Scalable Biomimetic Syntheses of Paeciloketal B, 1- epi-Paeciloketal B, and Bysspectin A

Article abstract of DOI:10.1021/acs.jnatprod.1c00502

The first total synthesis of the benzannulated 5,5-spiroketal natural products paeciloketal B and 1-epi-paeciloketal B has been achieved in 10 linear steps employing a biomimetic spiroketalization. This approach also furnished the related natural product

Full text of DOI:10.1021/acs.jnatprod.1c00502

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Scalable Biomimetic Syntheses of Paeciloketal B, 1-epi-Paeciloketal
B, and Bysspectin A
Emma K. Davison, Caitlin E. Shepperson, Zoe E. Wilson, and Margaret A. Brimble*
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sı Supporting Information
ABSTRACT: The first total synthesis of the benzannulated 5,5-
spiroketal natural products paeciloketal B and 1-epi-paeciloketal B
has been achieved in 10 linear steps employing a biomimetic
spiroketalization. This approach also furnished the related natural
product bysspectin A from the same putative biosynthetic
precursor as the paeciloketals. Alternatively, bysspectin A could
be accessed in only six steps using an improved route. This scalable
and efficient synthesis affords insight into the biosynthesis of these
natural products in nature.
enzannulated 5,5-spiroketals are relatively rare in nature
The structurally related benzofuran natural product
B
(<20 reported), and a number have exhibited promising
bioactivity, motivating research into their isolation and
bysspectin A (2) was isolated in 2018 from the endophytic
6
fungus Byssochlamys spectabilis (the teleomorph of P. variotii ),
1
−4
obtained from the leaves of Edgeworthia chrysantha, a shrub
synthesis.
The novel benzannulated 5,5-spiroketal paecilo-
7
commonly used in Chinese traditional medicine. Bysspectin A
ketal B (1a) and its epimer 1-epi-paeciloketal B (1b) were
discovered in 2015. These natural products were isolated from
the culture broth of Paecilomyces variotii, a fungus acquired
from the giant jellyfish Nemopilema nomurai, which was
(
2) failed to exhibit antibacterial activity against Escherichia
coli, S. aureus, Staphylococcus epidermidis, or Mycobacterium
−1
smegmatis (MICs > 128 μg mL ); however, 2 was found to
display reversible, selective inhibition of human carboxylester-
5
collected from the South Korean Sea. While the two epimers
ase 2 (hCE2) (IC = 2.01 μM) over hCE1 (IC > 100 μM).
5
0
50
were separable by HPLC, 1a was found to rapidly convert to
As hCE2 is an important intestinal enzyme in the metabolism
of certain anticancer drugs such as irinotecan, capecitabine,
flutamide, and gemcitabine’s prodrug LY2334737, selective
hCE2 inhibitors such as 2 hold potential clinical value as
combination therapies to improve the efficacy or ameliorate
the associated adverse effects of these anticancer chemo-
1
b in MeOH at room temperature to afford an equilibrium
5
mixture of 1a:1b (11:14). The absolute configuration of the
C-1 and C-3 stereocenters was assigned through an ECD
5
study. Preliminary antibacterial screening of the mixture of
paeciloketals 1a and 1b against Staphylococcus aureus,
Streptococcus iniae, and Vibrio ichthyoenteri showed no
8
therapies.
Bysspectin A (2) was proposed to be biosynthetically
derived from reactive ketoaldehyde precursor 3, which is
supported by the coisolation of 2 with bysspectin C (4), since
−
1
significant activity (MICs > 40 μg mL ).
4
3
could also be derived from 3 through benzylic reduction (or
could be derived from 4 through benzylic oxidation)
7
(
Scheme 1). Alternatively, pinacol dimerization of ketoalde-
hyde 3 followed by dehydration could forge ketone 5, which
following cyclization and dehydration, would provide by-
Received: May 25, 2021
©
XXXX American Chemical Society and
American Society of Pharmacognosy
https://doi.org/10.1021/acs.jnatprod.1c00502
A
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Scheme 1. Plausible Biosynthetic Pathway for Natural
Scheme 2. Retrosynthesis of Paeciloketals 1a and 1b and
Bysspectin A (2)
Products 1a, 1b, 2, and 4 Isolated from P. variotii and B.
5
,7,9
spectabilis
Scheme 3. Efficient Synthesis of Bysspectin A (2) and
Attempted Synthesis of 11
sspectin A (2) (Scheme 1, blue arrows). The similarities in
both the structure and natural source of 1a, 1b, and 2 make it
probable that they share a common biosynthetic pathway;
therefore, it is postulated that ketone 5 could alternatively
undergo spiroketalization and methylation to provide
paeciloketals 1a and 1b (Scheme 1, green arrows).
While bysspectin A (2) was synthesized in 2019 by Xie and
1
0
co-workers, paeciloketals 1a and 1b have yet to succumb to
total synthesis. Owing to our ongoing interest in benzannu-
1
,2
lated spiroketal natural products and biomimetic synthe-
1
1−13
ses,
we sought to pursue the synthesis of 1a and 1b, and
in doing so, we hoped to explore their putative biosynthetic
relationship to 2. We anticipated that completion of the
synthesis of 1a, 1b, and 2 would also enable investigation into
the antifungal activity of these compounds, which has not been
explored to date.
a
Reagents and conditions: (a) bis(TMS)acetylene (0.6 equiv),
RESULTS AND DISCUSSION
Pd(PPh ) Cl , CuI, H SiF (aq), diisopropylamine, 70 °C, 17 h,
3
2
2
2
6
■
6
1
4% (7 → 9), 11% (10 → 2); (b) BBr , CH Cl , −78 °C to rt, 4 h,
3
2
2
Accordingly, it was decided to target the synthesis of putative
biosynthetic precursor 5, which could be subjected to both
biomimetic spiroketalization and methylation (affording 1a
and 1b) and cyclodehydration (affording 2), thus providing
insight into the proposed biosynthetic pathway (Scheme 2). It
was envisaged that ketone 5 could be accessed through
hydration and methyl ether cleavage of diarylalkyne 6, which in
turn could be obtained through sila-Sonogashira coupling of
known aryl iodide 710 with bis(trimethylsilyl)acetylene.
2%; (c) AlCl , chlorobenzene, 75 to 85 to 100 °C, 2 h, 41%.
3
7
,14,15
details).
Pleasingly, it was found that sila-Sonogashira
dimerization of 7 with bis(trimethylsilyl)acetylene (0.6 equiv)
16
using Tolnai’s procedure yielded diarylalkyne 9 in good yield.
Initial attempts to remove the methyl ether protecting groups
of 9 using boron tribromide led to a complex mixture.
However, treatment of 9 with aluminum trichloride success-
fully mediated deprotection followed by rapid cyclization,
presumably through Lewis-acid-mediated activation of the
alkyne, thus affording bysspectin A (2) in moderate yield
To this end, aryl iodide 71 was prepared from 3-
methoxybenzyl alcohol (8) in four steps using literature
procedures (Scheme 3) (see Supporting Information for
0
B
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Scheme 4. Scalable Biomimetic Synthesis of Paeciloketals (± )-1a and (± )-1b and Byspectin A (2)
a
Reagents and conditions: (a) BBr , CH Cl , 0 °C to rt, 4 h, 59%; (b) MOMCl, DIPEA, CH Cl , 0 °C to rt, 1 h, 97%; (c) OctylMgBr, THF, 0 °C,
3
2
2
2
2
3
0 min, 14 (45%), 15 (52%); (d) DMP, NaHCO , CH Cl , rt, 1 h, 97% for 16, 92% for 17; (e) bis(TMS)acetylene (0.6 equiv), Pd(PPh ) Cl ,
3
2
2
3
2
2
CuI, H SiF (aq), diisopropylamine, 80 °C, 65 h, 68%; (f) HCl, MeOH, CH Cl , rt, 4 h, 65%; (g) Hg(OAc) , PPTS, H O, THF, 45 °C, 1.5 h, 69%;
2
6
2
2
2
2
(
h) TFA, CH Cl , rt, 1.5 h, 81%; (i) TMSBr, CH Cl , −10 °C, 20 min; (j) PPTS, MeOH, CH Cl , 0 °C, 40 min, 62% over 2 steps.
2
2
2
2
2
2
(
41%, unoptimized). Bysspectin A (2) has been previously
alongside reduced benzyl alcohol 15. Nevertheless, benzyl
alcohol 15 could be recycled back to benzaldehyde 16 through
oxidation with Dess-Martin periodinane (DMP), thus
providing alcohol 14 in 68% overall yield after one round of
recycling. Oxidation of 14 provided ketone 17, which
underwent gram-scale sila-Sonogashira dimerization with
bis(trimethylsilyl)acetylene using the previously identified
conditions to provide diarylalkyne 18 in 68% yield. Pleasingly,
18 then underwent MOM ether cleavage smoothly with
hydrochloric acid to afford 11 in good yield. With 11 in hand,
hydration of the alkyne could next be investigated. Hamze and
synthesized using a convergent route (13 steps total) with an
10
overall yield of 7.7%; however, this route represents a more
step-efficient synthesis of the natural product, requiring only 6
steps to access 2 in 5.8% overall yield.
While this result was a welcome digression, our attention
returned to the primary objective: biomimetic synthesis of
paeciloketals 1a and b from ketone 5. As the methyl ether
cleavage of iodide 7 using boron tribromide had previously
been reported by Xie and co-workers in their synthesis of
10
bysspectin A (2), this was attempted next. In our hands,
however, phenol 10 was obtained in poor yield (12%), in
co-workers reported the use of PtO and p-toluenesulfonic acid
(pTSA) in aqueous MeOH for the internal hydration of
2
10
contrast with the reported yield of 74%. Nevertheless,
sufficient material was obtained to attempt the sila-Sonogashira
coupling. Frustratingly, under the aforementioned conditions,
considerable degradation occurred, and only bysspectin A (2)
was isolated in poor yield (11%), indicating that desired
product 11 was prone to cyclization under the reaction
conditions and that this approach was unlikely to allow access
to ketone 5.
diarylalkynes and stated that omission of PtO was required for
2
substrates bearing ortho-heteroatoms to avoid indole/benzo-
19
furan formation. It was thought that subjecting 11 to these
conditions might not only effect hydration of the alkyne but
also mediate concomitant spiroketalization and MeOH
addition to the resulting hemiketal under the acidic methanolic
conditions to provide 1a and 1b in a one-pot biomimetic
20
A more labile protecting group was deemed necessary to
allow for late stage removal under milder conditions.
cascade. Disappointingly, however, under these conditions,
11 was only observed to form bysspectin A (2), indicating that
the alkyne was more prone to attack from the adjacent phenols
than from H O, even in the absence of PtO . MOM-protected
Methoxymethyl (MOM) ether was chosen, since Turkmen
̈
and Rawal have demonstrated that diphenylacetylene diols can
be prepared in excellent yield through deprotection of the
corresponding MOM ethers with hydrochloric acid at room
2
2
diarylalkyne 18 was also subjected to these conditions (PtO2,
pTSA, MeOH, H O, 70 → 90 °C); however, this led to
2
1
7
15
temperature. Deprotection of known benzaldehyde 12
previously prepared en route to 7) with boron tribromide
according to the procedure described by Kingston and co-
formation of a complex mixture suspected to result from partial
deprotection of the MOM ethers. Thus, it was thought that
milder conditions might be required to promote hydration of
18 without mediating deprotection and subsequent formation
of 2. While both sodium sulfide/hydrochloric acid in aqueous
(
1
8
workers afforded desired phenol 13 in good yield (2 g scale,
Scheme 4). MOM protection of phenol 13 (gram scale),
followed by a Grignard reaction with octylmagnesium bromide
21
22
MeOH and PdCl in aqueous dioxane promoted only
2
(
2 g scale), afforded desired alcohol 14 in moderate yield
MOM ether cleavage and degradation respectively, treatment
C
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of 18 with Hg(OAc)2 and pyridinium p-toluenesulfonate
thought to be prone to epimerization, attempting an
asymmetric spiroketalization of 19 using chiral acids was
expected to be unproductive.
23
(
PPTS) in aqueous THF pleasingly afforded desired ketone
9 in good yield on a half-gram scale (460 mg) with no
1
observable formation of 2. Ketone 19 was selected as the end
point of the scalable synthesis, since this compound serves as a
divergent point for exploration of the biomimetic trans-
formations of 19 into both paeciloketals 1a/b and bysspectin A
Natural products (±)-1a/b and 2 were evaluated for
antifungal activity using minimum inhibitory concentration
(MIC) assays against two species of yeast, Candida albicans
SC5314 (type strain) and Candida utiliz SVB-Y1 (clinical
isolate), and one species of mold, Aspergillus fumigatus SVB-
F136 (clinical isolate). Unfortunately, both compounds failed
to show inhibitory activity toward any of the tested strains
(MIC > 64 μM, Table S1, Supporting Information).
(
2). Thus, with ample quantities of 19 in hand, the final MOM
deprotection, spiroketalization, and methylation sequence
toward paeciloketals 1a and 1b could be attempted.
Again, it was thought that acidic methanolic conditions
20
could feasibly achieve this sequence in a biomimetic cascade.
In summary, a scalable and efficient first synthetic route to
paeciloketals (±)-1a and (±)-1b was established in 6.1% yield
over 10 steps using a biomimetic spiroketalization of ketone
19. Bysspectin A (2) was prepared in only six linear steps, a
Unfortunately, treating 19 with either aqueous hydrochloric
acid or pTSA in MeOH/CH Cl (1:1) led to formation of
2
2
complex mixtures. Interestingly, however, TFA was found to
mediate clean conversion of 19 to bysspectin A (2) in excellent
yield (81%), thus supporting the proposed biosynthetic
cyclodehydration of 5 to form 2 in nature. To our delight,
10
much more efficient route than that previously disclosed.
The facile formation of 2 from putative biosynthetic precursors
19 and 20 under several conditions lends evidence to support
the proposed biosynthetic pathway, and the successful
synthesis of (±)-1a and (±)-1b from putative biosynthetic
precursor 19 supports their proposed biosynthetic relationship
to bysspectin A (2). Additionally, observations on the stability
of hemiketal (rac)-20 and natural products 1a and 1b suggest
enzymatic catalysis is required for the selective formation of
the C-3R methyl ketal stereoconfiguration of the natural
products in nature.
24
bromotrimethylsilane smoothly effected deprotection and
spiroketalization of 19 in one pot, to provide hemiketal (rac)-
2
0 (as a 1:1 mixture of diastereomers). Interestingly, (rac)-20
was prone to hemiketal ring opening and dehydration to form
(as observed by TLC) either when the reaction was
2
quenched with brine or when the crude material was
concentrated in the presence of residual HBr, thus lending
further evidence to support the common biosynthetic pathway
of 1a/b and 2 in nature. However, careful neutralization of the
reaction with 50% aqueous sodium bicarbonate provided
EXPERIMENTAL SECTION
■
(
rac)-20, which could be isolated; however, owing to its
General Experimental Procedures. Infrared spectra were
obtained using a PerkinElmer spectrum One Fourier Transform
Infrared spectrometer. Absorption maxima are expressed in wave-
observed instability upon storage, the crude material was
treated directly with methanolic PPTS to provide paeciloketals
−
1
numbers (cm ). NMR spectra were recorded as indicated on either a
Bruker 400 MHz Avance III spectrometer or a Bruker 500 MHz
Avance III-HD spectrometer (equipped with a cryogenically cooled
(
±)-1a and (±)-1b in 62% yield over two steps. The
1
diastereomeric ratio (1a:1b, 11:14) along with the H and
1
3
C NMR data of synthetic (±)-1a and (±)-1b showed
1
probehead) as specified. H NMR chemical shifts are reported in parts
excellent agreement with that reported for the natural
per million (ppm) relative to the chloroform (δH 7.26) or methanol
5
1
products; however, careful analysis of the 2D NMR data of
signal (δH 3.31), except for the paeciloketals (±)-1a and (±)-1b H
synthetic (±)-1a/b led us to review the assignments for several
of the signals (see Supporting Information for details). The
authors of the isolation paper reportedly detected 1a and 1b in
MeOH-free extracts of the culture broth by both normal-phase
NMR spectrum, which was calibrated to the OCH signal for 1-epi-
3
1
3
paeciloketal B ((±)-1b, δ
3.12). The C NMR values were
H
referenced to the residual chloroform (δC 77.16) or methanol (δ
C
1
3
4
9.0) signal except for the paeciloketals (±)-1a and (±)-1b C NMR
5
spectrum, which was calibrated to the CH
2
(1′) signal for 1-epi-
and reversed-phase TLC analysis. While TLC analysis of an
paeciloketal B ((±)-1b, δ 40.9). NMR assignments were made with
C
extract containing a complex mixture of metabolites is not a
conclusive method of unequivocally confirming the presence of
methyl ketals 1a and 1b in the MeOH-free extract, the
observed facile dehydration of hemiketal (rac)-20 in this study
corroborates the postulate that the C-3 methyl ketal of 1a/b
occurs naturally and is not an artificial derivative of 20.
However, in the absence of conclusive evidence, it remains
plausible that 1a and 1b are isolation artifacts resulting from
reaction of hemiketal 20 with MeOH.
The authors of the isolation paper reported that 1a and 1b
existed as an 11:14 mixture of epimers (1R,3R/1S,3R) that
were prone to epimerization at the spiroketal C-1 stereocenter
but that no epimerization was observed at C-3. Since we
observed that putative biosynthetic intermediate hemiketal 20
is prone to ring opening and dehydration to form bysspectin A
the aid of DEPT 90, DEPT 135, COSY, and HSQC experiments. All
experiments were conducted at 298 K. Conventional NMR tubes (5
mm diameter, Wilmad) using a sample volume of 500 μL were used.
High-resolution mass spectra were obtained by electrospray ionization
in positive ion mode at a nominal accelerating voltage of 70 eV on a
microTOF mass spectrometer, except for phenol 13, which required
negative ion mode. Analytical thin layer chromatography was
performed using silica plates, and compounds were visualized at
2
54 and/or 365 nm ultraviolet irradiation followed by staining with
either alkaline permanganate or ethanolic vanillin solution.
Commercially available reagents were used throughout without
purification unless otherwise stated. Anhydrous solvents were used
as supplied. Tetrahydrofuran and CH Cl were dried using an LC
2
2
Technology Solutions Inc. SP-1 solvent purification system under an
atmosphere of dry nitrogen. All reactions were routinely carried out in
oven-dried glassware under a nitrogen atmosphere unless otherwise
stated. All reactions requiring heating were done so using IKA
magnetic hot plates with integrated temperature control sensors and
DrySyn heating blocks or sand baths.
(
2), this indicates that C-3 of hemiketal 20 would also be
subject to epimerization but that the methyl ketal (of 1a and
b) stabilizes this center and prevents C-3 epimerization.
1
General Procedure A: Sila-Sonogashira Coupling. A stirred
solution of aryl iodide in diisopropylamine (0.14 M) was degassed
with argon by sparging for 10 min, before Pd(PPh ) Cl (5 mol %),
Taken together, this suggests that in nature enzymatic control
is required to set the methyl ketal C-3R stereoconfiguration of
3
2
2
1
a and 1b. Furthermore, as 1a and 1b were characterized as a
copper(I) iodide (5 mol %), hexafluorosilicic acid (34% aqueous, 5
μL, 0.048 mmol), and bis(trimethylsilyl)acetylene (0.6 equiv) were
mixture, and both the C-1 and C-3 stereocenters of 20 were
D
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added in that order. The vessel was sealed under argon and heated to
the temperature stated for the specified time. The mixture was then
purified by flash chromatography on silica gel, eluted with petroleum
ether−EtOAc (3:1) to afford phenol 13 (1.49 g, 6.01 mmol, 59%) as
−
1
diluted with EtOAc and quenched with H O, the layers were
a pale yellow amorphous solid. v /cm (neat): 3128, 1660, 1562,
2
max
separated, and the aqueous layer was extracted with EtOAc. The
combined organic layers were then dried using anhydrous Na SO ,
filtered, and concentrated in vacuo. The crude product was purified by
flash chromatography on silica gel, eluted with petroleum ether−ethyl
acetate (4:1) to afford the desired product.
1455, 1290, 1274, 1252, 1216, 1164, 1015, 780; δH (400 MHz,
CDCl ) 10.03 (1H, s, CHO), 7.46 (1H, dd, J 7.5, 1.5, CH), 7.36 (1H,
2
4
3
t, J 7.8, CH), 7.27 (1H, dd, J 8.0, 2.0, CH), 5.80 (1H, s, OH); δ (100
C
MHz, CDCl ) 194.7 (CHO), 155.6 (C), 135.9 (C), 130.1 (CH),
3
124.2 (CH), 120.9 (CH), 91.2 (C); HRMSESIMS m/z: 246.9263 [M
−
General Procedure B: Dess-Martin Periodinane (DMP)
Oxidation. To a stirred solution of alcohol starting material in
− H] (calcd for C H IO , 246.9262); R 0.25 (petroleum ether−
7
4
2
f
EtOAc, 3:1).
CH Cl2 (0.064 M) was added sodium bicarbonate (6.8 equiv)
2-Iodo-3-(methoxymethoxy)benzaldehyde (16). To a stirred
2
followed by DMP (1.7 equiv), and the resulting mixture was stirred at
room temperature (rt) for 1 h. Upon complete consumption of the
starting material, saturated solutions of sodium bicarbonate (20 mL)
and sodium thiosulfate (20 mL) were added, and the mixture was
stirred vigorously for 10 min. The layers were then separated, and the
aqueous layer was extracted with CH Cl (3 × 30 mL). The
solution of phenol 13 (1.49 mg, 6.02 mmol) in CH Cl (65 mL) at 0
2
2
°C was added diisopropylethylamine (2.02 mL, 11.6 mmol) and
chloromethyl methyl ether (0.67 mL, 7.85 mmol). The resulting
solution was warmed to rt and stirred for 1 h, before being diluted
with CH Cl (50 mL) and quenched with H O (50 mL). The layers
2
2
2
2
2
were separated, and the organic layer was washed with H O/saturated
2
combined organic extracts were dried using anhydrous Na SO ,
aqueous sodium hydrogen carbonate (1:1, 60 mL). The combined
2
4
filtered, and concentrated in vacuo. The crude product was purified by
flash chromatography on silica gel, eluted with petroleum ether−
EtOAc (9:1) to afford the desired product.
organic layers were then dried using anhydrous Na SO , filtered, and
2
4
concentrated in vacuo. The crude product was purified by flash
chromatography on silica gel, eluted with petroleum ether−EtOAc
(9:1) to afford 16 (1.71 g, 5.86 mmol, 97%) as a colorless amorphous
1
0
Diarylalkyne 9. Aryl iodide 7 (20 mg, 0.0535 mmol) was
subjected to General Procedure A, with heating to 70 °C in a screw-
capped vial for 17 h to afford diarylalkyne 9 (8.8 mg, 0.0170 mmol,
−
1
solid. vmax/cm (neat): 3079, 2893, 1688, 1563, 1457, 1385, 1257,
1233, 1155, 1083, 1001, 921, 893, 785; δ (400 MHz, CDCl ) 10.19
H
3
−
1
6
1
4%) as an orange waxy solid. IR vmax/cm (neat): 2922, 2851, 1689,
568, 1466, 1270, 1294, 997, 787, 737; δ (400 MHz, CDCl ) 7.33
(1H, s, CHO), 7.55 (1H, dd, J 7.5, 1.5, CH), 7.36 (1H, td, J 7.8, 1.0,
H
3
CH), 7.31 (1H, dd, J 8.1, 1.6, CH), 5.30 (2H, s, CH ), 3.54 (3H, s,
2
(
2H, t, J 8.1, 2 × CH), 7.10 (2H, dd, J 7.5, 1.0, 2 × CH), 6.98 (2H, d,
OCH ); δ (100 MHz, CDCl ) 196.4 (CHO), 156.5 (C), 137.0 (C),
3
C
3
J 8.0, 2 × CH), 3.91 (6H, s, 2 × OCH ), 3.19 (4H, t, J 7.5, 2 × CH ),
1
(
129.6 (CH), 123.7 (CH), 120.2 (CH), 95.4 (CH ), 94.9 (C), 56.7
3
2
2
+
.68 (4H, pent, J 7.5, 2 × CH ), 1.32−1.20 (20H, m, 10 × CH ), 0.84
(OCH ); HRMSESIMS m/z: 314.9485 [M + Na] (calcd for
2
2
3
6H, t, J 7.0, 2 × CH ); δ (100 MHz, CDCl ) 205.3 (2 × C), 160.8
C H IO Na, 314.9489); R 0.43 (petroleum ether−EtOAc, 4:1).
3
C
3
9
9
3
f
(
2 × C), 144.5 (2 × C), 129.7 (2 × CH), 119.7 (2 × CH), 112.6 (2 ×
Benzyl Alcohol 15 and Alcohol 14. A two-necked flask attached
CH), 110.2 (2 × C), 93.6 (2 × C), 56.2 (2 × OCH ), 42.8 (2 ×
with a Vigreux condenser and charged with magnesium granules (768
mg, 31.6 mmol) was flame-dried and then allowed to cool under
vacuum. The apparatus was flushed with argon, and then, enough
THF was added to just cover the magnesium (∼5 mL). Dibromo-
ethane (∼0.1 mL) was then added, followed by trimethylsilyl chloride
(∼0.1 mL) until the mixture was bubbling vigorously and the flask
was hot to the touch, at which point 1-bromooctane (4.42 mL, 25.6
mmol) was added dropwise as a solution in THF (25 mL). The
resulting mixture was heated to reflux for 1.5 h under argon and then
left to cool to rt without stirring for 1 h to allow the residual
magnesium to settle.
3
CH ), 32.0 (2 × CH ), 29.6 (2 × CH ), 29.5 (2 × CH ), 29.3 (2 ×
2
2
2
2
CH ), 24.7 (2 × CH ), 22.8 (2 × CH ), 14.2 (2 × CH );
2
2
2
3
+
HRMSESIMS m/z: 541.3273 [M + Na] (calcd for C H O Na,
3
4
46
4
5
41.3288); R 0.33 (petroleum ether−EtOAc, 4:1).
f
Bysspectin A (2). Method A. To a stirred solution of diarylalkyne
9
(18 mg, 0.0347 mmol) in chlorobenzene (1.4 mL) was added
aluminum trichloride (23 mg, 0.135 mmol), and the resulting solution
was heated to 75 °C for 30 min, then 85 °C for 30 min, and then 100
°
C for 1 h. Upon completion, the reaction mixture was cooled to rt,
diluted with EtOAc (10 mL), and quenched with H O/saturated
2
aqueous sodium hydrogen carbonate (1:1, 10 mL). The layers were
separated, and the aqueous layer was extracted with EtOAc (10 mL).
The combined organic layers were dried using anhydrous Na SO ,
filtered, and concentrated in vacuo. The crude product was purified by
flash chromatography on silica gel, eluted with petroleum ether−
EtOAc (85:15) to afford bysspectin A (2) (7.0 mg, 0.0143 mmol,
To a stirred solution of benzaldehyde 16 (2.002 g, 6.86 mmol) in
THF (80 mL) at 0 °C was added the octylmagnesium bromide
solution (12 mL, estimated 8.93 mmol) dropwise, and the resulting
solution was stirred for 30 min. Upon complete consumption of the
starting material, the reaction mixture was quenched with saturated
aqueous ammonium chloride (40 mL), and the mixture was extracted
with diethyl ether (3 × 50 mL). The combined organic layers were
then dried using anhydrous Na SO , filtered, and concentrated in
2
4
4
1%) as a yellow oil.
Method B. To a stirred solution of 19 (10 mg, 0.0168 mmol) in
2
4
CH Cl (1 mL) was added trifluoroacetic acid (0.1 mL), and the
vacuo. The crude product was purified by flash chromatography on
silica gel, eluted with petroleum ether−EtOAc (4:1) to afford the
desired alcohol 14 (1.25 g, 3.07 mmol, 45%) as a colorless waxy solid,
alongside benzyl alcohol 15 (1.06 g, 3.59 mmol, 52%) as a colorless
amorphous solid. Benzyl alcohol 15 (1.05 g, 3.57 mmol) could be
recycled back to benzaldehyde 16 (1.01 g, 3.46 mmol, 97%)
according to General Procedure B, thus providing desired alcohol
14 in 68% overall yield after 1 round of recycling. Alcohol 14: vmax/
2
2
resulting solution was stirred at rt for 1.5 h. Upon complete
consumption of the starting material, the reaction was diluted with
CH Cl (10 mL) and washed with H O (5 mL). The organic layer
2
2
2
was dried using anhydrous Na SO , filtered, and concentrated in
2
4
vacuo. The crude product was purified by flash chromatography on
silica gel, eluted with petroleum ether−EtOAc (4:1) to afford
bysspectin A (2) (6.7 mg, 0.0137 mmol, 81%) as a yellow oil. v
/
max
−
1
−1
cm (neat): 3357, 2923, 2854, 1678, 1587, 1456, 1424, 1289, 1259,
cm (neat): 3389, 2923, 2853, 1458, 1252, 1153, 1007, 923, 786,
+
9
21, 793, 746; HRMSESIMS m/z: 513.2969 [M + Na] (calcd for
717; δ (400 MHz, CDCl ) 7.28 (1H, t, J 8.0, CH), 7.18 (1H, dd, J
H
3
1
C H O Na, 513.2975); R 0.41 (petroleum ether−EtOAc, 7:3). H
7.6, 1.5, CH), 6.97 (1H, dd, J 8.2, 1.5, CH), 5.26, 5.24 (2H, ABq, J
6.5, CH ), 5.03−5.00 (1H, m, CH), 3.52 (3H, s, OCH ), 1.96 (1H,
3
2
42
4
f
1
3
and C NMR, Tables S2 and S3 (Supporting Information).
-Hydroxy-2-iodobenzaldehyde (13). To a stirred solution of
aryl iodide 12 (2.68 g, 10.2 mmol) in CH Cl (100 mL) at 0 °C was
added boron tribromide (1 M solution in CH Cl , 25.5 mL, 25.5
2
3
3
brs, OH), 1.81−1.74 (1H, m, 1/2 × CH ), 1.66−1.56 (1H, m, 1/2 ×
2
15
CH ), 1.53−1.27 (12H, m, 6 × CH ), 0.88 (3H, t, J 6.7, CH ); δ
2
2
2
2
3
C
(100 MHz, CDCl ) 156.7 (C), 149.1 (C), 129.5 (CH), 120.5 (CH),
2
2
3
mmol) dropwise, and the resulting solution was warmed to rt for 4 h.
113.8 (CH), 95.3 (CH ), 91.6 (C), 77.8 (CH), 56.6 (OCH ), 37.9
2
3
The solution was then carefully quenched with H O (20 mL) and
(CH ), 32.0 (CH ), 29.68 (CH ), 29.63 (CH ), 29.4 (CH ), 26.1
2
2 2 2 2 2
diluted with CH Cl (50 mL). The layers were separated, and the
organic layer was washed with brine (50 mL), dried using anhydrous
(CH ), 22.8 (CH ), 14.3 (CH ); HRMSESIMS m/z: 429.0890 [M +
2 2 3
2
2
+
Na] (calcd for C H IO Na, 429.0897); R 0.31 (petroleum ether−
1
7
27
3
f
−1
Na SO , filtered, and concentrated in vacuo. The crude product was
EtOAc, 17:3). Benzyl alcohol 15: v /cm (neat): 3289, 2899,
2
4
max
E
https://doi.org/10.1021/acs.jnatprod.1c00502
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
pubs.acs.org/jnp
Article
1
459, 1253, 1147, 1003, 918, 766; δ (400 MHz, CDCl ) 7.29 (1H, t,
quenched with water (0.2 mL) followed by H O/saturated aqueous
H
3
2
J 8.0, CH), 7.13 (1H, dd, J 7.3, 1.2, CH), 7.01 (1H, dd, J 8.0, 1.2,
CH), 5.26 (2H, s, CH ), 4.72 (2H, d, J 6.5, CH ), 3.52 (3H, s,
sodium hydrogen carbonate (1:1, 3 mL). The mixture was diluted
with CH Cl (5 mL), and the layers were separated. The organic layer
2
2
2
2
OCH ), 2.04 (1H, t, J 6.5, OH); δ (100 MHz, CDCl ) 156.0 (C),
was washed with H O/saturated aqueous sodium hydrogen carbonate
3
C
3
2
1
44.8 (C), 129.5 (CH), 122.1 (CH), 114.1 (CH), 95.2 (CH ), 90.8
(1:1, 5 mL) and then dried using anhydrous Na SO , filtered, and
2
2
4
(
C), 69.9 (CH ), 56.6 (OCH ); HRMSESIMS m/z: 316.9638 [M +
concentrated in vacuo. The crude hemiketal intermediate (rac)-20
could be used directly in the next step or, for characterization
purposes, was purified by flash chromatography on silica gel, eluted
with petroleum ether−EtOAc (4:1) (78% yield) (see Supporting
Information for characterization data). The crude hemiketal
intermediate (rac)-20 was dissolved in a solution of MeOH−
2
3
+
Na] (calcd for C H IO Na, 316.9645); R 0.13 (petroleum ether−
EtOAc, 17:3).
-(2-Iodo-3-(methoxymethoxy)phenyl)nonan-1-one (17).
Carbonyl 17 (1.14 g, 2.82 mmol, 92%) was prepared as a yellow oil
from alcohol 14 (1.25 g, 3.08 mmol) according to General Procedure
B. vmax/cm (neat): 2924, 2854, 1701, 1563, 1457, 1421, 1255, 1154,
9
11
3
f
1
−
1
CH Cl (1:1, 1 mL) and cooled to 0 °C, and then, pyridinium p-
2 2
9
7
73, 923, 782; δ (400 MHz, CDCl ) 7.31 (1H, dd, J 8.5, 7.5, CH),
.09 (1H, dd, J 8.5, 1.5, CH), 6.88 (1H, dd, J 7.5, 1.5, CH), 5.26 (2H,
toluenesulfonate (1.5 mg, 0.00597 mmol) was added, and the solution
was stirred for 40 min. The solvent was removed under a gentle
stream of nitrogen, and then, the crude material was taken up in
H
3
s, CH ), 3.52 (3H, s, OCH ), 2.86 (2H, t, J 7.3, CH ), 1.72 (2H, pent,
2
3
2
J 7.4, CH ), 1.39−1.27 (10H, m, 5 × CH ), 0.88 (3H, t, J 7.0, CH );
CH
2
Cl
2
(10 mL) and washed with H
2
O (5 mL). The organic layer
2
2
3
δ (100 MHz, CDCl ) 206.3 (C), 156.4 (C), 148.6 (C), 129.7 (CH),
was dried using anhydrous Na
2
SO , filtered, and concentrated in
4
C
3
1
20.4 (CH), 115.6 (CH), 95.2 (CH ), 84.4 (C), 56.7 (OCH ), 42.8
vacuo. The crude product was purified by flash chromatography on
silica gel, eluted with petroleum ether−EtOAc (9:1) to afford an
∼11:14 mixture of (±)-1a and (±)-1b (3.3 mg, 0.00632 mmol, 62%
2
3
(
(
CH ), 32.0 (CH ), 29.5 (CH ), 29.32 (CH ), 29.28 (CH ), 24.1
CH ), 22.8 (CH ), 14.2 (CH ); HRMSESIMS m/z: 427.0731 [M +
2 2 2 2 2
2
2
3
+
−1
Na] (calcd for C H IO Na, 427.0741); R 0.44 (petroleum ether−
EtOAc, 17:3).
Diarylalkyne 18. Aryl iodide 17 (1.14 g, 2.82 mmol) was
subjected to General Procedure A, with heating to 80 °C in a sealed
tube for 65 h to afford diarylalkyne 18 (557 mg, 0.963 mmol, 68%) as
over two steps) as a pale yellow amorphous solid. vmax/cm (neat):
1
7
25
3
f
3364, 2922, 2852, 1683, 1609, 1588, 1450, 1353, 1294, 1260, 1155+,
1091, 1025, 885, 797, 748; HRMSESIMS m/z: 545.3224 [M + Na]
(calcd for C33H O Na, 545.3237); R 0.38 and 0.41 (petroleum
46 5 f
1
13
ether−EtOAc, 4:1). H and C NMR, Tables S4−S7.
−
1
a yellow amorphous solid. vmax/cm (neat): 2919, 2853, 1691, 1568,
464, 1249, 1150, 1037, 977, 788, 735; δ (400 MHz, CDCl ) 7.32
1
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jnatprod.1c00502.
H
3
■
(
2H, dd, J 8.5, 7.5, 2 × CH), 7.25 (2H, dd, J 1.0, 2 × CH)*, 7.15 (2H,
*
dd, J 7.5, 1.5, 2 × CH), 5.27 (4H, s, 2 × CH ), 3.51 (6H, s, 2 ×
OCH ), 3.14 (4H, t, J 7.4, 2 × CH ), 1.67 (4H, pent, J 7.4, 2 × CH ),
2
3
2
2
1
.31−1.20 (20H, m, 10 × CH ), 0.85 (6H, t, J 7.0, 2 × CH ); δ (100
2
3
C
MHz, CDCl ) 204.9 (2 × C), 158.7 (2 × C), 144.5 (2 × C), 129.7 (2
Experimental details and characterization data for the
synthesis of 7 (from 8 via 12), 10, 11, and (±)-20a/b;
experimental details and results of the antifungal assays
3
×
CH), 120.9 (2 × CH), 117.1 (2 × CH), 111.3 (2 × C), 95.2 (2 ×
CH ), 93.1 (2 × C), 56.5 (2 × OCH ), 42.7 (2 × CH ), 31.9 (2 ×
2
3
2
CH ), 29.6 (2 × CH ), 29.5 (2 × CH ), 29.3 (2 × CH ), 24.8 (2 ×
1
13
2
2
2
2
of 2 and (±)-1a/b; H and C NMR comparisons of
CH ), 22.8 (2 × CH ), 14.2 (2 × CH ), *Signal partially occluded by
2
2
3
synthetic bysspectin A (2), (±)-paeciloketal B (1a), and
+
residual CHCl peak; HRMSESIMS m/z: 601.3477 [M + Na] (calcd
3
(
±)-1-epi-paeciloketal B (1b) with that detailed in their
for C H O Na, 601.3500); R 0.26 (petroleum ether−EtOAc 4:1).
36
50
6
f
1 13
respective isolation reports; H and C NMR spectra
Ketone 19. To a stirred solution of diarylalkyne 18 (464 mg,
.802 mmol) in THF (19 mL) was added pyridinium p-
for compounds 9, 2, 13, 16, 14, 15, 17, 18, 11, 19, 20,
0
(
±)-21a/b, (±)-1a/b; COSY and edited HSQC NMR
toluenesulfonate (305 mg, 1.21 mmol) and H O (0.16 mL), followed
by mercury(II) acetate (80 mg, 0.251 mmol, 30 mol %), and the
resulting mixture was heated to 45 °C for 1.5 h. The mixture was then
cooled to rt, diluted with EtOAc (30 mL), and washed with H O/
saturated aqueous sodium hydrogen carbonate (1:1, 40 mL). The
organic layer was then dried using anhydrous Na SO , filtered, and
2
spectra for (±)-1a/b (PDF)
2
AUTHOR INFORMATION
Corresponding Author
■
2
4
Margaret A. Brimble − School of Chemical Sciences,
University of Auckland, Auckland 1010, New Zealand;
School of Biological Sciences, University of Auckland,
Auckland 1010, New Zealand; The Maurice Wilkins Centre
for Molecular Biodiscovery, The University of Auckland,
Auckland 1142, New Zealand; orcid.org/0000-0002-
concentrated in vacuo. The crude product was purified by flash
chromatography on silica gel, eluted with petroleum ether−EtOAc
(
4:1) to afford ketone 19 (332 mg, 0.557 mmol, 69%) as a yellow
−
1
amorphous solid. vmax/cm (neat): 2924, 2854, 1687, 1578, 1456,
1
2
258, 1152, 984, 922, 734; δ (400 MHz, CDCl ) 7.39−7.37 (2H, m,
H
3
× CH), 7.36−7.32 (1H, m, CH), 7.29−7.27 (2H, m, 2 × CH), 7.19
(
(
7
1H, dd, J 5.3, 3.6, CH), 5.22 (2H, s, CH ), 5.21 (2H, s, CH ), 4.59
2H, s, CH ), 3.51 (3H, s, OCH ), 3.49 (3H, s, OCH ), 2.96 (2H, t, J
2
2
7086-4096; Email: m.brimble@auckland.ac.nz
2
3
3
.6, CH ), 2.81 (2H, t, J 7.5, CH ), 1.71 (2H, pent, J 7.4, CH ), 1.61
2
2
2
Authors
(
(
2H, pent, J 7.4, CH ), 1.37−1.26 (20H, m, 10 × CH ), 0.89−0.86
6H, m, 2 × CH ); δ (100 MHz, CDCl ) 206.2 (C), 201.7 (C),
2
2
Emma K. Davison − School of Chemical Sciences, University
3 C 3
of Auckland, Auckland 1010, New Zealand; School of
Biological Sciences, University of Auckland, Auckland 1010,
New Zealand; The Maurice Wilkins Centre for Molecular
Biodiscovery, The University of Auckland, Auckland 1142,
New Zealand; orcid.org/0000-0002-9280-2742
Caitlin E. Shepperson − School of Chemical Sciences,
University of Auckland, Auckland 1010, New Zealand
Zoe E. Wilson − School of Chemical Sciences, University of
Auckland, Auckland 1010, New Zealand; The Maurice
Wilkins Centre for Molecular Biodiscovery, The University of
Auckland, Auckland 1142, New Zealand
200.7 (C), 156.8 (C), 154.5 (C), 142.9 (C), 138.0 (C), 132.3 (C),
130.2 (CH), 127.8 (CH), 121.85 (CH), 121.76 (C), 120.6 (CH),
118.6 (CH), 116.7 (CH), 95.3 (CH ), 94.8 (CH ), 56.5 (OCH ),
5
3
2
2
2
2
3
6.1 (OCH ), 42.3 (CH ), 40.5 (CH ), 39.8 (CH ), 32.03 (CH ),
3
2
2
2
2
1.99 (CH ), 29.8 (CH ), 29.6 (CH ), 29.5 (CH ), 29.43 (CH ),
2
2
2
2
2
9.40 (CH ), 29.3 (CH ), 24.2 (CH ), 24.1 (CH ), 22.82 (CH ),
2
2
2
2
2
2.80 (CH ), 14.2 (2 × CH ); HRMSESIMS m/z: 619.3599 [M +
2
3
+
Na] (calcd for C H O Na, 619.3605); R 0.23 (petroleum ether−
EtOAc, 4:1).
36
52
7
f
Paeciloketal B and 1-epi-Paeciloketal B (11:14, (± )-1a:1b).
To a stirred solution of ketone 19 (5 mg, 0.00838 mmol) in CH Cl
2
2
(
0.33 mL) at −10 °C was added bromotrimethylsilane (9 μL, 0.0682
mmol), and the resulting solution was stirred for 20 min before being
Complete contact information is available at:
F
https://doi.org/10.1021/acs.jnatprod.1c00502
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
pubs.acs.org/jnp
Article
https://pubs.acs.org/10.1021/acs.jnatprod.1c00502
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to thank Dr. A. J. Cameron (The University
of Auckland) for kindly conducting the antifungal assays. The
authors wish to acknowledge the Royal Society of New
Zealand Te Aparangi for a Rutherford Foundation Postdoc-
̅
toral Fellowship (EKD) and the Maurice Wilkins Centre for
Molecular Biodiscovery for financial support.
REFERENCES
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https://doi.org/10.1021/acs.jnatprod.1c00502
J. Nat. Prod. XXXX, XXX, XXX−XXX