Synthesis of the fungus metabolite cladosin C

Article abstract of DOI:10.1039/c7ob01795b

Cladosin C is one of the few known enaminotetramic acids, isolated from extracts of the deep sea fungus Cladosporium sphaerospermum. It was synthesised in ten steps and 14% overall yield by a late-stage amination of the corresponding 3-acyltetramic acid. This was obtained by a Dieckmann condensation of an N-β-ketoacylaminoester derived from dehydrovalinate and the thioester-terminated side chain containing the stereogenic centre which stemmed from poly-(R)-3-hydroxybutyrate.

Full text of DOI:10.1039/c7ob01795b

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Synthesis of the fungus metabolite cladosin C†
Cite this: DOI: 10.1039/c7ob01795b David Linder and Rainer Schobert
*
Cladosin C is one of the few known enaminotetramic acids, isolated from extracts of the deep sea fungus
Cladosporium sphaerospermum. It was synthesised in ten steps and 14% overall yield by a late-stage
amination of the corresponding 3-acyltetramic acid. This was obtained by a Dieckmann condensation of
an N-β-ketoacylaminoester derived from dehydrovalinate and the thioester-terminated side chain con-
taining the stereogenic centre which stemmed from poly-(R)-3-hydroxybutyrate.
Received 20th July 2017,
Accepted 25th August 2017
DOI: 10.1039/c7ob01795b
rsc.li/obc
3
acids. Also, their bioactivity appears to be less pronounced
than that of the far better investigated 3-acyl congeners. Of
Introduction
4
The cladosins A–D (1–4) (Fig. 1) were isolated as mixtures of the cladosins A–D, only cladosin C (3) showed moderate bio-
1
two geometric isomers from the deep sea derived fungus activity, namely an anti-influenza A H1N1 virus effect. Here,
Cladosporium sphaerospermum 2005-01-E3 and structurally we report a short first synthesis of cladosin C.
1
characterised by D. Li et al. in 2013. They are distinguished by
their 3-enamino side chains and their biosynthetic derivation
from valine, two features quite rare for naturally occurring
Results and discussion
2
tetramic acids. Synthetic 3-enaminotetramic acids were recently
shown by Moloney et al. to have a different mode of antibacter- Fig. 2 shows our retrosynthetic approach. Cladosin C was to be
ial action to the more common analogous 3-acyltetramic prepared by a late-stage amination of the corresponding 3-acyl-
tetramic acid 5 which in turn should be obtainable by a
Dieckmann condensation of the N-(β-ketoacyl)amino ester 6.
The latter was thought to be accessible by aminolysis of the
thioester-terminated side chain 7 with amino ester 8 according
5
to Ley et al.
The β-ketothioester 7 was prepared in six steps. The cheap,
commercially available poly-(R)-3-hydroxybutyrate (9) was de-
polymerised to give ethyl ester R-10 in 81% yield (Scheme 1).
Fig. 1 Structures of cladosins A–D (1–4).
Department of Chemistry, University Bayreuth, Universitaetsstr. 30, D-95440
Bayreuth, Germany. E-mail: Rainer.Schobert@uni-bayreuth.de
1
13
†
Electronic supplementary information (ESI) available: H and C NMR spectra
of new compounds. See DOI: 10.1039/c7ob01795b
Fig. 2 Retrosynthetic approach to cladosin C (3).
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Scheme 2 Synthesis of methyl dehydrovalinate hydrochloride.
Scheme 1 Synthesis of side chain thioester 7.
Its stereogenic centre was inverted by quantitatively mesylating
the OH group to leave 11 which was treated with CaCO3 in
N
water to undergo an S 2-type hydrolysis furnishing ethyl ester
6
S-10 in 82% yield. TBS protection of its hydroxy group with
TBSCl and imidazole afforded ester 12 in 90% yield. The latter
was reduced with DIBAL–H to the corresponding aldehyde 13
7
in 95% yield according to literature. An E-selective Horner–
Wadsworth–Emmons olefination of aldehyde 13 with Ley’s
5
t-butyl 4-diethylphosphono-3-oxobutanthioate (14), prepared
8
by a modified protocol, afforded β-ketothioester 7 in excellent
9
2% yield when n-BuLi was used as the base. Other bases,
such as NaH or KHMDS gave poor yields below 50%.
Scheme 3 Ley N-acylation, Dieckmann cyclisation and amination to
The required dehydrovaline methyl ester 8 was synthesised give 3.
in five steps starting from L-valine (15), which was converted to
2
the methyl ester hydrochloride with SOCl and then Boc-
protected with Boc O under basic conditions to give methyl 3-acyltetramic acid 5 in 99% yield without further purification.
2
N-Boc-valinate (16) in 98% yield over two steps. In order to Its conversion into the desired enamine turned out to be far
oxidise the amino group of 16 to an imine which can tautomerize from straightforward. Treatment of 5 with NH OH according
4
1
1
to the desired dehydrovaline, we chlorinated the nitrogen with to Beyer et al. gave very little product, while its reaction with
Ca(OCl) in the presence of Al O in 86% yield. Other methods HMDS failed completely even after three days refluxing in
9
2
2 3
were either cumbersome (t-BuOCl) or did not work at all such as dichloroethane. However, reaction of 5 with 2,4-dimethoxy-
the chlorination with NaOCl and Oxon® in the presence of NaCl benzylamine (DMB-NH ) afforded the DMB-enamine tetramic
2
12
or trichloroisocyanuric acid. This chlorination only works for acid 19 in 40% yield. Moloney et al. also showed that the
N-atoms being part of amides or carbamates.
yield of reactions of 3-acyltetramic acids with alkyl amines may
N-Chloroamino ester 17 was dehydrohalogenated with DBU vary from 25% to 95%. A simultaneous deprotection of
to afford the protected dehydrovaline methyl ester 18 in 97% hydroxy and amino groups of 19 with CH Cl /TFA 9 : 1 even-
2
2
10
yield. Since Ley’s aminolysis of thioesters does not work for tually afforded cladosin C (3) in 77% yield as a 2 : 1 mixture of
electron-poor amino groups we had to remove the electron 3E- and 3Z-isomers.
withdrawing Boc-group first. Treatment of 18 with acetyl chlor-
ide and MeOH furnished methyl dehydrovalinate hydro-
chloride 8 × HCl in 99% yield and thus in 81% over all five
steps (Scheme 2).
Conclusions
The N-acylation of 8 with thioester 7 in the presence of The marine fungus metabolite cladosin C (3) was synthesised
5
AgO
2
CCF
3
according to Ley et al. afforded the N-(β-ketoacyl) in ten steps (longest linear sequence) starting from cheap,
amino ester 6 in 89% yield when carried out under strict exclu- commercially available compounds. The synthetic approach
sion of moisture (Scheme 3). The Dieckmann cyclisation of 6 should be applicable also to other 3-enaminotetramic acids.
was carried out with five equivalents of NaOMe in methanol
Three details of this synthesis are remarkable and probably
under reflux for one hour to give the corresponding pure of a general nature. The N-chlorination of methyl valinate was
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possible only after a preceding Boc protection and only with
Ethyl (R)-3-mesyloxybutyrate (11). Ester R-10 (2.0 g,
Ca(OCl) in the presence of Al O , however in high yield and 15.13 mmol, 1 equiv.) was dissolved in CH Cl (50 mL), cooled
2
2
3
2
2
under mild conditions. The Dieckmann cyclisation proceeded to 0 °C and MsCl (1.41 mL, 18.16 mmol, 1.2 equiv.) and NEt
3
quantitatively since an excess of base and heating could be (2.52 mL, 18.16 mmol, 1.2 equiv.) were added dropwise via a
applied because of the absence of an acidic H-atom at C5 of syringe. The reaction was stirred for 2 h and slowly warmed up
the 3-acyltetramic acid 5. Finally, its amination with to room temperature. The suspension was filtered over Celite
2
DMB-NH , although of moderate yield in this particular case, and the filtrate was evaporated in vacuo. The residue was deso-
took place under mild and water-free conditions and offers lved in Et O (50 mL) and filtered a second time over Celite.
2
various debenzylation options.
After removing the solvent under reduced pressure, 11 (3.14 g,
9%) was obtained as a colourless oil. R = 0.39 (n-hexane/
9
f
2
5
13
EtOAc 2 : 1); [α] −26.0 (c 1.0 in CHCl ) (lit [α]D −32.1 (c 1.0
D
3
−
1
in CHCl
105, 1028, 974, 914, 895, 795, 730; H NMR (300 MHz, CDCl
δ 1.23 (3H, t, J = 7.2 Hz, CH CH ), 1.46 (3H, d, J = 6.1 Hz,
3
)); IR (νmax/cm ) 2984, 1731, 1346, 1300, 1194, 1170,
Experimental section
General information
1
1
3
)
2
3
1
13
H and C NMR spectra were recorded on a Bruker Avance 300 CHCH
300 MHz) or on a Bruker Avance DRX-500 spectrometer J = 16.4, 8.7 Hz, OCOCH), 2.99 (3H, s, SCH
500 MHz, with cryoprobe). Chemical shifts are given in parts J = 7.2 Hz, CH CH ), 5.15–5.03 (1H, m, CH); C NMR
3
), 2.53 (1H, dd, J = 16.4, 4.6 Hz, OCOCH), 2.73 (1H, dd,
(
(
3
), 4.12 (2H, q,
1
3
2
3
per million using the residual solvent peak as an internal (75 MHz, CDCl
3
) δ 14.0 (CH
CH
Abbreviations for the multiplicity used: s singlet, d doublet, (ESI) m/z [M + Na] calcd for C H O SNa 233.04542, found
2
CH
3 3 3
), 21.2 (CHCH ), 38.0 (SCH ),
standard. Coupling constants (J) are quoted in Hz. 41.0 (CH
2
CH), 60.7 (CH
2
3
), 75.8 (CH), 169.5 (CO); HRMS
+
7
14 5
t triplet, q quartet, sx sextet, and m multiplet. Mass spectra 233.04500.
were measured with a Varian MAT 8500 (EI, 70 eV). High Ethyl (S)-3-hydroxybutyrate (S-10). Mesylate 11 (12.61 g,
resolution mass spectra were obtained with a UPLC/Orbitrap 60.0 mmol, 1 equiv.) was dissolved in H O (120 mL) and
2
3
MS system in ESI mode. IR spectra were recorded with an CaCO (3.30 g, 33.0 mmol, 0.55 equiv.) was added. After stir-
FT-IR spectrophotometer equipped with an ATR unit. Optical ring at 80 °C for 3 h the mixture was extracted with ethyl
rotations were measured at 589 nm (Na-D line) on a acetate and the combined organic layers were washed with
values are given in 10−
1
PerkinElmer 241 Polarimeter; [α]
D
4
brine and dried over MgSO . After removal of the volatiles
2
−1
deg cm g . For chromatography silica gel 60 (230–400 mesh) under reduced pressure S-10 (6.51 g, 82%) was obtained as a
and RP-silica gel C 18 endcapped polygorep 100-50 were used. colourless oil. R = 0.34 (n-hexane/EtOAc 2 : 1); [α] +33.1 (c 3.0
All reagents were purchased from commercial sources and in CHCl
2
5
f
D
1
4
25
D
−1
3
) (lit [α] +32.8 (c 3.0 in CHCl
3
)); IR (νmax/cm )
were used without further purification. Reactions were 3428, 2979, 1731, 1373, 1296, 1176, 1116, 1067, 1028, 948, 845,
1
routinely carried out under an atmosphere of dry argon unless 594, 563; H NMR (500 MHz, CDCl ) δ 1.14 (3H, d, J = 6.2 Hz,
3
stated otherwise. All glassware was flame-dried before use. CHCH
Analytical TLC was carried out using Merck silica gel 60GF254 4.1 Hz, CH
pre-coated aluminium-backed plates or Merck 60RP-18 F254s 4.08 (2H, q, J = 7.2 Hz, CH CH ), 4.01–4.17 (1H, m, CH);
3
), 1.18 (3H, t, J = 7.2 Hz, CH
2 3
CH ), 2.35 (1H, d, J =
2
CH), 2.37 (1H, d, J = 1.3, CH CH), 3.22 (1H, br. s, OH),
2
2
3
13
foil plates.
Ethyl (R)-3-hydroxybutyrate (R-10). A suspension of poly-(R)- 42.8 (CH
-hydroxybutyrate (9) (6.0 g, 79.7 mmol, 1 equiv.) in 1,2- m/z [M + H] calcd for C H O 133.08592, found 133.08589.
C NMR (75 MHz, CDCl
3
) δ 14.1 (CH
2 3 3
CH ), 22.4 (CHCH ),
2
CH), 60.5 (CH CH
2
3
), 64.1 (CH), 172.5 (CO); HRMS (ESI)
+
3
6
13 3
dichlorethane (60 mL) was treated with H
2
SO
4
(1.3 mL,
Ethyl (S)-3-(t-butyldimethylsilyloxy)butyrate (12). A solution
Cl (120 mL) was
5 h at reflux. After adding brine (70 mL) the mixture was cooled to 0 °C and treated with imidazole (3.79 g, 55.7 mmol,
Cl 1.2 equiv.) and TBSCl (7.69 g, 51.0 mmol, 1.1 equiv.). The reac-
2
7
3.9 mmol, 0.3 equiv.) and ethanol p.a. (20 mL) and stirred for of S-10 (6.13 g, 46.4 mmol, 1 equiv.) in CH
2
2
stirred for a further 10 min and then extracted with CH
2
2
.
The combined organic layers were washed with brine, aqueous tion mixture was stirred at room temperature for 15 h and
NaHCO solution, and a second time with brine. After drying then quenched with NaHCO and washed with H O and brine.
3
3
2
over MgSO
4
the solvent was removed under reduced pressure The aqueous layer was reextracted with CH
2
Cl
2
. The combined
to give R-10 (8.52 g, 81%) as a light orange oil. R
f
= 0.30 organic layers were dried over MgSO
4
and concentrated under
reduced pressure. The crude product was purified by column
3
)); IR (νmax/cm ) 3340, 2978, 1732, chromatography, eluting with n-hexane/ethyl acetate 8 : 1, to
2
5
6
(
n-hexane/ethyl acetate 2 : 1); [α] −36.9 (c 1.0 in CHCl ) (lit
D
3
−
1
[α]
D
−44.5 (c 1.0 in CHCl
1
1
373, 1296, 1178, 1116, 1068, 1027, 947, 845, 593; H NMR give 12 (10.28 g, 90%) as a colourless oil. R = 0.49 (n-hexane/
f
2
5
15
25
(
300 MHz, CDCl ) δ 1.14 (3H, d, J = 6.3 Hz, CHCH ), 1.19 (3H, ethyl acetate 19 : 1); [α] +24.6 (c 1.0 in CHCl ) (lit [α] +24.1
3
3
D
3
D
−
1
t, J = 7.2 Hz, CH
2
CH
3
3
), 2.35 (1H, dd, J = 16.2, 8.3 Hz, OCOCH), (c 1.0 in CHCl )); IR (νmax/cm ) 2931, 2858, 1737, 1473, 1376,
1
2
4
.40 (1H, dd, J = 16.2, 4.3 Hz, OCOCH), 3.23 (1H, br. s, OH), 1254, 1182, 1081, 1001, 831, 809, 774, 659; H NMR (300 MHz,
.08 (2H, q, J = 7.2 Hz, CH CH ), 4.02–4.16 (1H, m, CH); CDCl ) δ 0.04 (3H, s, SiCH ), 0.06 (3H, s, SiCH ), 0.86 (9H, s,
2
3
3
3
3
1
3
C NMR (75 MHz, CDCl
CH CH), 60.4 (CH CH
M + H] calcd for C H O 133.08592, found 133.08596.
3
) δ 14.0 (CH
2
3
CH ), 22.4 (CHCH
3
), 42.8 SiC(CH
3
)
3
), 1.19 (3H, d, J = 6.2 Hz, CHCH
3
), 1.26 (3H, t, J =
(
[
2
2
3
), 64.0 (CH), 172.5 (CO); HRMS (ESI) m/z 7.2 Hz, CH
2 3
CH ), 2.36 (1H, dd, J = 14.4, 5.3 Hz, OCOH), 2.46
(1H, dd, J = 14.4, 7.2 Hz, OCOH), 4.17–4.06 (2H, m, CH CH ),
+
6
13
3
2
3
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1
3
4
.31–4.24 (1H, m, CH); C NMR (75 MHz, CDCl
3
) δ −5.0
Methyl
L-N-chloro-N-(t-butyloxocarbonyl)valinate
(17).
(
(
(
SiCH ), −4.4 (SiCH ), 14.3 (CH CH ), 18.1 (SiC(CH ) ), 24.0 Ca(OCl)2 (12.56 g, 87.8 mmol, 3 equiv.) and Al O (87.9 g,
3
3
2
3
3 3
2 3
CHCH
3
), 25.8 (SiC(CH
3
)
3
2 2 3 2 2
), 45.1 (CH CH), 60.4 (CH CH ), 65.9 Brockmann V) were suspended in CH Cl and 16 (6.77 g,
+
CH), 171.8 (CO); HRMS (ESI) m/z [M + H] calcd for 29.3 mmol, 1 equiv.) was added. The suspension was stirred
for 16 h at 40 °C. The reaction mixture was filtered over Celite
S)-3-(t-Butyldimethylsilyloxy)butanal (13). A solution of 12 and the filtrate was evaporated under reduced pressure to
4.0 g, 16.23 mmol, 1 equiv.) in CH Cl was cooled to −78 °C afford product 17 (66.4 g, 86%) as a colourless oil. R = 0.66
and slowly treated with DIBAL-H (17.86 mL, 1 M in Hexan, 1.1 (n-hexane/ethyl acetate 4 : 1); [α] −71.5 (c 1.0 in CHCl3);
C H O Si 247.17240, found 247.17202.
1
2
27 3
(
(
2
2
f
2
5
D
−
1
equiv.) by means of a syringe pump over a 90 min period. After IR (νmax/cm ) 2978, 1746, 1705, 1369, 1280, 1254, 1203, 1157,
this time the reaction was stirred for a further 30 min, treated 1131, 1008, 848, 750; H NMR (300 MHz, CDCl ) δ 0.93 (3H, d,
3
1
with methanol (5 mL) and stirred another 15 min. After J = 6.8 Hz, CHCH ), 0.98 (3H, d, J = 6.8 Hz, CHCH ), 1.44 (9H,
3
3
warming up to room temperature Rochelle salt (40 mg) was s, C(CH
3
)
3
), 2.38–2.24 (1H, m, (CH
3
)
2
3
CH), 3.69 (3H, s, OCH ),
1
3
added and the mixture was allowed to stir another 20 min. The 4.40 (1H, d, J = 10.0 Hz, NHCH); C NMR (75 MHz, CDCl
turbid reaction mixture was extracted with CH Cl and the 19.0 (CHCH3), 19.6 (CHCH3), 27.5 (CH(CH3)2), 27.9
O and brine and (C(CH ), 52.0 (OCH ), 68.0 (NCH), 83.4 (C(CH ), 154.7
. After the solvent had been removed under (NCO), 169.9 (CO CH
reduced pressure product 13 (3.12 g, 95%) was obtained as a C H ClNO Na 288.09731, found 288.09672.
3
)
δ
2
2
combined organic layers were washed with H
dried over MgSO
2
3
)
3
3
3 3
)
+
4
2
3
); HRMS (ESI) m/z [M + Na] calcd for
1
1
20
4
2
5
colourless oil. R
f
= 0.70 (n-hexane/ethyl acetate 9 : 1); [α]
Methyl N-(t-butyloxocarbonyl)dehydrovalinate (18). A solu-
tion of 17 (885 mg, 3.33 mmol, 1 equiv.) in Et O (30 mL) was
cm ) 2931, 2858, 1728, 1264, 1098, 1026, 833, 774, 680, 560; treated with DBU (547 μL, 3.66 mmoL, 1.1 equiv.) and stirred
D
1
6
25
+
16.7 (c 1.0 in CHCl
3
) (lit [α] +14 (c 1.0 in CHCl
3
)); IR (νmax
/
2
D
−
1
1
H NMR (300 MHz, CDCl
3
) δ 0.06 (3H, s, SiCH
3
), 0.07 (3H, s, at room temperature for 3 h. The mixture was filtered and the
SiCH ), 0.87 (9H, s, C(CH
3
3
)
3
), 1.24 (3H, d, J = 6.0 Hz, CHCH
3
), filtrate was concentrated in vacuo. The crude product was puri-
2
1
.46 (1H, ddd, J = 15.8, 5.1, 2.2 Hz, OHCCH), 2.55 (1H, ddd, J = fied by column chromatography, eluting with n-hexane/ethyl
5.8, 7.1, 2.8 Hz, OHCCH), 4.39–4.32 (1H, m, CH), 9.80 (1H, acetate 2 : 1, to give 18 (742 mg, 97%) as a white solid of
1
3
dd, J = 2.8, 2.2 Hz, CHO); C NMR (75 MHz, CDCl
3
f
) δ −5.1 m.p. 81 °C. R = 0.37 (n-hexane/ethyl acetate 5 : 1); IR (νmax/
−
1
(
(
(
SiCH ), −4.5 (SiCH ), 17.9 (C(CH ) ), 24.1 (CHCH ), 25.7 cm ) 3351, 1705, 1497, 1315, 1258, 1226, 1160, 1092, 1054,
C(CH
%) = 202 (2) [M ], 185 (9), 159 (100) [C
3
3
3 3
3
1
3
)
3
), 52.9 (OHCCH
2
), 64.6 (CH), 202.3 (CvO); MS: m/z 756, 574; H NMR (300 MHz, CDCl
3
) δ 1.43 (9H, s, C(CH
3
)
3
),
+
8
H
19OSi], 145 (79) 1.85 (3H, s, vCCH ), 2.10 (3H, s, vCCH
3
3
), 3.73 (3H, s, OCH
3
),
1
3
[
[
C H O Si], 119 (51), 101 (47), 75 (94) [C H OSi], 73 (55) 5.77 (1H, br. s, NH); C NMR (75 MHz, CDCl ) δ 21.2
C
6
13
2
2
7
3
3
H
5
O
2
], 57 (40), 41 (20).
(vCCH
3
), 22.4 (vCCH
3
), 28.2 (C(CH
3
)
3
3
), 51.8 (OCH ), 80.2
Methyl L-N-(t-butyloxocarbonyl)valinate (16). To a suspen- (C(CH
3 3
) ), 121.5 (NCvC), 145.0 (NCvC), 154.1 (NCO), 165.6
+
sion of L-valine (15.0 g, 0.128 mol, 1 equiv.) in methanol SOCl2 (CO CH ); HRMS (ESI) m/z [M + Na] calcd for C H NO Na
2
3
11 19
4
(9.29 mL, 0.13 mmol, 1 equiv.) was slowly added at 0 °C. 252.12063, found 252.12028.
Stirring was continued for 1 h at room temperature and then
Dehydrovaline methyl ester hydrochloride (8 × HCl). Acetyl
for another 22 h at boiling temperature. The solvent was chloride (9.3 mL, 0.13 mol, 30 equiv.) and dry methanol
reduced under vacuum to afford the methyl valinate hydro- (5.6 mL, 0.13 mol, 30 equiv.) were carefully added to dioxane
chloride as a white solid which was taken up in CH
2
Cl
2
at 0 °C and stirred for 5 min. Then carbamate 18 (1.00 g,
(
120 mL) and treated at 0 °C with NEt (34.9 mL, 0.25 mmol, 4.36 mmol, 1 equiv.) was added in one portion and the result-
3
2
equiv.) and Boc
2
O (30.3 g, 0.14 mmol, 1.1 equiv.) and stirred ing mixture was stirred for a further 90 min. The solvent was
for 17 h at room temperature. Water (100 mL) was added evaporated under reduced pressure to afford 8 × HCl (715 mg,
−
1
and the resulting mixture was extracted with CH Cl . 99%) as colourless crystals of m.p. 115–120 °C. IR (νmax/cm
)
2
2
To remove residual NEt
3
the organic layers were washed with 2980, 2785, 2594, 1986, 1729, 1622, 1436, 1304, 1228, 1146,
1
1
M citric acid and dried over MgSO . All volatiles were 1126, 1043, 877, 770, 617; H NMR (300 MHz, D O) δ 2.02 (3H,
4
2
1
3
removed under vacuum to obtain product 16 (29.0 g, 98%) as a s, CCH ), 2.25 (3H, s, CCH ), 3.84 (3H, s, OCH ); C NMR
3
3
3
2
5
colourless oil. R
f
= 0.57 (n-hexane/ethyl acetate 4 : 1); [α]
(75 MHz, D
2
O) δ 21.6 (CCH
3
), 22.0 (CCH
3
3
), 53.0 (OCH ), 114.8
D
1
7
25
D
+
10.6 (c 1.0 in CHCl
3
) (lit [α] +15.9 (c 1.0 in CHCl
3
)); IR (NCvC), 152.8 (NCvC), 164.1 (CO
2
3
CH ); HRMS (ESI) m/z [M +
−1
+
(
νmax/cm ) 3377, 2967, 1744, 1710, 1499, 1366, 1156, 1015, H] calcd for C H NO 130.08626, found 130.08632.
6
12
2
1
7
79; H NMR (300 MHz, CDCl
3
) δ 0.80 (3H, d, J = 6.6 Hz,
), 1.35 (9H, s, C(CH
S-t-Butyl (S,E)-7-(t-butyldimethylsilyloxy)-3-oxo-oct-4-enthio-
CHCH
3
), 0.86 (3H, d, J = 6.6 Hz, CHCH
3
3
)
3
), ate (7). A solution of S-t-butyl 4-diethylphosphonio-3-oxo-
5
2
.12–1.92 (1H, m, (CH ) CH), 3.64 (3H, s, OCH ), 4.17–4.08 butanthioate 14 (2.87 g, 9.3 mmol, 1.4 equiv.) in THF (90 mL)
3
2
3
1
3
(
(
1H, m, NCH), 5.03 (1H, br. d, J = 7.7 Hz, NH); C NMR was cooled to −78 °C and slowly treated with n-butyl lithium
75 MHz, CDCl 17.7 (CHCH ), 18.9 (CHCH ), (7.41 mL, 18.5 mmol, 2.5 M in hexane, 2.8 equiv.). The reaction
3
)
δ
3
3
2
8.3 (C(CH ) ), 31.2 ((CH ) CH), 51.8 (OCH ), 58.4 (NHCH), mixture was stirred for 30 min and then treated with aldehyde
3
3
3 2
3
7
9.7 (C(CH
3
)
3
), 155.7 (NCO), 172.9 (CO
2
CH
3
); HRMS (ESI) 13 (1.34 g, 6.6 mmol, 1 equiv.). After 3 h the reaction was
Cl, extracted with Et O and dried over
+
m/z [M + Na] calcd for C11
54.13628.
H
21NO
4
Na 254.13594, found quenched with NH
4
2
2
MgSO . The solvent was removed under reduced pressure to
4
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3 3 3 3 2
leave 7 (2.18 g, 92%) as a red oil and as a 4 : 1 mixture of enol (CvCH ), 24.0 (CHCH ); 25.9 (SiC(CH ) ), 42.9 (CHvCHCH ),
and keto tautomers. R = 0.26 (ketone), 0.41 (enol) (n-hexane/ 45.7 (COCH CO), 52.0 (OCH ), 67.5 (CHOSi), 121.2 (NCvC),
f
2
3
2
5
−1
Et
2
O 98 : 2); [α]D +7.4 (c 1.0 in CHCl
3
); IR (νmax/cm ) 2957, 131.9 (CHvCHCH
2
2
), 144.5 (NCvC), 148.1 (CHvCHCH ),
2
928, 2858, 1656, 1583, 1364, 1255, 1075, 1002, 968, 832, 773; 164.5 (COOCH ), 165.2 (NCO), 195.7 (COCH CO); enol: −4.7
3
2
1
H NMR (300 MHz, CDCl ) δ enol: 0.03 (3H, s, SiCH ), 0.04 (SiCH ), −4.3 (SiCH ), 21.3 (CvCH ), 22.4 (CvCH ), 24.0
3
3
3
3
3
3
(
3H, s, SiCH
CHCH
.89 (1H, sx, J = 6.1 Hz, CHOSi), 5.35 (1H, s, COCHCOH), 5.70 121.2 (NCvC), 126.9 (CHvCHCH ), 137.0 (CHvCHCH ),
3
), 0.88 (9H, s, SiC(CH
3
)
3
), 1.15 (3H, d, J = 6.1 Hz, (CHCH
3
);
25.9
(SiC(CH
3
)
3
),
39.7
(SiC(CH
3
)
3
),
49.4
3
), 1.52 (9H, s, SC(CH
3
)
3
), 2.24–2.35 (2H, m, CvCHCH
2
), (CHvCHCH
2 3
), 52.0 (OCH ), 67.5 (CHOSi), 91.2 (COCHCOH),
3
2
2
(
3
1H, dd, J = 15.4, 1.5 Hz, CHvCH), 6.68 (1H, dt, J = 15.4, 7.6 144.5 (NCvC), 164.5 (COOCH ), 165.2 (NCO), 195.7
+
2 5
Hz, CHvCH), 12.56 (1H, d, J = 1.5 Hz, OH); ketone: 0.04 (3H, s, (COCH CO); HRMS (ESI) m/z [M + H] calcd for C20H35NO Si
SiCH ), 0.05 (3H, s, SiCH ), 0.88 (9H, s, SiC(CH ) ), 1.16 (3H, 398.23573, found 398.23503.
3
3
3 3
d, J = 6.1 Hz, CHCH
3
), 1.55 (9H, s, SC(CH
3
)
3
), 2.31–2.38 (2H,
3-[(S,E)-5-(t-Butyldimethylsilyloxy)hex-2-enoyl]-5-(propan-2-
m, CvCHCH
2
), 3.92–3.98 (1H, m, CHOSi), 5.31 (2H, s, yliden)-pyrrolidin-2,4-dione (5). A solution of β-ketoamide 6
COCH CO), 5.62 (1H, dd, J = 12.1, 1.8 Hz, CHvCH), 6.91 (1H, (100 mg, 0.25 mmol, 1 equiv.) in methanol (10 mL) was treated
2
1
3
dt, J = 15.9, 7.5 Hz, CHvCH); C NMR (75 MHz, CDCl
δ enol: −4.8 (SiCH ), −4.6 (SiCH ), 24.0 (CHCH ), 26.0 (SiC The reaction was quenched with H
CH ) ), 29.9 (SiC(CH ) ), 30.3 (SC(CH ) ), 43.2 (CHvCHCH ), with Et O and washed with brine. After drying with MgSO the
3
)
with NaOMe (68 mg, 1.26 mmol, 5 equiv.) and refluxed for 1 h.
3
3
3
2
O and 10% HCl, extracted
(
3
3
3 3
3 3
2
2
4
4
8.4 (SC(CH
3
)
3
), 68.1 (CHOSi), 100.6 (COCHCOH), 126.3 solvent was removed under reduced pressure to afford 5
(
(
CHvCHCH
2
), 139.4 (CHvCHCH ), 166.6 (SCO), 196.6 (92 mg, 99%) as a yellow oil. R = 0.54 (tailing) (CH Cl /metha-
2
f
2
2
2
5
−1
CHCOH); ketone: −4.8 (SiCH ), −4.6 (SiCH ), 24.0 (CHCH ), nol 95 : 5); [α] +9.4 (c 1.0 in methanol); IR (νmax/cm ) 3195,
3
3
3
D
2
6.0 (SiC(CH
3
)
3
), 29.8 (SC(CH
3
)
3
3 3
), 29.9 (SiC(CH ) ), 42.9 2955, 2930, 2858, 1694, 1646, 1583, 1374, 1252, 1128, 1088,
1
(
(
(
2 3 3
CHvCHCH ), 48.9, (SC(CH ) ), 56.0 (COCHCOH), 67.6 991, 833, 806, 773, 662; H NMR (500 MHz, MeOD) δ 0.07 (3H,
CHOSi), 131.6 (CHvCHCH ), 147.3 (CHvCHCH ), 191.6 s, SiCH ), 0.08 (3H, s, SiCH ), 0.89 (9H, s, SiC(CH ) ), 1.20 (3H,
2
2
3
3
3 3
+
CHCOH), 192.6 (SCO); HRMS (ESI) m/z [M + Na] calcd for d, J = 6.1 Hz, CHCH
NaSSi 381.18901, found 381.18832. CvCH
Methyl (S,E)-N-[7-(t-butyldimethylsilyloxy)-3-oxo-oct-4-enoyl] Hz, CHOSi), 7.23 (2H, br. s, CHvCH); C NMR (75 MHz,
dehydrovalinate (6). A suspension of dehydrovaline methyl DMSO-d E-isomer: −4.8 (SiCH ), −4.5 (SiCH ), 17.9
ester hydrochloride (8 × HCl) (2.0 g, 12.1 mmol, 2.5 equiv.) (SiC(CH ), 18.2 (CvCCH ), 21.2 (CvCCH ), 24.0 (CHCH ),
3
), 1.89 (3H, s, CvCCH
3
), 2.25 (3H, s,
C
18
H
34
O
3
3 2
), 2.40–2.52 (2H, m, CHvCHCH ), 4.05 (1H, q, J = 6.1
1
3
6
)
δ
3
3
3
)
3
3
3
3
and powdered 4 Å molecular sieve in THF (130 mL) was cooled 25.8 (SiC(CH ) ), 42.8 (CHvCHCH ), 67.5 (CHOSi), 100.8
3
3
2
to 0 °C and treated first with NEt
3
(1.74 mL, 12.6 mmol, 2.6 (NCOC), 122.7 (CHvCHCH
2
), 125.0 (NCvC), 129.8 (NCvC),
equiv.) and after stirring for 5 min with β-ketothioester 7 147.0 (CHvCHCH
2
3
), 164.5 (NCO), 172.3 (COCvCCH ), 175.8
(
1.73 g, 4.84 mmol, 1 equiv.). The reaction flask was wrapped (CvCOH); Z-isomer: −3.1 (SiCH ), 17.9 (SiC(CH ) ), 18.9
3
3 3
in light-tight foil and then silver trifluoroacetate (2.14 g, (CvCCH
3
), 21.2 (CvCCH
3
), 24.0 (CHCH
3
3 3
), 25.8 (SiC(CH ) ),
9
.7 mmol, 2.0 equiv.) was added in one portion. After 3 h the 42.8 (CHvCHCH
reaction mixture was filtered over Celite and the residue was (CHvCHCH2), 126.2 (NCvC), 129.8 (NCvC), 147.6
washed with CH Cl . The solvent was evaporated in vacuo and (CHvCHCH ), 165.7 (NCO), 171.9 (COCvCCH ), 182.1
Si
2
), 65.3 (CHOSi), 103.1 (NCOC), 123.3
2
2
2
3
+
the remainder was purified by column chromatography, (CvCOH); HRMS (ESI) m/z [M + H] calcd for C19
eluting with n-hexane/ethyl acetate 9 : 1, to leave 6 (1.71 g, 366.20951, found 366.20935.
H32NO
4
8
2
9%) as a white solid of m.p. 81 °C with a ratio ketone/enol of
Cladosin C (3). Tetramic acid 5 (90 mg, 0.25 mmol, 1 equiv.)
2
5
: 1. R
f
= 0.46 (n-hexane/ethyl acetate 3 : 1); [α] +3.2 (c 1.0 in was dissolved in toluene (4 mL). 3 Å molecular sieve pellets
D
−
1
CHCl ); IR (νmax/cm ) 3243, 2928, 2856, 1721, 1608, 1542, and 2,4-dimethoxybenyzlamine (37 μL, 0.25 mmol, 1 equiv.)
1
3
1
307, 1220, 1088, 831, 774; H NMR (300 MHz, CDCl
3
)
were added and the mixture was refluxed for 80 min. The
δ ketone: 0.04 (3H, s, SiCH ), 0.05 (3H, s, SiCH ), 0.88 (9H, s, resulting solution was filtrated and washed with CH
3
3
2
2
Cl . Upon
SiC(CH ) ), 1.17 (3H, d, J = 6.2 Hz, CHCH ), 1.85 (3H, s, removal of the solvent, tetramic acid 19 (50 mg, 40%) was
3
3
3
CvCCH
CvCHCH
(
3
), 2.14 (3H, s, CvCCH
), 3.63 (2H, s, COCH
1H, q, J = 5.9 Hz, CHOSi), 6.18 (1H, d, J = 15.9 Hz,
CHvCHCH ), 7.01 (1H, dt, J = 15.9, 7.6 Hz, CHvCHCH ), 8.46 added to a solution of TFA (0.2 mL) in CH
1H, s, NH); enol: 0.03 (3H, s, SiCH ), 0.05 (3H, s, SiCH ), 0.88 the mixture was stirred at room temperature for 1 h. The
3
), 2.36–2.40 (2H, m, obtained as a yellow oil. The crude product was used for the
2
2
CO), 3.73 (3H, s, OCH
3
), 3.97 next step without further purification.
Crude compound 19 (40 mg, 0.08 mmol, 1 equiv.) was
Cl (1.8 mL) and
2
2
2
2
(
3
3
(
9H, s, SiC(CH ) ), 1.15 (3H, d, J = 6.1 Hz, CHCH ), 1.87 (3H, s, solvent was removed as an azeotrope with toluene under
3
3
3
CvCCH
CvCHCH
3
), 2.18 (3H, s, CvCCH
), 3.75 (3H, s, OCH
CHOSi), 4.94 (1H, s, CHCOH), 5.78 (1H, d, J = 15.4 Hz, 40 : 60, to leave cladosin C (3) (15 mg, 77%) as a pale yellow oil
3
), 2.23–2.31 (2H, m, reduced pressure and the resulting crude product was purified
), 3.90 (1H, q, J = 6.4 Hz, by RP–column chromatography, eluting with methanol/H O
3 2
2
CHvCHCH
CHvCHCH
2
), 6.31 (1H, s, NH), 6.60 (1H, dt, J = 15.4, 7.7 Hz, as a 2 : 1 mixture of 3E- and 3Z- isomers. R
f
= 0.25 (CH
2
Cl
2
/
1
3
25
1
25
2
); C NMR (75 MHz, CDCl
3
) δ ketone: −4.7 methanol 95 : 5); [α]D +7.9 (c 0.1 in methanol) (lit [α] +10.5
D
−
1
(
SiCH ), −4.3 (SiCH ), 18.2 (SiC(CH ) ), 21.3 (CvCH ), 22.4 (c 0.1 in methanol)); IR (νmax/cm ) 3204, 2973, 1659, 1615,
3
3
3 3
3
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Organic & Biomolecular Chemistry
1
5
512, 1457, 1381, 1289, 1203, 1137, 1034, 837, 801, 722, 618,
2 For examples of
3-enaminotetramic acids
see:
1
85, 557; H NMR (500 MHz, DMSO-d ) δ E-isomer: 1.10 (3H, d,
(a) L. Hagmann and F. Jüttner, Tetrahedron Lett., 1996, 37,
6539–6542; (b) C. W. Holzapfel, R. D. Hutchison and
D. C. Wilkins, Tetrahedron, 1970, 26, 5239–5246.
3 (a) Y.-C. Jeong, M. Anwar and M. G. Moloney, Bioorg. Med.
Chem. Lett., 2014, 24, 1901–1906; (b) S. W. B. Tan,
C. L. L. Chai and M. G. Moloney, Org. Biomol. Chem., 2014,
12, 1711–1716.
6
J = 6.2 Hz, CHCH
CvCCH ), 2.29–2.34 (2H, m, CHvCHCH
CHOH), 4.71 (1H, d, J = 5.0 Hz, OH), 6.87–6.98 (1H, m,
3
), 1.74 (3H, s, CvCCH
3
), 2.15 (3H, s,
3
2
), 3.72–3.79 (1H, m,
CHvCHCH
2
), 7.42 (1H, d, J = 16.2 Hz, CHvCHCH
2
), 8.70 (1H,
br. s, NH ), 9.21 (1H, s, NH), 9.88 (1H, br. s, NH
1
2
2
); Z-isomer:
.10 (3H, d, J = 6.2 Hz, CHCH ), 1.73 (3H, s, CvCCH ), 2.12
3
3
(
(
3H, s, CvCCH
3
), 2.29–2.34 (2H, m, CHvCHCH
2
), 3.72–3.79
4 (a) B. J. L. Royles, Chem. Rev., 1995, 95, 1981–2001;
(b) R. Schobert, Naturwissenschaften, 2007, 94, 1–11;
(c) R. Schobert and A. Schlenk, Bioorg. Med. Chem., 2008,
16, 4203–4221; (d) M. Petermichl and R. Schobert, Synlett,
2017, 654–663.
1H, m, CHOH), 4.71 (1H, d, J = 5.0 Hz, OH), 6.87–6.98 (1H, m,
CHvCHCH ), 7.42 (1H, d, J = 16.2 Hz, CHvCHCH ), 8.60 (1H,
br. s, NH
2
2
13
2
), 9.25 (1H, s, NH
2
), 9.39 (1H, br. s, NH); C NMR
(75 MHz, DMSO-d
6
)
δ
E-isomer: 18.3 (CvCCH ), 21.3
3
(
9
CvCCH ), 23.8 (CHCH ), 43.3 (CHvCHCH ), 66.0 (CHOH),
5 S. V. Ley, S. C. Smith and P. R. Woodward, Tetrahedron,
1992, 48, 1145–1174.
3
3
2
6.8 (CvCNH
2
), 118.3 (CvCCH
3
), 123.2 (CHvCHCH
2
), 130.2
(
(
(
CvCCH
3
), 142.1 (CHvCHCH
2
), 161.2 (CvCNH
2
), 168.3
6 A. J. Carnell, R. Head, D. Bassett and M. Schneider,
Tetrahedron: Asymmetry, 2004, 15, 821–825.
7 R. A. Fernandes and S. V. Mulay, J. Org. Chem., 2010, 20,
7029–7032.
NHCO), 187.2 (COCvCCH ); Z-isomer: 18.4 (CvCCH ), 21.2
3
3
CvCCH
3
), 23.8 (CHCH
3
), 43.4 (CHvCHCH
2
), 66.0 (CHOH),
9
6.5 (CvCNH ), 118.6 (CvCCH ), 123.5 (CHvCHCH ), 130.8
2 3 2
(
(
CvCCH ), 142.1 (CHvCHCH ), 161.4 (CvCNH ), 171.6
NHCO), 183.7 (COCvCCH ); HRMS (ESI) m/z [M + H] calcd
3
8 M. Petermichl, S. Loscher and R. Schobert, Angew. Chem.,
Int. Ed., 2016, 34, 10122–10125.
3
2
2
+
for C13
H
19
O
3
N
2
251.13902, found 251.13831.
9 O. V. Larionov, S. I. Kozhushkov and A. de Meijere,
Synthesis, 2003, 1916–1919.
1
1
1
1
0 S.-P. Lu and A. H. Lewin, Tetrahedron, 1998, 54, 15097–
15104.
Conflicts of interest
1 C. Beyer, K. Woithe, B. Lüke, M. Schindler, H. Antonicek
and J. Scherkenbeck, Tetrahedron, 2011, 67, 3062–3070.
2 Y.-C. Jeong, M. Anwar, Z. Bikadi, E. Hazai and
M. G. Moloney, Chem. Sci., 2013, 4, 1008–1015.
There are no conflicts to declare.
3 H. Urata, D. Goto and T. Fuchikami, Tetrahedron Lett.,
Acknowledgements
1991, 26, 3091–3094.
We thank Dr Ulrike Lacher (University Bayreuth) for GC-MS 14 J. S. Yadav, S. Nanda, P. T. Reddy and A. B. Rao, J. Org.
analyses.
Chem., 2002, 67, 3900–3903.
15 R. A. Fernandes and S. V. Mulay, J. Org. Chem., 2010, 75,
7
029–7032.
6 S. Wattanasereekul and M. E. Maier, Adv. Synth. Catal.,
004, 346, 855–861.
1
Notes and references
2
1
G. Wu, X. Sun, G. Yu, W. Wang, T. Zhu, Q. Gu and D. Li, 17 J. Zheng, B. Yin, W. Huang, X. Li, H. Yao, Z. Liu, J. Zhang
J. Nat. Prod., 2014, 77, 270–275.
and S. Jiang, Tetrahedron Lett., 2009, 50, 5094–5097.
Org. Biomol. Chem.
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