Transformations of the 2,7- Seco Aspidosperma alkaloid leuconolam, structure revision of epi -leuconolam, and partial syntheses of leuconoxine and leuconodines A and F

Article abstract of DOI:10.1021/np400922x

Several transformations of the seco Aspidosperma alkaloid leuconolam were carried out. The based-induced reaction resulted in cyclization to yield two epimers, the major product corresponding to the optical antipode of a (+)-meloscine derivative. The structures and relative configuration of the products were confirmed by X-ray diffraction analysis. Reaction of leuconolam and epi-leuconolam with various acids, molecular bromine, and hydrogen gave results that indicated that the structure of the alkaloid, previously assigned as epi-leuconolam, was incorrect. This was confirmed by an X-ray diffraction analysis, which revealed that epi-leuconolam is in fact 6,7-dehydroleuconoxine. Short partial syntheses of the diazaspiro indole alkaloid leuconoxine and the new leuconoxine-type alkaloids leuconodines A and F were carried out.

Full text of DOI:10.1021/np400922x

Article
pubs.acs.org/jnp
Transformations of the 2,7-Seco Aspidosperma Alkaloid Leuconolam,
Structure Revision of epi-Leuconolam, and Partial Syntheses of
Leuconoxine and Leuconodines A and F
Yun-Yee Low,† Fong-Jiao Hong,† Kuan-Hon Lim,‡ Noel F. Thomas,† and Toh-Seok Kam*,†
†Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
‡School of Pharmacy, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia
S
* Supporting Information
ABSTRACT: Several transformations of the seco Aspidosperma alkaloid
leuconolam were carried out. The based-induced reaction resulted in cyclization
to yield two epimers, the major product corresponding to the optical antipode of a
(+)-meloscine derivative. The structures and relative configuration of the products
were confirmed by X-ray diffraction analysis. Reaction of leuconolam and epi-
leuconolam with various acids, molecular bromine, and hydrogen gave results that
indicated that the structure of the alkaloid, previously assigned as epi-leuconolam,
was incorrect. This was confirmed by an X-ray diffraction analysis, which revealed
that epi-leuconolam is in fact 6,7-dehydroleuconoxine. Short partial syntheses of
the diazaspiro indole alkaloid leuconoxine and the new leuconoxine-type alkaloids
leuconodines A and F were carried out.
earlier report (KOH in EtOH/MeOH at rt for 6 h),
he 2,7-seco Aspidosperma alkaloid leuconolam (1) and its
TC-21 epimer, epi-leuconolam (2), which are related to the compounds 6 and 7 were formed in 12% and 3% yields,
known rhazinilam,1 were first isolated from the bark extract of
respectively, and these were also accompanied by unreacted 1
(20%).12
Leuconotis griffithii.2 Subsequently, the related diazaspiro
pentacyclic alkaloid leuconoxine (3) was reported from the
Indonesian L. eugenefolia,3 and other rhazinilam/leuconolam
alkaloids were also found to occur in other genera of the
Apocynaceae such as Kopsia.4 These alkaloids include rhazinal,5
rhazinicine,6 and arboloscine.7 Another alkaloid with a new
tetracyclic skeleton incorporating a tetrahydro-2H-azepine
moiety was mersicarpine (4), which was isolated from Kopsia8
and Leuconotis.9 A speculative biosynthetic pathway has been
suggested from a leuconolam precursor,8 and several total
syntheses of this alkaloid have also been reported.10 In addition,
a number of new rhazinilam, leuconolam, and leuconoxine
alkaloids (leuconodines A−E) have been reported from
Leuconotis.9 Since leuconolam (1) was available from our
ongoing work on Kopsia and Leuconotis alkaloids,8,9 it offered
the opportunity to explore the chemistry of this unusual indole
compound, characterized by a 2,7-seco Aspidosperma carbon
skeleton.
The major product 6 was obtained as a colorless oil and
subsequently as colorless block crystals (mp 266−268 οC) from
CCl4/MeOH, with [α]25 = −198 (c 0.06, CHCl3). The UV
D
spectrum showed absorption maxima at 210, 253, and 287 nm,
indicating the presence of a dihydroquinolone chromophore,11
while the IR spectrum showed the presence of OH (3226
cm−1) and lactam carbonyl functions (1667 cm−1). The ESIMS
of 6 showed an [M + H]+ ion at m/z 327, and HRESIMS
measurements gave the molecular formula as C19H22N2O3 + H.
The 1H and 13C NMR data of 6 were similar to those reported
earlier.2c The attachment of C-16 to C-7 was supported by the
observed three-bond correlation from H-16 to C-6 in the
HMBC spectrum, while the α-orientation of H-16 was assigned
from the NOE enhancement between H-6α and H-16. The
minor product 7 was obtained as a colorless oil and
subsequently as colorless block crystals (mp 250−252 °C)
from CH2Cl2/hexanes, with [α]25 = −150 (c 0.01, CHCl3).
D
The UV (210, 251, 306 nm) and IR data (3322, 1712, 1667
cm−1) were similar to those of 6, while the ESIMS showed that
7 was isomeric with 6. A major difference in the NMR data of 7
compared with those of the major product 6 was the notable
absence of an NOE between H-6α and H-16, which suggested
the β-orientation of H-16 in 7. Since suitable crystals of both 6
It was reported that the reaction of leuconolam (1) with
KOH in EtOH/MeOH gave the cyclized product 6 [the
enantiomer of a derivative of (+)-meloscine (5)]11 as the sole
product in high yield.2c As no evidence was presented to
support the stereochemical assignments, we reinvestigated this
transformation. When the reaction was repeated by the use of
stronger bases such as NaOMe/MeOH or NaHMDS/THF, the
reaction did not proceed and led only to the recovery of
starting material. When using the conditions employed in the
Received: November 4, 2013
Published: January 15, 2014
© 2014 American Chemical Society and
American Society of Pharmacognosy
327
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
and 7 were obtained, X-ray diffraction analyses were carried
out, which confirmed the structures and relative configurations
assigned based on the NMR data (Figure 1).
Table 1. Transformations of Leuconolam (1) and its
Supposed C-21 Epimer (epi-Leuconolam, 2) Under Various
Conditions
products
starting
entry material
reaction
conditions
1
2
compound A
8
1
1
5% HCl, rt, 8 h
no reaction (recovery of 1 on
basification)
2
1
5% HCl/CH2Cl2 35%
47%
1.4%
+ TEACl, rt,
a
14 h
3
4
5
1
1
1
HCl/MeOH, rt,
12 h
4%
63%
54%
CSA/CH2Cl2, rt, 10%
62%
19%
2%
a
14 h
CSA/CH2Cl2, rt,
11 h (4 equiv
MeOH added)
Figure 1. X-ray crystal structures of 6 and 7.
6
7
8
9
1
1
1
2
CSA/MeOH, rt,
14 h
4%
4%
3%
2%
94%
94%
PTSA/MeOH,
rt, 14 h
0.8%
42%
The formation of 6 and 7 can be rationalized based on an
intramolecular Michael addition from the presumably more
stable E-enolate, which approaches the si-face of C-7 to form
the major product 6, while the minor product 7 resulted from
attack by the presumably less stable Z-enolate to the same si-
face of C-7 (Scheme 1).
It was initially envisaged that treatment of leuconolam (1)
with acid should result in a facile transannular closure to give a
dehydroleuconoxine derivative, which could serve as a possible
starting compound for further elaboration to leuconoxine (3)
and its recently discovered congeners, leuconodines A−E,9 or
to mersicarpine (4).8,9
Treatment of leuconolam (1) with aqueous HCl (5%, rt, 12
h) did not result in any reaction, leading only to recovery of
starting material upon basification (Table 1). When the same
reaction was carried out in a two-phase medium in the presence
of a phase-transfer catalyst (tetraethylammonium chloride,
TEACl), both epi-leuconolam (2) (47%) and unreacted
leuconolam (1) (35%) were obtained. Examination of the
product mixture revealed the formation of a minor product
(compound A), with a yield of 1.4%. Repeating the two-phase
experiment (5% HCl/CH2Cl2, TEACl) with epi-leuconolam
PTSA/CH2Cl2,
rt, 14 h
5%
5% HCl/CH2Cl2 15%
84%
+ TEACl, rt,
a
12 h
b
10
11
12
2
2
8
CSA/CH2Cl2, rt, no reaction
15 h
PTSA/CH2Cl2,
rt, 10 h
1%
70%
c
PTSA/CH2Cl2,
rt, 10 h
no reaction
13
14
15
16
1
2
1
2
H2, Pd/C
H2, Pd/C
Br2/CHCl3
Br2/CHCl3
no reaction
3 (90%)
13 (86%)
13 (96%)
a
b
Prolonged reaction time leads to reduced overall yields. Traces of 1
and compound A detected from TLC. Traces of 1 and 2 detected
from TLC.
c
(2) resulted in the isolation of leuconolam (1) (15%) and epi-
leuconolam (2) (84%).
When leuconolam (1) was treated with 10-camphorsulfonic
acid (CSA) in anhydrous CH2Cl2, epi-leuconolam (2) was
Scheme 1. Formation of 6 and 7
328
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
reevaluation of the earlier structure elucidation for leuconolam
(1) and epi-leuconolam (2).
epi-Leuconolam was first isolated as a minor alkaloid from L.
griffithii and L. eugenefolia.2c It has subsequently been detected
as a minor alkaloid in Kopsia griffithii.13 The structure was
assigned as the C-21 epimer of leuconolam (i.e., 2) based on
EIMS and NMR data. In the initial report, the EIMS apparently
showed an [M]+ ion at m/z 326, which was also the base peak
and which analyzed for C19H22N2O3 by HREIMS, indicating an
isomeric relationship with leuconolam (1).2c,14 This was
confirmed by a subsequent independent EIMS measurement
on a different instrument, which also showed the [M]+ ion as a
base peak at m/z 326 and which also analyzed for
C19H22N2O3.13,15 In both instances, a strong [M − H2O]+
1
ion at m/z 308 was also detected. The H NMR spectrum
obtained in a yield of 62%, accompanied by 2% of the
previously noted minor product (compound A). Similar
treatment of 1 with CSA in anhydrous MeOH resulted in the
formation of O-methylleuconolam (8)2c in 94% yield,
accompanied by 2% of compound A. Treatment of 1 with
concentrated HCl (a few drops) in anhydrous MeOH gave only
8 in a reduced yield of 63%. Treatment of 1 with p-
toluenesulfonic acid (PTSA) in anhydrous MeOH also yielded
the O-methyl derivative 8 as the major product (94%) with
compound A detected as the minor product (1%).
When leuconolam (1) was treated with PTSA in anhydrous
CH2Cl2, an inversion in the product distribution was noted,
with compound A obtained as the major product (42%) and
epi-leuconolam (2) as the minor product (5%). These results
are summarized in Table 1.
The results of the transformations of leuconolam (1) and epi-
leuconolam under various conditions as summarized in Table 1
presented some puzzling features. The formation of epi-
leuconolam (2) and leuconolam (1) when leuconolam (1) or
epi-leuconolam (2) was treated with aqueous acid under two-
phase conditions (entries 2 and 9, Table 1) suggested the
possibility that the products are derived via reversible formation
of the N-4−C-21 iminium ion 9, followed by solvolysis to give a
mixture of 2 and 1, with 2 (epi-leuconolam) predominating in
both instances. This would appear to suggest that 2 was the
thermodynamically more stable product under such conditions.
When the acid-induced reaction was carried out in MeOH
(entries 3, 6, and 7, Table 1) in the presence of either HCl,
CSA, or PTSA, virtually quantitative conversion to O-
methylleuconolam (8, 21β-OMe) was observed, suggesting
efficient trapping of the iminium ion from the β-face by MeOH.
The exclusive formation of the C-21 β-oriented methyl ether is
puzzling, especially since the α-OH epimer (epi-leuconolam, 2)
appeared to be the thermodynamically preferred product.
Another discrepancy was noted when comparing entries 4 and
10, Table 1. The reaction of leuconolam (1) with CSA in
CH2Cl2 gave epi-leuconolam (2) as the major product, but
when epi-leuconolam (2) was exposed to the same conditions,
no reaction occurred (cf. entries 2 and 9, Table 1).
Other inconsistencies were subsequently noted for the
hydrogenation and bromination reactions of leuconolam (1)
and epi-leuconolam (2). For instance, while epi-leuconolam (2)
was smoothly hydrogenated, leuconolam (1) was, by
comparison, unreactive (entries 13 and 14, Table 1), whereas
in the case of the bromination reaction, both 1 and 2 reacted to
give the same bromine addition product (entries 15 and 16,
Table 1). Furthermore debromination (Zn/AcOH) of the
dibromide apparently yielded epi-leuconolam (2). These
puzzling and apparently inconsistent results led to a
showed features that in many ways indicated the isomeric
relationship with leuconolam (1). A sharp singlet at δH 6.02
showed the presence of an isolated olefinic proton correspond-
ing to H-6, while the triplet centered at δH 0.76 indicated the
presence of an ethyl substituent. A notable difference observed
in the 1H NMR spectrum of epi-leuconolam (2) when
compared with that of 1, however, was the absence of the
characteristic indolic NH and C-21-OH resonances. The 13C
NMR spectrum of epi-leuconolam accounted for the 19 carbons
and showed a close similarity to the spectrum of leuconolam
(1) except for small differences in the chemical shifts.2c
Since the structure of leuconolam (1) rested firmly on an X-
ray diffraction analysis, which we have repeated,16 we
reinvestigated the structure assignment of epi-leuconolam
using a natural sample from our concurrent work in the
alkaloid field.9
LC-ESIMS analysis of epi-leuconolam (2) gave an [M + H]+
ion at m/z 309, which indicated a molecular ion (m/z 308) 18
mass units less than that obtained previously by EIMS.
HRESIMS gave the formula C19H20N2O2 + H. Banwell and
co-workers have also reported syntheses of rhazinal, rhazinilam,
leuconolam (1), and epi-leuconolam (2).17,18 The latter two
compounds were obtained by oxidation of rhazinilam (excess
PCC, 18 °C, 4 Å molecular sieves), followed by aqueous
workup (EtOAc/MeOH/H2O) of the reaction mixture.18 The
EIMS of the synthetic epi-leuconolam (2) showed a base peak
at m/z 308, with the m/z 326 ion detected as a weak peak
(<1%). In the original report, it was noted that the IR spectrum
of epi-leuconolam (2) showed a strong broad absorption at
3400 cm−1 attributed to NH and OH.2c The IR spectra of epi-
leuconolam (2) and leuconolam (1) recorded by us indicated
that the IR spectrum of leuconolam (1) had a broad absorption
at ca. 3260 cm−1, but that epi-leuconolam (2) did not show any
significant absorption in the 3400 cm−1 region (the same result
was obtained by Banwell and co-workers18). In addition, the
UV spectra of leuconolam (1) (207, 220, 287 nm) and epi-
leuconolam (2) (203, 252, 350 nm) were markedly different,
indicating the presence of different chromophores.
The 1H and 13C NMR data of epi-leuconolam (2) have been
reported on a number of occasions and were consistently in
agreement with those of the original report.2c,13,18 We have also
carried out additional 2D NMR experiments (COSY, HMQC,
HMBC) for epi-leuconolam (2), which indicated the presence
of similar correlations to those in leuconolam (1).
In view of the above results, we carried out an X-ray
diffraction analysis of the alkaloid that has to date been assigned
as epi-leuconolam (2) (natural sample; suitable crystals were
obtained from a CH2Cl2/hexanes solution). The X-ray
329
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
diffraction analysis revealed that the alkaloid previously
assigned as “epi-leuconolam (2)” is in fact 6,7-dehydroleuco-
noxine (10) (Figure 2). The molecular ion at m/z 326 in EIMS
aromatic hydrogens (δH 6.65, 6.66, 6.96, and 7.09) correspond-
ing to an ortho-disubstituted aromatic moiety, one olefinic
proton (δH 6.14), and a broad two-proton singlet due to an
amino group, NH2 (δH 3.94, exchangeable with D2O). The
COSY and HMQC data showed the presence of
NCH2CH2CH2, COCH2CH2, and CH2CH3 partial struc-
tures, as well as an isolated olefinic hydrogen, corresponding to
H-6 (Figure 3). Comparison of the NMR data of compound A
Figure 2. X-ray crystal structure of 10.
was probably an artifact due to facile cleavage of the initially
formed molecular ion followed by facile capture by water
present as a contaminant in the sample. The presence of
adventitious water probably also accounts for the observation of
the broad absorption at 3400 cm−1 in the IR spectrum, which
was attributed to the presence of NH/OH groups, while the
revised structure, 6,7-dehydroleuconoxine (10), is now
compatible with the UV spectrum. The revised structure also
accounted for Banwell’s transformation of rhazinilam to
leuconolam (1) and “epi-leuconolam” (or 6,7-dehydroleuco-
noxine (10)),18 since the use of excess PCC followed by the
aqueous workup resulted in an acidic medium, which triggered
the transannular cyclization of leuconolam (1) to 6,7-
dehydroleuconoxine (10).
Figure 3. Selected HMBCs and NOE of 11.
with those of the starting leuconolam (1) indicated that the N-
4−C-5−C-6, N-4−C-3−C-14−C-15, and C-16−C-17−CO
partial structures, as well as the C-20 ethyl side chain, have
remained intact. The attachment of C-5, C-3, and C-21 to N-4
was supported by the observed correlations (HMBC, Figure 3)
from H-6 and H-3 to C-21 (low-field quaternary resonance at
δC 102.1). The observed three-bond correlations from H-15 to
C-17, C-19, and C-21 indicated attachment of C-15, C-17, and
C-19 to the quaternary C-20, as well as the attachment of C-20
to C-21. The assembly of the molecule is completed by
cleavage of the N-1 amide function (e.g., in 1) to a free primary
amine and attachment of the carboxylic oxygen to C-21, to
reveal the amino lactam-lactone as shown in 11.
In order to provide proof of the proposed structure, X-ray
diffraction analysis was carried out, which confirmed the
structure proposed and defined the absolute configuration, as
shown in Figure 4. The crystal structure showed that the NH2
With the problem regarding the misassigned structure of
“epi-leuconolam” resolved, we subsequently focused on the
structure of compound A, obtained in the acid-induced
transformations of leuconolam (1).
Compound A, eventually assigned structure 11, was obtained
as a yellowish oil and subsequently as yellowish block crystals
Figure 4. X-ray crystal structure of 11.
from CH2Cl2/hexanes (mp 179−182 °C) with [α]25 = +116
D
(c 0.5, CHCl3). The UV spectrum showed absorption maxima
at 212, 240, and 340 nm, while the IR spectrum showed the
presence of NH2 (3483 and 3397 cm−1) and carbonyl functions
(1743 and 1709 cm−1). The EIMS of compound A showed an
[M]+ ion at m/z 326, while HREIMS measurements gave the
molecular formula C19H22N2O3.
group is oriented away from the lactone moiety and proximate
to the olefinic H-6, which was also supported by the observed
reciprocal NOEs observed between NH2 and H-6 (Figure 3).
With the structure of 6,7-dehydroleuconoxine (10),
previously misassigned as epi-leuconolam (2), and that of
compound A (11) firmly established, the results of the
transformations of leuconolam (1) and 6,7-dehydroleuconoxine
(10) under various conditions become intelligible (Table 2).
The formation of 6,7-dehydroleuconoxine (10) with
recovered leuconolam (1), when leuconolam (1) was treated
with aqueous acid under two-phase conditions (entry 2, Table
2), presumably derives from reversible formation of the N-4−
The 13C NMR spectrum accounted for all 19 carbon
resonances and confirmed the presence two carbonyl functions
at δC 166.8 (lactam carbonyl) and 170.6 (lactone carbonyl), in
addition to a low-field quaternary resonance (δC 102.1) due to
C-21, which is α to both a nitrogen and an oxygen atom. The
1H NMR spectrum showed resonances due to four contiguous
330
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
With the larger and more nucleophilic MeOH, approach from
the less hindered convex β-face is overwhelmingly favored, and
the nucleophilic addition step is virtually irreversible; the O-
methylleuconolam (8) once formed was stable under the
reaction conditions (8 did not react when exposed to acid,
entry 12, Table 2).
Table 2. Transformations of Leuconolam (1) and 6,7-
Dehydroleuconoxine (10) Under Various Conditions
products
starting
entry material
reaction conditions
5% HCl, rt, 8 h
1
10 11
8
1
1
no reaction (recovery of 1 on
basification)
When the reaction was carried out in PTSA/CH2Cl2, a
change in the product distribution was observed, with the
amino lactam-lactone 11 obtained as the major product (42%)
and 6,7-dehydroleuconoxine (10) as the minor product (5%)
(entry 8, Table 2). TLC monitoring of the progress of the
reaction showed that the amino lactam-lactone 11 was formed
subsequent to the formation of 10, suggesting that 11
originated from the first-formed 10. Further confirmation was
provided by the observation that treatment of 10 with PTSA/
CH2Cl2 resulted in the formation of the amino lactam-lactone
11 as the major product in 70% yield (entry 11, Table 2).
A possible pathway for this transformation is shown in
Scheme 2, involving acidic hydrolysis of the N-1 lactam,
followed in succession by fragmentation to the iminium ion 12,
and finally facile intramolecular capture of the iminium ion 12
by the carboxylic acid group, leading eventually to the amino
lactam-lactone 11. The fact that the starting 6,7-dehydroleuco-
noxine (10) is more strained than the product 11 constitutes
additional support for the proposed amide hydrolysis under
relatively mild conditions.
In view of the facile acid-induced transannular cyclization of
leuconolam (1) to 6,7-dehydroleuconoxine (10), a two-step
sequence involving acid-induced cyclization (CSA/CH2Cl2)
followed by hydrogenation (H2, Pd/C) yielded leuconoxine (3)
in ca. 55% overall yield from leuconolam (1). This trans-
formation represents a partial synthesis of leuconoxine (3) from
leuconolam (1). Leuconoxine (3) was previously obtained by
bioconversion of rhazinilam with Beauveria bassiana LMA
(ATCC 7159), but in low yield (0.6%).19
2
1
5% HCl/CH2Cl2 +
a
35%
47%
1.5%
TEACl, rt, 14 h
3
4
5
1
1
1
HCl/MeOH, rt, 12 h
CSA/CH2Cl2, rt, 14 h
4%
63%
54%
a
10%
62%
19%
2%
CSA/CH2Cl2, rt, 11 h
(4 equiv MeOH
added)
6
7
8
9
1
CSA/MeOH, rt, 14 h
4%
4%
2%
94%
94%
1
PTSA/MeOH, rt, 14 h
0.8%
42%
1
PTSA/CH2Cl2, rt, 14 h 3%
5%
10
5% HCl/CH2Cl2 +
15%
84%
a
TEACl, rt, 12 h
b
10
11
12
13
14
15
16
10
10
8
CSA/CH2Cl2, rt, 15 h
PTSA/CH2Cl2, rt, 10 h
no reaction
1%
c
70%
PTSA/CH2Cl2, rt, 10 h no reaction
1
H2, Pd/C
H2, Pd/C
Br2/CHCl3
Br2/CHCl3
no reaction
3 (90%)
10
1
13 (86%)
13 (96%)
10
a
b
Prolonged reaction time leads to reduced overall yields. Traces of 1
c
and compound A detected from TLC. Traces of 1 and 9 detected
from TLC.
C-21 iminium ion 9, followed by transannular cyclization to the
spirocyclic dehydroleuconoxine (10). The reversible nature of
this reaction is indicated by the formation of 1 with recovered
10, when 10 was subjected to the same reaction conditions
(entry 9, Table 2). When the acid-induced reaction was carried
out in the polar, protic, nucleophilic MeOH (entries 3, 6, and 7,
Table 2) in the presence of either HCl, CSA, or PTSA, virtually
quantitative conversion to O-methylleuconolam (8) was
observed, suggesting efficient trapping of the iminium ion
from the β-face by the larger and more nucleophilic MeOH.
During the course of the present study, an alkaloid
corresponding to 6,7-dehydroleuconoxine (10) (NMR data
identical to “epi-leuconolam” or 6,7-dehydroleuconoxine) was
reported as a minor alkaloid from the stem-bark extract of
Scheme 2. Possible Pathway for the Formation of 11 from 10
331
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
Melodinus henryi.20 In view of the above, the possibility that this
alkaloid is an artifact due to the action of traces of acid on
leuconolam (1) cannot be discounted.
Compound 15 was obtained as a yellowish oil and
subsequently as yellowish needles from MeOH (mp 128−132
°C), with [α]25 = +584 (c 0.4, CHCl3). The UV spectrum
D
Treatment of leuconolam (1) with Br2 in CHCl3 gave the
dibromoleuconoxine derivative 6β,7β-dibromoleuconoxine
(13) as the sole product in about 90% yield (Table 2).2c
Monitoring of the progress of the bromination reaction by TLC
indicated that two products, in addition to the starting
leuconolam (1), were detected at an early stage of the reaction.
These were 6β,7β-dibromoleuconoxine (13) and 6,7-dehydro-
leuconoxine (10). This observation suggested a two-step
sequence involving transannular cyclization to 10, followed by
bromine addition to furnish 13. This was supported by the
observation that treatment of 6,7-dehydroleuconoxine (10)
with Br2/CHCl3 proceeded smoothly to yield the same
dibromoleuconoxine product 13. Monitoring of the reaction
progress by TLC showed only the presence of 6,7-
dehydroleuconoxine (10) and the dibromoleuconoxine addi-
tion product, 13. Furthermore, debromination (Zn/AcOH) of
the dibromo addition product led smoothly to 6,7-dehydro-
leuconoxine (10).
showed absorption maxima at 209, 246, and 388 nm, while the
IR spectrum showed a conjugated lactam carbonyl at 1641 and
1682 cm−1. The ESIMS of 15 showed an [M + H]+ ion at m/z
295, in agreement with the molecular formula C19H22N2O + H.
1
A notable difference in the H NMR spectrum of 15 when
compared with that of 6,7-dehydroleuconoxine (10) was the
presence of two additional proton resonances due to a
methylene group adjacent to a heteroatom at δH 3.55 and
3.81, attributable to H-2 (based on HMQC). Also, the
characteristic C-2 lactam carbonyl resonance observed in the
13C NMR spectrum of 15 was replaced by a resonance at δC
40.8 attributed to C-2 in 15. These observations indicated
deoxygenation at C-2 of 15. Compound 15 is therefore 2-
dihydro-6,7-dehydroleuconoxine.
Compound 16 was obtained as a fluorescent yellowish oil
and subsequently as fluorescent yellowish rods (mp 198−200
°C), with [α]25 = +667 (c 0.3, CHCl3). The UV spectrum
D
showed absorption maxima at 209, 245, and 394 nm, while the
IR spectrum showed an OH band at 3343 cm−1 and a
conjugated lactam carbonyl at 1666 cm−1. The ESIMS of 16
showed an [M + H]+ ion at m/z 311, in agreement with the
molecular formula C19H22N2O2 + H. Notable differences in the
1H NMR spectrum of 16 when compared with that of 6,7-
dehydroleuconoxine (10) were the presence of a low-field
proton resonance at δH 5.52 due to H-2 and a broad OH
resonance at δH 4.02. The 13C NMR spectrum showed the
absence of the characteristic C-2 lactam resonance, while
displaying an additional resonance at δC 76.1, attributed to C-2.
These observations indicated that the C-2 carbonyl in 16 has
been reduced to an OH. The C-2 configuration was assigned as
S, based on the observed NOE between C-2 and C-12.
Compound 16 is therefore 2α-hydroxy-6,7-dehydroleuconox-
ine.25 The structures of 15 and 16 were both confirmed by X-
ray diffraction analysis (Figure 5).
6β,7β-Dibromoleuconoxine (13) was obtained as a white,
amorphous solid, with [α]25 = −38 (c 0.6, CHCl3). The UV
D
spectrum showed absorption maxima at 208, 227, and 292 nm,
while the IR spectrum showed the presence of two lactam
carbonyls at 1691 and 1709 cm−1. The ESIMS of 1 showed an
[M + H]+ ion at m/z 467, and HRESIMS measurements gave
1
the molecular formula C19H21N2O279Br2 + H. The H and 13C
NMR data of 1 were similar to those reported by Goh and co-
workers.2c The 6β,7β-dibromo configuration of 13 was assigned
by analogy to leuconoxine and its congeners, where H or OH
substituents attached to C-7 in the diazaspiro leuconoxine
skeleton has to be β-oriented (7α-substituted analogues are
highly strained, and none are known). In addition, the observed
NOE between H-6 and H-9 is only possible if H-6 is α-
oriented.
The bromination of alkenes is a well-known reaction, which
usually yields trans-dibromo products as a consequence of anti-
addition of bromine. The generally accepted mechanism
invokes the intermediacy of a bromonium ion intermediate.
In this instance however, a cis-dibromo addition product was
clearly obtained as the sole product. Deviations from trans
selectivity, usually giving rise to cis/trans mixtures of addition
products, have been observed, e.g., in acenaphthylene.21
Deviations from trans selectivity are explained by the
intermediacy of nonbridged cationic species such as the β-
bromocarbocation22 or more recently by the intermediacy of
the tribromide adduct.23
The exclusive formation of a cis-dibromo addition product 13
may be explained by acid-catalyzed epimerization of the trans
addition product (formed with exclusive trans selectivity via the
bromonium ion) or from cis/trans mixtures formed via the
intermediacy of the β-bromocarbocation or the tribromide
adduct.
It was at first envisaged that a hydroboration reaction on 6,7-
dehydroleuconoxine (10) might lead to 6-hydroxyleuconoxine
(or leuconodine A, 14), a new leuconoxine-type alkaloid from
L. griffithii.9 However, when 10 was treated with BH3·SMe2 (5
equiv) in THF at room temperature,24 a complex mixture of
products was obtained from which two leuconoxine-type
derivatives arising from reduction of the C-2 lactam carbonyl,
viz., 15 (completely reduced product, 37%) and 16 (partially
reduced product, 6%), were isolated.
Figure 5. X-ray crystal structures of 15 and 16.
Since hydroboration of 10 did not furnish leuconodine A
(14), a direct α-oxygenation of leuconoxine (3) at C-6, via
enolate-mediated oxidation, was next attempted. However,
treatment of leuconoxine (3) with lithium diisopropylamide
(LDA) in THF at 0 °C, followed by oxidation of the lactam
enolate with O2,27 gave compound 17 as the sole product
(21%), accompanied by a significant amount of unreacted 3
(69%). The enolate-mediated oxidation occurred at C-16
instead of at C-6, possibly due to the formation of the more
stable six-membered enolate.
Compound 17 was obtained as a colorless oil and
subsequently as colorless needles from CH2Cl2/hexanes (mp
332
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
184−186) with [α]25 = −29 (c 0.2, CHCl3). The UV
reactions (HCl in two-phase medium, CSA in CH2Cl2) resulted
in transannular cyclization to give 6,7-dehydroleuconoxine
(10). A two-step sequence from leuconolam (1), comprising
acid-induced cyclization, followed by catalytic hydrogenation,
provided a concise semisynthesis of leuconoxine (3). When the
acid-induced reaction of leuconolam (1) or 6,7-dehydroleuco-
noxine (10) was carried out with PTSA in CH2Cl2, the product
was the amino lactam-lactone 11, while the acid-induced
reactions in MeOH afforded O-methylleuconolam (8) as the
sole product in high yields. The original assignment of the
structure of epi-leuconolam (2) was revised to 6,7-dehydroleu-
conoxine (10) based on X-ray diffraction analysis, which was
prompted by inconsistencies observed in the various trans-
formations of leuconolam and its supposed C-21 epimer, “epi-
leuconolam”. Bromination (Br2/CHCl3) of leuconolam (1)
proceeds in two steps via intermediacy of 6,7-dehydroleuco-
noxine (10) to furnish the 6β,7β-dibromoleuconoxine adduct
(13). Concise semisyntheses of the new leuconoxine-type
alkaloids leuconodines A (14) and F (19) were achieved by
transformation of leuconolam (1) with excess TFA into
leuconodine A (14) and 10 and subsequently by oxidation of
14 to leuconodine F (19).
D
spectrum showed absorption maxima at 210, 241, and 374 nm,
while the IR spectrum showed the presence of an OH (3417
cm−1) and carbonyl functions (1691 cm−1, broad). The ESIMS
of 17 showed an [M + H]+ ion at m/z 327, in agreement with
the molecular formula C19H22N2O3 + H. Notable differences in
1
the H NMR spectrum of 17 when compared with that of 3
include the downfield shift of H-16 from δH 2.78 and 2.49 in 3
to δH 4.45 in 17 and the presence of an OH resonance at δH
3.28 (exchangeable with D2O) in 17. The 13C NMR data
showed that the resonance due to C-16 had shifted downfield
(δC 64.9) when compared with that of 3. These results strongly
suggested that oxidation had occurred at C-16. The relative
configuration at C-16 was assigned as R, based on the observed
NOE between H-16 and H-15α. Compound 17 is therefore
16β-hydroxyleuconoxine. Suitable crystals of 17 were obtained
from CH2Cl2/hexanes, and an X-ray diffraction analysis
confirmed the structure assignment (Figure 6).
EXPERIMENTAL SECTION
■
General Experimental Procedures. Melting points were
measured on a Mel-Temp melting point apparatus and were
uncorrected. Optical rotations were recorded on a JASCO P-1020
digital polarimeter. IR spectra were recorded on a Perkin-Elmer
Spectrum 400 spectrophotometer or on a Perkin-Elmer 1600 Series
FT-IR spectrophotometer. UV spectra were obtained on a Shimadzu
UV-3101PC spectrophotometer. 1H and 13C NMR spectra were
recorded in CDCl3 using TMS as internal standard on a JEOL JNM-
LA 400, JNM-ECA 400, or Bruker Avance III 400 spectrometer, at 400
and 100 MHz, respectively, or on a Bruker Avance III 600
spectrometer at 600 and 150 MHz, respectively. ESIMS and
HRESIMS were obtained on an Agilent 6530 Q-TOF mass
spectrometer. EIMS and HREIMS were obtained at Organic Mass
Spectrometry, Central Science Laboratory, University of Tasmania,
Tasmania, Australia. All air/moisture-sensitive reactions were carried
out under N2 in oven-dried glassware. THF was freshly distilled from
Na/benzophenone under N2, MeOH was freshly distilled from Mg
turnings under N2, and CH2Cl2 was distilled from CaH2 under N2. All
other reagents were used without further purification.
Figure 6. X-ray crystal structure of 17.
Leuconodine A (14) was eventually obtained by treatment of
leuconolam (1) with excess trifluoroacetic acid (TFA).
Treatment of leuconolam (1) with TFA (2 equiv) resulted in
transannular cyclization to 6,7-dehydroleuconoxine (10) via the
iminium ion 9. The use of excess TFA (20 equiv) gave a
mixture of two products, viz., 10 (30% yield) and leuconodine
A (14) (25% yield).
The formation of 14 and 10 in the presence of excess TFA is
rationalized in Scheme 3. The possibility that conjugate
addition by the TFA anion to the conjugated iminium ion 9
competes with transannular cyclization to 10, leading eventually
to leuconodine A (14) (Scheme 3, path a), is rendered less
likely on account of the poor nucleophilicity of the
trifluoromethylacetate anion. A preferred pathway is via a
[3,3] sigmatropic shift (analogous to the Overman rearrange-
ment of allylic trichloroacetimidates)28 from the ester 19,
formed by the reaction of 1 with excess TFA (Scheme 3, path
b). This pathway would also account for the stereoselectivity
observed (6β-OH). Subsequently, Dess−Martin periodinane
(DMP) oxidation of leuconodine A (14) afforded the new
leuconoxine alkaloid leuconodine F (19).9,29
Source of Compounds 1 and 10. Compounds 1 and 10 were
previously isolated from Leuconotis griffithii.9
Leuconolam (1): colorless block crystals from MeOH; mp 178−
180 °C [lit.2 263−264 °C]; [α]25 −303 (c 0.8, CHCl3) [lit.2 [α]D
D
−28.3 (c 0.7 CHCl3)]; UV (EtOH) λmax (log ε) 205 (4.00), 220
(3.22), and 292 (3.96) nm; IR (dry film) νmax 3263, 1683, and 1650
cm−1; 1H NMR (CDCl3, 400 MHz) δ 0.55 (1H, t, J = 7.5 Hz, H-18),
1.23 (1H, dq, J = 13.6, 7.5 Hz, H-19b), 1.40 (1H, br t, J = 14.5 Hz, H-
17b), 1.48 (2H, m, H-14a, H-14b), 1.57 (1H, m, H-15b), 1.60 (1H, td,
J = 14.5, 7.3 Hz, H-17a), 1.60 (1H, m, H-19a), 1.79 (1H, td, J = 13.5,
4.5 Hz, H-15a), 1.99 (1H, td, J = 14, 1.7 Hz, H-16b), 2.12 (1H, dd, J =
14, 7.3 Hz, H-16a), 2.94 (1H, td, J = 12.5, 4.5 Hz, H-3b), 3.98 (1H,
dd, J = 12.5, 4.5 Hz, H-3a), 4.99 (1H, br s, 21-OH), 5.77 (1H, s, H-6),
7.18 (1H, dd, J = 7.5, 1.5 Hz, H-9), 7.33 (1H, td, J = 7.5, 1.5 Hz, H-
11), 7.36 (1H, td, J = 7.5, 1.5 Hz, H-10), 7.71(1H, br s, NH), 7.91
(1H, td, J = 7.5, 1.5 Hz, H-12); 13C NMR (CDCl3, 100 MHz) δ 6.9
(CH, C-18), 19.7 (CH2, C-14), 24.5 (CH2, C-15), 25.4 (CH2, C-17),
27.3 (CH2, C-19), 32.1 (CH2, C-16), 35.3 (CH2, C-3), 44.9 (C, C-20),
93.6 (C, C-21), 126.3 (CH, C-10), 126.6 (CH, C-12), 128.1 (CH, C-
6), 129.3 (CH, C-9), 129.4 (CH, C-11), 133.1 (C, C-8), 135.0 (C, C-
13), 155.6 (C, C-7), 166.5 (C, C-5), 177.8 (C, C-2); ESIMS m/z 327
[M + H]+ (C19H22N2O3 + H).
CONCLUSION
■
Several transformations of the ring-opened Aspidosperma
alkaloid leuconolam (1) were investigated. The based-induced
reaction of leuconolam (1) resulted in enolate-mediated
transannular cyclization to give two epimeric pentacyclic
meloscine-like products, 6 and 7, while the acid-induced
6,7-Dehydroleuconoxine (10) (formerly epi-leuconolam): color-
less block crystals from CH2Cl2/hexanes; mp 164−168 °C; [α]25
D
333
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
Scheme 3. Formation of 14 and 18
+271 (c 0.1, CHCl3); UV (EtOH) λmax (log ε) 203 (4.32), 252 (4.33),
and 350 (3.70) nm; IR (dry film) νmax 1691, 1649, and 1595 cm−1; 1H
NMR (CDCl3, 400 MHz) δ 0.76 (1H, t, J = 7.4 Hz, H-18), 1.10 (1H,
td, J = 14, 7 Hz, H-15b), 1.35 (1H, dq, J = 13.6, 7.4 Hz, H-19b), 1.45
(1H, dq, J = 13.6, 7.4 Hz, H-19a), 1.66 (1H, ddd, J = 14, 6, 1.5 Hz, H-
15a), 1.71 (1H, td, J = 15, 5 Hz, H-17b), 1.79 (1H, m, H-14b), 2.04
(1H, m, H-14a), 2.09 (1H, ddd, J = 15, 6, 2 Hz, H-17a), 2.62 (1H, J =
15, 5, 2 Hz, H-16b), 3.09 (1H, td, J = 15, 6 Hz, H-16a), 3.22 (1H, ddd,
J = 15, 9.6, 6 Hz, H-3b), 4.46 (1H, ddd, J = 15, 12, 4 Hz, H-3a), 6.22
(1H, s, H-6), 7.12 (1H, td, J = 7.5, 1 Hz, H-10), 7.33 (1H, td, J = 7.5, 1
Hz, H-11), 7.46 (1H, ddd, J = 7.5, 1, 0.6 Hz, H-9), 8.16 (1H, ddd, J =
7.5, 1, 0.6 Hz, H-12); 13C NMR (CDCl3, 100 MHz) δ 8.3 (CH, C-18),
16.8 (CH2, C-14), 26.0 (CH2, C-15), 30.4 (CH2, C-17), 33.1 (CH2, C-
16), 34.1 (CH2, C-19), 37.0 (CH2, C-3), 44.6 (C, C-20), 93.7 (C, C-
21), 115.9 (CH, C-12), 118.2 (CH, C-6), 121.6 (CH, C-9), 123.5 (C,
C-8), 124.3 (CH, C-10), 131.6 (CH, C-11), 148.6 (C, C-13), 164.2
(C, C-7), 173.5 (C, C-5), 176.1 (C, C-2); ESIMS m/z 309 [M + H]+;
HRESIMS m/z 309.1590 [M + H]+ (calcd for C19H20N2O2 + H,
309.1598).
1.08 (1H, dq, J = 14, 7.6 Hz, H-19a), 1.44 (1H, m, H-15b), 1.59 (2H,
m, H-14a, H-14b), 1.81 (1H, dt J = 14.5, 4.5 Hz, H-15a), 2.20 (1H,
ddd, J = 14, 10.5, 2 Hz, H-17b), 2.32 (1H, dd, J = 14, 2.5 Hz, H-17a),
2.38 (1H, br s, 21-OH), 2.69 (1H, d, J = 17.7 Hz, H-6β), 2.91 (1H, m,
H-16), 2.94 (1H, m, H-3b), 3.03 (1H, d, J = 17.7 Hz, H-6α), 4.23 (1H,
dt, J = 13, 7.5 Hz, H-3a), 6.76 (1H, dd, J = 8, 1.5 Hz, H-12), 7.10 (1H,
td, J = 8, 1.5 Hz, H-11), 7.23 (1H, td, J = 8, 1.5 Hz, H-10), 7.39 (1H,
dd, J = 8, 1.5 Hz, H-9), 8.41 (1H, br s, NH); 13C NMR (CDCl3, 100
MHz) δ 7.4 (CH, C-18), 19.6 (CH, C-14), 26.3 (CH2, C-19), 28.0
(CH2, C-15), 32.2 (CH2, C-17), 37.2 (CH2, C-3), 46.7 (C, C-20), 50.0
(CH2, C-6), 50.6 (C, C-8), 51.9 (CH, C-16), 100.8 (C, C-21), 116.1
(CH, C-12), 122.0 (C, C-8), 123.8 (CH, C-10), 129.0 (CH, C-11),
129.1 (CH, C-9), 136.0 (C, C-13), 170.5 (C, C-2), 171.0 (C, C-5);
ESIMS m/z 327 [M + H]+; HRESIMS m/z 327.1712 [M + H]+ (calcd
for C19H22N2O3 + H, 327.1703).
Compound 7: colorless oil and subsequently as colorless block
crystals from MeOH; mp 250−252 °C; [α]25D −150 (c 0.01, CHCl3);
UV (EtOH) λmax (log ε) 210 (4.46), 251 (4.22), and 306 (3.00) nm;
IR (dry film) νmax 3322, 1712, and 1681 cm−1; 1H NMR (CDCl3, 400
MHz) δ 0.89 (1H, t, J = 7.3 Hz, H-18), 1.20 (1H, td, J = 14.5, 6.8 Hz,
H-15b), 1.41 (1H, m, H-19b), 1.56 (1H, m, H-17b), 1.59 (1H, m, H-
19a), 1.66 (2H, m, H-14a, H-14b), 1.82 (1H, dt, J = 14.5, 4.5 Hz, H-
15a),2.17 (1H, dd, J = 13.7, 5 Hz, H-17a), 2.35 (1H, d, J = 18 Hz, H-
6β), 2.75 (1H, d, J = 18 Hz, H-6α), 2.90 (1H, br s, 21-OH), 3.07 (1H,
dd, J = 13.7, 5.5 Hz, H-16), 3.08 (1H, m, H-3b), 4.03 (1H, dt, J = 13,
7.5 Hz, H-3a), 6.85 (1H, br d, J = 7.8 Hz, H-9), 7.01 (1H, td, J = 7.8,
1.5 Hz, H-11), 7.19 (1H, td, J = 7.8, 1.5 Hz, H-10), 7.69 (1H, d, J = 7.8
Hz, H-12), 7.65 (1H, br s, NH); 13C NMR (CDCl3, 100 MHz) δ 8.8
(CH, C-18), 24.0 (CH2, C-19), 30.0 (CH2, C-15), 30.4 (CH2, C-17),
36.6 (CH2, C-3), 41.5 (CH2, C-6), 46.3 (CH, C-16), 50.0 (C, C-7),
50.3 (C, C-20), 99.2 (C, C-21), 117.0 (CH, C-12), 123.6 (CH, C-10),
123.9 (CH, C-9), 127.9 (CH, C-11), 133.0 (C, C-8), 137.2 (C, C-13),
170.7 (C, C-2), 171.1 (C, C-5); ESIMS m/z 327 [M + H]+;
Reaction of Leuconolam (1) with KOH, MeOH/EtOH.
Leuconolam (1) (50 mg, 0.15 mmol) was dissolved in methanolic
ethanol (9:1, 50 mL). Two pellets of KOH were added, and the
solution was stirred at rt for 6 h, quenched with 5% HCl (20 mL), and
basified with 10% NaHCO3 (30 mL). The mixture was extracted with
CH2Cl2 (4 × 100 mL), washed with H2O, dried (Na2SO4), and
concentrated in vacuo, and the residue purified by centrifugal
preparative TLC (SiO2, 10% MeOH/Et2O, NH3-saturated) to give 5
(6 mg, 12%) and 7 (1.5 mg, 3%) and recovered 1 (10 mg, 20%).
Compound 6: colorless oil and subsequently as colorless block
crystals from CCl4/MeOH; mp 266−268 °C [lit.2c 175−177 °C];
[α]25D −198 (c 0.06, CHCl3) [lit.2c [α]D −14.3 (c 0.35, CHCl3)]; UV
(EtOH) λmax (log ε) 210 (4.50), 253 (4.01), and 287 (3.38) nm; IR
1
(dry film) νmax 3226 and 1667 cm−1; H NMR (CDCl3, 400 MHz) δ
0.67 (1H, t, J = 7.6 Hz, H-18), 0.96 (1H, dq, J = 14, 7.6 Hz, H-19b),
334
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
HRESIMS m/z 327.1710 [M + H]+ (calcd for C19H22N2O3 + H,
327.1703).
mmol) and CH2Cl2 (5 mL) was added leuconolam (1) (14.3 mg,
0.044 mmol). The mixture was stirred for 12 h at rt, quenched with
10% K2CO3 (10 mL), and extracted with CH2Cl2 (3 × 5 mL). The
combined organic extract was washed with H2O (3 × 10 mL), dried
(Na2SO4), and concentrated in vacuo, and the residue purified by
centrifugal preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated)
to give 6,7-dehydroleuconoxine (10) (8.2 mg, 62%), amino lactam-
lactone 11 (0.1 mg, 2%), and recovered 1 (1.4 mg, 10%).
Reaction of Leuconolam (1) with 5% HCl. To a stirred solution
of 5% HCl (5 mL) was added 1 (11 mg, 0.034 mmol). The mixture
was stirred for 12 h at rt, quenched with 10% Na2CO3 (10 mL),
extracted with CH2Cl2 (3 × 10 mL), washed with H2O (3 × 20 mL),
dried (Na2SO4), and concentrated in vacuo. TLC of the residue
showed only the presence of 1 (8.9 mg, 81% recovery).
Reaction of Leuconolam (1) with 5% HCl/CH2Cl2 in the
Presence of Tetraethylammonium Chloride. Leuconolam (1)
(14.5 mg, 0.044 mmol) was added to a two-phase system comprising
5% HCl (5 mL), CH2Cl2 (5 mL), and TEACl (7 mg, 0.044 mmol).
The mixture was stirred for 12 h at rt, quenched with 10% Na2CO3
(10 mL), and extracted with CH2Cl2 (3 × 5 mL). The combined
organic extract was washed with H2O (3 × 20 mL), dried (Na2SO4),
and concentrated in vacuo, and the residue purified by centrifugal
preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated) to give 6,7-
dehydroleuconoxine (10) (6.5 mg, 47%), amino lactam-lactone 11
(0.2 mg, 1.4%), and recovered 1 (5.1 mg, 35%).
Reaction of Leuconolam (1) with CSA in Anhydrous CH2Cl2/
MeOH. To a stirred solution of CSA (13.2 mg, 0.057 mmol) and
CH2Cl2 (5 mL) was added leuconolam (1) (11.8 mg, 0.038 mmol).
The mixture was stirred for 30 min, and MeOH (6 μL, 0.152 mmol)
was added. The mixture was stirred for another 11 h at rt, quenched
with 10% K2CO3 (10 mL), and extracted with CH2Cl2 (5 × 10 mL).
The combined organic extract was washed with H2O (3 × 10 mL),
dried (Na2SO4), and concentrated in vacuo, and the residue purified by
centrifugal preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated)
to give O-methylleuconolam (8) (6.6 mg, 54%) and 6,7-
dehydroleuconoxine (10) (2.2 mg, 19%).
Amino Lactam-lactone 11: yellowish oil and subsequently as
yellowish block crystals from CH2Cl2/hexanes; mp 179−182 °C;
[α]25D +116 (c 0.5, CHCl3); UV (EtOH) λmax (log ε) 212 (4.87), 240
(4.83), and 342 (4.01) nm; IR (dry film) νmax 3483, 3397, 1743, and
1709 cm−1; 1H NMR (CD2Cl2, 400 MHz) δ 0.68 (1H, t, J = 87.6 Hz,
H-18), 1.26 (1H, m, H-19b), 1.28 (1H, m, H-17b), 1.43 (1H, m, H-
15b), 1.45 (1H, m, H-17a), 1.51 (1H, m, H-19a), 1.53 (1H, m, H-
15a), 1.58 (1H, m, H-14), 2.20 (1H, ddd, J = 19, 10, 1.2 Hz, H-16b),
2.44 (1H, ddd, J = 19, 6, 1.5 Hz, H-16a), 2.82 (1H, ddd, J = 13, 4, 2
Hz, H-3b), 3.94 (2H, br s, NH2), 4.09 (1H, ddd, J = 13, 11, 4 Hz, H-
3a), 6.14 (1H, s, H-6), 6.65 (1H, td, J = 8, 1.5 Hz, H-10), 6.65 (1H,
dd, J = 8, 1.5 Hz, H-12), 6.96 (1H, dd, J = 8, 1.5 Hz, H-9), 7.09 (1H,
td, J = 8, 1.5 Hz, H-11); 13C NMR (CD2Cl2, 100 MHz) δ 7.1 (CH, C-
18), 19.8 (CH, C-14), 25.0 (CH2, C-19), 25.5 (CH2, C-15), 25.6
(CH2, C-17), 26.3 (CH2, C-16), 35.9 (CH2, C-3), 37.9 (C, C-20),
102.1 (C, C-21), 116.6 (CH, C-12), 118.0 (C, C-8), 118.4 (CH, C-
10), 121.9 (CH, C-6), 128.9 (CH, C-9), 130.8 (CH, C-11), 144.1 (C,
C-13), 155.7 (C, C-7), 166.8 (C, C-5), 170.6 (C, C-2); EIMS m/z (rel
int) 326 [M]+ (100), 299 (5), 280 (10), 267 (12), 239 (20), 225 (5),
209 (7), and 185 (8); HREIMS m/z [M]+ 326.1629 (calcd for
C19H22N2O3, 326.1630).
Reaction of Leuconolam (1) with Concentrated HCl in
MeOH. Leuconolam (1) (12.9 mg, 0.040 mmol) was dissolved in a
minimal amount of MeOH (ca. 0.1 mL). Concentrated HCl (2 drops)
was added dropwise. The mixture was stirred for 16 h at rt, quenched
with 10% Na2CO3 (10 mL), and extracted with CH2Cl2 (3 × 5 mL).
The combined organic extract was washed with H2O (3 × 20 mL),
dried (Na2SO4), and concentrated in vacuo, and the residue purified by
centrifugal preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated)
to give O-methylleuconolam (8) (8.6 mg, 63%) and recovered 1 (0.5
mg, 4%).
Reaction of Leuconolam (1) with CSA in Anhydrous MeOH.
To a stirred solution of CSA (11.8 mg, 0.051 mmol) and MeOH (5
mL) was added leuconolam (1) (11 mg, 0.034 mmol). The mixture
was stirred for 12 h at rt, quenched with 10% K2CO3 (10 mL), and
extracted with CH2Cl2 (5 × 10 mL). The combined organic extract
was washed with H2O (3 × 10 mL), dried (Na2SO4), and
concentrated in vacuo, and the residue purified by centrifugal
preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated) to give O-
methylleuconolam (8) (10.9 mg, 94%), amino lactam-lactone 11 (0.1
mg, 2%), and recovered 1 (0.4 mg, 4%).
Reaction of Leuconolam (1) with p-Toluenesulfonic Acid in
Anhydrous MeOH. To a stirred solultion of PTSA (9.5 mg, 0.056
mmol) and MeOH (5 mL) was added leuconolam (1) (12 mg, 0.037
mmol). The mixture was stirred for 12 h at rt, quenched with 10%
K2CO3 (10 mL), and extracted with CH2Cl2 (5 × 10 mL). The
combined organic extract was washed with H2O (3 × 10 mL), dried
(Na2SO4), and concentrated in vacuo, and the residue purified by
centrifugal preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated)
to give O-methylleuconolam (8) (11.8 mg, 94%), amino lactam-
lactone 11 (0.1 mg, 0.8%), and recovered 1 (0.4 mg, 4%).
Reaction of Leuconolam (1) with PTSA in Anhydrous CH2Cl2.
To a stirred solution of PTSA (8.6 mg, 0.05 mmol) and CH2Cl2 (5
mL) was added leuconolam (1) (11.7 mg, 0.036 mmol). The mixture
was stirred for 15 h at rt, quenched with 10% Na2CO3 (10 mL), and
extracted with CH2Cl2 (3 × 5 mL). The combined organic extract was
washed with H2O (3 × 10 mL), dried (Na2SO4), and concentrated in
vacuo, and the residue purified by centrifugal preparative TLC (SiO2,
Et2O, NH3-saturated) to give 6,7-dehydroleuconoxine (10) (0.6 mg,
5%), amino lactam-lactone 11 (5 mg, 42%), and recovered 1 (0.4 mg,
3%).
Reaction of 6,7-Dehydroleuconoxine (10) with 5% HCl/
CH2Cl2 in the Presence of TEACl. 6,7-Dehydroleuconoxine (10)
(19.5 mg, 0.063 mmol) was added into a two-phase system comprising
5% HCl (5 mL), CH2Cl2 (5 mL), and TEACl (10 mg, 0.063 mmol).
The mixture was stirred for 12 h at rt, quenched with 10% Na2CO3
(10 mL), and extracted with CH2Cl2 (3 × 5 mL). The combined
organic extract was washed with H2O (3 × 20 mL), dried (Na2SO4),
and concentrated in vacuo, and the residue purified by centrifugal
preparative TLC (SiO2, 5% MeOH/Et2O, NH3-saturated) to give
leuconolam (1) (2.9 mg, 15%) and recovered 6,7-dehydroleuconoxine
(10) (16.3 mg, 84%).
O-Methylleuconolam (8): colorless oil and subsequently as
colorless block crystals from MeOH; mp 214−218 °C [lit.2 155−
156 °C]; [α]25 −240 (c 0.6, CHCl3); UV (EtOH) λmax (log ε) 238
D
(3.99) and 348 (3.03) nm; IR (dry film) νmax 3477 and 1693 cm−1; 1H
NMR (CDCl3, 400 MHz) δ 0.55 (1H, t, J = 7.5 Hz, H-18), 1.28 (1H,
dq, J = 13.6, 7.5 Hz, H-19b), 1.49 (2H, m, H-14a, H-14b), 1.50 (3H,
m, H-15b, H-16b, H-17b), 1.54 (1H, m, H-19a), 1.75 (1H, m, H-17a),
2.05 (1H, ddd, J = 15, 5, 2 Hz, H-15a), 2.17 (1H, td, J = 15, 6 Hz, H-
16a), 2.61 (1H, td, J = 12.5, 4 Hz, H-3b), 3.15 (3H, s, 21-OMe), 4.18
(1H, td, J = 12.5, 4 Hz, H-3a), 6.33 (1H, s, H-6), 7.26 (1H, ddd, J =
7.5, 1, 0.5 Hz, H-9), 7.34 (1H, m, H-10), 7.41 (1H, m, H-11), 7.42
(1H, m, H-12), 8.25 (1H, br s, NH); 13C NMR (CDCl3, 100 MHz) δ
7.3 (CH, C-18), 19.6 (CH2, C-14), 24.1 (CH2, C-19), 26.2 (CH2, C-
17), 28.0 (CH2, C-16), 32.5 (CH2, C-15), 35.9 (CH2, C-3), 45.5 (C,
C-20), 49.9 (CH3, 21-OMe), 97.4 (C, C-21), 126.7 (CH, C-9), 127.0
(CH, C-10), 128.6 (CH, C-12), 129.9 (CH, C-11), 131.9 (CH, C-6),
133.1 (C, C-8), 135.7 (C, C-13), 151.0 (C, C-7), 166.8 (C, C-5),
178.6 (C, C-2); ESIMS m/z 341 [M + H]+ (C20H24N2O3 + H).
Reaction of Leuconolam (1) with 10-Camphorsulfonic Acid
in Anhydrous CH2Cl2. To a stirred solution of CSA (15 mg, 0.066
Reaction of 6,7-Dehydroleuconoxine (10) with CSA in
Anhydrous CH2Cl2. To a stirred solution of CSA (11.8 mg, 0.051
mmol) and CH2Cl2 (5 mL) was added 6,7-dehydroleuconoxine (10)
(11 mg, 0.034 mmol). TLC of the reaction mixture after 15 h showed
traces of leuconolam (1) and amino lactam-lactone 11, in addition to
the starting material 10.
Reaction of 6,7-Dehydroleuconoxine (10) with PTSA in
Anhydrous CH2Cl2. To a stirred solution of PTSA (9.2 mg, 0.054
mmol) and CH2Cl2 (5 mL) was added 6,7-dehydroleuconoxine (10)
(10.3 mg, 0.036 mmol). The mixture was stirred for 10 h at rt,
335
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
quenched with 10% Na2CO3 (10 mL), and extracted with CH2Cl2 (3
× 5 mL). The combined organic extract was washed with H2O (3 × 10
mL), dried (Na2SO4), and concentrated in vacuo, and the residue
purified by centrifugal preparative TLC (SiO2, Et2O, NH3-saturated)
to give amino lactam-lactone 11 (7.1 mg, 70%) and recovered 10 (0.3
mg, 1%).
Reaction of O-Methylleuconolam (8) with PTSA in Anhy-
drous CH2Cl2. To a stirred solution of PTSA (8 mg, 0.044 mmol) and
CH2Cl2 (5 mL) was added O-methylleuconolam (8) (10 mg, 0.029
mmol). TLC of the mixture after 10 h showed traces of leuconolam
(1) and amino lactam-lactone (11), in addition to the starting material
8.
Hydrogenation of Leuconolam (1). Leuconolam (1) (20 mg,
0.061 mmol) was dissolved in CH2Cl2 (5 mL) and stirred over 10%
Pd/C (12.4 mg) under a hydrogen atmosphere at rt. TLC of the
mixture every 1 h for 6 h showed only the presence of the stating
material, 1.
Na2CO3 (10 mL), extracted with CHCl3 (3 × 5 mL), washed with
H2O, and dried (Na2SO4), the solvent removed in vacuo, and the
residue purified by centrifugal preparative TLC (SiO2, 5% MeOH/
CHCl3, NH3-saturated) to give 6β,7β-dibromoleuconoxine (13) (9.6
mg, 96%).
Debromination of 6β,7β-Dibromoleuconoxine (13). To a
solution of 6β,7β-dibromoleuconoxine (13) (13 mg, 0.028 mmol) in
HOAc (5 mL) was added freshly activated Zn (91 mg, 0.139 mmol).
The mixture was stirred for 2 h, after which the mixture was poured
into saturated Na2CO3 (30 mL), extracted with CH2Cl2 (3 × 20 mL),
washed with H2O (3 × 20 mL), and dried (Na2SO4), the solvent
removed in vacuo, and the residue purified by centrifugal preparative
TLC (SiO2, 5% MeOH/CHCl3, NH3-saturated) to give 6,7-
dehydroleuconoxine (10) (3.7 mg, 41%).
Reaction of 6,7-Dehydroleuconoxine (10) with BH3·SMe2.
BH3·SMe2 (75 μL, 1 M in THF) was added to 6,7-dehydroleuconox-
ine (10) (16 mg, 0.051 mmol) in THF (5 mL), and the mixture was
stirred for 24 h at rt. The progress of the reaction was monitored by
TLC, and the reaction was quenched with NH4Cl solution when >95%
of the starting material had been consumed. The mixture was extracted
with CH2Cl2 (3 × 10 mL), washed with H2O (3 × 20 mL), dried over
Na2SO4, and filtered, the solvent removed in vacuo, and the residue
purified by centrifugal preparative TLC (SiO2, 5% MeOH/CHCl3,
NH3-saturated) to give compounds 15 (5.6 mg, 37%) and 16 (1 mg,
6%).
Hydrogenation of 6,7-Dehydroleuconoxine (10). 6,7-Dehy-
droleuconoxine (10) (20 mg, 0.061 mmol) was dissolved in CH2Cl2
(5 mL) and stirred over 10% Pd/C (12.4 mg) under a hydrogen
atmosphere at rt for 1 h. The catalyst was removed by filtration over
Celite. Evaporation of the solvent in vacuo, followed by chromatog-
raphy of the resulting residue (SiO2, 5% MeOH/Et2O, NH3-
saturated), gave leuconoxine (3) (18.1 mg, 90%) as a colorless oil
and subsequently as colorless block crystals from MeOH; mp 210−
215 °C (lit.3 238−242 °C); [α]25 −86 (c 0.7, CHCl3) [lit.3 [α]25
Compound 15: yellowish oil and subsequently as yellowish needles
D
D
−88 (c 1.2, MeOH)]; 1H NMR (CDCl3, 400 MHz) δ 0.93 (1H, t, J =
7.4 Hz, H-18), 1.37 (1H, dq, J = 13.4, 7.4 Hz, H-19b), 1.60 (4H, m, H-
14a, H-14b, H-15b), 1.78 (1H, dq, J = 13.4, 7.4 Hz, H-19a), 1.86 (1H,
ddd, J = 14, 6.5, 1.4 Hz, H-17b), 1.97 (1H, ddd, J = 14, 12, 5 Hz, H-
15a, H-17a), 2.49 (1H, ddd, J = 19, 6, 1.4 Hz, H-16b), 2.68 (1H, d, J =
17 Hz, H-6b), 2.78 (1H, ddd, J = 19, 14, 6.5 Hz, H-16a), 2.80 (1H, m,
H-3b), 2.87 (1H, dd, J = 17, 7.3 Hz, H-6a), 3.82 (1H, d, J = 7.3 Hz, H-
7), 3.95 (1H, ddt, J = 13, 4.4, 2.3 Hz, H-3a), 7.14 (1H, td, J = 7.6, 1
Hz, H-10), 7.17 (1H, dd, J = 7.6, 1 Hz, H-9), 7.25 (1H, td, J = 7.6, 1
Hz, H-11), 7.77 (1H, dd, J = 7.6, 1 Hz, H-12); 13C NMR (CDCl3, 100
MHz) δ 7.3 (CH, C-18), 20.1 (CH2, C-14), 26.2 (CH2, C-15), 26.6
(CH2, C-17), 26.9 (CH2, C-19), 29.4 (CH2, C-16), 36.8 (CH2, C-3),
37.6 (CH2, C-6), 38.1 (C, C-20), 41.9 (CH, C-7), 92.5 (C, C-21),
120.1 (CH, C-12), 123.8 (CH, C-9), 125.5 (CH, C-10), 128.0 (CH,
C-11), 135.4 (C, C-8), 142.1 (C, C-13), 170.8 (C, C-5), 172.9 (C, C-
2); ESIMS m/z 311 [M + H]+ (C19H22N2O2 + H).
from MeOH; mp 128−132 °C; [α]25 +584 (c 0.4, CHCl3); UV
D
(EtOH) λmax (log ε) 209 (3.65), 246 (3.86), and 388 (3.02) nm; IR
1
(dry film) νmax 1682 and 1641 cm−1; H NMR (CDCl3, 400 MHz) δ
0.67 (1H, t, J = 7.6 Hz, H-18), 1.14 (1H, dq, J = 13.2, 7.6 Hz, H-19b),
1.15 (1H, m, H-17b), 1.38 (1H, dq, J = 13.2, 7.6 Hz, H-19a), 1.53
(1H, m, H-17a), 1.56 (2H, m, H-14a, H-14b), 1.69 (2H, m, H-15b, H-
16b), 2.00 (2H, m, H-15a, H-16a), 3.05 (1H, ddd, J = 13.5, 4.5, 2 Hz,
H-3b), 3.55 (1H, ddd, J = 15.4, 11, 7.8 Hz, H-2b), 3.81 (1H, dd, J =
15.4, 7.8 Hz, H-2a), 4.31 (1H, ddd, J = 13, 11, 4.5 Hz, H-3a), 6.16
(1H, s, H-6), 6.75 (1H, dd, J = 7.5, 1 Hz, H-12), 6.83 (1H, td, J = 7.5,
1 Hz, H-10), 7.24 (1H, td, J = 7.5, 1 Hz, H-11), 7.36 (1H, dd, J = 7.5,
1 Hz, H-9); 13C NMR (CDCl3, 100 MHz) δ 8.3 (CH, C-18), 17.0
(CH2, C-16), 20.1 (CH2, C-14), 25.4 (CH2, C-17), 27.4 (CH2, C-15),
29.6 (CH2, C-19), 39.0 (CH2, C-3), 40.8 (CH2, C-2), 41.4 (C, C-20),
94.5 (C, C-21), 109.7 (CH, C-12), 116.9 (CH, C-6), 119.7 (CH, C-
10), 122.4 (CH, C-9), 122.5 (C, C-8), 131.3 (CH, C-11), 157.0 (C, C-
13), 166.1 (C, C-7), 173.7 (C, C-5); ESIMS m/z 295 [M + H]+;
HRESIMS m/z [M + H]+ 295.1792 (calcd for C19H22N2O + H,
295.1805).
Compound 16: fluorescent yellowish oil and subsequently as
fluorescent yellowish rods from CH2Cl2/hexanes; mp 198−200 °C;
[α]25D +667 (c 0.3, CHCl3); UV (EtOH) λmax (log ε) 209 (4.14), 245
(4.42), and 394 (3.64) nm; IR (dry film) νmax 3343, 1666, and 1644
cm−1; 1H NMR (CDCl3, 400 MHz) δ 0.55 (1H, t, J = 7.4 Hz, H-18),
0.87 (1H, m, H-15b), 0.97 (1H, dq, J = 13.1, 7.4 Hz, H-19b), 1.27
(1H, dq, J = 13.1, 7.4 Hz, H-19a), 1.31 (1H, m, H-17b), 1.54 (1H, m,
H-14b), 1.73 (2H, m, H-15a, H-16b), 1.81 (1H, m, H-16a), 1.84 (1H,
m, H-14a), 3.67 (1H, ddd, J = 14, 4, 2 Hz, H-3b), 3.99 (1H, ddd, J =
14, 11, 4 Hz, H-3a), 4.02 (1H, br s, OH), 5.52 (1H, br s, H-2), 5.67
(1H, s, H-6), 6.60 (1H, br d, J = 8.2 Hz, H-12), 6.68 (1H, br t, J = 7.8
Hz, H-10), 6.99 (1H, dd, J = 7.8, 1 Hz, H-9), 7.15 (1H, td, J = 8.2, 1
Hz, H-11); 13C NMR (CDCl3, 100 MHz) δ 8.3 (CH, C-18), 18.1
(CH2, C-14), 21.5 (CH2, C-17), 23.8 (CH2, C-16), 24.0 (CH2, C-15),
29.6 (CH2, C-19), 35.8 (CH2, C-3), 42.3 (C, C-20), 76.1 (CH, C-2),
94.7 (C, C-21), 108.2 (CH, C-12), 117.3 (CH, C-6), 119.3 (CH, C-
10), 120.0 (C, C-8), 122.7 (CH, C-9), 131.3 (CH, C-11), 153.7 (C, C-
13), 166.1 (C, C-7), 177.1 (C, C-5); ESIMS m/z 311 [M + H]+;
HRESIMS m/z [M + H]+ 311.1750 (calcd for C19H22N2O2 + H,
311.1754).
Bromination of Leuconolam (1). Leuconolam (1) (11 mg, 0.034
mmol) was dissolved in CHCl3 (4 mL), and Br2 (2.6 μL, 0.051 mmol)
was added dropwise at rt. After being stirred for 14 h, the mixture was
quenched with 10% NaHSO3 or Na2CO3 (10 mL), extracted with
CHCl3 (3 × 5 mL), washed with H2O, and dried (Na2SO4), the
solvent removed in vacuo, and the residue purified by centrifugal
preparative TLC (SiO2, 5% MeOH/CHCl3, NH3-saturated) to give
6β,7β-dibromoleuconoxine (13) (13.7 mg, 86%) as a white,
amorphous solid; [α]25 −38 (c 0.62, CHCl3) [lit.2 [α]25 −32 (c
D
D
0.5, CHCl3)]; UV (EtOH) λmax (log ε) 208 (4.32), 227 (4.22), and
1
292 (3.35) nm; IR (dry film) νmax 1709 and 1691 cm−1; H NMR
(CDCl3, 400 MHz) δ 0.94 (1H, t, J = 7 Hz, H-18), 1.56 (1H, m, H-
14b), 1.60 (1H, m, H-14a), 1.62 (1H, m, H-15b), 1.73 (1H, m, H-
19b), 1.98 (1H, m, H-19a), 2.03 (1H, m, H-17b), 2.23 (1H, m, H-
17a), 2.64 (1H, m, H-16b), 2.73 (1H, m, H-3b), 2.75 (1H, m, H-15a),
2.82 (1H, m, H-16a), 4.08 (1H, ddd, J = 13.5, 4, 2 Hz, H-3a), 5.17
(1H, s, H-6), 7.24 (1H, dt, J = 7.2, 1 Hz, H-10), 7.33 (1H, m, H-9),
7.36 (1H,m, H-11), 7.80 (1H, dd, J = 7.2, 1 Hz, H-12); 13C NMR
(CDCl3, 100 MHz) δ 7.0 (CH, C-18), 19.6 (CH2, C-14), 24.5 (CH2,
C-15), 25.5 (CH2, C-17), 28.0 (CH2, C-19), 29.4 (CH2, C-16), 38.7
(CH2, C-3), 39.2 (C, C-20), 50.6 (CH, C-6), 63.7 (C, C-7), 100.5 (C,
C-21), 120.9 (CH, C-12), 123.8 (CH, C-10), 126.5 (CH, C-9), 130.4
(CH, C-11), 136.9 (C, C-8), 139.2 (C, C-13), 164.3 (C, C-5), 172.4
(C, C-2); ESIMS m/z 467 [M + H]+.
Attempted Enolate-Mediated C-6 Oxidation of Leuconoxine
(3). A solution of 3 (11 mg, 0.035 mmol) in THF (5 mL) was added
to a solution of LDA (27 μL, 2 M in THF) in THF (10 mL) at 0 °C,
and the resulting mixture was stirred for 30 min. Dry O2 was bubbled
into the solution for 10 min. A Na2SO3 solution (1 M, 2 mL) was
Bromination of 6,7-Dehydroleuconoxine (10). 6,7-Dehydro-
leuconoxine (10) (7 mg, 0.021 mmol) was dissolved in CHCl3 (4
mL), Br2 (1.2 μL, 0.032 mmol) was added dropwise at rt, and the
mixture was stirred for 13 h. The mixture was quenched with 10%
336
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
added, and the mixture extracted with CH2Cl2 (3 × 10 mL), dried
(Na2SO4), and concentrated in vacuo. The resulting residue was
purified by centrifugal preparative TLC (SiO2, 5% MeOH/Et2O, NH3-
saturated) to afford compound 17 (2.4 mg, 21%) and recovered 3 (7.6
mg, 69%).
(c 0.03, CHCl3)); 1H NMR (CDCl3, 400 MHz) δ 0.92 (1H, t, J = 7.4
Hz, H-18), 1.23 (1H, dq, J = 13, 7.4 Hz, H-19b), 1.49 (1H, dq, J = 13,
7.4 Hz, H-19a), 1.66 (1H, td, J = 14, 6 Hz, H-17b), 1.71 (3H, m, H-
14a, H-14b, H-15b), 1.98 (1H, ddd, J = 14, 6.5, 1.4 Hz, H-17a), 2.05
(1H, m, H-15a), 2.59 (1H, ddd, J = 19, 6, 1.4 Hz, H-16b), 2.86 (1H,
ddd, J = 19, 14, 6.5 Hz, H-16a), 3.10 (1H, ddd, J = 13, 11, 4 Hz, H-
3b), 4.11 (1H, ddt, J = 13, 5, 2.3 Hz, H-3a), 4.23 (1H, s, H-7), 7.16
(1H, td, J = 7.6, 1 Hz, H-10), 7.22 (1H, dd, J = 7.6, 1 Hz, H-9), 7.37
(1H, td, J = 7.6, 1 Hz, H-11), 7.82 (1H, dd, J = 7.6, 1 Hz, H-12); 13C
NMR (CDCl3, 100 MHz) δ 7.3 (CH, C-18), 20.1 (CH2, C-14), 26.3
(CH2, C-15), 26.6 (CH2, C-17), 27.7 (CH2, C-19), 29.5 (CH2, C-16),
37.6 (C, C-20), 37.8 (CH2, C-3), 53.4 (CH, C-7), 88.0 (C, C-21),
121.0 (CH, C-12), 125.1 (CH, C-9), 125.9 (CH, C-10), 126.2 (C, C-
8), 129.9 (CH, C-11), 142.6 (C, C-13), 157.5 (C, C-5), 172.2 (C, C-
2), 192.5 (C, C-6); ESIMS m/z [M + H]+ 325.
X-ray Crystallographic Analysis of 1, 6, 7, 10, 11, 15, 16, and
17. X-ray diffraction analysis was carried out on a Bruker SMART
APEX II CCD area detector system equipped with a graphite
monochromator and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å),
at 100 or 298 K. The structure was solved by direct methods
(SHELXS-97) and refined with full-matrix least-squares on F2
(SHELXL-97). All non-hydrogen atoms were refined anisotropically,
and all hydrogen atoms were placed in idealized positions and refined
as riding atoms with relative isotropic parameters. Crystallographic
data for compounds 1, 6, 7, 10, 11, 15, 16, and 17 have been deposited
with the Cambridge Crystallographic Data Centre. Copies of the data
can be obtained, free of charge, on application to the Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 (0)1223-336033,
or e-mail: deposit@ccdc.cam.ac.uk).
Crystallographic data of 1: colorless block crystals, C19H22N2O3·
CH3OH, Mr = 358.43, orthorhombic, space group P212121, a =
8.1771(1) Å, b = 10.8938(2) Å, c = 19.8901(3) Å, V = 1771.80(5) Å3,
T = 100 K, Z = 4, Dcalcd = 1.344 g cm−3, crystal size 0.25 × 0.32 × 0.43
mm3, F(000) = 768.0. The final R1 value is 0.0310 (wR2 = 0.0891) for
3896 reflections [I > 2σ(I)]. CCDC number: 970038.
Crystallographic data of 6: colorless block crystals, 2C19H22N2O3·
3H2O, Mr = 704.80, triclinic, space group P1, a = 10.1848(2) Å, b =
10.3620(2) Å, c = 10.3848(2) Å, α = 71.7420(10)°, β = 67.3780(10)°,
γ = 60.6470(10)°, V = 871.11(3) Å3, T = 100 K, Z = 2, Dcalcd = 1.344 g
cm−3, crystal size 0.10 × 0.21 × 0.6102 mm3, F(000) = 376.0. The final
R1 value is 0.0562 (wR2 = 0.1185) for 6370 reflections [I > 2σ(I)].
CCDC number: 970039.
Crystallographic data of 7: colorless block crystals, C19H22N2O3,
Mr = 326.39, orthorhombic, space group P212121, a = 9.6652(5) Å, b =
16.1799(9) Å, c = 21.1959(11) Å, V = 3314.7(3) Å3, T = 100 K, Z = 8,
Dcalcd = 1.308 g cm−3, crystal size 0.15 × 0.27 × 0.31 mm3, F(000) =
1392.0. The final R1 value is 0.0356 (wR2 = 0.0877) for 4999
reflections [I > 2σ(I)]. CCDC number: 970040.
Crystallographic data of 10: colorless block crystals, C19H20N2O2,
Mr = 308.37, orthorhombic, space group P212121, a = 8.8855(4) Å, b =
11.3940(5) Å, c = 14.8635(7) Å, V = 1504.80 (12) Å3, T = 100 K, Z =
4, Dcalcd = 1.361 g cm−3, crystal size 0.26 × 0.34 × 0.48 mm3, F(000) =
656.0. The final R1 value is 0.0374 (wR2 = 0.0929) for 2316 reflections
[I > 2σ(I)]. CCDC number: 970041.
Crystallographic data of 11: yellowish block crystals,
2C19H22N2O3·CH2Cl2, Mr = 737.70, monoclinic, space group P21, a
= 8.00860(10) Å, b = 14.9302(3) Å, c = 15.3044(3) Å, α = γ, β =
94.6480(10)°, V = 1823.93(6) Å3, T = 100 K, Z = 2, Dcalcd = 1.447
gcm−3, crystal size 0.04 × 0.17 × 0.63 mm3, F(000) = 780.0. The final
R1 value is 0.0460 (wR2 = 0.0998) for 10151 reflections [I > 2σ(I)].
CCDC number: 970042.
Crystallographic data of 15: yellowish needles, C19H22N2O, Mr =
294.39, orthorhombic, space group P212121, a = 11.2107(6) Å, b =
11.5443(6) Å, c = 12.1199(7) Å, V = 1568.55(15) Å3, T = 100 K, Z =
4, Dcalcd = 1.247 g cm−3, crystal size 0.02 × 0.60 × 0.90 mm3, F(000) =
632.0. The final R1 value is 0.0379 (wR2 = 0.0985) for 2057 reflections
[I > 2σ(I)]. CCDC number: 970043.
Compound 17: colorless oil and subsequently as colorless needles
from CH2Cl2/hexanes; mp 184−186 °C; [α]25 −29 (c 0.2, CHCl3);
D
UV (EtOH) λmax (log ε) 210 (4.10), 241 (3.88), and 274 (3.23) nm;
IR (dry film) νmax 3417 and 1675 cm−1; 1H NMR (CDCl3, 400 MHz)
δ 0.88 (1H, t, J = 7.3 Hz, H-18), 1.38 (1H, de, J = 14.5, 7.1 Hz, H-
19b), 1.50 (1H, m, H-17b), 1.51 (1H, m, H-14b), 1.56 (1H, m, H-
14a), 1.66 (2H, m, H-15b, H-19a), 1.82 (1H, ddd, J = 14.5, 11, 4 Hz,
H-15a), 2.26 (1H, dd, J = 13, 6 Hz, H-17a), 2.56 (1H, d, J = 17 Hz, H-
6b), 2.69 (1H, ddd, J = 13.5, 4.5, 1.5 Hz, H-3b), 2.77 (1H, dd, J = 17,
7.8 Hz, H-6a), 3.81 (1H, m, H-3a), 3.83 (1H, d, J = 7.8 Hz, H-7), 4.45
(1H, dd, J = 13, 6 Hz, H-16), 7.13 (1H, m, H-10), 7.21 (1H, m, H-11),
7.22 (1H, m, H-9), 7.60 (1H, br d, J = 7.8 Hz, H-12); 13C NMR
(CDCl3, 100 MHz) δ 7.5 (CH, C-18), 20.0 (CH2, C-14), 28.1 (CH2,
C-15), 29.3 (CH2, C-19), 35.9 (CH2, C-17), 36.8 (CH2, C-3), 37.4
(CH2, C-6), 38.9 (C, C-20), 42.4 (CH, C-7), 64.9 (CH, C-16), 93.7
(C, C-21), 120.9 (CH, C-12), 124.0 (CH, C-9), 126.3 (CH, C-10),
128.0 (CH, C-11), 135.3 (C, C-8), 140.9 (C, C-13), 171.0 (C, C-5),
175.0 (C, C-2); ESIMS m/z 327 [M + H]+; HRESIMS m/z [M + H]+
327.1710 (calcd for C19H22N2O3 + H, 327.1703).
Reaction of Leuconolam (1) with Trifluoroacetic Acid. To a
stirred solution of 1 (11 mg, 0.034 mmol) and CH2Cl2 (5 mL) was
added TFA (9.5 μL, 0.068 mmol). The mixture was stirred for 13 h at
rt, quenched with 10% Na2CO3 (10 mL), and extracted with CH2Cl2
(3 × 5 mL). The combined organic extract was washed with H2O (3 ×
10 mL), dried (Na2SO4), and concentrated in vacuo, and the residue
purified by centrifugal preparative TLC (SiO2, 5% MeOH/Et2O, NH3-
saturated) to give 6,7-dehydroleuconoxine (2) (4.1 mg, 37%) and
recovered leuconolam (1) (5.8 mg, 53%).
Reaction of Leuconolam (1) with Excess Trifluoroacetic
Acid. To a stirred solution of 1 (13 mg, 0.04 mmol) and CH2Cl2 (5
mL) was added TFA (60 μL, 0.8 mmol). The mixture was stirred for
12 h at rt, quenched with 10% Na2CO3 (10 mL), and extracted with
CH2Cl2 (3 × 5 mL). The combined organic extract was washed with
H2O (3 × 10 mL), dried (Na2SO4), and concentrated in vacuo, and the
residue purified by centrifugal preparative TLC (SiO2, 5% MeOH/
Et2O, NH3-saturated) to 6,7-dehydroleuconoxine (2) (3.9 mg, 30%),
leuconodine A (14) (3.3 mg, 25%), and recovered 1 (1.2 mg, 9%).
Leuconodine A (14): colorless oil and subsequently as colorless
block crystals from EtOH; mp 134−136 °C (lit.9 135−138 °C); [α]25
D
9
1
−18 (c 0.03, CHCl3) (lit. −20 (c 0.26, CHCl3)); H NMR (CDCl3,
400 MHz) δ 0.90 (1H, t, J = 7.3 Hz, H-18), 1.49 (1H, dq, J = 13, 7.3
Hz, H-19b), 1.60 (1H, m, H-17b), 1.64 (1H, m, H-15b), 1.70 (2H, m,
H-14a, H-14b), 1.92 (1H, m, H-15a), 1.94 (1H, m, H-17a), 1.96 (1H,
m, H-19a), 2.53 (1H, ddd, J = 19, 6, 1.4 Hz, H-16b), 2.78 (1H, ddd, J
= 19, 14, 6.5 Hz, H-16a), 2.89 (1H, ddd, J = 13, 11, 4 Hz, H-3b), 3.90
(1H, s, H-7), 3.99 (1H, ddd, J = 13, 5, 2 Hz, H-3a), 4.51 (1H, s, H-6),
7.13 (1H, td, J = 7.8, 1 Hz, H-10), 7.25 (1H, td, J = 7.8, 1 Hz, H-11),
7.27 (1H, dd, J = 7.8, 1 Hz, H-9), 7.87 (1H, dd, J = 7.8, 1 Hz, H-12);
13C NMR (CDCl3, 100 MHz) δ 7.7 (CH, C-18), 19.4 (CH2, C-14),
27.3 (CH2, C-15), 27.5 (CH2, C-17), 28.5 (CH2, C-19), 30.2 (CH2, C-
16), 36.7 (C, C-20), 36.8 (CH2, C-3), 49.6 (CH, C-7), 75.1 (CH, C-
6), 93.5 (C, C-21), 119.6 (CH, C-12), 124.5 (CH, C-9), 125.4 (CH,
C-10), 128.3 (CH, C-11), 132.1 (C, C-8), 141.9 (C, C-13), 172.0 (C,
C-5), 173.1 (C, C-2); ESIMS m/z [M + H]+ 327.
Oxidation of Leuconodine A (14). A solution of 14 (7 mg, 0.021
mmol) in CH2Cl2 (5 mL) was treated with the Dess−Martin
periodinane (82 μL, 0.3 M in CH2Cl2), and the mixture was stirred at
rt for 30 min. Et2O (25 mL) and NaOH (10 mL, 1.3 M) were added,
and the mixture was stirred for another 15 min. The aqueous layer was
removed, the organic layer was washed with 1.3 M NaOH (2 × 10
mL) and dried with Na2SO4, the solvent was removed in vacuo, and
the residue was purified by centrifugal preparative TLC (SiO2, 5%
MeOH/Et2O, NH3-saturated) to give leuconodine F (19) (5.3 mg,
76%) as a colorless oil and subsequently as colorless block crystals
from MeOH: mp 246−250 °C; [α]25D +94 (c 0.05, CHCl3) (lit.29 +75
Crystallographic data of 16: fluorescent yellowish rods,
C19H24N2O2, Mr = 312.40, trigonal, space group P31, a = b =
11.4602(2) Å, c = 10.0616(2) Å, α = β, γ = 120.00°, V = 1144.41(4)
337
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338

Journal of Natural Products
Article
Å3, T = 100 K, Z = 3, Dcalcd = 1.351 g cm−3, crystal size 0.20 × 0.20 ×
0.70 mm3, F(000) = 504.0. The final R1 value is 0.1315 (wR2 = 0.3669)
for 3270 reflections [I > 2σ(I)]. CCDC number: 970044.
Crystallographic data of 17: colorless needles, C19H22N2O3, Mr =
326.39, orthorhombic, space group P212121, a = 7.1721(4) Å, b =
26.1619(13) Å, c = 27.9882(15) Å, V = 5251.6(5) Å3, T = 298 K, Z =
12, Dcalcd = 1.238 g cm−3, crystal size 0.02 × 0.08 × 0.68 mm3, F(000)
= 2088.0. The final R1 value is 0.0508 (wR2 = 0.1153) for 3171
reflections [I > 2σ(I)]. CCDC number: 970045.
(16) See Supporting Information.
(17) Banwell, M. G.; Edwards, A. J.; Jolliffe, K. A.; Smith, J. A.;
Hamel, E.; Verdier-Pinard, P. Org. Biomol. Chem. 2003, 1, 296−305.
(18) Banwell, M. G.; Beck, D. A. S.; Willis, A. C. Arkivoc 2006, iii,
163−174.
(19) Dec
Ouazzani, J.; Cresteil, T.; Guer
2006, 14, 1558−1564.
́
or, A.; Bellocq, D.; Thoison, O.; Lekieffre, N.; Chiaroni, A.;
́
itte, F.; Baudoin, O. Bioorg. Med. Chem.
(20) Feng, T.; Cai, X. H.; Liu, Y. P.; Li, Y.; Wang, Y. Y.; Luo, X. D. J.
Nat. Prod. 2010, 73, 22−26.
ASSOCIATED CONTENT
(21) (a) Anikin, V. F.; Veduta, V. V.; Merz, A. Monatsh. Chem. 1999,
130, 681−690. (b) Bellucci, G.; Chiappe, C.; Bianchini, R.; Lemmen,
P.; Lenoir, D. Tetrahedron 1997, 53, 785−790.
(22) (a) Ruasse, M. F. Adv. Phys. Org. Chem. 1993, 28, 207−291.
(b) Melloni, G.; Modena, G.; Tonellato, U. Acc. Chem. Res. 1981, 14,
227−233.
■
S
* Supporting Information
1H and 13C NMR spectra for compounds 1, 3, 6−8, 10−11,
13−17, and 19. X-ray crystallographic data in CIF format for
compounds 1, 6, 7, 10, 11, 15, 16, and 17. This material is
available free of charge via the Internet at http://pubs.acs.org.
(23) Herges, R.; Papafilippopoulos, A.; Hess, K.; Chiappe, C.; Lenoir,
D.; Detert, H. Angew. Chem., Int. Ed. 2005, 44, 1412−1416.
(24) (a) Brown, H. C.; Kramer, G. W.; Levy, A. B.; Midland, M. M.
In Organic Syntheses via Boranes; John Wiley and Sons: New York;
1975. (b) Braun, L. M.; Braun, R. A.; Crissman, H. R.; Opperman, M.;
Adams, R. M. J. Org. Chem. 1982, 36, 2388−2389. (c) Brown, H. C.;
Choi, Y. M.; Narasimhan, S. J. Org. Chem. 1982, 47, 3153−3163.
(25) A possible origin of 16 derives from hydrolysis (due to traces of
water) of the alkaloid-borane complex, formed after the initial hydride
transfer.
AUTHOR INFORMATION
Corresponding Author
*Tel: 603-79674266. Fax: 603-79674193. E-mail: tskam@um.
edu.my.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the University of Malaya and MOHE Malaysia (HIR-
005) for financial support.
(26) Hu, W. H.; Tsai, P. C.; Hsu, M. K.; Wang, J. J. J. Org. Chem.
2004, 69, 3983−3985.
■
(27) Wasserman, H. H.; Lipshutz, B. H. Tetrahedron Lett. 1975, 16,
1731−1734.
(28) Overman, L. E. J. Am. Chem. Soc. 1974, 96, 597−599.
(29) Lim, S. H.; Sim, K. M.; Abdullah, Z.; Hiraku, O.; Hayashi, M.;
Komiyama, K.; Kam, T. S. J. Nat. Prod. 2007, 70, 1380−1383.
REFERENCES
■
(1) (a) Linde, H. H. A. Helv. Chim. Acta 1965, 48, 1822−1842.
(b) Banerji, A.; Majumder, P. L.; Chatterjee, A. G. Phytochemistry
1970, 9, 1491−1493. (c) De Silva, K. T.; Ratcliffe, A. H.; Smith, G. F.;
Smith, G. N. Tetrahedron Lett. 1972, 13, 913−916.
(2) (a) Goh, S. H.; Chen, W.; Ali, A. R. M. Tetrahedron Lett. 1984,
25, 3483−3484. (b) Chen, W.; Mohd. Ali, A. R.; Goh, S. H.; Sinn, E.;
Butcher, R. J. Acta Crystallogr. 1986, C42, 349−351. (c) Goh, S. H.;
Ali, A. R. M.; Wong, W. H. Tetrahedron 1989, 45, 7899−7920.
(3) Abe, F.; Yamauchi, T. Phytochemistry 1994, 35, 169−171.
(4) Kam, T. S.; Lim, K. H. In The Alkaloids: Chemistry and Biology;
Cordell, G. A., Ed.; Academic Press: The Netherlands, 2008; Vol. 66,
pp 1−111.
(5) Kam, T. S.; Tee, Y. M.; Subramaniam, G. Nat. Prod. Lett. 1998,
12, 307−310.
(6) Kam, T. S.; Subramaniam, G.; Chen, W. Phytochemistry 1999, 51,
159−169.
(7) Lim, K. H.; Kam, T. S. Helv. Chim. Acta 2007, 90, 31−35.
(8) Kam, T. S.; Subramaniam, G.; Lim, K. H.; Choo, Y. M.
Tetrahedron Lett. 2004, 45, 5995−5998.
(9) Gan, C. Y.; Low, Y. Y.; Thomas, N. F.; Kam, T. S. J. Nat. Prod.
2013, 76, 957−964.
(10) (a) Magolan, J.; Carson, C. A.; Kerr, M. A. Org. Lett. 2008, 10,
1437−1440. (b) Biechy, A.; Zard, S. Z. Org. Lett. 2009, 11, 2800−
2803. (c) Nakajima, R.; Ogino, R.; Yokoshima, S.; Fukuyama, T. J. Am.
Chem. Soc. 2010, 132, 1236−1237. (d) Iwama, Y.; Okano, K.;
Sugimoto, K.; Tokuyama, H. Org. Lett. 2012, 14, 2320−2322.
(e) Iwama, Y.; Okano, K.; Sugimoto, K.; Tokuyama, H. Chem.
Eur. J. 2013, 19, 9325−9334.
(11) (a) Bernauer, K.; Englert, G.; Vetter, W.; Weiss, E. Helv. Chim.
Acta 1969, 52, 1886−1905. (b) Oberhansli, W. E. Helv. Chim. Acta
̈
1969, 52, 1905−1911.
(12) The original report claimed formation of a single product, 5,
with a yield of 80%. We were not able to reproduce the reported yield.
(13) Sim, K. M. Alkaloids from Holarrhena, Kopsia, and
Tabernaemontana. Ph.D. Thesis, University of Malaya, Kuala Lumpur,
March 2001.
(14) Measured on a Kratos MS 3074 mass spectrometer.
(15) Measured on a VG ProSpec mass spectrometer.
338
dx.doi.org/10.1021/np400922x | J. Nat. Prod. 2014, 77, 327−338