equivalent methyl groups in each dimethylamino moiety.In Table 2, twenty-one 13 C chemical shifts and their multiplicity as determined by DEPT (DEPT90 and DEPT135), are listed.Firstly, we used HMQC (Heteronuclear Multiple Quantum Correlation) 4 , by which the six correlations summarized in Table 3 were established.Peaks 7, 12 and 13 were readily assigned to C4, C8 and C9, respectively, based solely on chemical shifts and multiplicity.Peak 5 is automatically assigned to 4-N(CH 3 ) 2 since peak 6 is correlated with another N(CH 3 ) 2 moiety.The assignment of 13 C peaks 2 and 4 (either C4a or C5a) by HMQC was difficult since the H4a and H5a peaks are nearly overlapping.The assignment of these two peaks, together with all other quaternary carbon nuclei, can be achieved with the aid of HMBC (Heteronuclear Multiple Bond Correlation) 5 .Figure 2 is the HMBC spectrum of 1 in DMSO-d 6 .

Figure 2. HMBC spectrum of 1.
It is rather unfortunate for HMBC that the H4a and H5a peaks, and H5" and H6' peaks are overlapping, making the assignment rather ambiguous.However, with the aid of some chemical shift considerations, the assignment was carried out as given below. (1) The H4 peak correlates with 13 C peak 4d but not with peak 2d; hence peak 4d can be assigned to C4a.Consequently, the remaining doublet 13 C peak 2d is assigned to C5a. ( 2) The H4 peak correlated with peak 5q, one of the N(CH 3 ) 2 moieties.Hence peak 5q should be assigned to 4-N(CH 3 ) 2 , to confirm the assignment based on HMQC.(3)  It is expected that C12a should resonate at a relatively high field since it is an sp 3 hybridized species.Peak 8 is highest among the remaining twelve singlets and, hence, is assigned to C12a, This assignment can be confirmed by correlation with H5 peaks.(4) One of the NH 2 peaks (δ 9.50) correlates only with 13 C peak 9s, while another NH 2 peak (δ 9.04) correlates with 13 C peak 17s.From chemical shift considerations, 13 C peak 9s should be C2, and 13 C peak 17s should be due to -CONH 2 . (5)

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C peak 16s correlates with H8, H9 and 10-OH peaks.Hence peak 16s can be assigned to C10. (6)  The 10-OH peak shows a correlation with 13 C peak 11 (there is little possibility of correlation with peak 12 (C8) ) which should be either C10a or C11a.The latter possibility is excluded, given the considerable distance between them and, accordingly, 13 C peak 10 is assigned to C11a.(7) This assignment was confirmed by the correlation of the 10s peak with H4a, H5a, H5 and H6.(8) One interesting correlation is the one between the 7-N(CH 3 ) 2 peak and 13 C peak 15, which is also correlated with the H8 peak, indicating that 13 C peak 15 is assignable to C7. (9) 13 C peak 14s is not due to a carbonyl carbon; hence this peak is assigned to C6a, and this assignment is consistent with the correlation with proton peaks (H5"/H6' and H6").(10) 13 C peak 14s is in relatively high field among singlet carbon nuclei.From chemical shift considerations, the only possible assignment is to C10a.Now, remaining is the assignment of the signals for 13 C peaks 18-21, two β-carbons (peaks 18s and 19s) of the β-hydroxy-α,β-unsaturated ketone moiety and two carbonyl carbons (peaks 20s and 21s), (11) The correlation between the H4 peak and 13 C peak 19 clearly indicates that peak 19 should be assigned to C3 and hence peak 18 to C12. (12) The assignment of the two carbonyl peaks 20 and 21 was difficult because of overlapping of the H4a/H5a peaks.HMBC does not work in such a case, and thus we felt a deuterium isotope shift might be useful.In a mixed solvent of CD 3 OD/DMSO-d 6 , OH and NH 2 protons are exchanged slowly, and the chemical shifts of carbon nuclei nearby will be moved, in most cases, to high field. 6There is an exchangeable proton three bonds apart from C1, that is, 12a-OH.From C11, all exchangeable protons (10-OH and 12-OH) are four bonds apart, and no isotope shift is expected.Indeed, peak 21 showed a small, but distinct, high field shift of ca.0.2 ppm, while peak 20 did not show any such shift.Based on these observations, we assigned peaks 20 and 21 to C11 and C1, respectively, to complete the assignment of twenty-one 13 C peaks of 1.
Our assignment together with that by Mazzola et al. 3 is given in Table 4. Table 4. 13  There are a few disagreements between the two assignments.We believe our assignment, based as it is on newer techniques, will be more dependable and we propose that the assignment of at least the following pairs, 4-N(CH 3 ) 2 /7-N(CH 3 ) 2 , C5/C6, C6a/C7 and C8/C9, should be reversed.
We are continuing our efforts to re-examine the NMR spectra, and modification with germanium moieties, of a variety of antibiotics.
General Procedures. 1 H and 13 C NMR spectra of 1 were determined with a JEOL ECP-500 spectrometer in a 5 mm o.d.tube containing CD 3 OD or DMSO-d 6 solution operating at 500 MHz and 125 MHz, respectively, using tetramethylsilane (TMS) as an internal standard.0.050g of 1 was dissolved in 0.5 ml of CD 3 OD or 0.014 g of 1 in 0.5 ml of DMSO-d 6 .All the measurements were carried out in the manner described in the literature (HMQC 4 , HMBC 5 ) with the aid of the installed software.J values are recorded in Hz.

Table 1 .
1H NMR Spectral Data for 1 a Data in CD 3 OD.

Table 2 .
Chemical shifts and multiplicity of 13 C peaks of minomycin(1)

Table 2
demonstrates that there are cases where many carbon nuclei with the same multiplicity have close chemical shift values.More advanced techniques are thus required for the unambiguous assignment of all carbon nuclei.

Table 3 .
Correlation established by HMQC in CD 3 OD C NMR Spectral Data for 1 in DMSO-d 6