Total synthesis of phorbazole B

Article abstract of DOI:10.3390/molecules25204848

Phorbazoles are polychlorinated heterocyclic secondary metabolites isolated from a marine sponge and several of these natural products have shown inhibitory activity against cancer cells. In this work, a synthesis of the trichlorinated phorbazole B using late stage electrophilic chlorination was developed. The synthesis relied on the use of an oxazole precursor, which was protected with an iodine in the reactive 4-position, followed by complete chlorination of all pyrrole positions. Attempts to prepare phorbazole A and C, which contain a 3,4-dichlorinated pyrrole, were unsuccessful as the desired chlorination pattern on the pyrrole could not be obtained. The identities of the dichlorinated intermediates and products were determined using NMR techniques including NOESY/ROESY, 1,1-ADEQUATE and high-resolution CLIP-HSQMBC.

Full text of DOI:10.3390/molecules25204848

molecules
Article
Total Synthesis of Phorbazole B
1
2
2
1
Yngve Guttormsen
, Magnus E. Fairhurst , Sunil K. Pandey , Johan Isaksson
,
2
,
1,
Bengt Erik Haug * and Annette Bayer *
1
Department of Chemistry, UiT The Arctic University of Norway, Hansine Hansens veg 54,
037 Tromsø, Norway; yngve.guttormsen@uit.no (Y.G.); johan.isaksson@uit.no (J.I.)
Department of Chemistry and Centre for Pharmacy, University of Bergen, Allégaten 41,
007 Bergen, Norway; magnus.fairhurst@uib.no (M.E.F.); sunil.pandey@uib.no (S.K.P.)
9
2
5
*
Correspondence: bengt-erik.haug@uib.no (B.E.H.); annette.bayer@uit.no (A.B.);
Tel.: +47-55-58-34-68 (B.E.H.); +47-77-64-40-69 (A.B.)
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Magne Olav Sydnes and Joanne Harvey
Received: 19 September 2020; Accepted: 18 October 2020; Published: 21 October 2020
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: Phorbazoles are polychlorinated heterocyclic secondary metabolites isolated from a marine
sponge and several of these natural products have shown inhibitory activity against cancer cells.
In this work, a synthesis of the trichlorinated phorbazole B using late stage electrophilic chlorination
was developed. The synthesis relied on the use of an oxazole precursor, which was protected with an
iodine in the reactive 4-position, followed by complete chlorination of all pyrrole positions. Attempts
to prepare phorbazole A and C, which contain a 3,4-dichlorinated pyrrole, were unsuccessful as
the desired chlorination pattern on the pyrrole could not be obtained. The identities of the dichlorinated
intermediates and products were determined using NMR techniques including NOESY/ROESY,
1
,1-ADEQUATE and high-resolution CLIP-HSQMBC.
Keywords: phorbazole; oxazole; pyrrole; late-stage chlorination
1
. Introduction
Phorbazoles A–D (1–4, Figure 1) are polychlorinated heterocyclic secondary metabolites isolated
from the marine sponge Phorbas aff. Clathrata off the coast of South Africa in 1994 [1].
Figure 1. Phorbazoles A–D (1–4).
0
The phorbazoles were the first natural compounds isolated with the 2-(2 -pyrrolyl)oxazole
fragment. They were also the first examples of chlorinated pyrroles found in nature. Later on,
a N-methylated analogue of
mollusk Aldisa andersoni [ ]. No bioactivity has so far been reported for
analogue and the 9-chlorinated analogue of have been found to have a feeding deterrence effect,
which suggests that they are involved in chemical defense [ ]. The two latter compounds have also
1
and the 9-chlorinated analogue of
4
were isolated from the marine
2
2–
4
, while and its N-methylated
1
4
2
been found to inhibit growth of cancer cells in vitro
[
2
]. While the phorbazoles are biosynthetically
Molecules 2020, 25, 4848; doi:10.3390/molecules25204848
www.mdpi.com/journal/molecules

Molecules 2020, 25, 4848
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produced by cyclization of dipeptides of tyrosine and proline, a synthesis of
3
reported in 2001 relied on
a Robinson–Gabriel cyclodehydration of a precursor carrying the appropriately chlorinated pyrrole [
Breitfussin A–H ( 12, Figure 2) constitute the second class of marine natural products containing
the pyrrole-oxazole motif [ ]. Like the phorbazoles, the breitfussins are highly halogenated natural
products derived from dipeptide precursors, but with tryptophan instead of tyrosine.
3].
5–
4,5
R2
R5
R3
R6
N
N
H
N
N
H
R1
O
O
O
O
R4
N
N
Br
Br
H
H
5
6
8
9
1
R1 = I, R2 = R3 = R4 = H
R1 = H, R2 = R4 = H, R3 = Br
R1 = R2 = R3 = R4 = H
R1 = R3 = R4 = H, R2 = H
0 R1 = R2 = R3 = H, R4 = I
7 R5 = R6 = H
11 R5 = H, R6 = Br
12 R5 = Br, R6 = H
Figure 2. Breitfussins A–H (5–12).
We have earlier reported on the total syntheses of
palladium-catalyzed cross couplings to install the indole and pyrrole onto the oxazole core, followed
by halogenation of the pyrrole at a late stage in the case of ]. An alternative synthesis of (and by
coincidence also ) based on application of the Robinson–Gabriel cyclodehydration to form the oxazole
ring combined with a late stage bromination strategy [7], has also been reported.
5–8, which were based on the utilization of two
6
[5
,
6
6
8
In this work, we report the total synthesis of
2 via late-stage chlorination and attempts at
synthesizing 1 and 3 using the same strategy.
2
. Results and Discussion
2
.1. Retrosynthetic Analysis of 2
We envisioned that the phorbazoles should be accessible via chlorination at a late stage, and a
retrosynthetic analysis of 2, which has not been prepared before, is shown in Scheme 1.
Cl
I
I
3
N
Cl
N
N
2
4
I
O
O
1
5
O
N
N
B(OH)2
N
Boc
H
Cl
13
H
14
2
TsO
TsO
HO
CHO
C
1
4
Ts
N
TsO
15
Scheme 1. Retrosynthetic analysis for 2.
Due to the intrinsic propensity of both the oxazole and the pyrrole to undergo electrophilic
aromatic substitution, the regioselectivity is difficult to control. The Chen group nicely illustrated
the subtle inherent reactivity differences in the synthesis of
acetone to tetrahydrofuran (THF)/pyridine altered the regioselectivity of the halogenation from
the oxazole 4-position to the pyrrole 5-position [ ]. Our strategy is based on our realization from
the breitfussin syntheses that the iodine in the oxazole 4-position is acid-labile and may act as a
protecting group also toward synthesizing phorbazoles. We thus envisioned that could be obtained
6, where changing the solvent from
7
2
by chlorination of compound 13, where the oxazole 4-position is protected. Access to 13 was planned
through Suzuki Miyaura coupling on diiodide 14, which we planned to obtain from the Van Leusen
methyl isocyanide synthon [
8] after cyclisation to the oxazole and subsequent deiodination as we have
reported for the breitfussin syntheses [5,6].

Molecules 2020, 25, 4848
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2
.2. Synthesis of 2
The synthesis of the common intermediate 13 commenced with a modified Van Leusen oxazole
synthesis on the previously reported aldehyde 15 (Scheme 2) [9].
Scheme 2. Synthesis of 2 by late-stage chlorination.
Protection of the phenol is required both for the lithiation step and to avoid chlorination on
the phenol later in the synthesis [10]. In order to avoid deprotection of the tosyl group, 1,8-diazabicyclo
[
5.4.0] undec-7-en (DBU) was used as base instead of K CO . Heating the aldehyde 15 and the TosMIC
2 3
◦
reagent with DBU in 1,2-dimethoxyethane (DME) (70 C, 5 h) gave oxazole 16 in 70% yield. This material
was dilithiated by treatment with lithium bis(hexamethylsilyl)amide (LiHMDS) in THF at
for 1 h before addition of iodine to give the diiodinated oxazole 14 in 79% yield. While this protocol
also resulted in the formation of the monoiodinated 2-iodo oxazole 17 in 12% yield, it does not
require prolonged reaction times as has been reported for other methods [11]. The diiodo oxazole
◦
−
78 C
14 was subsequently coupled to Boc-protected (1H-pyrrol-2-yl)boronic acid in a Suzuki-Miyaura
reaction. The coupling reaction proceeded with excellent regioselectivity favoring the 2-substituted
oxazolyl-pyrrole 18 in 75% yield after 68 h at rt. We were not able to detect any product stemming from
coupling at the oxazole 4-position, although this result cannot be completely ruled out. Next, the Boc
group was removed using trimethylsilyl trifluoromethanesulfonate (TMSOTf) under basic conditions
to give the common intermediate 13 in high yield (89%). Chlorination of the common intermediate 13
using Palau’chlor [12], gave a mixture of di- and trichlorinated products, while the recently developed
procedure of Gustafson [13] using N-chlorosuccinimide (NCS)/Ph PS resulted in complete conversion.
3
At first 3.07 equiv. of NCS and 20 mol% of Ph PS was added; however, when HRMS analysis revealed
3
uncomplete reaction (see experimental part for details), a further 0.2 equiv. of NCS was added.
Thus, using a total of 3.27 equiv. of NCS, we obtained the desired trichlorinated pyrrole 19 in 73%
yield. No chlorination on the phenyl ring was observed. Next, we opted to deiodinate 19 using
the conditions identified through our total syntheses of breifussins A–D (5–9, Figure 2); however,
treatment of 19 under acidic conditions did not give any traces of deiodinated product. On the other
hand, simultaneous deprotection of the tosyl and iodine protecting groups of 19 was achieved using
zinc in refluxing ethanolic sodium hydroxide, to give
2 in 87% yield. The NMR spectra of synthetic
material in (CD ) SO (see Figures S1 and S2) matched the reported spectra of the natural phorbazole
3
2
B [1] (see Table S1 for a comparison).
2
.3. Attempted Synthesis of 1 and 3
We envisioned that it might be possible to access natural products 1 and 3 by shielding the pyrrole
5-position with a bulky triisopropylsilyl (TIPS) group on the pyrrole N-atom, giving 20a and 20b
as intermediates for the synthesis of
1
and , respectively (Scheme 3). The literature suggests that
3
the extent of shielding that TIPS protection of the pyrrole can infer on C-5 (our numbering, see Scheme 3)
of the pyrrole, varies depending on the chlorination reagent and solvent that is employed; however,

Molecules 2020, 25, 4848
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conditions that result in full shielding of the 2-position of pyrrole itself have not been achieved [14].
On the other hand, for bromination and iodination, the TIPS group is reported to offer excellent
shielding as both exposure of N-TIPS-protected pyrrole to NBS or NIS resulted in clean formation of
the 3-bromo or 3-iodo derivative [14–17].
Cl
R
R
I
3
N
Cl
N
N
2
4
O
: R = Cl
O
O
1
5
N
N
N
H
1
H
TIPS
13
2
0a: R = H
0b: R = I
HO
TsO
TsO
3
: R = H
2
Scheme 3. Retrosynthetic analysis for 1 and 3.
The TIPS protection of 13 (Scheme 4) turned out to be a delicate step, as a reaction using THF as
solvent was successful with 96% yield the first time it was tested; however, all later attempts using
this solvent only gave back the starting material. The use of DMF as a solvent and NaH as a base [18],
and quenching the reaction by addition of crushed ice consistently gave the crude product of 20b
in around 94% yield and with high purity. Unfortunately, 20b proved to be somewhat unstable upon
flash chromatography purification and was therefore, for the most part, used in the next step without
purification. Upon prolonged storage, the TIPS group was cleaved off to give back the starting material.
Scheme 4. Attempted synthesis of 3.
Chlorination of 20b was performed analogously to the chlorination of the unprotected pyrrole
in 13 (see above). By using 2.05 equivalents of NCS, one major dichlorinated product was formed
1
as evident by H-NMR analysis of the crude product. The clean conversion of 20b to one major
dichlorinated product was very encouraging; however, upon global deprotection (vide infra), the NMR
spectra of the material obtained did not match those reported for natural product 3 [1]. It turned out
that, instead of the desired chlorination of the pyrrole 3- and 4-positions, selective dichlorination on
the pyrrole 4- and 5-positions had occurred to give regioisomer 21 in an 84% isolated yield (see NMR
discussion below for the structure assignment of 21).
Upon flash column purification of the crude product following the chlorination reaction,
we experienced that the material decomposed when ethyl acetate (0–20%) in hexanes was used
as the eluent, to give TIPS-deprotected 22 as the major product. We speculate that the presence of
acetic acid in the ethyl acetate might provide an explanation for the observed decomposition. By using
CH Cl (0–100%) in heptane as the eluent, decomposition was avoided completely.
2
2
Complete deprotection—i.e., removal of the iodine, the tosyl and the TIPS protecting groups—could
be facilitated using Zn in refluxing ethanolic NaOH (vide supra) to give 23 in high yield (88%).
Under acidic conditions, using Zn in refluxing ethanolic HCl, the tosyl group was retained providing
compound 24—the tosyl-protected analogue of 23. In the face of the undesired regioselectivity for
the dichlorination step and the lability of the TIPS group of 20b, further attempts to synthesize
1 and 3
were suspended.

Molecules 2020, 25, 4848
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The unambiguous determination of the chlorination pattern of fully protected 21 and partially
deprotected 24 was not trivial because of the absence of protons in the area. The spectral assignments
1
13
were made by a combination of H-1D, C-1D, HSQC, HMBC, COSY, NOESY/ROESY, 1,1-ADEQUATE
and high-resolution CLIP-HSQMBC (see Figures S36–S48). In order to establish the chlorination
pattern, it was necessary to unambiguously determine the position of the pyrrole CH-proton.
From the spectral data obtained of 24, we could exclude that this compound contained a proton
on the pyrrole 5-position due to the absence of any NOESY or ROESY correlation to the pyrrole NH
proton (Figure S48). The experimental chemical shift for the pyrrole CH was determined to be 111.3
ppm, which is not consistent with this carbon atom being next to a nitrogen, for which a chemical shift
of around 130 ppm would have been expected. Thus, in order to differentiate between positions 3
and 4 on the pyrrole, we had to establish which carbon atoms carried chlorine and differentiate these
from C-2 of the pyrrole, which experimentally resonates at 117.3 ppm—inconveniently near the two
chlorinated carbon atoms C-5 (117.1 ppm) and C-4 (111.9 ppm). The pyrrole C-4 signal was sharp
and a ~0.6 Hz 3 Cl isotope shift could be directly observed at 214 MHz field for C. The pyrrole
5,37
13
13
C-5 resonance was however broad and the isotope shift could not be observed directly in C-NMR
nor indirectly in a high-resolution selective CLIP-HSQMBC. A weak long-range J was observed
1
n
CH
between the proton and the carbon in the 2-position of the oxazole, and supports the proton sitting
in position 3 of the pyrrole, but since the coupling was very weak it could not be used alone to rule out
a potential four-bond coupling from the pyrrole 4 position.
In intermediate 21, the 3 Cl isotope shift could however be observed for both the C-4 (113.8 ppm)
5,37
13
and the C-5 (123.4 ppm) of the pyrrole, in both C-NMR and selective CLIP-HSQMBC (Figure S41),
presumably because the absence of the exchangeable pyrrole NH no longer causes line broadening
of the neighboring carbon resonances. The presence of the N-TIPS group in 21 did, however, affect
the chemical shifts significantly. On the other hand, the long range coupling constants between
the oxazole C-2 and the pyrrole H-3 (2.0 Hz), the pyrrole C-2 and the pyrrole H-3 (5.4 Hz) and the pyrrole
C-5 and H-3 (9.5 Hz) were not significantly changed by the TIPS group, and the measurements of these
couplings for all intermediates were used to ensure that the chemical shift assignment of the chlorinated
2
pyrrole carbons could be transferred from 21 to 24. Knowing the unambiguous assignments of pyrrole
carbons C-2 and C-5, 1,1-ADEQUATE correlations from pyrrole H-3 to C-2 and C-4 could be used
to conclusively determine the chlorination pattern.
3
. Materials and Methods
3
.1. General Information
Chemicals and solvents were purchased from Sigma-Aldrich and used as delivered unless otherwise
◦
stated. All moisture sensitive reactions were carried out under argon atmosphere in oven-dried (130 C)
equipment that has been cooled down under vacuum. Anhydrous THF was either obtained from a
sodium/benzophenone still or an anhydrous solvent delivery system (SPS-800 system from M. Braun
GmbH, Garching, Germany). Flash column chromatography was performed using silica gel from
Merck (Silica gel 60, 0.040–0.063 mm). Thin layer chromatography (TLC) analyses were performed
on aluminum sheets coated with Merck TLC silica gel 60 F254 and visualization was achieved by
using ultraviolet light (254 nm) or a solution of sodium permanganate. The NMR experiments were
recorded on a Bruker 400 MHz, a Bruker BioSpin AV500 or a Bruker BioSpin Ascend spectrometer
1
operating at 850 MHz for H equipped with an inverse-detected triple resonance (TCI) cryoprobe.
1
13
H and C chemical shifts (
δ) are reported in ppm with reference to the solvent residual peak (CDCl3:
δ
H 7.26 and C 77.16; (CD ) SO:
δ
δ
H 2.50 and C 39.98). All coupling constants are given in Hz.
δ
3
2
Positive and negative ion electrospray ionization mass spectrometry was conducted on a Thermo
electron LTQ Orbitrap XL spectrometer (Thermos, Bremen, Germany) with methanol as solvent.
Flash chromatography was performed on a Biotage SP1 HPFC system (Biotage, Uppsala, Sweden) or a
PuriFlash XS 420 system (Interchim, Montlucon Cedex, France).

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3
.2. Methods
3
.2.1. Synthesis of Phorbazole B (2)
The trichlorinated pyrrole 19 (7.65 mg, 12.5
µmol) was dissolved in ethanol (0.2 mL) and 6 M
aqueous NaOH (50
µL) was added. The reaction mixture was heated to reflux, zinc (12 mg, 190 µmol)
was added and heating was continued for 1 h. To the reaction mixture 1 mL 10% NaHCO was
3
added and extracted with 3
The solution was evaporated to give the title compound.
×
1 mL CHCl on a Biotage phase separator with a Na SO filter plug.
3 2 4
1
Colorless solid; yield 3.58 mg (87%); H-NMR (400 MHz, (CD ) SO)
δ
= 13.57 (s, 1H), 9.86 (s, 1H),
= 158.0, 151.6,
3
2
13
7.63 (d, J = 7.7, 2H), 7.60 (s, 1H),6.89 (d, J = 7.7, 2H); C-NMR (101 MHz, (CD ) SO)
δ
3
2
35
1
50.3, 125.7, 121.1, 118.3, 115.9, 115.8, 115.0, 110.0, 108.5; HRMS (ESI) m/z calcd. for C H O N Cl3
13 6 2 2
−
H] : 326.9500; found: 326.9496. The NMR spectra in (CD ) SO (Table S1) matched the original
3 2
[M
−
spectra [1].
3
.2.2. Synthesis of 4-(4-Iodo-2-(1H-pyrrol-2-yl)oxazol-5-yl)phenyl 4-methyl-benzenesulfonate (13)
To a solution of Boc-protected pyrrole 18 (530 mg, 0.87 mmol) in anhydrous CH Cl (5 mL) at
2
2
◦
0
C was slowly added Et N (1.20 mL, 8.74 mmol) and TMSOTf (1.58 mL, 8.74 mmol). The reaction
3
mixture was stirred at room temperature for 16 h, after which no starting material could be observed
by TLC analysis. The reaction mixture was diluted with ethyl acetate, quenched with cold water
and extracted with ethyl acetate (3
×
20 mL). The organic phase was concentrated and dissolved in a
small amount of CH Cl before adsorption on a Biotage snaplet precolumn and purified by flash
2
2
chromatography on a Biotage SNAP Ultra column using an eluent with 0–80% ethyl acetate in heptane
to give the title compound.
◦
1
Orange solid; yield 395 mg (89%); mp 53.4–54.7 C; H-NMR (400 MHz, CDCl )
δ
= 9.58 (bs, 1H),
.93 (d, J = 8.8, 2H), 7.74 (d, J = 8.2, 2H), 7.34 (d, J = 8.0, 2H), 7.08 (d, J = 8.8, 2H), 7.00–6.96 (m, 1H),
.91–6.87 (m, 1H), 6.34–6.30 (m, 1H), 2.46 (s, 3H); 13C-NMR (101 MHz, CDCl3)
= 157.5, 149.6, 147.4,
45.7, 132.3, 130.0, 128.7, 127.0, 126.4, 122.9, 122.6, 119.1, 111.7, 110.8, 79.7, 29.9 (grease), 21.9: HRMS
3
7
6
1
δ
+
(
ESI) m/z calcd. for C H O N IS [M + H] : 506.9870; found: 506.9865.
20 16 4 2
3
.2.3. Synthesis of 4-(2,4-diiodooxazol-5-yl)phenyl 4-methylbenzenesulfonate (14)
Oxazole 16 (5.00 g, 15.9 mmol) was dissolved in anhydrous THF (75 mL) and cooled to
◦
78 C.
−
Freshly prepared LiHMDS (1M in THF/hexanes, 47.6 mL, 47.6 mmol. Prepared by adding a 2.5 M n-BuLi
solution in hexanes into HMDS in THF) was added dropwise. The resulting mixture was stirred at
◦
−
78 C for 1 h, during which the solution turned very viscous. A solution of iodine (12.10 g, 47.6 mmol)
in anhydrous THF (10 mL) was added very slowly under thorough stirring. The resulting mixture
was heated to room temperature for 1 h before 10% aqueous Na S O (100 mL) was added to quench
2
2
3
the reaction and the resulting two layers were separated. The aqueous layer was extracted with ethyl
acetate (2 50 mL) and the combined organic layers were washed with saturated NaCl (50 mL), dried over
Na SO and evaporated. The resulting residue was purified by flash chromatography on a Biotage
×
2
4
SNAP Ultra column using an eluent with 5–40% ethyl acetate in heptane to give the title compound.
◦
1
Yellow solid; yield 6.99 g (79%); mp 57.3–58.7 C; H-NMR (400 MHz, CDCl ):
δ
= 7.86 (d, J = 8.9,
3
1
3
2
H), 7.73 (d, J = 8.4, 2H), 7.33 (d, J = 8.0, 2H), 7.09 (d, J = 8.9, 2H), 2.46 (s, 3H); C-NMR (101 MHz,
CDCl3):
δ
= 156.0, 150.3, 145.8, 132.3, 130.0, 128.7, 127.4, 125.3, 123.0, 101.4, 80.00, 21.9; HRMS (ESI) m/z
+
calcd for C H O KNI S [M + K] : 605.8130; found: 605.8129.
16
11
4
2
3
.2.4. Synthesis of 4-Formylphenyl 4-methylbenzenesulfonate (15)
4
-Hydroxy-benzaldehyde (20.33 g, 166 mmol) and DMAP (68 mg, 0.61 mmol) were dissolved in a
◦
mixture of CH Cl (200 mL) and Et N (75 mL) and cooled to 0 C. Tosyl chloride (31.7 g, 166 mmol)
2
2
3
was dissolved in CH Cl (60 mL) and added via an addition funnel for 20 min. The resulting mixture
2
2
was left to heat to room temperature overnight and was then poured into water (200 mL). The phases

Molecules 2020, 25, 4848
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were separated and the organic phase was washed with 1M aqueous HCl (2x100 mL), water (100 mL)
and saturated NaCl (100 mL) before drying over MgSO and evaporation. The residue was dissolved in a
4
◦
small amount of ethyl acetate and heated to reflux before cooling to -20 C overnight. The precipitated
crystals were filtered off and dried to afford 25.90 g of the title compound. The mother liquid was
evaporated and the crystallization was repeated to give an additional 2.81 g of the title compound.
The remaining mother liquid was evaporated and crystallized from 10–15% water in ethanol to give an
additional 9.10 g of the title compound.
◦
1
Colorless solid; yield 37.81 g (82%); mp 72.5–73.5 C; H-NMR (400 MHz, CDCl ):
δ = 9.96 (s, 1H),
3
13
7
.82 (d, J = 8.2, 2H), 7.71 (d, J = 7.9, 2H), 7.32 (d, J = 8.0, 2H), 7.16 (d, J = 8.2, 2H), 2.44 (s, 3H); C-NMR
(
101 MHz, CDCl3)
δ
= 190.7, 154.0, 146.0, 134.9, 132.1, 131.4, 130.1, 128.6, 123.2, 21.8; HRMS (ESI) m/z
−
calcd for C H O S [M − H] : 375.0384; found: 375.0384.
14
11
4
3
.2.5. Synthesis of 4-(Oxazol-5-yl)phenyl 4-methylbenzenesulfonate (16)
Aldehyde 15 (2.0 g, 7.24 mmol) and TosMIC (1.48 g, 7.60 mmol) were dissolved in DME (10 mL).
DBU (1.14 mL, 7.60 mmol) was added using a syringe and the resulting mixture was heated to reflux
until TLC analysis showed that the starting material had been consumed (approx. 5 h). The reaction
mixture was evaporated, re-dissolved in a small amount of acetone, adsorbed onto a Biotage snaplet
precolumn and purified by flash chromatography on a Biotage SNAP Ultra column using an eluent
with 15–100% ethyl acetate in heptane to give the title compound.
◦
1
Pale yellow solid; yield 1.59 g (70%); mp 112.2–114.1 C; H-NMR (400 MHz, CDCl ) δ = 7.90 (s, 1H),
3
13
7
(
.72 (d, J = 8.3, 2H), 7.57 (d, J = 8.8, 2H), 7.32 (m, 3H), 7.05 (d, J = 8.8, 2H), 2.45 (s, 3H); C-NMR
101 MHz, CDCl3) = 150.9, 150.5, 149.6, 145.7, 132.3, 130.0, 128.7, 126.8, 125.8, 123.2, 122.2, 21.9;
HRMS (ESI) m/z calcd for C H NO S [M + H] : 354.0197, found 354.0201.
δ
+
+
16
14
4
3
.2.6. Synthesis of 4-(2-Iodooxazol-5-yl)phenyl 4-methylbenzenesulfonate (17)
The title compound was isolated as a side-product during the synthesis of 14.
◦
1
Yellow solid; yield 837 mg (12%); mp 39.1–41.4 C; H-NMR (400 MHz, CDCl ):
δ = 7.71 (d, J = 8.4,
3
1
3
2
H), 7.51 (d, J = 8.8, 2H), 7.31 (d, J = 8.0, 2H), 7.24 (s, 1H), 7.04 (d, J = 8.8, 2H), 2.46 (s, 3H); C-NMR
= 152.7, 149.9, 146.8, 145.8, 132.3, 130.0, 128.7, 126.0, 125.5, 124.0, 123.4, 29.8
(101 MHz, CDCl3):
δ
+
(
grease), 21.9; HRMS (ESI) m/z calcd for C H O NIS [M + H] : 441.9604; found: 441.9604.
16
13
4
3
.2.7. Synthesis of t-Butyl 2-(4-iodo-5-(4-(tosyloxy)phenyl)oxazol-2-yl)-1H-pyrrole-1-carboxylate (18
)
Diiodooxazol 16 (3.22 g, 5.77 mmol), Cs CO (5.64 g, 17.3 mmol), (N-Boc-pyrrol-2-yl)boronic acid
2
3
(
1.46 g, 6.93 mmol) and PdCl (dppf) CH Cl (235 mg, 0.289 mmol) were dissolved in a mixture of
·
2
2 2
degassed dioxane (18 mL) and degassed water (6 mL). The solution was stirred at room temperature for 6
h under argon upon which TLC analysis showed some remaining starting material. Additional boronic
acid (0.5 equiv.) and catalyst (5 mol%) were added and the resulting mixture was stirred for 42 h after
which TLC still showed starting material left. Additional boronic acid (0.5 equiv.) and catalyst (5 mol%)
were added and the mixture was stirred for 18 h, after which TLC showed no sign of starting material.
The mixture was filtered through a plug of celite, diluted with ethyl acetate, washed with water (20 mL)
and saturated NaCl (20 mL), dried over Na SO and evaporated. The residue was purified by flash
2
4
chromatography on a Biotage SNAP Ultra column using an eluent with 0–80% ethyl acetate in heptane
to give the title compound.
◦
1
Orange solid; yield 2.64 g (75%); mp 73.1–73.3 C; H-NMR (400 MHz, CDCl ):
δ
= 7.94 (d, J = 8.8,
H), 7.73 (d, J = 7.9, 2H), 7.44 (s, 1H), 7.32 (d, J = 8.1, 2H), 7.26 (s, 1H), 7.15–7.02 (m, 2H), 6.75 (s, 1H),
.29 (d, J = 3.5, 1H), 2.45 (s, 3H), 1.44 (s, 9H); 13C-NMR (101 MHz, CDCl3):
= 156.6, 149.8, 149.2, 148.2,
3
2
6
δ
1
45.7, 132.4, 130.0, 128.7, 127.2, 126.4, 125.3, 122.9, 119.7, 119.7, 111.3, 85.1, 79.3, 27.8, 21.9; HRMS (ESI)
+
m/z calcd for C H O N IS [M + H] : 607.0394; found: 607.0394.
25
24
6
2

Molecules 2020, 25, 4848
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3
4
.2.8. Synthesis of 4-(4-Iodo-2-(3,4,5-trichloro-1H-pyrrol-2-yl)oxazol-5-yl)phenyl
-methylbenzenesulfonate (19)
2
-(4-Iodooxazole)pyrrole 13 (101 mg, 0.20 mmol) and triphenylphosphine sulfide (12 mg, 40
µ
mol)
◦
were dissolved in CH Cl (0.8 mL) and cooled to
portion and the reaction mixture was allowed to heat to room temperature for 10 min. A small aliquot
−20 C. NCS (82 mg, 0.61 mmol) was added in one
2
2
was subjected to HRMS analysis, which revealed that the trichlorination was not complete. The reaction
◦
mixture was again cooled to
−
20 C and additional NCS (5 mg, 37
µ
mol) was added. After heating
to room temperature no traces of dichlorinated compound(s) could be observed, and the reaction
mixture was directly applied onto a Biotage SNAP Ultra column and purified by flash chromatography
using 40–100% CH Cl in heptane to give the title compound. This material was used in the next
2
2
1
step without further purification; however, H-NMR analysis revealed the presence of minor amounts
of residual solvent/and or grease. A small sample was purified further by normal and C18 flash
1
13
chromatography for characterization by H- and C-NMR analysis.
◦
1
Pale orange solid; yield 88 mg (73%); mp 97.7–99.4 C; H-NMR (400 MHz, (CD ) SO)
d, J = 8.9, 2H), 7.78 (d, J = 8.2, 2H), 7.50 (d, J = 8.2, 2H), 7.24 (d, J = 8.9, 2H), 2.44 (s, 3H); C-NMR (101
δ
= 8.02
3
1
2
3
(
MHz, CDCl3) δ = 154.7, 149.4, 147.4, 146.5, 131.7, 130.8, 128.8, 127.3, 126.4, 123.3, 117.1, 115.0, 111.8, 109.2,
35
37
−
8
3.6, 21.7; HRMS (ESI) m/z calcd. for C H O N S Cl ClI [M − H] : 608.8526; found: 608.8514.
20
11
4
2
2
3
4
.2.9. Synthesis of 4-(4-Iodo-2-(1-(triisopropylsilyl)-1H-pyrrol-2-yl)oxazol-5-yl)-phenyl
-methylbenzenesulfonate (20b)
2
-(4-Iodo-oxazole)pyrrole 13 (479 mg, 0.95 mmol) was dissolved in anhydrous DMF (20 mL)
◦
and the solution was cooled at 0 C in an ice/water bath. NaH (60% in mineral oil, 92 mg, 2.28 mmol)
was added slowly and the color of the solution changed from green to orange. The flask was flushed
with argon and stirring was continued for 15 min before TIPS-Cl (0.25 mL, 1.14 mmol) was added
dropwise upon which the reaction mixture changed to a peachy color. Stirring was continued for
◦
1
5 min at 0 C before the cooling bath was removed and stirring continued at room temperature for 2 h
when TLC analysis showed complete conversion of the starting material (TLC sample quenched with
ice and extracted with ethyl acetate before spotting). The reaction mixture was cooled on ice, diluted
with ethyl acetate (20 mL) and quenched by slow addition of crushed ice. Phases were separated,
and the aqueous phase was extracted with ethyl acetate (2
washed with water (25 mL) and saturated NaCl (25 mL), dried over Na SO , filtered and evaporated
×
25 mL). The combined organic phases were
2
4
under reduced pressure. The residue was purified by flash chromatography using a Biotage SNAP
Ultra column using 0–20% ethyl acetate in heptane to give the title compound. For the most part,
the crude product from this step was used in the next step without further purification.
1
Colorless oil; 558 mg (89%); H-NMR (400 MHz, CDCl )
δ
= 7.94 (d, J = 8.8, 2H), 7.74 (d, J = 8.4,
3
2
1
1
H), 7.33 (d, J = 8.1, 2H), 7.12–7.02 (m, 4H), 6.35 (t, J = 3.1, 1H), 2.46 (s, 3H), 1.86 (hept, J = 7.6, 3H),
.13 (d, J = 7.6, 18H); 13C-NMR (101 MHz, CDCl3)
= 158.0, 149.4, 147.0, 145.7, 132.4, 131.1, 130.0, 128.7,
26.9, 126.7, 124.4, 122.8, 117.6, 111.1, 79.3, 29.84 (impurity), 21.9, 18.4, 13.5; HRMS (ESI) m/z calcd.
δ
+
for C H O N SiSINa [M + Na] : 685.1024; found: 685.1022.
29
35
4
2
3
4
.2.10. Synthesis of 4-(2-(4,5-Dichloro-1-(triisopropylsilyl)-1H-pyrrol-2-yl)-4-iodo-oxazol-5-yl)phenyl
-methylbenzenesulfonate (21)
TIPS-protected pyrrole 20b (100 mg, 0.161 mmol) and triphenylphosphine sulfide (10 mg, 32
µ
mol)
◦
were dissolved in CH Cl (0.8 mL) and cooled to
−
20 C. NCS (44 mg, 0.330 mmol) was added in one
2
2
batch, and the reaction mixture was allowed to warm to room temperature for 10 min. A small
aliquot was subjected to HRMS analysis, which showed that complete dichlorination had occurred.
The reaction mixture was directly applied onto a Biotage SNAP Ultra column and purified using flash
chromatography using 0–100% CH Cl in heptane to give the title compound.
2
2
1
Colorless oil; yield 99 mg (84%); H-NMR (400 MHz, CDCl3)
δ = 7.91 (d, J = 8.8, 2H),
7
.74 (d, J = 8.1, 2H), 7.34 (d, J = 8.1, 2H), 7.09 (d, J = 8.8, 2H), 6.75 (s, 1H), 2.46 (s, 3H), 1.72 (hept, J = 7.5,

Molecules 2020, 25, 4848
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3
H), 1.10 (d, J = 7.5, 18H); 13C-NMR (101 MHz, CDCl3)
δ
= 157.1, 149.9, 148.7, 145.8, 132.4, 130.0, 128.7,
1
27.2, 126.1, 123.8, 123.4, 123.0, 118.5, 113.8, 79.1, 29.9 (imp.), 21.9, 18.5, 13.7; HRMS (ESI) m/z calcd. for
35
+
C H N O N SiS Cl IK [M + K] : 768.9984; found: 768.9987.
29
33
2
4
2
2
3
.2.11. Synthesis of 4-(2-(4,5-Dichloro-1H-pyrrol-2-yl)oxazol-5-yl)phenol (23)
TIPS-protected dichlorinated pyrrole 21 (48 mg, 66
µ
mol) was dissolved in ethanol (0.7 mL)
◦
at 70 C and 20% aqueous NaOH (0.3 mL) was added, upon which the color changed from clear
to orange. The mixture was heated to reflux and, after 5 min, zinc (64 mg, 0.984 mmol) was added
and heating was continued for 45 min. The reaction mixture was filtered through a pad of Celite
and partitioned between ethyl acetate (10 mL) and water (10 mL). The aqueous phase was extracted
with ethyl acetate (2
×
10 mL) and the combined organic phases were washed with saturated NaCl,
dried over Na SO and evaporated. The residue was adsorbed onto a Biotage snaplet precolumn using
2
4
CH Cl and purified by flash chromatography using a Biotage SNAP Ultra column using 0–100% ethyl
2
2
acetate in heptane to give the title compound.
Colorless solid; yield 17 mg (88%); mp 246 C (dec.); H-NMR (400 MHz, (CD ) SO)
(
◦
1
δ
= 13.12
s, 1H), 9.82 (s, 1H), 7.62 (d, J = 8.4, 2H), 7.51 (s, 1H), 6.94–6.75 (m, 3H); 13C-NMR (101 MHz, (CD3)2SO)
= 157.8, 153.0, 150.1, 125.6, 121.1, 118.9, 118.5, 115.8, 114.7, 109.12, 109.11; HRMS (ESI) m/z calcd.
3
2
δ
35
+
for C H O N Cl [M + H] : 295.0036; found: 295.0037.
13
9
2
2
2
4
. Conclusions
In conclusion, we have achieved the total synthesis of phorbazole B in six steps with an overall
yield of 23%, showing the utility of a simple catalyzed chlorination and demonstrating the use of iodine
as a protection group for oxazoles. Furthermore, we have concluded that phorbazoles A and C were not
accessible through our synthetic approach based on previous literature reports and our own observation
of predominantly 4,5-dichlorination of the pyrrole instead of the desirable 3,4-dichlorination pattern of
phorbazole A and C.
Supplementary Materials: The following are available online, Figures S1–S22: 1D NMR spectra for
2
and 13
–24,
Table S1: Comparison of spectral data for synthetic and isolated
2, synthesis protocol for 24, structural assignment
of 21 and 24, Figures S25–S37: 1D and 2D NMR spectra for assignment of chlorination pattern for 21 and 24.
Author Contributions: Conceptualization, Y.G., J.I., B.E.H. and A.B.; funding acquisition, B.E.H. and A.B.;
methodology, Y.G., M.E.F., S.K.P. and J.I.; writing—original draft, Y.G., J.I., B.E.H. and A.B.; writing—review
and editing, Y.G., M.E.F., S.K.P., J.I., B.E.H. and A.B. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was funded by the Research Council of Norway (grants 224790/O30 and 244264/O30)
and the University of Bergen (PhD funding M.E.F.). The Research Council of Norway is further acknowledged for
its support through the Norwegian NMR Platform (NNP) 226244/F50. The publication charges for this article
have been funded by a grant from the publication fund of UiT The Arctic University of Norway
Acknowledgments: The authors would like to thank Jarl Underhaug for the assistance with running
NMR experiments.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study;
in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish
the results.
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