Structure of the lycorinine alkaloid nobilisitine A

Article abstract of DOI:10.1021/jo2016899

The structure 3 recently proposed, on the basis of computed NMR chemical shifts, for the natural product nobilisitine A has been synthesized from its C5-epimer (+)-clividine (4). The spectral data derived from compound 3 match those reported for the natural product.

Full text of DOI:10.1021/jo2016899

NOTE
pubs.acs.org/joc
Structure of the Lycorinine Alkaloid Nobilisitine A
Brett D. Schwartz, Lorenzo V. White, Martin G. Banwell,* and Anthony C. Willis
Research School of Chemistry, Institute of Advanced Studies, The Australian National University, Canberra ACT 0200, Australia
S Supporting Information
b
ABSTRACT: The structure 3 recently proposed, on the basis of
computed NMR chemical shifts, for the natural product nobi-
lisitine A has been synthesized from its C5-epimer (+)-clividine
(4). The spectral data derived from compound 3 match those
reported for the natural product.
he natural product nobilisitine A was isolated from the
T
ornamental plant Clivia nobilis found growing in Egypt.1
On the basis of the derived 1H and 13C NMR spectroscopic data,
Evidente and co-workers assigned structure 1 (Figure 1) to this
compound, thus implying it is a member of the marasmane or
lycorinine subclass of Amaryllidaceae alkaloid.1 While almost no
biological evaluation of nobilisitine A appears to have been
undertaken, other compounds within the subclass and other
natural products possessing related structures display various
interesting biological effects, including antifungal, antitumor, and
apoptosis-inducing activities.2
Figure 1. Structures of compounds 1, ent-1, and 2.
For the purposes of checking Evidente’s structural assignment,
which incorporates an all-cis arrangement of substituents about
the central C-ring, we recently completed3 a synthesis of
compound ent-1 using the enantiomerically pure cis-1,2-dihy-
drocatechol 2 as starting material.4 As a result, we determined
that the derived spectral data did not match those recorded for
nobilisitine A and concluded, therefore, that its structure had
been assigned incorrectly.3
Prompted by our revelation of this erroneous assignment and
following certain suggestions we made about the true structure of
the natural product, Lodewyk and Tantillo5 conducted a series of
DFT calculations to predict the 1H and 13C NMR chemical shifts
of the various diastereoisomeric forms of compound 1. As a
consequence, they suggested that the correct structure for
nobilisitine A is likely to be 3 or its enantiomer (Figure 2).
Coincidentally, and using chemistry related to that employed in
the synthesis of compound ent-1, we recently completed a
synthesis of (+)-clividine (4),6 the C5-epimer of the new
structure proposed for nobilisitine A. Accordingly, it seemed
appropriate to pursue the conversion 4 f 3 in an effort to
determine if the revised structure proposed for nobilisitine A is
correct. The outcomes of relevant studies are reported here.
Our initial attempts to convert (+)-clividine (4) into epimer 3
involved treating the former compound under Mitsunobu
conditions7 using α-chloroacetic acid as the nucleophile. How-
ever, no reaction was observed, an outcome in keeping with the
known sensitivity of this reaction to steric effects.8 In another
approach, compound 4 was oxidized to the corresponding
Figure 2. Structures of compounds 3 and 4.
ketone but when this was, in turn, subjected to reaction with
zinc borohydride a complex mixture of products, including small
quantities of the original alcohol 4, was obtained. The ultimately
successful means for preparing compound 3 is shown in
Scheme 1. The pivotal step involved exploiting a novel but
underutilized protocol developed by Zard and co-workers9 for
the inversion of secondary alcohols. Thus, the starting material 4
was converted into the corresponding propargyl xanthate 5
(67%) by successive treatment of the former compound with
sodium hydride, carbon disulfide, and propargyl bromide. Ester 5
was, in turn, heated in refluxing chlorobenzene in the presence of
benzoic acid, thereby effecting, via sigmatropic rearrangement,
Received: August 11, 2011
Published: September 07, 2011
r
2011 American Chemical Society
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|

The Journal of Organic Chemistry
NOTE
Scheme 1. Synthesis of Compound 3
the lycorinine alkaloid nobilisitine A. The present work rein-
forces the notion that the de novo prediction of NMR chemical
shifts using DFT-based techniques represents a powerful tool for
assisting with the assignment of molecular structures.13 This
study also suggests that Zard’s protocol9 for the inversion of
configuration of secondary alcohols deserves more attention than
it has received thus far.
’ EXPERIMENTAL SECTION
General Experimental Procedures. Unless otherwise specified,
proton (1H) and carbon (13C) NMR spectra were recorded at 18 °C in
base-filtered CDCl3 at 400 MHz for proton and 100 MHz for carbon
1
nuclei. For H NMR spectra, signals arising from the residual protio
forms of the solvent were used as the internal standards. 1H NMR data
are recorded as follows: chemical shift (δ) [multiplicity, coupling
constant(s) J (Hz), relative integral] where multiplicity is defined as
s = singlet; d = doublet; t = triplet; q = quartet; m = multiplet or
combinations of the above. The signal due to residual CHCl3 appearing
at δH 7.26 and the central resonance of the CDCl3 “triplet” appearing at
δC 77.0 were used to reference 1H and 13C NMR spectra, respectively.
Infrared spectra (νmax) were analyzed as thin films on KBr plates.
Melting points are uncorrected. Analytical thin-layer chromatography
(TLC) was performed on aluminum-backed 0.2 mm thick silica gel 60
F254 plates. Eluted plates were visualized using a 254 nm UV lamp and/
or by treatment with a suitable dip followed by heating. These dips
included phosphomolybdic acid/ceric sulfate/sulfuric acid (conc)/
water (37.5 g/7.5 g/37.5 g/720 mL) or potassium permanganate/
potassium carbonate/5% sodium hydroxide aqueous solution/ water
(3 g/20 g/5 mL/300 mL). Flash chromatographic separations were
carried out following protocols defined by Still et al.14 with silica gel 60
(40À63 μm) as the stationary phase and using the AR- or HPLC-grade
solvents indicated. Starting materials and reagents were generally
available from commercial suppliers and were used as supplied. Drying
agents and other inorganic salts were purchased from commercial
suppliers. Tetrahydrofuran (THF), methanol, and dichloromethane
(DCM) were dried using a solvent purification system that is based
upon a technology originally described by Grubbs et al.15 Where
necessary, reactions were performed under an argon atmosphere.
Compound 5. A magnetically stirred solution of ent-clividine (4)6
(237 mg, 0.75 mmol) in THF (8 mL) maintained at 18 °C under an
atmosphere of nitrogen was treated with NaH (30 mg of a 60%
dispersion in mineral oil, 0.90 mmol). After 1 h, the reaction mixture
was treated with carbon disulfide (54 μL, 0.90 mmol), stirred at 18 °C
for0.5h, andthen treated with propargyl bromide (100μL ofan 80%w/v
solution in toluene, 0.90 mmol). After a further 0.5 h, the reaction
mixture was quenched with NaHCO3 (10 mL of a saturated aqueous
solution) and extracted with CH2Cl2 (3 Â 15 mL). The combined
organic phases were dried (Na2SO4), filtered, and concentrated under
reduced pressure at 40 °C. The brown oil so obtained was subjected to
flash chromatography (silica, 1:20 v/v ammonia saturated methanol/
CH2Cl2 elution), and concentration of the appropriate fractions (Rf =
0.8) afforded xanthate 5 (217 mg, 67%) as a viscous orange oil: [α]D
À2.5 (c 0.4, CDCl3); 1H NMR (CDCl3, 400 MHz) δ 7.51 (s, 1H), 6.92
(s, 1H), 6.06 (d, J = 1.3 Hz, 1H), 6.05 (d, J = 1.3 Hz, 1H), 5.93 (ddd, J =
11.3, 4.9, and 2.4 Hz, 1H), 4.84 (m, 1H), 3.89 (dd, J = 16.6 and 2.7 Hz,
1H), 3.84 (dd, J = 16.6 and 2.7 Hz, 1H), 3.29À3.23 (complex m, 1H),
2.75 (dd, J = 8.9 and 2.5 Hz, 1H), 2.64 (m, 1H), 2.53À2.45 (complex m,
2H), 2.40 (dd, J = 17.6 and 9.1 Hz, 1H), 2.23 (t, J = 2.7 Hz, 1H),
2.20À2.15 (complex m, 1H), 2.17 (s, 3H), 1.95À1.89 (complex m, 2H);
13C NMR (CDCl3, 100 MHz) δ 212.0, 164.4, 151.9, 147.4, 139.6, 117.8,
109.7, 107.6, 101.9, 77.6, 76.3, 71.9, 67.0, 55.7, 44.9, 42.5, 36.2, 29.9,
29.6, 25.1, 24.3; IR (KBr) νmax 3293, 2930, 1717, 1616, 1503, 1478,
1448, 1384, 1258, 1218, 1110, 1038, 939 cmÀ1; MS (ESI) m/z 454
the formation of a betaine9 that reacted with the added acid to
give a chromatographically separable mixture of the 7-azabicyclo-
[2.2.1]heptane 6 (37%) and the desired benzoate 7 (21%).
Compound 6 is thought to arise via a transannular nucleophilic
displacement of the betaine by the D-ring nitrogen followed by
fragmentation of the ensuing quaternary ammonium salt with
benzoic acid in a von Braun-type process. Its structure was
established by a single-crystal X-ray analysis (see the Supporting
Information for details).10 In the final step of the reaction
sequence, benzoate 7 was treated with potassium carbonate in
methanol, and in this manner the target alcohol 3 was obtained in
91% yield and as a white, crystalline solid. The derived spectral
data were in complete accord with the assigned structure, but
final confirmation of this followed from a single-crystal X-ray
analysis (see the Supporting Information for details).10
The melting range of the synthetically derived material
(195À196 °C) matched reasonably well with that reported1
for nobilisitine A (186À187 °C). Comparisons of the 13C and 1H
NMR spectral data obtained on compound 3 with those reported
for nobilisitine A and compound ent-1 are presented in the
Supporting Information in Tables S1 and S2, respectively.
Similar comparisons of the infrared and mass spectral data are
presented in Table S3 of the Supporting Information. All of these
quite clearly demonstrate that compound 3 does indeed corre-
spond to the structure of nobilisitine A. Unfortunately, the
specific rotation of the natural product has not been reported,
so it has not been possible to establish the absolute configuration
of this alkaloid.11 However, given that nobilisitine A is the C5-
epimer of clividine (which occurs naturally in the form ent-412),
there is some likelihood this is represented by structure ent-3.
The results reported here clearly show that compound 3 or
(more likely) its enantiomer corresponds to the true structure of
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The Journal of Organic Chemistry
NOTE
[(M + Na)+, 10], 432 [(M + H)+, 100], 318 (20), 300 (92); HRMS
(M + H)+ calcd for C21H21NO5S2 432.0939, found 432.0934.
Compounds 6 and 7. A magnetically stirred solution of xanthate 5
(217 mg, 0.50 mmol) in freshly distilled chlorobenzene (10 mL)
maintained under a nitrogen atmosphere was treated with benzoic acid
(184 mg, 1.51 mmol) and the ensuing mixture heated at reflux for 5 h.
The cooled reaction mixture was quenched with NaHCO3 (10 mL of a
saturated aqueous solution) and the separated aqueous layer extracted
with CH2Cl2 (3 Â 15 mL). The combined organic phases were then
dried (Na2SO4), filtered, and concentrated under reduced pressure at
40 °C. The malodorous residue obtained in this way was subjected to
flash chromatography (silica, 1:19 v/v methanol/CH2Cl2 elution), thus
affording three fractions, A, B, and C.
this material gave compound 31 (28 mg, 91%) as small, white crystalline
masses: mp 195À196 °C (lit.1 mp 186À187 °C); [α]D À47.0 (c 0.2,
CDCl3); 1H NMR (CDCl3, 400 MHz) δ see Table S2 in the Supporting
Information; 13C NMR (CDCl3, 100 MHz) δ see Table S1 in the
Supporting Information; IR (KBr) νmax 3389, 2931, 1710, 1616, 1503,
1480, 1447, 1385, 1263, 1122, 1073, 1034 cmÀ1; MS (EI, 70 eV) m/z
317 (M+•, 48), 301 (3), 273 (2), 242 (5), 228 (5), 172 (8), 126 (8), 96
(100); HRMS M+• calcd for C17H19NO5 317.1263, found 317.1265.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files, crystallographic data,
b
and anisotropic displacement ellipsoid plots derived from the
single-crystal X-ray analyses of compounds 3 and 6; Tables
S1ÀS3 comparing spectral data derived from compound ent-1,
Concentration of fraction A (Rf = 0.8 in 1:20 v/v ammonia saturated
MeOH/CH2Cl2) afforded xanthate 5 (26 mg, 12% recovery) that was
identical, in all respects, with an authentic sample.
nobilistine A, and compound 3; H and 13C NMR spectra for
1
Concentration of fraction B (Rf = 0.7 in 1:20 v/v ammonia saturated
MeOH/CH2Cl2) afforded benzoate 7 (45 mg, 21%) as an orange foam:
[α]D À47.8 (c 0.8, CDCl3); 1H NMR (CDCl3, 400 MHz) δ 8.04À8.01
(complex m, 2H), 7.59À7.52 (complex m, 1H), 7.55 (s, 1H), 7.43 (t, J =
7.7 Hz, 2H), 6.97 (s, 1H), 6.06 (d, J = 1.2 Hz, 1H), 6.06 (d, J = 1.2 Hz,
1H), 5.33 (m, 1H), 4.86 (dd, J = 6.4 and 4.3 Hz, 1H), 3.35À3.26
(complex m, 1H), 3.25 (m, 1H), 2.79À2.76 (complex m, 1H),
2.43À2.31 (complex m, 2H), 2.22 (s, 3H), 2.20À2.14 (complex m,
1H), 2.03À1.95 (complex m, 1H), 1.89 (dt, J = 13.6 and 9.1 Hz, 1H),
1.75 (broad s, 1H); 13C NMR (CDCl3, 100 MHz) δ 165.7, 163.7, 152.3,
147.3, 133.1, 129.9, 129.7, 128.4, 118.8, 110.0, 106.7, 102.0, 78.0, 70.9,
66.0, 55.4, 42.6, 37.8, 35.0, 30.2, 29.7 (one signal obscured or over-
lapping); IR (KBr) νmax 2939, 1717, 1616, 1503, 1480, 1449, 1383, 1264,
1111, 1071, 1035, 934, 713 cmÀ1; MS (EI, 70 eV) m/z 421 (M+•, 50),
316 (16), 299 (26), 96 (100); HRMS M+• calcd for C24H23NO6
421.1525, found 421.1526.
compounds 3 and 5À7. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: mgb@rsc.anu.edu.au.
’ ACKNOWLEDGMENT
We thank the Australian Research Council and the Institute of
Advanced Studies for generous financial support. Dr. Laurent
Petit is warmly thanked for drawing our attention to the inversion
protocol used here.
’ REFERENCES
Concentration of fraction C (Rf = 0.4 in 1:20 v/v ammonia saturated
methanol/CH2Cl2) afforded 7-azabicyclo[2.2.1]heptane 6 (78 mg,
37%) as a white crystalline solid: mp 145À147 °C; [α]D À50.3 (c 1.2,
CDCl3); 1H NMR (CDCl3, 400 MHz) δ 7.81À7.77 (complex m, 2H),
7.56À7.51 (complex m, 1H), 7.52 (s, 1H), 7.42À7.37 (complex m, 2H),
6.46 (s, 1H), 5.87 (d, J = 1.4 Hz, 1H), 5.52 (d, J = 1.4 Hz, 1H), 5.18 (ddd,
J = 11.0, 5.0, and 1.4 Hz, 1H), 4.22 (m, 1H), 4.11 (dt, J = 11.0 and 5.7 Hz,
1H), 3.68 (td, J = 4.6 and 0.8 Hz, 1H), 3.63 (dd, J = 11.0 and 4.9 Hz, 1H),
3.37 (dd, J = 4.9 and 0.9 Hz, 1H), 2.55 (s, 3H), 2.01 (dd, J = 12.8 and
8.8 Hz, 1H), 1.82À1.74 (complex m, 1H), 1.71À1.62 (complex m, 1H),
1.57À1.50 (complex m, 1H), 1.30 (m, 1H); 13C NMR (CDCl3, 100 MHz)
δ 166.3, 162.7, 152.8, 147.0, 135.0, 132.8, 130.1, 129.3, 128.3, 116.8,
109.2, 106.4, 101.7, 77.3, 70.5, 66.6, 62.8, 35.6, 35.0, 34.1, 33.0, 30.1; IR
(KBr) νmax 2947, 1710, 1619, 1503, 1481, 1449, 1410, 1274, 1251, 1126,
1035 cmÀ1; MS (EI, 70 eV) m/z 421 (M+•, 12), 299 (19), 232 (100),
231 (98), 162 (25), 126 (77); HRMS M+• calcd for C24H23NO6
421.1525, found 421.1522.
Compound 3. A magnetically stirred solution of benzoate 7 (50 mg,
0.12 mmol) in methanol (10 mL) was treated with anhydrous KHCO3
(20 mg, 0.13 mmol) and the resulting mixture stirred at 18 °C for 18 h
while being maintained under an atmosphere of nitrogen. The reaction
mixture thus obtained was acidified with glacial acetic acid (∼1 mL) and
the aqueous layer extracted with diethyl ether (10 mL). The separated
aqueous layer was then brought to pH = 8 using sodium bicarbonate
(saturated aqueous solution) and extracted with dichloromethane (3 Â
10 mL). The combined organic phases were washed with brine (1 Â
10 mL) and dried (Na2SO4) before being filtered and concentrated
under reduced pressure at 40 °C. The residue thus obtained was
subjected to flash chromatography (silica, 2:4:5 v/v/v ammonia satu-
rated methanol/ethyl acetate/hexane elution), and concentration of the
appropriate fractions (Rf = 0.2 in 1:20 v/v ammonia saturated methanol/
CH2Cl2 elution) afforded a white foam. Crystallization (ethyl acetate) of
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