Synthesis and Absolute Stereochemical Reassignment of Mukanadin F: A Study of Isomerization of Bromopyrrole Alkaloids with Implications on Marine Natural Product Isolation

Article abstract of DOI:10.1002/ejoc.201800422

Synthesis of both enantiomers of mukanadin F was achieved by using a seven step synthesis. Comparison of the optical rotation data of synthetic samples to that reported for the isolated natural product determined that the absolute configuration of the natural product is 9S and not the reported 9R. Further studies established that the reported low magnitude of optical rotation in the isolated sample is due to compounds of this type undergoing isomerization and racemization under benign laboratory conditions. Additionally the synthetic methods developed were applied to synthesize mukanadins B and D.

Full text of DOI:10.1002/ejoc.201800422

A Journal of
Accepted Article
Title: Synthesis and absolute stereochemical reassignment of
mukanadin F: A study of isomerization of bromopyrrole alkaloids
with implications on marine natural product isolation
Authors: Michelle van Rensburg, Brent Copp, and David Barker
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800422
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800422
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
Synthesis and absolute stereochemical reassignment of mukanadin F:
A study of isomerization of bromopyrrole alkaloids with implications
on marine natural product isolation.
Michelle van Rensburg, Brent R. Copp and David Barker*
School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
E-mail: d.barker@auckland.ac.nz https://unidirectory.auckland.ac.nz/people/profile/d-barker
Abstract
Synthesis of both enantiomers of mukanadin F (1) was achieved using a seven step synthesis.
Comparison of the optical rotation data of synthetic samples to that reported for the isolated natural
product determine that the absolute configuration of the natural product is 9S and not the reported 9R.
Further studies established that the reported low magnitude of optical rotation in the isolated sample is
due to compounds of this type undergoing isomerization and racemization under benign laboratory
conditions. Additionally the synthetic methods developed were applied to synthesize mukanadins B (6)
and D (7).
Bromopyrrole alkaloids are a diverse family of marine natural products that have been identified as
secondary metabolites of marine sponges.[1] Since the mid-1970’s, there have been roughly 140
compounds classified as bromopyrrole alkaloids with structures ranging from simple ‘linear’ to more
complicated polycyclic derivatives. These natural products have been attributed with a wide range of
biological activities and as such have been of great interest to both natural product and synthetic
chemists.
A number of C-9 substituted ‘linear’ bromopyrrole alkaloids (Figure 1) have been isolated from
sponges of the orders Agelasida and Axinellida. It is interesting to note that these alkaloids were mostly
[
2–6]
reported as either racemic or scalemic mixtures.
Previous studies investigating the racemization of
[5]
compounds of this type were attempted using debromomukanadin A (3) but were inconclusive.
Mukanadin F (1) is unique amongst this set in that it was isolated as a single enantiomer with 9R
configuration established using Mosher’s esters.[2] Due to the unique stereochemical nature of
mukanadin F (1), preparation of both enantiomers was undertaken firstly to confirm its absolute
stereochemistry and secondly to give insight into why it alone, in this compound class, has been isolated
non-racemically.
1
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
Figure 1. Selected examples of marine bromopyrrole alkaloids.
Both enantiomers of mukanadin F, (S)-1 and (R)-1, were successfully prepared following the
synthesis depicted in Scheme 1. The synthesis of (S)-mukanadin F (S)-1 began with Boc protection of
(
R)-amino diol (R)-8 to give carbamate (R)-9, followed by selective primary alcohol protection giving
TBDMS ether (R)-10.[ PMB protection of the remaining alcohol in (R)-10 gave the desired tri-
protected (R)-11 as well as, unexpectedly, the deprotected primary alcohol (R)-12 and side products,
oxazolidinones 13 and 14. TBDMS-ether (R)-11 was converted into further alcohol (R)-12, which
underwent Swern oxidation, to give an unstable aldehyde, followed by a Horner-Wadsworth-Emmons
reaction with hydantoin phosphonate 15 to give hydantoin (S)-16 as a mixture of E/Z isomers (E/Z,
7,8]
1:2). Next, simultaneous Boc and PMB deprotection was accomplished under acidic conditions to give
amine salt (S)-17, which was used immediately in the subsequent pyrrole coupling reaction with
dibromopyrrole 18,[ giving (S)-mukanadin F (S)-1 as a mixture of E/Z isomers (E/Z, 1:1.3). (R)-
Mukanadin F (R)-1 was synthesized, as a 1:2 mixture of E/Z isomers, following the same procedure
described for (S)-1, starting from (S)-amino diol (S)-8 (Scheme 2).
9]
2
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
Scheme 1. Synthesis of (S)-mukanadin F (S)-1.
Scheme 2. Synthesis of (R)-mukanadin F (R)-1.
Purification of the E/Z isomers of 1 using flash chromatography proved troublesome with samples
containing only the suspected E-isomer being found to contain trace amounts of the Z-isomer upon
NMR analysis, suggesting instability of the E-isomer (see below). As a result, only samples of (S,E)-,
(
(
S,Z)- and (R,Z)-mukanadin F were able to be obtained in pure form. The olefin geometry of (S,E)- and
S,Z)-mukanadin F was confirmed by analysis of JC-H values (obtained via proton coupled C NMR
3
13
acquisition). In the case of (S,E)-mukanadin F, the H-10 - C-12 coupling constant was determined to be
.9 Hz, while for (S,Z)-mukanadin F it was 5.6 Hz (Figure 2); these values fall within the appropriate
9
[5]
ranges for E- and Z- isomers respectively. Comparison of NMR data of synthetic Z samples of
mukanadin F (1) matched those reported for the natural product, confirming the assigned alkene
geometry.[ Comparing the [α]
2]
values, neither synthetic R,Z-1 (+22.2 (c = 0.3, MeOH)) nor S,Z-1
D
(
-26.7 (c = 0.3, MeOH)) had rotations that matched the reported value for the natural product
3
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
[2]
(
-3.9 (c = 0.3, MeOH)), and both were notably higher in magnitude. The negative rotation value for
synthetic S,Z-1 suggests the natural product, albeit with its lower negative value, has 9S stereochemistry,
which is opposite to that proposed for the isolated natural sample.
Figure 2. Long range coupling constants determining E- and Z-mukanadin F (S)-1.
The noted difficultly in purifying samples of E-1, combined with inconsistencies in optical rotation
data between synthetic and natural samples, suggested that isomerization and racemization are facile
amongst this compound class. To determine the extent to which either/both of these processes occur
during natural product isolation studies, we initially examined the propensity of simpler, achiral
bromopyrrole alkaloids containing this ene-hydantoin to undergo E/Z isomerization, noting that all have
been isolated as Z-isomers.
Using similar methodology to that used to prepare 1, the synthesis of 24, debromo-dehydroxy-
mukanadin F, commenced with Boc protection of amino alcohol 19 followed by Swern oxidation giving
aldehyde 20 (Scheme 3). Aldehyde 20 then underwent a Horner-Wadsworth-Emmons reaction with
phosphonate 15 to give hydantoin 21 as a 1:1 mixture of E and Z isomers. The 1:1 E:Z mixture of 21
then underwent Boc deprotection, followed by immediate coupling with pyrrole 23 to give 24 as a
mixture of E/Z isomers (1:1.5, E:Z). Using this same method we prepared mukanadin B (6) and
mukanadin D (7) using pyrroles 22 or 18, respectively.
All were obtained as mixtures of isomers, with Z always being the major isomer. It was noted that
the isomeric ratio became enriched in Z after chromatographic purification. After separation the
1
3
assignment of E and Z geometry was again confirmed by performing proton coupled C NMR
experiments. Mukanadin D (7) has been synthesized previously but as a mixture of isomers, whilst
mukanadin B (6) has not been previously prepared. These syntheses along with successful separation
of their respective Z-isomers, enables confirmation of the geometry of the Z-alkene originally assigned
to the natural products.[
3,10,11]
4
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
Scheme 3. Synthesis of mukanadin B (6).
Isomerization studies were carried out on the simplest prepared mukanadin, 24. A
chromatographically obtained, E-enriched sample, of 24 was dissolved in deuterated methanol and
stored in a NMR tube exposed to sunlight. Time course analysis showed that the proportion of Z-isomer
increased over time, highlighting facile olefin isomerization under these conditions (Figure 3) which
are commonly encountered in the extraction and purification of marine natural products. Interestingly,
when a similar time course experiment was conducted on a sample of Z-24, no Z to E isomerization was
observed (data not shown). Calculations using Gaussian software show that Z-24 has a lower Gibbs free
-1
energy compared to the E-24 (Δ 27.29 kJ mol ), and hence, is the more stable isomer (see
Supplementary for details).
5
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
1
Figure 3. Isomerization of 24. H NMR spectra excerpts of E- and Z-24 highlighting changes in H-9 and H-10 ratios over
time. (A) T = 0 days or (B) T = 53 days after initial purification.
Having observed the isomerization of E-24 to Z-24 in methanol, similar isomerization studies were
performed on mukanadin F (1), to establish if this process also occurs for 9-substituted bromopyrrole
alkaloids. (S,E)-Mukanadin F was dissolved in deuterated methanol and exposed to a range of
conditions, described in Table 1. As observed for 24, (S,E)-mukanadin F underwent E/Z isomerization
in methanol and sunlight as well as with the other three conditions (acidic, basic, exclusion of light)
trialed, with all the samples becoming enriched with Z-isomer over time. It was observed that the rate
of isomerization was greatest in acidic conditions. Again, no Z to E isomerization was observed for
(
S,Z)-mukanadin F.
Table 1. Isomerization of (S,E)-mukanadin F (S)-1.
E/Z ratio
Day 7
Br
O
Day 0
Day 39
3.0:1.0
2.4:1.0
0.7:1.0
NH
OH
H
Without sunlighta
With sunlighta
TFAa,b,c
10.0:1.0
10.0:1.0
10.0:1.0
10.0:1.0
3.1:1.0
3.0:1.0
0.9:1.0
2.5:1.0
Br
O
N
N
N
H
H
O
(
S,E)-mukanadin F (S)-1
Et
3
Na,b
2.4:1.0
1
a Samples were approx. 4.4 mM in deuterated methanol. b one drop of each reagent was added to a 2.5 mL sample. c H NMR
analysis also indicated the formation of a unidentifiable new species (1%) as well as isomerization.
6
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
Based on the isomerization findings, further experiments were performed to study the effects of the
four conditions on the optical rotation of the prepared samples (Figure 4). Four samples each of (S,Z)-
and (S,E)-mukanadin F (1) (4.4 mM) were prepared in deuterated methanol, and exposed to the same
four conditions; sunlight, protected from light, presence of TFA, and presence of Et N (Figure 4). Under
3
all conditions (S,Z)-mukanadin F was found to racemize over time to give a scalemic mixture, with
sunlight having the greatest effect on the rate of racemization. This change in the optical rotation can
only be due to the racemization at C-9, as no Z to E isomerization was observed for Z-isomers. Similarly,
(
S,E)-mukanadin F also racemizes to a scalemic mixture over time, with acidic conditions having the
greatest effect on the rate of racemization. In this case, the change in optical rotation can be attributed
to both the racemization at C-9 as well as E/Z isomerization. The added factor of E/Z isomerization
accounts for the observed change in sign for the optical rotation value. Overall, both E and Z isomers
were found to racemize, with optical rotation values in all cases approaching 0.
Figure 4. Racemization studies of (S,Z)- and (S,E)-mukanadin F (S)-1.
Proposed mechanisms of isomerization and racemization were formulated to further understand the role
methanol could be having on the results we had observed (Figure 5). Isomerization of the alkene in
(
S,E)-mukanadin F (1) may occur through the addition of a nucleophile, such as methanol, to C-10,
giving an enol which could then undergo C-10,11 bond rotation, ultimately allowing the formation of
S,Z)-mukanadin F (1). Racemization of (S,Z)-mukanadin F (1) could proceed through the removal of
(
the acidic proton at C-9. This would lead to the formation of a planar enolate, which upon protonation
would racemize the chiral center at C-9. There is evidence of this second mechanism observed in the
proton NMR where it was observed that relative integral of H-9 to H-10 decreased over time when left
in deuterated methanol solvent, implying that the proton was being replaced by deuterium.
7
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
a)
b)
Figure 5. a) Proposed isomerization mechanism of (S,E)-mukanadin F (1) in methanol. b) Proposed racemization mechanism
of (S,Z)-mukanadin F (1) in methanol.
The results from the isomerization and racemization studies can be used to explain the differences
observed in optical rotation values of isolated natural product to that of synthetic samples of mukanadin
F (1). The isomerization and racemization experiments performed on synthetic 1 suggest that the
isolated natural product was characterized as a scalemic mixture enriched in the 9S enantiomer.[12] These
results establish the ability of C-9 to undergo racemization under normal laboratory conditions. The
potential reactivity at C-9 has recently been observed with the isolation of 9-O-ethyl-mukanadin F, the
8
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
authors suggesting that this compound could in fact be an artefact arising from extraction and isolation
procedures utilizing ethanol.[ Similarly, 9-O-methyl-debromomukanadin F was recently isolated,
using extraction methods utilizing methanol, however, the authors do not speculate as to whether the
new compound could in fact be an artefact.[14] It should be noted that we did not observe any 9-O-
methyl products during our isomerization studies.
13]
The occurrence of artefacts and modifications to natural products as a result of isolation and/or
purification processes have been well documented.[15] Little has been investigated within this group of
alkaloids, with structures reported without mention of possible effects from these processes. These
findings have implications on the isolation and characterization of C-9 functionalized ene-
hydantoin/imidazole marine natural products. The results presented herein suggest that not only
isomerization but also racemization occurs under conditions regularly used during isolation and
purification procedures, and as such, caution needs to be exercised when assigning the C-9
configuration of marine natural products containing this α,β-unsaturated system. In the racemization
studies, extrapolation of the trends see the samples racemizing to give scalemic mixtures with optical
rotation values approaching 0. While many of the chiral marine natural products shown in Figure 1
have been isolated and reported as racemic mixtures, the isolation of mukanadin A (2) as both a
[3]
[4]
scalemic and racemic mixture leads to the possibility that this class of compounds do exist in nature
as pure enantiomers, which upon exposure to isolation and/or purification procedures undergo C-9
racemization. Therefore, care should be taken when assigning the configuration of these types of marine
natural products.
Experimental
General Experimental Procedures. Oven-dried glassware and freshly distilled solvents were used
when carrying out reactions, with reaction vessels purged with an inert gas such as nitrogen unless
stated otherwise. Chemical reagents were not subjected to quality analysis and were used as purchased.
A MicroTOF-Q mass spectrometer was used to perform mass spectrometry analysis on compounds
(
low- and high-resolution), using methods of electrospray ionization (ESI) or chemical ionization (CI).
Nuclear magnetic resonance (NMR) spectroscopic analysis was performed on Bruker 300, 400 or 500
1
MHz spectrometers. H NMR analysis is presented as position (δ), relative integration, multiplicity,
1
3
coupling constant (J, Hz), followed by the assignment of the atom. C NMR analysis is presented as
position (δ) followed by the assignment of the atom. The position (δ) of signals in the NMR are reported
1
13
relative to the signal of either chloroform (δ 7.26 for H and δ 77.00 for C), dimethyl sulfoxide (δ 2.50
1
13
1
13
for H and δ 39.52 for C), or methanol (δ 3.31 for H and δ 49.00 for C) where stated. Assignments
of atoms were performed using 2D NMR techniques of HSQC, COSY and HMBC. All reported
theoretical energy calculations were performed using the Gaussian 09 package. The level of theory
employed for the theoretical calculation was RB3LYP level and DGDZVP level of theory. Due to the
use of methanol in the calculations, SCRF was used in the DFT calculation in order to model the reaction
under solvation conditions. Furthermore, an implicit solvent model, PCM was used. The Gibbs free
-1
energies were reported in hartrees and converted to kJ mol using the conversion 1 hartree = 2625.50
-1
kJ mol .
tert-Butyl (R)-(2,3-dihydroxypropyl)carbamate (R)-9. To a stirred solution of (R)-3-amino-1,2-diol
(
R)-8 (2.5 g, 27 mmol) in dry methanol (75 mL) at 0 ºC was added first triethylamine (3.80 mL, 41
mmol) and then Boc O (9.46 mL, 41 mmol) in portions. The reaction mixture was allowed warm to
2
room temperature and stirred for 21 h before the solvent was removed in vacuo to give the crude product
which was purified by gradient flash chromatography (firstly with 1:4 n-hexanes:ethyl acetate then ethyl
acetate) to afford the title compound (R)-9 (5.38 g, quant.) as a clear colorless oil which solidifies to a
9
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
[7]
white crystalline solid after being frozen. m.p. 51-53 ºC. (lit. m.p. 53-55 ºC). R
f
= 0.30 (ethyl acetate).
[
8]
[
α]
D
-8.9 (c = 1.46, CHCl
3
). (lit. [α]
D
-8.3(c = 1.4, CHCl
3
)). δ
H
(300 MHz, CDCl
3
) 1.43 (9H, s, CMe ),
3
2
4
.90 (2H, br s, 2 x OH), 3.23-3.28 (2H, m, H-1), 3.57 (2H, br d, J = 3.9 Hz, H-3), 3.72 (1H, br p, J =
.8 Hz, H-2), 4.95 (1H, br s, NH). These values were in agreement with literature values.[16] tert-Butyl
(
S)-(2,3-dihydroxypropyl)carbamate (S)-9 was prepared using the same procedure as above, starting
[7]
with (S)-8, giving the title compound as a white solid. m.p. 49-51 ºC. (lit. m.p. 51-52 ºC). [α]
D
+10.8
[7]
(
c = 0.5, CHCl
previously prepared (R)-9.
tert-Butyl (R)-(3-((tert-butyldimethylsilyl)oxy)-2-hydroxypropyl)carbamate (R)-10. To a stirred
solution of diol (R)-9 (5.28 g, 28 mmol) in dry CH Cl (130 mL) at room temperature was added
3
). (lit. [α]
D
+6.7 (c = 0.5, CHCl
3
)). All spectroscopic data was identical to that of the
2
2
imidazole (5.64 g, 83 mmol) and then TBSCl (4.58 g, 30 mmol) in one portion. The reaction mixture
was stirred for 1 h before being quenched with sat. aq. ammonium chloride (30 mL) and extracted with
CH
2
Cl
2
(3 x 30 mL). The combined organic extracts were then dried (MgSO
4
) before the solvent was
removed in vacuo to give the crude product which was purified by flash chromatography (3:1 n-
hexanes:ethyl acetate) to afford the title compound (R)-10 (7.08 g, 84%) as a clear colorless oil. R
f
=
)).[
8]
0
.60 (1:1 n-hexanes:ethyl acetate). [α]
(300 MHz, CDCl ) 0.06 (6H, s, SiMe
OH), 3.11 (1H, ddd, J = 13.8, 6.3, 4.8 Hz, 1-H
Hz, 3-H ), 3.63 (1H, dd, J = 9.9, 4.8 Hz, 3-H
D
+8.2 (c = 0.916, CHCl
), 0.89 (9H, s, SiCMe ), 1.43 (9H, s, OCMe
), 3.31-3.35 (1H, br m, 1-H ), 3.52 (1H, dd, J = 9.9, 6.3
), 3.68-3.77 (1H, m, H-2), 4.94 (1H, br s, NH). These
3
). (lit. [α]
D
+9.5 (c = 0.9, CHCl
3
δ
H
3
2
3
3
), 2.80 (1H, br s,
B
A
B
A
[8]
values were in agreement with literature values. tert-Butyl (S)-(3-((tert-butyldimethylsilyl)oxy)-2-
hydroxypropyl)carbamate (S)-10 was prepared using the same procedure as above, starting with (S)-
9
, giving the title compound as a clear colorless oil. [α]
was identical to that of the previously prepared (R)-10.
tert-Butyl (R)-(3-((tert-butyldimethylsilyl)oxy)-2-((4-methoxybenzyl)oxy)propyl)carbamate (R)-
D
-7.9 (c = 0.92, CHCl ). All spectroscopic data
3
1
1, (R)-5-(((tert-butyldimethylsilyl)oxy)methyl)-3-(4-methoxybenzyl)oxazolidin-2-one (R)-13,
R)-3-(4-methoxybenzyl)-5-(((4-methoxybenzyl)oxy)methyl)oxazolidin-2-one (R)-14 and tert-butyl
R)-(3-hydroxy-2-((4-methoxybenzyl)oxy)propyl)carbamate (R)-12. To a stirred mixture of alcohol
R)-10 (1.0 g, 3.3 mmol), sodium hydride (60% dispersion in oil w/w, 0.14 g, 3.4 mmol) and
(
(
(
tetrabutylammonium iodide (0.12 g, 0.33 mmol) in dry THF (10 mL) at 0 ºC was added PMBCl (0.49
mL, 3.6 mmol) slowly. The reaction mixture was stirred for 1 h at 0 ºC before being allowed to warm
to room temperature and stirred for a further 5 d before being quenched with sat. aq. ammonium chloride
(
5 mL) followed by extraction with ether (3 x 5 mL). The combined organic extracts were then washed
with sat. aq. sodium thiosulphate (10 mL) and dried (MgSO ) before the solvent was removed in vacuo
4
to give the crude product which was purified by flash chromatography (gradient elution first with n-
hexanes then 9:1 n-hexanes:ethyl acetate) to afford the title compound (R)-11 (0.28 g, 20%) as a yellow
oil. R
f
= 0.67 (3:1 n-hexanes:ethyl acetate). [α]
D
+18.6 (c = 4.78, CHCl
) -0.02 and -0.01 (6H, s, SiMe ), 0.01 (6H, s, SiMe *), 0.81 (9H, s, SiCMe
), 1.36 (9H, s, OCMe *), 3.06-3.34 (4H, m, H-1 and H-3*), 3.45
), 3.81-3.84 (1H, m, H-
Ar), 4.46 (1H, d, J = 11.4 Hz, CH Ar), 4.78-
.81 (1H, m, NH*), 4.89-4.92 (1H, m, NH), 6.77-6.80 (4H, m, Ar-H and Ar-H*), 7.15 -7.19 (4H, m, Ar-
(75 MHz, CDCl ) -5.7 and -5.7 (Si(CH and Si(CH *), 17.9 (SiC(CH *), 18.0
), 25.6 (SiC(CH *), 25.7 (SiC(CH ), 28.2 (OC(CH ), 41.4 (C-1), 54.9 (OCH ), 63.7 (C-
and C-1*), 70.1 (C-2*), 71.4 (CH Ar), 72.1 (C-3*), 72.8 (CH Ar*), 77.5 (C-2), 78.6 (OC(CH ),
13.5 (Ar-CH*), 113.6 (Ar-CH), 129.0 (Ar-CH*), 129.1 (Ar-CH), 130.0 (Ar-C*), 130.3 (Ar-C), 155.7
3
). (* denotes minor rotamer)
δ
H
(400 MHz, CDCl
3
2
2
3
*), 0.83
(
(
9H, s, SiCMe
3
), 1.35 (9H, s, OCMe
3
3
1H, p, J = 5.2 Hz, H-2), 3.53-3.64 (4H, m, H-3 and H-1*), 3.69 (3H, s, OCH
3
2
4
*), 4.36 (2H, s, CH
2
Ar*), 4.44 (1H, d, J = 11.4 Hz, CH
B
A
H and Ar-H*). δ
SiC(CH
C
3
3
)
2
3
)
2
)
3 3
(
3
1
3
)
3
3
)
3
3
)
3
3
)
3
3
2
2
)
3 3
-1
(
C=O*), 155.8 (C=O), 159.0 (COCH
3
*), 159.0 (COCH
3
). νmax(ATR)/cm 3364, 2930, 2857, 1715,
+
+
+
1513, 1464, 1365, 1247, 1171, 834. m/z (ESI ): 448 (MNa , 100%), 392 (12%); HRMS (ESI ) found
1
0
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
+
(
MNa ): 448.2476, C22
H
39NNaO
5
Si requires 448.2490. In a separate fraction 5-(((tert-
butyldimethylsilyl)oxy)methyl)-3-(4-methoxybenzyl)oxazolidin-2-one (R)-13 (0.23 g, 20%) was
also isolated, as a clear colourless oil. R = 0.17 (3:1 n-hexanes:ethyl acetate). [α] -28.6 (c = 0.41,
CHCl ). δ (400 MHz, CDCl ) 0.04 (6H, s, SiMe ), 0.84 (9H, s, CMe ), 3.29-3.40 (2H, m, H-4), 3.66
1H, dd, J = 11.0, 3.6 Hz, 6-H ), 3.74 (1H, dd, J = 11.0, 4.8 Hz, 6-H ), 3.79 (3H, s, OCH ), 4.35 (2H,
d, J = 4.0 Hz, H-7), 4.45-4.50 (1H, m, H-5), 6.86 (2H, d, J = 8.6 Hz, Ar-H), 7.20 (2H, d, J = 8.6 Hz, Ar-
H). δ (100 MHz, CDCl ) -5.5 and -5.5 (Si(CH ), 18.2 (C(CH ), 25.7 (C(CH ), 45.3 (C-4), 47.6
C-7), 55.2 (OCH ), 63.4 (C-6), 72.9 (C-5), 114.1 (Ar-CH), 127.8 (Ar-C), 129.5 (Ar-CH), 157.9 (C-2),
f
D
3
H
3
2
3
(
B
A
3
C
3
3
)
2
3
)
3
)
3 3
(
1
1
3
-
1
+
+
59.3 (COCH ). νmax(ATR)/cm 2929, 1744, 1514, 1440, 1361, 1245, 834. m/z (ESI ): 374 (MNa ,
3
+
+
+
00%), 352 (MH , 10%); HRMS (ESI ) Found (MNa ): 374.1794, C18
H
29NNaO Si requires 374.1758.
4
+
Found (MH ): 352.1961, C18
H
30NO Si requires 352.1939. In a separate fraction 3-(4-methoxybenzyl)-
4
5
-(((4-methoxybenzyl)oxy)methyl)oxazolidin-2-one (R)-14 (78 mg, 7%) was also isolated, as a
yellow solid. m.p. 74-76 ºC. R = 0.03 (3:1 n-hexanes:ethyl acetate). [α] -39.0 (c = 0.20, CHCl ).
(400 MHz, CDCl ) 3.25 (1H, dd, J = 8.6, 6.0 Hz, 4-H ), 3.40 (1H, t, J = 8.6 Hz, 4-H ), 3.51-3.59
2H, m, H-6), 3.78 (3H, s, OCH ), 3.79 (3H, s, OCH ), 4.29-4.39 (2H, m, H-7), 4.43-4.51 (2H, m, O-
CH Ar), 4.55-4.61 (1H, m, H-5), 6.80-6.88 (4H, m, ArH), 7.15-7.21 (4H, m, ArH). δ (100 MHz,
CDCl
f
D
3
δ
H
3
B
A
(
3
3
2
C
3
) 45.8 (C-4), 47.5 (C-7), 70.0 (C-6), 71.8 (C-5), 73.2 (OCH
2
Ar), 113.8 and 114.0 (Ar-CH), 127.6
-
(
1
ArC), 129.3 and 129.3 (Ar-CH), 129.5 (ArC), 157.7 (C-2), 159.2 and 159.3 (COCH
3
). νmax(ATR)/cm
+
+
+
2
960, 1728, 1513, 1448, 1347, 1250, 810. m/z (ESI ): 380 (MNa , 100%); HRMS (ESI ) found
+
(
MNa ): 380.1474, C20
H
23NNaO requires 380.1468. In a separate fraction alcohol (R)-12 (0.36 g, 36%)
5
was also isolated.
tert-Butyl (S)-(3-((tert-butyldimethylsilyl)oxy)-2-((4-methoxybenzyl)oxy)propyl)carbamate (S)-
1 was prepared using the same procedure as above, starting with (S)-10, giving the title compound as
a clear colorless oil. [α] -14.4 (c = 4.8, CHCl ). All spectroscopic data was identical to that of the
previously prepared (R)-11.
1
D
3
tert-Butyl (R)-(3-hydroxy-2-((4-methoxybenzyl)oxy)propyl)carbamate (R)-12. To a stirred
solution of silyl (R)-11 (0.41 g, 0.96 mmol) in THF (10 mL) at 0 ºC was added a solution of TBAF
(
0.61 g, 1.9 mmol) in THF (1M). The reaction mixture was allowed to warm to room temperature and
stirred for 4 h before being washed with sat. aq. ammonium chloride (5 mL) and extracted with ethyl
acetate (3 x 5 mL). The organic extracts were then washed with brine (10 mL) and dried (MgSO
4
)
before the solvent was removed in vacuo to give the crude product which was purified by flash
chromatography (gradient 3:1 n-hexanes:ethyl acetate then 1:1 n-hexanes:ethyl acetate) to afford the
title compound (R)-12 (0.31 g, quant.) as a colorless oil. [α]
hexanes:ethyl acetate). (* denotes minor rotamer) δ (400 MHz, CDCl
1H, m, OH), 3.22-3.42 (2H, m, H-1), 3.37-3.48 (2H, m, H-1*), 3.50-3.55 (1H, m, H-2), 3.57-3.61 (3H,
D
-25.0 (c = 0.58, CHCl
3
). R
f
= 0.06 (3:1 n-
H
3
) 1.43 (9H, s, CMe
3
), 3.0-3.15
(
m, H-3 and H-2*), 3.79 (3H, s, OCH
3
), 4.46 (2H, s, CH
2
Ar*), 4.51 (2H, s, CH
(100 MHz, CDCl
Ar), 71.5 (C-1*), 73.0 (CH Ar*), 77.6 (C-
*), 113.76 and 113.82 (Ar-CH and Ar-CH*), 129.4 (Ar-CH and
Ar-CH*), 129.8 and 130.0 (Ar-C and Ar-C*), 156.6 and 157.0 (C=O and C=O*), 159.2 and 159.3
2
Ar), 5.00-5.06 (1H, m,
NH and NH*), 6.86-6.88 (2H, m, Ar-H), 7.23-7.27 (2H, m, Ar-H). δ
C(CH ), 40.2 (C-1), 55.2 (OCH ), 61.0 (C-3), 71.3 (CH
), 79.4 and 79.6 (C(CH and C(CH
C
3
) 28.2
(
3
)
3
3
2
2
2
3
)
3
3 3
)
-1
(
8
3
COCH
3
and COCH
3
*). νmax(ATR)/cm 3357, 2932, 2874, 1689, 1513, 1455, 1366, 1246, 1170, 1033,
+
+
+
+
21. m/z (ESI ): 334 (MNa , 100%); HRMS (ESI ) found (MNa ): 334.1617, C16
34.1625. tert-Butyl (S)-(3-hydroxy-2-((4-methoxybenzyl)oxy)propyl)carbamate (S)-12 was
H
25NNaO requires
5
prepared using the same procedure as above, starting with (S)-11, giving the title compound as a clear
colorless oil. [α] +20.6 (c = 0.50, CHCl ). All spectroscopic data was identical to that of the previously
prepared (R)-12.
D
3
1
1
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
tert-Butyl (R)-(2-((4-methoxybenzyl)oxy)-3-oxopropyl)carbamate. To a stirred solution of oxalyl
chloride (0.45 mL, 5.3 mmol) in CH Cl (13 mL) at -78 ºC under nitrogen was added DMSO (0.56 mL,
.9 mmol) dropwise. After 15 min of stirring, alcohol (R)-12 (0.82 g, 2.6 mmol) in CH Cl (11 mL)
2
2
7
2
2
was added to the reaction mixture which was maintained at -78 ºC and stirred for a further 30 min.
Triethylamine (2.2 mL, 16 mmol) was then added and the reaction stirred for a further 30 min at -78 ºC
followed by a further 30 min at 0 ºC. The reaction was then quenched with sat. aq. ammonium chloride
(
10 mL) and extracted with ethyl acetate (3 x 10 mL). The combined organic extracts were then washed
with water (20 mL), sat. aq. sodium bicarbonate (20 mL), brine (20 mL) and dried (MgSO ) before the
4
solvent was removed in vacuo to give the title compound which was used in the next reaction without
further purification. tert-Butyl (S)-(2-((4-methoxybenzyl)oxy)-3-oxopropyl)carbamate was prepared
using the same procedure as above, starting with (S)-12, giving the title compound which was used in
the next reaction without further purification.
tert-Butyl
(S,E)-(3-(2,5-dioxoimidazolidin-4-ylidene)-2-((4-
methoxybenzyl)oxy)propyl)carbamate (S,E)-16 and tert-butyl (S,Z)-(3-(2,5-dioxoimidazolidin-4-
ylidene)-2-((4-methoxybenzyl)oxy)propyl)carbamate (S,Z)-16. To a solution of sodium methoxide
(
1
prepared from sodium (73 mg, 3.2 mmol) in dry methanol (7 mL)) was added hydantoin phosphonate
5 (0.75 g, 3.2 mmol) and the reaction placed under an atmosphere of nitrogen. After 5 min a solution
of tert-butyl (R)-(2-((4-methoxybenzyl)oxy)-3-oxopropyl)carbamate (0.82 g, 2.6 mmol) in dry
methanol (3 mL) was added and the reaction stirred for 40 h before being quenched with sat. aq.
ammonium chloride (5 mL). The solvent was removed in vacuo and the remaining aqueous mixture
was diluted with water (5 mL) and extracted with ethyl acetate (3 x 5 mL). The combined organic
extracts were washed with sat. aq. sodium bicarbonate (10 mL), brine (10 mL), dried (MgSO
solvent was removed in vacuo to give the crude product which was purified by flash chromatography
2:1 ethyl acetate:n-hexanes) afford the title compounds (S,Z)-16 and (S,E)-16 (0.68 g, 66% over two
steps) as a 2:1 mixture of Z to E isomers, as a cream foam. Further purification to obtain an analytical
sample of each isomer afforded (S,Z)-16 as a yellow gum (* denotes minor rotamer). R = 0.56 (3:1
ethyl acetate:n-hexanes). δ (400 MHz, CDCl ) 1.42 (9H, s, CMe ), 3.15-3.43 (3H, m, H-1 and H-2*),
.81 (3H, s, OCH ), 4.05 (2H, d, J = 6.4 Hz, H-1*), 4.31 (1H, br s, H-2), 4.38-4.58 (4H, m, CH Ar and
CH Ar*), 4.95 (1H, br s, 1-NH*), 5.30 (1H, t, J = 6.6 Hz, 1-NH), 5.76 (1H, d, J = 5.1 Hz, H-3), 6.89
2H, dd, J = 8.6, 2.7 Hz, Ar-H), 7.24 (2H, dd, J = 8.6, 2.0 Hz, Ar-H), 8.19 (1H, br s, NH), 8.26-8.36
3H, m, 2 x NH* and NH). δ (100 MHz, CDCl ) 28.3 (C(CH ), 37.2 (C-1*), 44.0 (C-1), 50.9 (C-2*),
5.3 (OCH ), 71.7 (ArCH ), 73.3 (ArCH *), 75.5 (C-2), 79.7 and 80.2 (C(CH and C(CH *), 110.6
C-3), 114.0, 114.1 and 114.2 (Ar-C), 125.8 and 126.4 (C-4' and C-4'*), 128.7, 129.6 and 129.8 (Ar-C),
4
) and the
(
f
H
3
3
3
3
2
2
(
(
C
3
)
3 3
5
3
2
2
3
)
3
)
3 3
(
1
1
1
30.9 (Ar-C), 152.4 and 153.2 (C-5' and C-5'*), 156.3 (C=O), 159.4 and 159.7 (COCH
62.9 and 163.5 (C-2' and C-2'*). νmax(ATR)/cm 3213, 2976, 1725, 1678, 1512, 1458, 1366, 1162,
3
and COCH
3
*),
-1
+
+
+
+
031, 763. m/z (ESI ): 414 (MNa , 100%); HRMS (ESI ) found (MNa ): 414.1620, C19
H N
25 3
NaO
= 0.44
), 3.20-3.23 (2H, m, H-1), 3.77
Ar), 5.20-5.26 (1H, m, H-2), 5.31 (1H, d, J = 9.6 Hz, H-3), 6.85-
.92 (2H, m, Ar-H), 7.24-7.31 (2H, m Ar-H). δ (100 MHz, CDCl ) 28.4 (C(CH ), 44.6 (C-1), 55.3
OCH ), 71.0 (ArCH ), 71.8 (C-2), 89.1 (C(CH ), 113.8 and 114.1 (Ar-CH), 117.1 (C-3), 127.8 (C-
'), 129.4 and 129.5 (Ar-CH), 130.1 (Ar-C), 156.2 (C=O), 159.1 (COCH
6
requires 414.1636. In a separate fraction (S,E)-16 was also isolated, as a yellow semisolid. R
f
(
(
3:1 ethyl acetate:n-hexanes). δ
3H, s, OCH ), 4.46-4.53 (2H, m, CH
H
(500 MHz, CD
3
OD) 1.42 (9H, s, CMe
3
3
2
6
(
4
C
3
)
3 3
3
2
)
3 3
3
), 159.3 (C-2'), 163.1 (C-5').
-1
+
ν
max(ATR)/cm 3212, 2980, 2930, 1725, 1677, 1514, 1457, 1367, 1159, 1031, 762. m/z (ESI ): 414
+
+
+
(
C
MNa , 70%), 390 (33%), 358 (100%), 314 (26%); HRMS (ESI ) found (MNa ): 414.1627,
NaO requires 414.1636. tert-Butyl (R,E)-(3-(2,5-dioxoimidazolidin-4-ylidene)-2-((4-
H N
19 25 3
6
methoxybenzyl)oxy)propyl)carbamate (R,E)-16 and tert-butyl (R,Z)-(3-(2,5-dioxoimidazolidin-4-
ylidene)-2-((4-methoxybenzyl)oxy)propyl)carbamate (R,Z)-16 was prepared using the same
1
2
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
procedure as above, starting with tert-butyl (S)-(2-((4-methoxybenzyl)oxy)-3-oxopropyl)carbamate,
giving the title compounds as a 1.0:1.5 mixture of Z to E isomers, as a yellow foam. All spectroscopic
data was identical to that of the previously prepared (S,E)-16 and (S,Z)-16.
(
S,E)- and (S,Z)-5-(3-Amino-2-hydroxypropylidene)imidazolidine-2,4-dione (S,E)-17 and (S,Z)-
7. To a solution of carbamate (S)-16 (0.42 g, 1.1 mmol) in CH Cl (1.0 mL) at 0 ºC was added TFA
0.8 mL) slowly. The reaction mixture was allowed to warm to room temperature and stirred for 1 h
before the solvent was removed in vacuo to give the crude product which was used immediately in the
next reaction without further purification. (R,E)- and (R,Z)-5-(3-Amino-2-
1
2
2
(
hydroxypropylidene)imidazolidine-2,4-dione (R,E)-17 and (R,Z)-17 was prepared using the same
procedure as above, starting with a mixture of (R,E)- and (R,Z)-16, giving the title compounds which
were used in the next reaction without further purification.
1-(1H-Pyrrol-2-yl)-2,2,2-trichloroethanone 23. To a stirring solution of trichloroacetyl chloride (16
mL, 0.146 moles) in dry ether (26 mL), under an atmosphere of nitrogen, was added dropwise a solution
of pyrrole (10 mL, 0.143 mol) in dry ether (78 mL) over 2 h. After 18 h, a solution of potassium
carbonate (13 g, 0.1 mol) in water (40 mL) was added carefully. The organic layer was separated, dried
(
MgSO ), filtered and then stirred with activated charcoal for 10 min. The organic layer was then
4
filtered again and the solvent was removed in vacuo to give the crude product which was recrystallized
from n-hexanes to afford the title compound 23 (25 g, 82%) as a purpley-white metallic solid. m.p. 71-
17]
7
7
2 ºC. (lit. m.p. 73-75 ºC).[
δ
H
(300 MHz, CDCl ) 6.37-6.40 (1H, m, H-4), 7.15-7.17 (1H, m, H-5),
3
.37-7.39 (1H, m, H-3), 9.46 (1H, br s, NH). The NMR values were in agreement with literature
values.[
17]
1-(4,5-Dibromo-1H-pyrrol-2-yl)-2,2,2-trichloroethanone 18. To a stirring solution of pyrrole 23 (1
g, 4.71 mmol) in acetic acid (5 mL), under an atmosphere of nitrogen, was added a solution of bromine
(
0.49 mL, 9.4 mmol) in acetic acid (4.5 mL) at such a rate as to maintain the reaction temperature at 18
ºC. After the addition the reaction was heated at 60 ºC for 2 h before being cooled to room temperature.
The solvent was removed in vacuo to afford the title compound 18 (1.75 g, 100%) as a dark brown solid.
m.p. 135-136 ºC. (lit. m.p. 136-139 ºC).[9]
(
δ
(400 MHz, CDCl
1H, br s, NH). The spectroscopic data was in agreement with literature values.
General Procedure A: To a solution of amine salt (1 equiv.) and pyrrole (1 equiv.) in dry DMF at 0
) 7.34 (1H, d, J = 2.4 Hz, H-3), 9.81
H
3
[9]
ºC, under an atmosphere of nitrogen, was added triethylamine (2 equiv.) dropwise. The mixture was
allowed to warm to room temperature and stirred until completion indicated by TLC. The solvent was
removed in vacuo to give the crude product which was purified by flash chromatography, using
LiChroprep NH
2
silica, to afford the desired product.
(
S,E)- and (S,Z)-Mukanadin F (1).
The reaction was carried out following General Procedure A using salt (S)-17 (0.31 g, 1.1 mmol),
bromopyrrole 18 (0.40 g, 1.1 mmol) and triethylamine (0.30 mL, 2.2 mmol) in DMF (15 mL) stirring
for 48 h before the solvent was removed in vacuo to give the crude product which was purified by flash
chromatography (9:1 CH
mg, 21% over 2 steps) as a white solid, m.p. 151-153 ºC. R
c = 0.3, CH OH). δ (500 MHz, (CD SO) 3.24-3.40 (2H, m, H-8), 4.41-4.46 (1H, m, H-9), 5.33 (1H,
d, J = 5.0 Hz, OH), 5.45 (1H, d, J = 8.0 Hz, H-10), 6.94 (1H, s, H-3), 8.06 (1H, t, J = 6.0 Hz, NH-7),
0.03 (1H, s, NH-13), 11.03 (1H, br s, NH-15), 12.69 (1H, br s, NH-1). δ (125 MHz, (CD SO) 44.4
C-8), 65.6 (C-9), 97.8 (C-3), 104.6 (C-2), 111.6 (C-10), 112.9 (C-4), 128.0 (C-5), 130.4 (C-11), 154.7
2
Cl
2 3
:CH OH) using LiChroprep NH
2
to afford the title compound (S,Z)-1 (94
f
= 0.26 (9:1 CH Cl :CH OH). [α] -26.7
2
2
3
D
(
3
H
)
3 2
1
C
)
3 2
(
(
7
3
-
1
C-14), 159.0 (C-6), 164.6 (C-12). νmax(ATR)/cm 3184, 2923, 1716, 1561, 1387, 1326, 1067, 779,
51, 650. m/z (ESI ): 447 (81,81Br
+
MNa , 53%), 445 (79,81Br
+
MNa , 100%), 443 (79,79Br
+
MNa , 48%),
+
2
2
2
+
81,81
+
81,81
12 (50%); HRMS (ESI ) found ( Br
2
MNa ): 446.8902, C11
H
10
Br
2
N
4
NaO requires 446.8922.
4
7
9,81
+
79,81
79,79
+
Found ( Br
2
MNa ): 444.8939, C11
H
10
Br
2
N
4
NaO
4
requires 444.8941. Found ( Br MNa ):
2
1
3
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.201800422
442.8963, C11
H
10 9,79Br
7
2
N
4
NaO requires 442.8961. The H and C NMR data was in agreement with
4
1
13
[2]
literature values. In a separate fraction title compound (S,E)-1 (71 mg, 16 % over 2 steps) was also
isolated, as a white solid, m.p. 163-164 ºC. R = 0.17 (9:1 CH Cl -CH OH). [α] +6.7 (c = 0.3, CH OH).
(500 MHz, (CD SO) 3.16-3.40 (2H, m, H-8), 5.15-5.26 (2H, m, H-9 and OH), 5.29 (1H, d, J = 9.0
Hz, H-10), 6.92 (1H, s, H-4), 8.05 (1H, t, J = 5.0 Hz, NH-7), 10.01 (1H, s, NH-13), 11.01 (1H, br s,
NH-15), 12.62 (1H, br s, NH-1). δ (125 MHz, (CD SO) 44.7 (C-8), 63.6 (C-9), 97.7 (C-3), 104.3
C-2), 112.8 (C-4), 117.2 (C-10), 128.2 (C-5), 129.7 (C-11), 154.0 (C-14), 158.9 (C-6), 164.3 (C-12).
f
2
2
3
D
3
δ
H
)
3 2
C
)
3 2
(
ν
5
(
C
max(ATR)/cm 3147, 2925, 1718, 1564, 1389, 1328, 1059, 748, 650. m/z (ESI ): 447 (81,81Br MNa+,
-
1
+
2
7
9,81
+
79,79
+
+
2%), 445 ( Br
2
MNa , 100%), 443 ( Br
MNa ): 446.8911, C11 NaO
NaO
2
MNa , 51%), 413 (39%), 367 (22%); HRMS (ESI ) found
8
1,81
+
1081,81Br
79,81
+
Br
2
H
2
N
4
4
requires 446.8922. Found ( Br
2
MNa ): 444.8940,
1079,81Br
N
requires 444.8941. Found (79,79Br
MNa ): 442.8958, C11
+
H
10
79,79
Br
N
4
NaO
11
H
2
4
4
2
2
4
requires 442.8961. (R,E)- and (R,Z)-Mukanadin F (1) was prepared using the same procedure as
above, starting with a mixture of (R,E)- and (R,Z)-16, giving the title compounds as a 0.5:1.0 mixture
of E to Z isomers. Purification by flash chromatography (9:1 CH
afforded (R,Z)-Mukanadin F (1) only, as a cream solid, m.p. 152-154 ºC. [α]
All spectroscopic data was identical to that of the previously prepared (S,Z)-1.
tert-Butyl 3-hydroxypropylcarbamate. To a stirring solution of 3-amino-propan-1-ol (2.0 g, 26.6
mmol) in dry CH Cl (50 mL), under an atmosphere of nitrogen, was added di-tert-butyl dicarbonate
9.65 g, 39.9 mmol). Triethylamine (5.56 mL, 39.9 mmol) was added and the mixture was stirred for
2
Cl
2
:CH
3
OH) using LiChroprep NH
2
D
+22.2 (c = 0.30, CH OH).
3
2
2
(
3
1
5 h before being quenched with sat. aq. ammonium chloride (20 mL) and extracted with CH
5 mL). The combined organic extracts were washed with brine (30 mL) and dried (MgSO
2
Cl
2
(3 x
4
). The
solvent was removed in vacuo to give the crude product which was purified by flash chromatography
gradient elution firstly with 4:1 n-hexanes:ethyl acetate and then with 1:1 n-hexanes:ethyl acetate) to
afford the title compound (3.48 g, 75%) as a pale yellow oil. R = 0.26 (1:1 n-hexanes:ethyl acetate).
(300 MHz, CDCl ) 1.45 (9H, s, C(CH ), 1.67 (2H, p, J = 6.0 Hz, H-2), 3.01 (1H, br s, OH), 3.28
(
f
δ
H
3
)
3 3
(
2H, t, J = 6.0 Hz, H-1), 3.66 (2H, t, J = 6.0 Hz, H-3), 4.82 (1H, br s, NH). These values were in
agreement with literature values.[
18]
tert-Butyl 3-oxopropylcarbamate 20. To a stirred solution of DMSO (1.2 mL, 17.1 mmol) in dry
CH
mmol) and the mixture was stirred for 15 min before a solution of tert-butyl 3-hydroxypropylcarbamate
1 g, 5.7 mmol) in dry CH Cl (5 mL) was added drop wise. After stirring for a further 1 h at -78 ºC,
2
Cl
2
(54 mL), under an atmosphere of nitrogen, at -78 ºC was added oxalyl chloride (0.73 mL, 8.6
(
2
2
triethylamine (4 mL, 28.5 mmol) was added and the reaction was allowed to warm to room temperature.
After stirring for a further 30 min, the reaction was quenched with 10% aqueous HCl (10 mL) and
extracted with ethyl acetate (3 x 20 mL). The combined organic extracts were washed with sat. aq.
sodium bicarbonate (20 mL), brine (20 mL) and dried (MgSO
afford the title compound 20 (0.91 g, 92%) as a pale yellow oil. δ
C(CH ), 2.70 (2H, t, J = 6.0 Hz, H-2), 3.42 (2H, t, J = 6.0 Hz, H-1), 4.92 (1H, br s, NH), 9.81 (1H, s,
H-3). The H NMR values were in agreement with literature values.
E/Z)-tert-Butyl 3-(2,5-dioxoimidazolidin-4-ylidene)propylcarbamate 21. To a solution of sodium
4
). The solvent was removed in vacuo to
H
(300 MHz, CDCl ) 1.43 (9H, s,
3
)
3 3
1
[19,20]
(
ethoxide (prepared from sodium (0.13 g, 5.5 mmol) in dry ethanol (13 mL)) was added hydantoin
phosphonate 15 (1.3 g, 5.5 mmol) and the reaction placed under an atmosphere of nitrogen. After 5
min a solution of aldehyde 20 (0.8 g, 4.6 mmol) in dry ethanol (2 mL) was added and the reaction stirred
for a further 90 min before being quenched with 10% aq. HCl (5 mL). The solvent was removed in
vacuo and the remaining aqueous mixture was diluted with water (5 mL) and extracted with ethyl
acetate (3 x 5 mL). The combined organic extracts were washed with sat. aq. sodium bicarbonate (10
mL), brine (10 mL), dried (MgSO
4
) and the solvent was removed in vacuo to afford the title compound
(300 MHz, CD OD, E/Z-
21 (0.62 g, 53%) as a pale yellow foam in a 1:1 mixture of E:Z isomers. δ
H
3
1
4
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
isomer ~ 0.85:1) 1.47 (18H, s, C(CH
3
)
3, E and Z), 2.41 (2H, dt, J = 9.0, 6.0 Hz, H-2, Z), 2.86 (2H, dt, J
=
=
9.0, 6.0 Hz, H-2, E), 3.17-3.25 (4H, m, H-1, E and Z), 5.59 (1H, t, J = 9.0 Hz, H-3, E), 5.75 (1H, t, J
9.0 Hz, H-3, Z). δ (75 MHz, CD OD) 27.8 and 28.6 (C-2), 28.8 (C(CH ), 40.3 and 41.0 (C-1), 80.0
C
3
)
3 3
and 80.2 (C(CH
3
)
3
), 111.4 and 116.8 (C-3), 131.7 and 132.9 (C=CH), 156.3 and 156.9, 158.5, 166.3
-1
+
+
(
(
C=O). νmax(ATR)/cm 3349, 1764, 1725, 1683, 1168. m/z (ESI ): 278 (MNa , 100%), 222 (31%), 200
25%), 178 (12%); HRMS (ESI ) found (MNa ): 278.1101, C11
+
+
H
17
N
3
NaO
4
requires 278.1111.
3-(2,5-Dioxoimidazolidin-4-ylidene)propan-1-amine trifluoroacetate salt. To a stirred solution of
carbamate 21 (0.1 g, 0.39 mmol) in dry CH Cl (0.35 mL) at 0 ºC was added TFA (0.28 mL) slowly.
2
2
The reaction was placed under an atmosphere of nitrogen and allowed to warm to room temperature.
After 2 h the solvent was removed in vacuo to afford the title compound (0.11 g, quantitative) as a dark
orange which was used in the next step without further purification.
1-(4-Bromo-1H-pyrrol-2-yl)-2,2,2-trichloroethanone 22. To a solution of pyrrole 23 (2.13 g, 10
mmol) in chloroform (10 mL) at 0 ºC, was added bromine (1.71 g, 11 mmol) dropwise. The mixture
was allowed to warm to room temperature and stirred for 10 min before being poured onto water and
the organic layer extracted with chloroform (3 x 10 mL). The combined organic extracts were washed
with sat. aq. sodium bicarbonate (20 mL), water (20 mL) and dried (MgSO
4
). The solvent was removed
in vacuo to give the crude product which was recrystallized from n-hexanes to afford the title compound
[21]
22 (2.0 g, 69%) as a black solid. m.p. 133-134 ºC. (lit. m.p. 134-136 ºC).
δ
H
(400 MHz, CDCl ) 7.14-
3
7
.16 (1H, m, H-5), 7.35-7.36 (1H, m, H-3), 9.48 (1H, br s, NH). The spectroscopic data was in
[22,23]
agreement with literature values.
E)- and (Z)-Mukanadin B[ (6). The reaction was carried out following General Procedure Ausing
24]
(
3-(2,5-dioxoimidazolidin-4-ylidene)propan-1-amine trifluoroacetate salt (53 mg, 0.20 mmol), pyrrole
22 (58 mg, 0.20 mmol) and triethylamine (54.0 µL, 0.40 mmol) in DMF (2 mL) stirring for 2.5 h to
give the crude product which was purified by flash chromatography (gradient elution firstly with 9:1
CH Cl :CH OH and then with 9:1 CH OH:NH ) to afford the title compounds (Z)-6 and (E)-6 (0.049
g, 77%) as a 1:2 mixture of E to Z isomers. Further purification by flash chromatography (9:1
CH Cl :CH OH) using LiChroprep NH to obtain pure samples of each isomer afforded (Z)-mukanadin
B (Z)-6 (5.4 mg, 8%) as a white solid, m.p. 131 133 ºC. R = 0.44 (9:1 CH Cl :CH OH). δ (400 MHz,
CD OD) 2.46 (2H, dt, J = 7.6, 7.2 Hz, H-9), 3.43 (2H, t, J = 7.2 Hz, H-8), 5.73 (1H, t, J = 7.6 Hz, H-
0), 6.75 (1H, d, J = 1.6 Hz, H-4), 6.91 (1H, d, J = 1.6 Hz, H-2). δ (100 MHz, CD OD) 28.2 (C-9),
9.2 (C-8), 97.5 (C-3), 110.9 (C-10), 113.2 (C-4), 122.9 (C-2), 127.4 (C-5), 133.0 (C-11), 156.9 (C-14),
2
2
3
3
3
2
2
3
2
-
f
2
2
3
H
3
1
3
1
5
C
3
-
1
62.8 (C-6), 166.4 (C-12). νmax(ATR)/cm 3237, 2926, 1724, 1687, 1625, 1567, 1389, 1327, 1129, 674,
+
81
+
79
+
81. m/z (ESI ): 381 (100%), 353 (16%), 351 ( BrMNa , 9%), 349 ( BrMNa , 10%), 301 (12%), 227
+
+
81
(
18%), 159 (12%); HRMS (ESI ) found (MNa ): 350.9882, C11
H11 BrN
4
NaO
requires 348.9907. In a separate fraction (E)-mukanadin
B (E)-6 (1.7 mg, 3%) was also isolated as a white film. R = 0.41 (9:1 CH Cl :CH OH). δ (400 MHz,
CD OD) 2.93 (2H, dt, J = 8.0, 6.8 Hz, H-9), 3.44 (2H, t, J = 6.8 Hz, H-8), 5.58 (1H, t, J = 8.0 Hz, H-
0), 6.74 (1H, d, J = 1.6 Hz, H-4), 6.90 (1H, d, J = 1.6 Hz, H-2). δ (100 MHz, CD OD) 27.3 (C-9),
0.0 (C-8), 97.4 (C-3), 113.1 (C-4), 116.3 (C-10), 122.7 (C-2), 127.6 (C-5), 131.9 (C-11), 156.3 (C-14),
3
requires 350.9887.
+
79
Found (MNa ): 348.9898, C11
H
11 BrN
4
NaO
3
f
2
2
3
H
3
1
4
1
C
3
-1
66.3 (C-6), 170.9 (C-12). νmax(ATR)/cm 3421, 3204, 2920, 1720, 1678, 1618, 1564, 1387, 1325,
+
81
+
79
+
+
1133, 769, 655. m/z (ESI ): 351 ( BrMNa , 100%), 349 ( BrMNa , 99%); HRMS (ESI ) found
+
81
+
(
C
MNa ): 350.9910,
NaO requires 348.9907.
E)- and (Z)-Mukanadin D
C
11
H
11 BrN
4
NaO
3
requires 350.9887.
Found (MNa ): 348.9917,
1179BrN
11
H
4
3
[
10,11]
(
(7). The reaction was carried out following General Procedure A
using 3-(2,5-dioxoimidazolidin-4-ylidene)propan-1-amine trifluoroacetate salt (47 mg, 0.30 mmol),
pyrrole 18 (112 mg, 0.30 mmol) and triethylamine (84.0 µL, 0.60 mmol) in DMF (2 mL) stirring for 22
h to give the crude product which was purified by flash chromatography (gradient elution CH
2
Cl then
2
1
5
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
9
:1 CH
2
Cl
2
:CH
3
OH) to afford the title compounds (Z)-7 and (E)-7 (0.057 g, 46%) as a 1:2.5 mixture of
(300 MHz, CD OD, E/Z-isomer ~ 1:2.5) 2.48 (2H, dt, J = 7.8, 6.9 Hz, H-9, Z), 2.93
2H, dt, J = 8.4, 6.9 Hz, H-9, E), 3.45 (2 x 2H, t, J = 6.9 Hz, H-8, E and Z), 5.60 (1H, t, J = 8.4 Hz, H-
E to Z isomers. δ
(
H
3
1
0, E), 5.74 (1H, t, J = 7.8 Hz, H-10, Z), 6.81 (1H, d, J = 3.0 Hz, H-4, E), 6.83 (1H, d, J = 5.4 Hz, H-4,
(75 MHz, CD OD) 27.3 and 28.2 (C-9), 39.3 and 40.0 (C-8), 100.0 and 100.1 (C-3), 106.0 and
06.2 (C-2), 111.1 and 116.5 (C-10), 114.3 and 114.4 (C-4), 128.7 (C-5), 131.8 and 133.0 (C-11), 156.3
and 156.9 (C-14), 161.7 and 161.9 (C-6), 166.3 (C-12). Further purification by flash chromatography
9:1 CH Cl :CH OH) using LiChroprep NH to obtain pure samples of each isomer afforded only (Z)-
mukanadin D (Z)-7 (9 mg, 7%) as a pale yellow solid, m.p. decomp. at 228 ºC. R = 0.51 (9:1
CH Cl :CH OH). δ (400 MHz, CD OD) 2.45 (2H, dt, J = 7.8, 6.9 Hz, H-9), 3.41 (2H, t, J = 6.9 Hz,
H-8), 5.71 (1H, t, J = 7.8 Hz, H-10), 6.78 (1H, s, H-4). δ (100 MHz, CD OD) 28.1 (C-9), 39.3 (C-8),
9.9 (C-3), 106.3 (C-2), 110.7 (C-10), 114.3 (C-4), 128.8 (C-5), 133.2 (C-11), 157.3 (C-14), 162.0 (C-
Z). δ
1
C
3
(
2
2
3
2
f
2
2
3
H
3
C
3
9
6
5
-1
), 166.7 (C-12). νmax(ATR)/cm 3134, 2920, 1707, 1674, 1629, 1563, 1389, 1320, 1224, 676, 636,
87. m/z (ESI ): 431 ( Br
+
81,81
+
79,81
+
79,79
+
2
MNa . 50%), 429 ( Br
2
MNa , 100%), 427 ( Br MNa , 50%), 408
2
+
81,81
(
35%), 233 (35%); HRMS (ESI+) found (MNa ): 430.8982, C11
H
10
Br
2
N
4
NaO
3
requires 430.8971.
+
79,81
+
Found (MNa ): 428.8997, C11
NaO requires 426.9012.
E)- and (Z)-Debromo-dehydroxymukanadin F (24). The reaction was carried out following
General Procedure A using 3-(2,5-dioxoimidazolidin-4-ylidene)propan-1-amine trifluoroacetate salt
H
10
Br
2
N
4
NaO
3
requires 428.8992. Found (MNa ): 426.9021,
C
11
H
1079,79Br
2
N
4
3
(
(
(
53 mg, 0.20 mmol), pyrrole 23 (42 mg, 0.20 mmol) and triethylamine (54.0 µL, 0.40 mmol) in DMF
2 mL) stirring for 2.5 h to give the crude product which was purified by flash chromatography (gradient
elution CH
3%) as a 1:1.5 mixture of E to Z isomers. δ
m, H-9, Z), 2.91-2.98 (2H, m, H-9, E), 3.46 (2H, t, J = 6.9 Hz, H-8, Z), 3.47 (2H, t, J = 6.6 Hz, H-8, E),
2
Cl
2
then 9:1 CH
2
Cl
2 3
:CH OH) to afford the title compounds (Z)-24 and (E)-24 (0.036 g,
5
H
(300 MHz, CD OD, E/Z-isomer ~ 1:1.5) 2.45-2.53 (2H,
3
5
6
.61 (1H, t, J = 8.4 Hz, H-10, E), 5.76 (1H, t, J = 8.0 Hz, H-10, Z), 6.15-6.18 (1H, m, H-3, E and Z),
.75-6.78 (1H, m, H-4, E and Z), 6.90-6.93 (1H, m, H-2, E and Z). δ (75 MHz, CD OD) 27.5 and 28.3
C
3
(
C-9), 39.2 and 39.9 (C-8), 110.2 and 110.2 (C-3), 111.1 and 116.6 (C-10), 111.6 and 111.7 (C-4), 122.9
and 123.0 (C-2), 126.7 and 129.3 (C-5), 131.8 and 133.0 (C-11), 156.9 (C-14), 164.0 (C-6), 166.4 (C-
2). Further purification by flash chromatography (9:1 CH Cl :CH OH) using LiChroprep NH to
obtain pure samples of each isomer afforded only (Z)-24 (14 mg, 29%) as a white solid, m.p. 240-242
ºC. R = 0.36 (9:1 CH Cl :CH OH). δ (400 MHz, CD OD) 2.47 (2H, dt, J = 8.0, 6.9 Hz, H-9), 3.43
2H, t, J = 6.9 Hz, H-8), 5.75 (1H, t, J = 8.0 Hz, H-10), 6.15-6.16 (1H, m, H-3), 6.75-6.77 (1H, m, H-
1
2
2
3
2
f
2
2
3
H
3
(
4
), 6.90-6.91 (1H, m, H-2). δ
C
(100 MHz, CD OD) 28.3 (C-9), 39.2 (C-8), 110.2 (C-3), 111.0 (C-10),
3
1
11.6 (C-4), 122.9 (C-2), 126.7 (C-5), 133.0 (C-11), 157.0 (C-14), 164.0 (C-6), 166.4 (C-12).
-1
+
+
ν
1
2
max(ATR)/cm 3288, 2978, 1723, 1688, 1620, 1566, 1408, 1331, 1125, 748. m/z (ESI ): 248 (M , 6%),
23 (23%), 94 (100%), 86 (95%); HRMS (ESI ) found (M ): 248.09021, C11
48.09094. The spectroscopic data was in agreement with literature values.
+
+
H
12
N
4
3
O requires
[25]
ASSOCIATED CONTENT
Supporting Information. NMR spectra for all novel compounds.
AUTHOR INFORMATION
Corresponding Author
*Email: d.barker@auckland.ac.nz
1
6
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.201800422
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the University of Auckland Faculty Research Development Fund for funding this project.
We thank Shi Min Tan for her assistance with DFT calculations.
Keywords: Bromopyrrole alkaloid; isomerization; racemization; marine natural product;
stereoselective synthesis
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European Journal of Organic Chemistry
10.1002/ejoc.201800422
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 2:
FULL PAPER
Natural Products
Michelle van Rensburg, Brent R. Copp
and David Barker*
Page No. – Page No.
Synthesis and absolute
stereochemical reassignment of
mukanadin F: A study of
isomerization of bromopyrrole
alkaloids with implications on marine
natural product isolation.
Spectroscopic analysis of synthetic, enantiopure, bromopyrrole alkaloid
mukanadin F determined that the reported absolute stereochemistry of the natural
product was misassigned. Furthermore, mukanadin F and similar compounds
undergo racemization and alkene-isomerization under benign conditions such as
sunlight and solvent.
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