A Complete and Unambiguous 1 H and 13 C NMR Signals Assignment of para-Naphthoquinones , ortho-and para-Furanonaphthoquinones

A complete and unambiguous assignment of H and C nuclear magnetic resonance (NMR) signals of 29 naphthoquinones is reported on the basis of oneand two-dimensional NMR techniques (H, C, H-H correlated spectroscopy (COSY) and H-C heteronuclear multiple-bond correlation (HMBC)). This is the first report distinguishing data between para-naphthoquinones, orthoand para-furanonaphthoquinones isomers.

The NMR experiments were performed in CDCl 3 solution at 25 ºC on a Bruker Avance DRX400 spectrometer.Tetramethylsilane (TMS) was used as internal reference and the program the Bruker's TopSpin 3.5 TM software package was used to process the NMR row data.
The 1 H spectra were acquired using the spectrometer frequency of 400 MHz, spectrum resolution of 0.12 Hz, zg 30 pulse program with ns 16, d1 1s, acquisition time 4.0894465 s and spectral width 20.0264 ppm.The phase and baseline were manually corrected and the TMS signal calibrated at 0.00 ppm.Integration regions of signal were selected manually.
The 13 C spectra were acquired using the spectrometer frequency of 100 MHz, spectrum resolution of 0.73 Hz, zgpg 30 pulse program with ns 1024, d1 2s, acquisition time 0.6815744 s and spectral width 238.9086 ppm.
The 1 H-1 H COSY contour maps were obtained with a 2 s relaxation delay, acquisition time 0.1024000 s and spectral width 24.9930 ppm.The 1 H- 13 C HMBC contour maps were recorded with a 1.5 s relaxation delay in a 24.9930 ppm spectral width in F2 and 260.0000 ppm in F1 and acquisition time 0.1024000 s (F2) and 0.0048928 s (F1).

Results and Discussion
The 1 H and 13 C NMR spectra of all the naphthoquinones were registered at 400 and 100 MHz, respectively.Signals assignments were based on chemical shifts (d, ppm) of 1 H and 13 C, on the multiplicity patterns of proton resonances depicted by the J couplings (Hz), and on data of homonuclear 1 H-1 H COSY and heteronuclear 1 H- 13 C HMBC.The NMR experiments and the signals assignments were made for all compounds (1, 4a-h, 5a-h and 6a-h), and the naphthoquinones 4d, 5d and 6d were focused for illustrating the 1 H and 13 C assignments.
To assess more information about the structure of naphthoquinone 4d, an 1 H- 13 C HMBC (Figures 5 and 6)   Although carbonyl carbon signals are close they were assigned with 1 H- 13 C HMBC support (Figure 5) once H-1' (d 6.60) shows a long-range coupling to the carbonyl carbon at d 184.50 which is thus attributed to C-4 and, therefore, the other carbonyl carbon signal at d 181.56 corresponds undoubtedly to C-1.Opposite assignments were previously proposed for lapachol (1) that discloses similar situation on the naphthoquinone moiety. 6,22Long range couplings of the carbonyl carbons C-1 and C-4 supported the assignment of the aromatic hydrogens.There is a clear coupling of C-1 to H-8 (d 8.04, d, 3  Assignments of the heteronuclear correlations observed in the 1 H- 13 C HMBC contour map for compound 4d are shown in Table 1. The main difference between NQ and FNQ is the change of spectral features in 1 H NMR spectra around d 7-6.In the NQ's (4d) 1 H NMR spectra the signal of H-1', a hydrogen of alkenyl moiety, is a doublet, while in the FNQs H-1' (5d and 6d), the only hydrogen in the furan ring, is a singlet (Figure 7).
The ortho-FNQ 1 H NMR spectrum of 5d (Figure 8) exhibits a triplet at All these hydrogens were assigned on the basis of the 1 H-1 H COSY spectrum (Figure 9), where can be observed that H-3' is coupled to H-4', H-4' to H-5', and H-5' to H-6'.A apparent doublet at d 8.00 and a apparent triplet  8) and from 1 H-1 H COSY contour map these hydrogens are coupling to each other (Figure 9).A multiplet at d 7.64-7.58 is related to H-5 and H-6.
Aiming to confirm the 5d chemical shifts assignments a heteronuclear correlation contour map 1 H- 13 C HMBC was recorded (Figure 10) and the following informations were derived from it.From the singlet at d 6.42 corresponding to H-1', the only hydrogen in the furan ring, it is possible to differentiate between the two carbonyl carbons because H-1' shows long-range coupling to C-2 at d 174.60 (        correlations observed in the 1 H-13 C HMBC contour map for compound 5d are shown in Table 2.
Finally, in the 1 H NMR spectrum of para-FNQ 6d (Figure 12) the singlet at d 6.60 is related to H-1', the only hydrogen in the furan ring and it shows long-range correlation ( 1 H- 13 C HMBC contour map) (Figures 13 and 14    All the hydrogens and heteronuclear correlations observed in the HMBC for compound 6d are shown in Table 3. The isomers ortho-and para-FNQ can be distinguished   H-8 shows long-range correlation of carbon resonance for carbonyl carbon C-1 (Figure 10).Therefore in NQs the chemical shifts of carbonyl carbon C-4 ranged from d 184.31 to 184.53 and were higher than those of C-1 (d 181.17-181.61)while in the para-FNQs the differences in chemical shifts of C-4 ( 180.88-181.10)and C-1 (d 171.64-173.30)are still higher.These data might be related to the resonance effect between the oxygen bonded to C-2 and the C-4 carbonyl (Figure 15).However in the ortho-FNQs the chemical shift of the carbonyl carbon C-1 (d 180.76-183.91)was higher than that of C-2 (d 174.44-180.92)probably because the resonance effect is more effective between the C-1 carbonyl and the oxygen bonded to C-4 (Figure 15).From the chemical shifts of para-and ortho-FNQs it is clear that the substituents at C-2' in the furan ring have practically no influence in the shielding of the hydrogens and carbons of the naphthoquinone moiety.

Conclusions
In this work we have shown the complete and unambiguous assignments of 1 H and 13 C chemical shifts of 2-hydroxy-3-(1'-alkenyl)-1,4-naphthoquinones (NQs), naphtho [1,2-b]furan-4,5-diones (ortho-FNQs) and naphtho[2,3-b]furan-4,9-diones (para-FNQs), as discussed for 4d, 5d and 6d, respectively.It was observed that the nature of the substituents at C-3 in the NQs and at C-2' in the furan ring in the FNQs do not affect significantly the 1 H and 13 C chemical shifts of the naphthoquinone system.As far as we are concerned, this is the first report on the distinction between NQs and FNQs by NMR data.The results described for ortho-and para-FNQs isomers can be used as a model for 1 H and 13 C assignments of compounds possessing the naphthoquinones and furanonaphthoquinones system.
J H,H 7.60), and of C-4 to H-5 (d 8.11, d, 3 J H,H 7.60) ( 3 J C,H ).Furthermore the H-8 signal shows a correlation with the carbon resonances of C-10 at d 132.87 ( 3 J C,H ).H-5 is coupled to C-7 at 133.16 and to C-9 at d 129.62 ( 3 J C,H ).The two apparent triplets at d 7.73 and 7.64 were assigned to H-6 and H-7, in this order.The H-6 signal shows long-range correlation with the carbon resonances of C-10 at d 132.87 and C-8 at d 126.08 ( 3 J C,H ), while the H-7 signal is correlated with the ones of C-9 at d 129.62 and C-5 at 127.19 ( 3 J C,H ) (Figure 5).

by 1 H
-13 C HMBC data.In para-FNQ (6d) signals of the two carbonyl carbons C-1 and C-4 exhibit long-range correlation to the hydrogen resonances for H-8 and H-5 (Figure 13), respectively, whereas in ortho-FNQ (5d) only