Relative stereochemical assignment of C-33 and C-35 in the antibiotic gladiolin

Article abstract of DOI:10.1016/j.tet.2018.06.060

Gladiolin is a macrolide antibiotic isolated from Burkholderia gladioli BCC0238 with promising activity against Mycobacterium tuberculosis, including several multidrug resistant strains. The configuration of all but one of the stereogenic centers of gladiolin has previously been elucidated using a combination of NOESY NMR experiments and predictive sequence analysis of the polyketide synthase responsible for its assembly. However, it was not possible to assign the configuration of the C-35 methyl group using such methods. Here we report the synthesis of C-33/C-35-syn and C-33/C-35-anti mimics of the C-30 to C-38 fragment of gladiolin from (R) and (S)-citronellol, respectively. Comparison of HSQC NMR data for the mimics and the natural product showed that the C-35 methyl is anti to the C-33 hydroxyl group, indicating that gladiolin has the 35S configuration.

Full text of DOI:10.1016/j.tet.2018.06.060

Accepted Manuscript
Relative stereochemical assignment of C-33 and C-35 in the antibiotic gladiolin
Christopher Perry, Jacob R. Sargeant, Lijiang Song, Gregory L. Challis
PII:
S0040-4020(18)30775-0
10.1016/j.tet.2018.06.060
TET 29656
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 7 March 2018
Revised Date: 25 June 2018
Accepted Date: 26 June 2018
Please cite this article as: Perry C, Sargeant JR, Song L, Challis GL, Relative stereochemical
assignment of C-33 and C-35 in the antibiotic gladiolin, Tetrahedron (2018), doi: 10.1016/
j.tet.2018.06.060.
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Relative stereochemical assignment of C-33
and C-35 in the antibiotic gladiolin
Leave this area blank for abstract info.
Christopher Perry, Jacob R. Sargeant, Lijiang Song and Gregory L. Challis
Department of Chemistry and Warwick Integrative Synthetic Biology Centre, University of Warwick, UK
Department of Biochemistry and Molecular Biology, Monash University, Australia

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Relative stereochemical assignment of C-33 and C-35 in the antibiotic gladiolin
Christopher Perrya, Jacob R. Sargeanta, Lijiang Songa, and Gregory L. Challisa, b, c
a Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK
b Warwick Integrative Synthetic Biology Centre, University of Warwick, Coventry CV4 7AL, UK
c Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Gladiolin is a macrolide antibiotic isolated from Burkholderia gladioli BCC0238 with promising
activity against Mycobacterium tuberculosis, including several multidrug resistant strains. The
configuration of all but one of the stereogenic centers of gladiolin has previously been elucidated
using a combination of NOESY NMR experiments and predictive sequence analysis of the
polyketide synthase responsible for its assembly. However, it was not possible to assign the
configuration of the C-35 methyl group using such methods. Here we report the synthesis of C-
33/C-35-syn and C-33/C-35-anti mimics of the C-30 to C-38 fragment of gladiolin from (R)
and (S)-citronellol, respectively. Comparison of HSQC NMR data for the mimics and the natural
product showed that the C-35 methyl is anti to the C-33 hydroxyl group, indicating that
gladiolin has the 35S configuration.
Available online
Keywords:
Antibiotic
Stereochemistry
Asymmetric allylation
Polyketide
2009 Elsevier Ltd. All rights reserved
.
NMR spectroscopy
———
Corresponding author. Tel.: +44-247-657-4024; fax: +44-247-652-4112; e-mail: g.l.challis@warwick.ac.uk, gregory.challis@monash.edu

2
Tetrahedron
ACCEPTED MANUSCRIPT
1
4
1. Introduction
and H coupling constant data. Sequencing of the
B. gladioli BCC0238 genome led to identification
of the trans-acyltransferase modular polyketide
synthase (PKS) responsible for gladiolin 1
assembly.4 Sequence analysis of the ketoreductase
(KR) domains in this PKS allowed the absolute
configurations of the stereogenic centres in the C-1
to C-33 portion of gladiolin 1 to be predicted.4
These predictions independently confirmed the
NMR-based relative stereochemical assignment of
the C-1 to C-31 portion of gladiolin 1. However,
the configuration of the C-35 stereogenic centre in
gladiolin 1, which appears to arise from the action
of an enoyl reductase domain in the PKS of
undetermined stereospecifity,4 remains to be
elucidated.
The emergence of wide-spread antimicrobial
resistanace in pathogenic microoganisms represents
a major global health threat.1 To combat this, new
antimicrobials that overcome resistance to current
drugs are urgently needed. Most antimicrobials in
clinical use are natural products originating from
Gram-positive Actinobacteria and filamentous
fungi, or semi-synthetic derivatives.2 Screening
such organisms for antimicrobial activity frequently
results in the rediscovery of known compounds. In
recent years, Gram-negative bacteria have become
increasingly recognised as underexplored producers
of antibiotics and other bioactive natural products.2
We are pursuing a programme of natural product
discovery in the Burkholderia cepacia complex
(BCC),3 a group of opportunisitic Gram-negative
pathogens that freqeuntly infect the lungs of cystic
fibrosis patients.
In 2017, we reported the discovery of the novel
macrolide antibiotic gladiolin 1 from Burkholderia
gladioli BCC0238, a clinical isolate from the lung
of a boy suffering from cyctic fibrosis (Figure 1).4
Gladiolin 1 has promising activity against isoniazid
and rifampicin resistant clinical isolates of
Mycobacterium tuberculosis.4 It inhibits RNA
polymerase,4 the same target as rifampicin, which is
widely used for the treatment of tuberculosis.
Rifampicin is becoming increasingly ineffective
due to resistance-conferring mutations in RNA
polymerase. However, the binding sites for
rifampicin and gladiolin 1 appear to be distinct,
because mutations in RNA polymerase that confer
rifampicin resistance do not appear to significantly
affect the activity of gladiolin.4 Moreover, gladiolin
1 shows low activity towards mamalian cell lines
and no toxicity was observed in a Galleria wax
moth model.4
The macrolide core of gladiolin 1 is similar to
that of etnangien 2 (Figure 1), an antibiotic isolated
from Sorangium cellulosum that is also active
against Mycobacteria.5 However, the structures of
the C-21 side chains of gladiolin 1 and etnangien 2
differ markedly. The C-24 to C-35 portion of the
etangien 2 side chain contains a conjugated hexaene
that is highly unstable.5 The corresponding C-24 to
C-31 region of the gladiolin 1 side chain does not
suffer such stability problems due to its shorter
length and higher degree of saturation.4
Here we report stereoselective synthesis of syn-
and anti-configured mimics of the C-30 to C-38
portion of the gladiolin C-21 side chain.
Comparison of HSQC NMR data for the mimics
and the natural product allowed the C-35
stereochemistry of gladiolin 1 to be assigned.
2. Results and discussion
We envisaged synthesising the C-30 to C-38
mimics 3 and 4 of the gladiolin C-21 side chain
(with anti and syn relative configurations) via
catalytic asymmetric allylation of the 3S and 3R
isomers of tert-butyldimethylsilyl (TBS)-protected
3-methyl-6-hydroxyhexanal, 5 and 6, respectively
(Scheme 1).
The relative stereochemistry of the C-1 to C-31
portion of gladiolin 1 was initially assigned by
NMR spectoscopy using a combination of NOESY
Burke et al. have reported a five step synthesis of
aldehyde 6 from (R)-citronellol 8 and we postulated

3
ACCEPTED MANUSCRIPT
that aldehyde 5 could be prepared from (S)- diasteromers (Scheme 3). Mosher’s ester analysis
citronellol 7 via the same route.6 Thus, acetylation
confirmed that the the newly created stereogenic
of (S)-citronellol 7, followed by ozonolytic centre in the major diastereomer of 16 is also S-
cleavage employing a reductive work up gave configured (see supplementary material).
alcohol 9, which was converted to the Elaboration of 16 to 4, via intermediates 18, 20 and
corresponding TBS ether 11 (Scheme 2). Based- 22, was accomplished using the same sequence of
catalysed transesterification removed the acetyl
group from 11 and the resulting alcohol 13 was
oxidised to the corresponding aldehyde 5 (Scheme
2). Starting from (R)-citronellol 8, aldehyde 6 was
reactions as that employed for the conversion of 15
to 3 (Scheme 3).
The signals due to C-34, C-35, C-36, C-37 and
the C-35 methyl group, and their associated
prepared using analogous procedures via hydrogen atoms, in the HSQC NMR spectrum of
intermediates 10, 12 and 14 (Scheme 2).6
gladiolin 1 all have essentially identical chemical
shifts to the corresponding signals in synthetic C-30
to C-38 mimic 3 (Figure 1). In contrast, the signals
in the HSQC spectrum of gladiolin due to C-34, C-
35, C-36 and the C-35 methyl group, and their
associated hydrogen atoms, are all significantly
shifted relative to the syn-configured mimic 4
(Figure 1). We thus assign the anti relative
configuration to the C-33 hydroxyl group and the
C-35 methyl group of gladiolin.
Reaction of aldehyde 5 with allytributyltin in the
presence of a catalytic (S)-BINAP.Ag(I) complex,
under the conditions reported by Yamamoto and
coworkers,7 yielded allylic alcohol 15 as a 9:1
mixture of diasteromers (Scheme 3). To confirm
the absolute configuration of the newly created
stereogenic centre in the major diastereomer, 15
was separately coupled with (S) and (R)-α-
methoxy-trifluorophenylacetic acid.8 Analysis of
the resulting Mosher’s esters using 1H NMR
spectroscopy confirmed that this stereogenic centre
is S-configured (see supplementary material).
Acetylation of 15 gave 17, which was deprotected
to yield 19. Oxidation of this alcohol gave the
corresponding carboxylic acid 21, which was
saponified to give 3 (Scheme 3).
Figure 1: Comparison of HSQC spectra for gladiolin 1 (blue) with syn-configured C-30
to C-38 mimic 4 (red, top panel) and anti-configured C-30 to C-38 mimic 3 (red, bottom
panel). Relevant signals due to carbon atoms of gladiolin and their associated hydrogen
atoms are labelled with blue numbers.
3. Conclusion
Stereocontrolled syntheses of syn and anti-
configured mimics of the C-30 to C-38 fragment of
gladiolin have been accomplished in 11 steps from
(R)- and (S)-citronellol, respectively. Comparison
of NMR spectroscopic data for the synthetic
Reaction of aldehyde 6 with allytributyltin in the
presence of a catalytic (S)-BINAP.Ag(I) complex
gave allylic alcohol 16, also as a 9:1 mixture of

4
Tetrahedron
and the resulting mixture was stirred for 10 minutes
ACCEPTED MANUSCRIPT
mimics and gladiolin led us to conclude that the
at room temperature. Tert-butyldimethylsilyl
chloride (3.95 g, 26.00 mmol) was added in one
portion and stirring was continued at room
temperature overnight. Diethyl ether (20 mL) and
water (20 mL) were added. The organics were
separated and washed with water (20 mL), brine
(20 mL), then dried (MgSO4) and concentrated in
vacuo. The crude mixture was purified by flash
column chromatography (diethyl ether: petroleum
ether, 1:9) to give 11 as a colourless oil (6.10 g,
C-33 hydroxyl and C-35 methyl groups have the
anti relative configuration in the natural product. C-
33 of gladiolin has previously been assigned the R
absolute configuration on the basis of the predicted
stereospecificity of the KR domain in module 2 of
the PKS responsible for its assembly. We thus
propose that C-35 of gladiolin is S-configured,
implying that the enoyl reductase domain
responsible for creation of this stereogenic centre
(in module 1 of the PKS) delivers hydride from
NAD(P)H to the re face of its β-methyl-α, β-
unsaturated thioester substrate.
96%).
(R)-6-((tert-butyldimethylsilyl)oxy)-3-
methyl-hexyl acetate 12 (1.98 g, 86%) was
synthesized from 10 using the same procedure.
1H NMR (500 MHz, CDCl3) δ: 4.13-4.05 (m, 2H,
CH2COCH3), 3.59 (t, J = 6.5 Hz, 2H, CH2OSi),
2.04 (s, 3H, COCH3), 1.69-1.10 (m, 7H,
CH2CH(CH3)CH2CH2), 0.91 (d, J = 6.5 Hz, 3H,
CH(CH3), 0.89 (s, 9H, OSi(CH3)3C(CH3)3), 0.04 (s,
6H OSi(CH3)2C(CH3)3); HRMS (ESI+): Found
311.2020 for 11 and 311.2016 for 12,
C15H32O3Si(Na)+ requires 311.2013. These data
match those reported previously for (R)-6-((tert-
4. Experimental section
4.1 (S)-6-hydroxy-3-methylhexyl acetate 9
To (S)-citronellol 7 (5.0 g, 32.00 mmol) in
dichloromethane (DCM) (30 mL) was added
pyridine (3.36 mL, 42.00 mmol) under an argon
atmosphere. The reaction mixture was cooled to -10
°C. Acetic anhydride (3.62 mL, 38.00 mmol) was
added drop-wise and the solution was left to warm
to room temperature over 16 hours. The reaction
mixture was diluted with DCM, and washed with
NaHCO3 (10 mL), 1 M HCl (10 mL) and brine (10
mL). The organics were dried (MgSO4) and
concentrated in vacuo and the resulting product was
dissolved in DCM (20 mL) then cooled to -78 °C.
Ozone was bubbled through the resulting solution
until it displayed a blue colour. Compressed air was
then bubbled through the solution until the blue
colour disappeared. The solution was warmed to 0
°C and diluted methanol (40 mL). Sodium
borohydride (4.77 g, 0.126 mol) was added
cautiously in small portions and the mixture was
left to stir for 90 minutes. DCM and water was then
added, the organic layer was separated, then
washed with brine (20 mL), dried (MgSO4) and
concentrated in vacuo. The crude product was
purified by flash column chromatography (ethyl
acetate: diethyl ether, 2:1) to give 9 as a colourless
oil (3.80 g, 68%). (R)-6-hydroxy-3-methylhexyl
acetate 10 (1.40 g, 42%) was synthesized from (R)-
citronellol 8 using the same procedure.
butyldimethyl-silyl)oxy)-3-methyl-hexyl
acetate
12.6
4.3 (S)-6-((tert-butyldimethylsilyl)oxy)-3-
methylhexan-1-ol 13
A mixture of 11 (6.10 g, 21 mmol) and potassium
carbonate (250 mg) in methanol (50 mL) was
stirred at room temperature for 24 hours. The
methanol was removed in vacuo and the resulting
residue was partitioned between ethyl acetate and
water. The organics were separated, dried (MgSO4)
and concentrated in vacuo. The crude product was
purified by flash column chromatography
(petroleum ether: diethyl ether, 9:1) to give 13 as a
colourless oil (4.04 g, 78%). (R)-6-((tert-
butyldimethylsilyl)oxy)-3-methylhexan-1-ol
14
(1.48 g, 87%) was synthesized from 12 using the
same procedure.
1H NMR (500 MHz, CDCl3) δ: 3.73-3.64 (m, 2H,
CH2OH), 3.59 (t, J = 6.5 Hz, 2H, CH2OSi), 1.64-
1.12 (m, 7H, CH2CH(CH3)CH2CH2), 0.91 (d, J =
6.5 Hz, 3H, CH(CH3)), 0.89 (s, 9H, OSi(CH3)2
C(CH3)3), 0.05 (s, 6H, OSi(CH3)3C(CH3)3); HRMS
(ESI+): Found 269.1906 for 13 and 269.1907 for
14, C13H30O2Si(Na)+ requires 269.1907. These data
match those reported previously for (R)-6-((tert-
butyldimethylsilyl)oxy)-3-methylhexan-1-ol 14.6
1H NMR (500 MHz, CDCl3) δ: 4.15-4.05 (m, 2H,
CHCO(CH3)), 3.63 (t, J = 6.5 Hz, 2H, CH2OH),
2.04 (s, 3H, COCH3), 1.70-1.36 and 1.25-1.17 (m,
7H, CH2CH(CH3)CH2CH2), 0.93 (d, J = 6.5 Hz,
3H, CH2CH(CH3)); HRMS (ESI+): Found 197.1147
for 9 and 197.1149 for 10, C9H18O3(Na)+ requires
197.1148. These data match those reported
previously for (R)-6-hydroxy-3-methylhexyl acetate
10.6
4.4 (S)-6-((tert-butyldimethylsilyl)oxy)-3-
methylhexanal 5
To oxalyl chloride (0.697 mL, 8.12 mmol) in DCM
(15 mL) at -78 °C was added dimethyl sulphoxide
(1.15 mL, 16.0 mmol) drop-wise and the resulting
solution was stirred for 20 minutes. Alcohol 13
(1.00 g, 4.06 mmol) in DCM (5 mL) was added and
the solution was stirred for 45 minutes.
Triethylamine (3.40 mL, 24.0 mmol) was added
drop-wise and the reaction mixture was stirred for a
further 15 minutes before being allowed to warm to
4.2 (S)-6-((tert-butyldimethylsilyl)oxy)-3-
methylhexyl acetate 11
To 9 (3.80 g, 22.00 mmol) in dimethylformamide
(20 mL) was added imidazole (2.23 g, 33.00 mmol)

5
room temperature. Water (20 mL) was added, and
(4S,
6R)-9-((tert-butyldimethylsilyl)oxy)-6-
ACCEPTED MANUSCRIPT
the organic layer was separated and extracted using methylnon-1-en-4-ol 16
DCM (2 x 20 mL). The organics were washed with
The same procedure as that employed for the
synthesis of 15 was used starting from aldehyde 6.
This yielded 16 as a colorless oil (170 mg, 48 %).
Mosher’s ester analysis showed that this material is
an approximately 9:1 mixture of diastereomers and
that the newly created stereogenic centre in the
major diastereomer is S-configured (see
supplememtary material).
brine (20 mL), dried (MgSO4) and concentrated in
vacuo. The crude product was purified by flash
column chromatography (petroleum ether: diethyl
ether, 1:1) to give 5 as a colourless oil (885 mg,
89%).
(R)-6-((tert-butyldimethylsilyl)oxy)-3-
methylhex-anal 6 (840 mg, 85%) was synthesized
from 14 using the same procedure.
1H NMR (500 MHz, CDCl3) δ: 9.76-9.75 (m, 1H,
CHOCH2), 3.59 (t, J = 6.5 Hz, 2H, CH2OSi), 2.43-
2.38 (ddd, J = 1.5, 5.5, 7.0 Hz, 1H, CHOCH2),
2.26-2.21 (ddd, J = 2.5, 8.0, 10.0 Hz, 1H,
CHOCH2), 2.10-2.04 (m, 1H, CH2CH(CH3)CH2),
1.60-1.46 (m, 2H, CH2CH2CH2OSi), 1.41-1.34 (m,
1H, CH2CH2CH2OSi), 1.31-1.23 (m, 1H,
CH2CH2CH2OSi), 0.98 (d, J = 6.5 Hz, 3H,
CH(CH3)), 0.89 (s, 9H, OSi(CH3)2 C(CH3)3), 0.04
(s, 6H, OSi(CH3)2 C(CH3)3); HRMS (ESI+): Found:
1H NMR (500 MHz, CDCl3) δ: 5.87-5.79 (m, 1H,
H2C=CHCH2), 5.15-5.12 (m, 2H, H2C=CHCH2),
3.77-3.72 (m, 1H, H2C=CHCH2CH(OH)), 3.61-
3.58 (t, J = 7.0 Hz, CH2OSi(CH3)2(CH3)3, 2.32-2.25
(m, 1H, H2C=CHCHH), 2.17-2.08 (m, 1H,
H2C=CHCHH),
1.68-1.60
1.53-1.45 (m, 2H),
1.44-1.38 (m, 2H,
1.39-1.31 (m, 1H,
1.16-1.09 (m, 1H,
(m,
1H,
CH(OH)CH2CH(CH3)),
CH2CH(CH3)CH2)),
CH2CH2OSi(CH3)2(CH3)3),
CH(OH)CH2CH(CH3)),
267.1751 for
5
and 267.1749 for 6,
C13H28O2Si(Na)+ requires 267.1751. These data
matched thosed reported previously for (R)-6-((tert-
butyldimethylsilyl)oxy)-3-methylhexan-al 6.6
CH(OH)CH2CH(CH3), 0.93 (d, J = 6.5 Hz, 3H,
CH2CH(CH3)CH2), 0.89 (s, 9H, OSi(CH3)2(CH3)3),
0.05 (s, 6H, OSi(CH3)2(CH3)3); 13C NMR (125
MHz, CDCl3) δ: 135.0, 118.4, 68.8, 63.7, 44.5,
42.3, 32.6, 30.2, 29.5, 26.1, 20.5, 18.5, -5.1; HRMS
(ESI+): Found 309.2226; C16H34O2Si(Na)+ requires
309.2220.
4.5 (4S, 6S)-9-((tert-butyldimethylsilyl)oxy)-6-
methylnon-1-en-4-ol 15
Silver triflate (32.0 mg, 0.12 mmol) and (S)-
BINAP (77.0 mg, 0.12 mmol) were added to THF
4.6 (4S,6S)-9-((tert-butyldimethylsilyl)oxy)-6-
(3 mL) under an argon atmosphere at room methylnon-1-en-4-yl acetate 17
temperature. Aldehyde 5 (260 mg, 1.23 mmol) was
To 15 (120 mg, 0.42 mmol) in DCM (3 mL) was
added and the resulting mixture was stirred for 10
minutes. After cooling to -20 °C, allytributyl
stannane (0.76 mL, 2.46 mmol) was added drop-
wise and the mixture was stirred at -20 °C for 8
hours. Water was added and the organics were
separated, extracted using THF (2x 5 mL), dried
(MgSO4) and concentrated in vacuo. The crude
product was purified by flash column
chromatography using a potassium carbonate: silica
(1:9) stationary phase, eluting with diethyl ether:
petroleum ether (1:9), to give 15 as a colorless oil
(162 mg, 46%). Mosher’s ester analysis showed
that this material is an approximately 9:1 mixture of
diastereomers and that the newly created
stereogenic centre in the major diastereomer is S-
configured (see supplememtary material).
1H NMR (500 MHz, CDCl3) δ: 5.87-5.79 (m, 1H,
H2C=CHCH2), 5.15 (d, J = 13.0 Hz, 2H,
H2C=CHCH2), 3.78-3.72 (m, 1H, CH(OH)), 3.59 (t,
J = 6.5 Hz, 2H, CH2OSi), 2.30-2.25 (m, 1H,
H2C=CHCHH), 2.17-2.11 (m, 1H, H2C=CHCHH),
1.73-1.61 (m, 1H, CH2CH(CH3)CH2), 1.52-1.46
(m, 2H, CH(CH3)CH2CH2),1.36-1.28 (m, 2H,
added pyridine (44.0 ꢀL, 0.55 mmol). The resulting
mixture was stirred at room temperature for 5
minutes, then cooled to -10 °C. Acetic anhydride
(48.0 ꢀL, 0.53 mmol) was added and the mixture
was stirred at room temperature for 16 hours. DCM
(3 mL) was added and the resulting solution was
washed with NaHCO3 (5 mL) and brine (5 mL),
then dried (MgSO4) and concentrated in vacuo. The
crude product was purified by flash column
chromatography (ethyl acetate: petroleum ether,
1:9) to give 17 as a colourless oil (70 mg, 51%).
1H NMR (500 MHz, CDCl3) δ: 5.78-5.70 (m, 1H,
H2C=CHCH2), 5.08-5.02 (m, 3H, H2C=CHCH2 and
CH2CH(O-COCH3), 3.58 (t, J = 6.5 Hz, 2H, CH2
OSi(CH2)2C(CH3)3, 2.29 (t, J = 6.5 Hz, 2H,
H2C=CHCH2), 2.02 (s, 3H, CH(O-COCH3), 1.62
(ddd,
J
=
4.0,
5.5,
7.0
Hz,
1H,
CH2CH(OCOCH3)CHH), 1.51-1.44 (m, 2H,
CH2CH(CH3)CH2CH2),
CH(CH3)CHHCH2),
CH(CH3)CH2CH2),
1.37-1.31
1.30-1.27
1.26-1.22
(m,
(m,
(m,
1H,
1H,
1H,
CH2CH(OCOCH3)CHH), 1.19-1.13 (m, 1H,
CH(CH3)CHHCH2), 0.89 (s, 12H, CH(CH3)CH2
CH(OH)CH2),
1.22-1.16
(m,
2H,
and
OSi(CH3)2C(CH3)3,
0.05
(s,
6H,
CH(CH3)CH2CH2), 0.92 (d, J = 6.5 Hz, 3H,
OSi(CH3)2C(CH3)3; 13C NMR (125 MHz, CDCl3)
δ: 170.9, 133.9, 117.8, 71.4, 63.6, 41.2, 39.7, 33.7,
30.4, 29.2, 27.5, 26.1, 21.4, 19.5, -5.2; HRMS
(ESI+): Found 351.2327, C18H36O3Si(Na)+ requires
351.2326; [α]D25 -0.40° (c 0.12, CHCl3).
CH(CH3), 0.89 (s, 9H, OSi(CH2)2C(CH3)3, 0.05 (s,
13
6H, OSi(CH2)(CH3); C NMR (125 MHz, CDCl3)
δ: 135.0, 118.3, 68.5, 63.7, 44.4, 42.9, 33.9, 30.4,
29.2, 26.1, 19.4, 13.9, -5.1; HRMS (ESI+): Found
309.2221; C16H34O2Si(Na)+ requires 309.2220.

6
Tetrahedron
(4S,6R)-9-((tert-butyldimethylsilyl)oxy)-6-
CH2CH(CH3)CH2CH2CH2OH), 0.92 (d, J = 6.0 Hz,
ACCEPTED MANUSCRIP
T
13
methylnon-1-en-4-yl acetate (14)
3H, CH2CH(CH3); C NMR (125 MHz, CDCl3) δ:
171.1, 133.8, 117.9, 71.7, 63.0, 41.3, 39.2, 32.17,
30.0, 29.2, 21.4, 20.2; HRMS (ESI+): Found
The same procedure as that employed for the
synthesis of 17 was used starting from compound
14. This yielded 18 as a colorless oil (100 mg,
73%).
25
237.1460, C12H22O3(Na)+ requires 237.1461; [α]D
+20.6 (c 0.09, CHCl3).
1H NMR (500 MHz, CDCl3) δ: 5.79-5.71 (m, 1H,
H2C=CHCH2), 5.08-4.98 (m, 3H, H2C=CHCH2 and
4.8 (4S,6R)-6-acetoxy-4-methylnon-8-enoic acid
CH2CH(CH3)CH2),
3.58-3.56
2.35-2.29
(m,
(m,
2H, 21
1H,
CH2OSi(CH3)2(CH3)3),
To
19
(41.0 mg,
0.19 mmol) in
H2C=CHCHH), 2.29-2.22 (m, 1H, H2C=CHCHH),
2.02 (s, 3H, CH2CH(OCOCH3)CH2), 1.52-1.40 (m,
dimethylsulphoxide (0.50 mL) was added
iodoxybenzoic acid (64.0 mg, 0.23 mmol). The
resulting mixture was stirred at room temperature
for 5 hours, then diluted with ethyl acetate (5 mL)
and filtered through celite. The solvent was
removed in vacuo and the crude product was
purified by flash column chromatography (diethyl
ether: petroleum ether, 2:1). The resulting aldehyde
was dissolved in dimethylformamide (1 mL),
Oxone (151 mg, 0.25 mmol) was added and the
solution was stirred at room temperature under an
argon atmosphere for 2 hours. Water (3 mL) was
added and the resulting mixture was extracted with
ethyl acetate (2 x 3 mL). The organic phase was
dried (MgSO4) and concentrated in vacuo. The
crude product was purified by flash column
chromatography (ethyl acetate: petroleum ether,
1:2) to give 21 as a colorless oil (26.0 mg, 60%).
4H,
CH2CH2CH2OSi(CH3)2(CH3)3
and
CH2CH2CH2OSi(CH3)2(CH3), 1.40-1.29 (m, 1H,
CH(OH)CHH), 1.15-1.08 (m, 1H, CH(OH)CHH),
0.91 (d, J = 5.5 Hz, 3H, CH(CH3)CH2), 0.89 (s, 9H,
OSi(CH3)2(CH3)3), 0.04 (s, 6H, OSi(CH3)2(CH3);
13C NMR (125 MHz, CDCl3) δ: 170.8, 133.9,
117.8, 71.9, 63.7, 41.2, 39.0, 32.8, 30.2, 29.6, 26.1,
21.4, 20.1, 18.5, -5.1; HRMS (ESI+): Found
351.2334, C18H36O3Si(Na)+requires 351.2337;
[α]D25 +4.20 (c 0.18, CHCl3).
4.7 (4S,6S)-9-hydroxy-6-methylnon-1-en-4-yl
acetate 19
Compound 17 (70 mg, 0.21 mmol) was stirred at
room temperature in a 3:1:1 mixture of acetic acid,
THF and H2O (3 mL) for 16 hours. Saturated
NaHCO3 (5 mL) was added and the mixture was
extracted using DCM (2 x 3 mL). The organic
phase was dried (MgSO4) and concentrated in
vacuo to yield 19 as a colorless oil (41 mg, 90%).
1H NMR (500 MHz, CDCl3) δ: 5.78-5.70 (m, 1H,
H2C=CHCH2), 5.08-5.01 (m, 3H, H2C=CHCH2 and
CH2CH(OCOCH3)), 3.63 (t, J = 6.60 Hz, 2H,
CH2OH), 2.29 (t, J = 6.20 Hz, 2H, H2C=CHCH2),
2.03 (s, 3H, CH(OCOCH3), 1.66-1.62 (m, 1H,
1H NMR (500 MHz, CDCl3) δ: 5.78-5.70 (m, 1H,
H2C=CHCH2), 5.09-5.01 (m, 3H, H2C=CHCH2 and
CH2CH(OCOCH3)),
2.43-2.33
(m,
2H,
CH2COOH), 2.30-2.28 (m, 2H, H2C=CHCH2), 2.10
(s, 3H, CH(OCOCH3)), 1.68-1.63 (m, 1H,
CHHCH2COOH),
CH(OCOCH3)CHH),
CH2CH(CH3)),
1.63-1.60
1.55-1.52
1.52-1.49
1.31-1.24
(m,
(m,
(m,
(m,
1H,
1H,
1H,
1H,
CHHCH2COOH),
CH(OCOCH3)CHH), 0.92 (d, J = 6.0 Hz, 3H,
CH2CH(CH3) ; 13C NMR (125 MHz, CDCl3) δ:
177.0, 171.0, 133.7, 118.0, 71.3, 40.8, 39.6, 32.2,
CH(OCOCH3)CHH),
CH2CH2CH2OH),
CH2CH(CH3)CH2),
CHHCH2CH2OH),
CH(OCOCH3)CHH),
1.62-1.60
1.51-1.46
1.41-1.32
1.27-1.24
1.23-1.19
(m,
2H,
1H,
1H,
1H,
1H,
(m,
31.2, 29.0, 21.3, 19.0; HRMS (ESI+): Found
(m,
(m,
(m,
251.1254, C12H20O4(Na)+ requires 251.1254; [α]D
25
-46.4 (c 0.06, CHCl3).
CHHCH2CH2OH) 0.90 (d, J = 6.60 Hz, 3H,
CH2CH(CH3)); 13C NMR (125 MHz, CDCl3) δ:
171.0, 133.9, 117.9, 71.4, 63.4, 41.1, 39.7, 33.5,
(4S,6R)-6-acetoxy-4-methylnon-8-enoic acid 22
30.2, 29.3, 21.4, 19.5; HRMS (ESI+): Found
237.1461, C12H22O3(Na)+ requires 237.1461; [α]D
25
The same procedure as that employed for the
synthesis of 21 was used starting from alcohol 20.
This yielded 22 as a colorless oil (26 mg, 50%).
-22.7 (c 0.06, CHCl3).
1H NMR (500 MHz, CDCl3) δ: 5.79-5.70 (m, 1H,
(4S,6R)-9-hydroxy-6-methylnon-1-en-4-yl acetate
H2C=CH), 5.09-5.00 (m, 3H, H2C=CH and
20
CH2CH(OCOCH3),
2.41-2.23
(m,
4H,
H2C=CHCH2 and CH2COOH), 2.03 (s, 3H,
The same procedure as that employed for the
synthesis of 19 was used starting from compound
18. This yielded 20 as a colorless oil (54.0 mg,
83%).
1H NMR (500 MHz, CDCl3) δ: 5.79-5.70 (m, 1H,
H2C=CHCH2), 5.08-5.02 (m, 3H, H2C=CHCH2 and
CH2CH(OCOCH3)CH2), 3.64-3.57 (m, 2H,
CH2CH2OH), 2.34-2.23 (m, 2H, H2C=CHCH2),
2.03 (s, 3H, CH(OCOCH3), 1.65-1.13 (m, 7H,
CH(OCOCH3)),
1.79-1.72
1.67-1.58
1.58-1.50
(m,
(m,
(m,
1H,
1H,
1H,
1H,
1H,
CH2CH(CH3)CHH),
CH(OCOCH3)CHH),
CH2CH(CH3),
1.45-1.39
(m,
CH2CH(CH3)CHH),
1.33-1.31
(m,
CH(OCOCH3)CHH), 0.94 (d, J = 6.5 Hz, 3H,
CH(CH3)CH2); 13C NMR (125 MHz, CDCl3) δ:
177.6, 170.9, 133.7, 118.0, 71.5, 40.8, 39.0, 31.3,
29.8, 29.2, 21.4, 19.7; HRMS (ESI+): Found

7
25
251.1250, C12H20O4(Na)+ requires 251.1254; [α]D
ACCEPTED MANUSCRIPT
177.9, 136.4, 117.3, 69.9, 45.0, 43.3, 32.5, 32.5,
30.3, 20.4; HRMS (ESI+): Found 209.1143,
+17.3° (c 0.06, CHCl3).
C10H18O3(Na)+ requires 209.1148; [α]D +3.5° (c
25
0.10, CHCl3).
4.10 (4S,6S)-6-hydroxy-4-methylnon-8-enoic
acid 3
Acknowledgments
Carboxylic acid 21 (20.0 mg, 0.09 mmol) and
lithium hydroxide (4.00 mg, 0.18 mmol) were
stirred in 2:1 THF/H2O (1.2 mL) at room
temperature for 16 hours. The THF was removed in
vacuo and 1 M HCl (2 mL) was added to the
residue. The resulting mixture was extracted with
ethyl acetate (2 x 2 mL), and the organic phase was
dried (MgSO4) and concentrated in vacuo. The
crude product was purified by flash column
chromatography (diethyl ether: petroleum ether,
1:2) to give 3 as a thick colorless oil (10.0 mg,
61%).
We thank Dr Lona Alkhalaf for critical reading of the
manuscript, Dr Douglas Roberts for helpful discussions and Dr
Ivan Prokes for assistance with the acquisition of NMR
spectroscopic data. This research was supported by the EPSRC
through the University of Warwick’s doctoral training account.
References and notes
1.
2.
3.
Neu, H. C. Source Sci. New Ser. 1992, 257,
5073, 1064–1073.
1H NMR (500 MHz, d4-MeOH) δ: 5.90-5.82 (m,
Masschelein, J.; Jenner, M.; Challis, G. L.
Nat. Prod. Rep. 2017, 34, 7, 712–783.
Mahenthiralingam, E.; Song, L.; Sass, A.;
White, J.; Wilmot, C.; Marchbank, A.;
Boaisha, O.; Paine, J.; Knight, D.; Challis, G.
L. Chem. Biol. 2011, 18, 5, 665–677.
Song, L.; Jenner, M.; Masschelein, J.; Jones,
C.; Bull, M. J.; Harris, S. R.; Hartkoorn, R.
C.; Vocat, A.; Webster, G.; Dunn, M.;
Weiser, R.; Paisey, C.; Cole, S. T.; Parkhill,
J.; Mahenthiralingam, E.; Challis, G. L. J.
Am. Chem. Soc. 2017, 139, 23, 7974-7981.
Irschik, H.; Schummer, D.; Höfle, G.;
Reichenbach, H.; Steinmetz, H.; Jansen, R. J.
Nat. Prod. 2007, 70, 6, 1060–1063.
1H,
H2C=CHCH2),
5.08-5.03
(m,
(m,
(m,
2H,
1H,
2H,
H2C=CHCH2),
3.72-2.67
H2C=CHCH2CH(OH)),
2.36-2.25
CH2COOH), 2.21-2.19 (t, J = 6.5 Hz, 2H,
H2C=CHCH2), 1.75-1.68 (m, 1H, CH2CH(CH3),
1.66-1.59 (m, 1H, CHHCH2COOH), 1.51-1.45 (m,
1H, CHHCH2COOH), 1.44-1.40 (m, 1H,
4.
CH2CH(OH)CHH),
1.23-1.19
(m,
1H,
CH2CH(OH)CHH), 0.92 (d, J = 6.5 Hz, 3H,
CH2CH(CH3)CH2); 13C NMR (125 MHz, d4-
MeOH) δ: 178.0, 136.5, 117.2, 69.7, 44.9, 43.9,
34.0, 32.8, 30.0, 19.2; HRMS (ESI+): Found
209.1140, C10H18O3(Na)+ requires 209.1148; [α]D
25
-0.80 (c 0.18, CHCl3).
5.
6.
7.
8.
(4S,6R)-6-hydroxy-4-methylnon-8-enoic acid 4
The same procedure as that employed for the
synthesis of 3 was used starting from compound 22.
This yielded 4 as a thick colorless oil (14.0 mg,
66%).
Burke, L. T.; Dixon, D. J.; Ley, S. V;
Rodríguez, F. Org. Biomol. Chem. 2005, 3,
274-280.
Yanagisawa, A.; Nakashima, H.; Ishiba, A.;
Yamamoto, H. J. Am. Chem. Soc. 1996, 118,
19, 4723-4724.
1H NMR (500 MHz, d4-MeOH) δ: 5.90-5.83 (m,
1H, H2C=CH), 5.08-5.03 (m, 2H, H2C=CH), 3.74-
3.69 (m, 1H, CH2CH(OH)), 2.36-2.25 (m, 2H,
CH2COOH), 2.24-2.14 (m, 2H, H2C=CHCH2),
1.77-1.70 (m, 1H, CHHCH2COOH), 1.69 – 1.64
(m, 1H, CH2CH(CH3)), 1.44-1.35 (m, 2H,
CHHCH2COOH and CH(OH)CHH) 1.33-1.28 (m,
1H, CH(OH)CHH), 0.94 (d, J = 6.5 Hz, 3H,
CH2CH(CH3)) ; 13C NMR (125 MHz, CDCl3) δ:
Hoye, T. R.; Jeffrey, C. S.; Shao, F. Nat.
Protoc. 2007, 2, 2451-2458.
Supplementary Material
Procedure for the preparation of Mosher’s ester derivatives of
1
alcohols 15 and 16, 1H NMR spectra of the products, and H and
13C NMR spectra of all compounds synthesised.