information suggested that 1 was a derivative of falcarindiol (5) [11] with the presence of a furan ring consisted of C-8 (C 134.4), C-9
(H 6.68, C 119.9), C-10 (H 6.01, C 106.7), and C-11 (C 159.9), which accounted for the one remaining indice of hydrogen deficiency.
The HMBC correlations of H-9/C-8 and C-11, H-10/C-8 and C-11, and H2-12/C-10 and C-11 and the NOESY correlation of H-10/H2-
12 (Fig. S3 in Supporting information) supported the above deduction. Thus, the 2D structure of 1 was defined as shown.
To establish the absolute configuration of 1, the modified Mosher’s method was carried out. However, attempts to prepare two
Mosher’s esters of 1 failed due to the limitation of its amount (1.1 mg). Therefore, the total synthesis of racemic ()-1 should be
conducted, and its retrosynthetic analysis is depicted in Scheme 1. ()-1 could be constructed by Sonogashira reaction [12] from diyne
15 and iodofuran 16. Disconnection at the conjugated acetylenic bond of 15 would give two alkynes 17 and 18 as intermediates for
Cadiot-Chodkiewicz coupling [13], while the former subunit could be further disassembled into aldehyde 21 and TMS-protected
acetylene 18 as substrates of 1,2-addition. Fragment 16 would be prepared by substitution reaction from furan 19 and alkyl iodide 20.
The synthesis of ()-1 is summarized in Scheme 2. 1,2-Addition of acrolein (21) with trimethylsilylacetylene (18) in the presence
of n-BuLi followed by protection of the resulting hydroxyl in 22 with t-butyldiphenylsilyl (TBDPS) group afforded 23 as a silyl ether.
Treatment of 23 with N-bromosuccinic imide (NBS) and catalytic amount of AgNO3 gave brominated alkyne 17, which was coupled
with 18 subsequently via Cadiot-Chodkiewicz reaction [13] to furnish the conjugated diyne 24. The terminal TMS group of 24 was then
selectively removed in K2CO3/MeOH to obtain 15. The coupling reaction of 15 with 16, which was prepared by alkylation of furan (19)
with 1-iodohexane (20) using n-BuLi followed by iodination, under Sonogashira reaction condition [12] yielded 26 smoothly. Finally,
deprotection of the TBDPS group of 26 by tetra-n-butylammonium fluoride (TBFA) provided ()-1 in 20.6% overall yield for 7 steps
from 21.
HPLC chiral resolution of ()-1 afforded two enantiomers, ()-1 at 8.7 min and ()-1 at 11.4 min, respectively.
Both enantiomers were subjected to esterification with (R)- and (S)--methoxyphenylacetic acid (MPA) to obtain
the corresponding esters, and analyses of their RS (R S) values led to the assignment of 3R configuration for
()-1 and 3S configuration for ()-1 (Fig. S1 in Supporting information). By comparison of the HPLC chromatogram
and ECD cruve of 1 with those of the synthetic products (Figs.S4 and S5 in Supporting information), the absolute
configuration of 1 was unambiguously defined as 3R.
A plausible biogenetic pathway to 1 is proposed with falcarindiol (5), a known co-isolated compound with 3R
and 8S configuration determined by the modified Mosher’s method (Fig. S2 in Supporting information), as the
precursor (Scheme S1 in Supporting information). Allylic oxidation of 5 resulted in the generation of carbonyl group
at C-11 followed by nucleophilically attacked by hydroxyl group at C-8. Then the removal of H2O induced the
electrons transfer and formation of a furan ring, and finally converted to 1.
All isolated compounds 1−14, as well as the synthetic product ()-1, were evaluated for their cytotoxicity
against three cancer cell lines, MCF-7, H1299, and HepG2, using the sulforhodamine B (SRB) assay [14], and their
IC50 values are presented in Table 2. Of these, the synthetic compound ()-1 exhibited the most potent cytotoxic
activity against the three cancer cell lines with IC50 values ranging from 0.6 μmol/L to 1.4 μmol/L, at least 24-fold
lower than those of its enantiomer 1, indicating the importance of 3S configuration for the cytotoxic effect.
Panaxydiol-type polyacetylenes (2−4), with IC50 values of 10.7−24.9 μmol/L, displayed stronger inhibitory effects on
the test cancer cells than those of most of falcarindiol-type polyacetylenes (5−12) and their reduction products (13
and 14), suggesting that the conjugated system enlarged by 8E-double bond may play a positive role in their
cytotoxicity.
a IC50 100 μmol/L.
b Positive control.
c Values presented in nmol/L.
In summary, a new polyacetylene (1), together with thirteen analogues (2−14), was isolated from the roots and rhizomes of N.
incisum. Its absolute configuration was determined as 3R by applying the modified Mosher’s method to synthetic enantiomers ()-1 and
()-1 followed by comparing their HPLC retention times and ECD spectra. Interestingly, the synthetic product ()-1, the enantiomer of
1, displayed the most significant cytotoxic activity against three cancer cell lines (MCF-7, H1299, and HepG2).
Acknowledgment
This wok was financially supported by the National Key Technology R&D Program “New Drug Innovation” of China (No.
2018ZX09711001-008-003).
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Zheng, Xikang
