A revision of the 13 C NMR spectral assignment of globulol

Complete assignment of the C NMR spectrum of the aromadendrane-type sesquiterpene alcohol, globulol was reported, however, it still remained to be ambiguous. We report here the unambiguous and complete assignment of the H and C NMR spectra of ( +)-globulol, which was isolated from the liverworts Plagiochila ovalifolia, Neotrichocolea bissetii and Pallavicinia subciliata.


Introduction
The first isolation of (−)-globulol was reported from Eucalyptus globulus [1].Our previous work of the terpenoid constituents of liverworts resulted in the first isolation of (+)-globulol (1) from Plagiochila yokogurensis [2][3][4].Further isolation of 1 from the liverwort Mylia taylorii [5] was reported.Although the structure of 1 was drawn erroneously [2,3], it was reported that many species of Plagiochila (Jungermanniales) contained 1.Whereas the 13 C NMR spectra of globulol have been reported [6,7], the assignments of the data remained to be ambiguous.We report here the unambiguous and complete assignment of the 1 H and 13 C NMR spectra of 1, which was isolated from the liverworts Plagiochila ovalifolia, Neotrichocolea bissetii and Pallavicinia subciliata.

Nuclear magnetic resonance instrumental conditions
The 1 H and 13 C NMR spectra were recorded at 600 and 150 MHz, respectively, on a Varian UNITY 600 spectrometer using CDCl 3 with TMS as the internal standard.Measurements were performed at 25 • C using 5 mm o.d.sample tubes.For the 1 H-13 C correlation experiment, pulsed field gradient heteronuclear single-quantum correlation (GHSQC) was used [8,9].The spectra were acquired with 1024 data points and 256 time increments with 8 transients per increment.The relaxation delay was 1.5 s and average 1 J(C-H) was set to 140 Hz.

Results and discussion
The first report of the assignment of the 13 C NMR spectrum of 1 found in the liverwort was demonstrated by Matsuo et al. [5].Miyazawa et al. [6] further reported the assignment of commercial (−)globulol (from Fluka).Recently, the assignment [6] has been revised by Wu et al. [7].Their assignments were given in Table 1.Our assignment of 1 isolated from the liverworts and of commercial (−)-globulol (from Fluka) was not identical to that of the latest reported assignment, although it was satisfactorily identical to the assignment [6] which has been revised by Wu et al.The latest reported assignment was therefore questionable.In order to clarify the assignment of 1, further experiment seemed to be necessary.
Table 1 The assignments of the 13 C NMR spectral data for globulol

Atom
Ref. [5] Ref. [6] Ref. [7] Present      The assignment of all protons of 1 (Fig. 1) was carried out by analysis of 1 H-1 H COSY and NOESY spectra as shown in Figs 2 and 3. Particularly, the connective correlation of H-4, -5, -6 and -7 was carefully assigned, since the carbons connected with those protons were different order (Table 1) in comparison with assignment of the 13 C NMR spectral data for 1 reported by Wu et al.Although there are no correlation between H-4 and -3 in Fig. 2, correlations for spin-spin coupling between H-2 and -3 (Fig. 2), and for NOEs between H-4 and -3α, and H-15 and -3β (Fig. 3) were observed in 1 H-1 H COSY and NOESY spectra.The assignment of all protons was apparent from above spectral evidences.The 1 H-13 C chemical shift correlation experiment allowed the assignment of all protonated carbons (Fig. 4).Two quaternary carbons C-10 (δ c 75.3 ppm) and 11 (δ c 19.4 ppm) were clearly distinguishable from their chemical shifts.Further confirmation of the assignment was provided by the HMBC spectrum of 1 (Fig. 5).The 13 C NMR chemical shift values are summarized in Table 1.