Synthesis of methoxyfumimycin with 1,2-addition to ketimines

Article abstract of DOI:10.1021/jo902026s

(Chemical Equation Presented) The synthesis of (±)-methoxyfumimycin, a potential new bacterial peptide deformylase (PDF) inhibitor, is reported. To generate the stereogenic fully substituted carbon, the key step is a 1,2-addition of amethylGrignard reagent to a ketimine. The overall synthetic strategy involves a Dakin oxidation of a vanillin derivative, Friedel-Crafts acylation, Claisen rearrangement, lactonization, and rhodium-catalyzed olefin isomerization.

Full text of DOI:10.1021/jo902026s

pubs.acs.org/joc
not essential in mammals. Therefore PDF-inhibitors like
Synthesis of Methoxyfumimycin with 1,2-Addition to
Ketimines
fumimycin and structurally related analogues like methoxy-
fumimycin might lead to novel broad-spectrum antibacterial
agents.6
Patrick J. Gross,† Caroline E. Hartmann,† Martin Nieger,‡
,†
€
and Stefan Brase*
†Institute of Organic Chemistry, Karlsruhe Institute of
Technology, Fritz-Haber-Weg 6, D-76131 Karlsruhe,
Germany and ‡Laboratory of Inorganic Chemistry, University
of Helsinki, 00014 Helsinki, Finland
braese@kit.edu
FIGURE 1. Fumimycin, a potential lead to antibacterial agents.
Received September 22, 2009
The structure of fumimycin was determined by ESI-MS,
IR, and two-dimensional NMR spectroscopy. The absolute
configuration is not yet known. Due to its fully substituted
stereogenic center, fumimycin is a challenging synthetic
target. So far no synthesis has been reported.7
Due to our longstanding interest in constructing R-trisub-
stituted amines8 we became interested in the synthesis of
methoxyfumimycin (2). We decided first to synthesize meth-
oxyfumimycin to elucidate the structure activity relation-
ship. Will the substitution of a methoxy group for a hydroxyl
group affect the PDF-inhibition ability?
The synthesis of (()-methoxyfumimycin, a potential new
bacterial peptide deformylase (PDF) inhibitor, is reported.
To generate the stereogenic fully substituted carbon, the
key step is a 1,2-addition of a methyl Grignard reagent to a
ketimine. The overall synthetic strategy involves a Dakin
oxidation of a vanillin derivative, Friedel-Crafts acyla-
tion, Claisen rearrangement, lactonization, and rhodium-
catalyzed olefin isomerization.
To build up the stereogenic center, we decided to use a 1,2-
addition of organometallic reagents to ketimines. Simulta-
neously we also pursued an organocatalytic approach via
electrophilic amination.9
Our retrosynthetic strategy employing alkylation of
ketimine 4 as the key step is outlined in Scheme 1. Foremost,
we intended to complete the racemic synthesis by using
Grignard reagents for the 1,2-addition to 4. For an asym-
metric synthesis, we envision the use of an enantioselective
addition of methyl zinc reagents that is currently under
investigation in our laboratories.10 Methoxyfumimycin (2)
derives from amine 3 through amidation, lactonization,
Claisen rearrangement, and olefin isomerization. The pre-
cursor for amine 3, the ketimine 4, can be built via Friedel-
Crafts acylation, Dakin oxidation, and allylation by
employing vanillin (7) as readily available starting material.
We began the synthesis with a sequence of allylation/
oxidative degradation (Scheme 2). To install the third oxygen
group at the aromatic core, vanillin (7) was converted with
Due to bacterial resistance to common classes of antibio-
tics, a continuing discovery of antibiotics with new modes of
action is critical.1 One target of special interest is the bacterial
peptide deformylase (PDF),2 a metalloenzyme catalyzing the
removal of the formyl group at the N-terminus of bacterial
proteins. In the course of screening for PDF inhibitors,
fumimycin3,4 (1, Figure 1) was isolated from cultures of the
mildew Aspergillus fumisynnematus F746.5 Fumimycin ex-
hibits potent inhibition of Staphylococcus aureus PDF with
an IC50 of 4.1 μM and also shows activity against methicillin-
resistant S. aureus (MRSA) and quinoline-resistant S. aureus
(QRSA). PDF is required for bacterial development, but is
(6) (a) Jain, R.; Chen, D.; White, R. J.; Patel, D. V.; Yuan, Z. Curr. Med.
Chem. 2005, 12, 1607–1621. (b) Chen, D.; Yuan, Z. Expert Opin. Invest.
Drugs 2005, 14, 1107–1116.
(7) For the syntheses of related scaffolds, see: (a) Royo, E.; Lopez, P.;
Cativiela, C. ARKIVOC 2005, 46–61. (b) de la Hoz, A.; Diaz-Ortiz, A.;
Victoria Gomez, M.; Antonio Mayoral, J.; Moreno, A.; Sanchez-Migallon,
A. M.; Vazquez, E. Tetrahedron 2001, 57, 5421–5428. (c) Verboom, W.; Bos,
H. J. T. Recl. Trav. Chim. Pays-Bas 1979, 98, 559–566. (d) Becker, H. D.;
Gustafsson, K. J. Org. Chem. 1977, 42, 2966–2973.
(1) Fischbach, M. A.; Walsh, C. T. Science 2009, 325, 1089–1093.
(2) (a) Yuan, Z.; Trias, J.; White, R. J. Drug Discovery Today 2001, 6,
954–961. (b) Waller, A. S.; Clements, J. M. Curr. Opin. Drug Discovery Dev.
2002, 5, 785–792.
€
(8) (a) Brase, S.; Baumann, T.; Dahmen, S.; Vogt, H. Chem. Commun.
€
2007, 1881–1890. (b) Baumann, T.; Vogt, H.; Brase, S. Eur. J. Org. Chem.
(3) Kwon, Y.-J.; Sohn, M.-J.; Zheng, C.-J.; Kim, W.-G. Org. Lett. 2007,
9, 2449–2451.
(4) For the related sorbicillalactones, see: Bringmann, G.; Lang, G.;
Gulder, T. A. M.; Tsuruta, H.; Muhlbacher, J.; Maksimenka, K.; Steffens,
S.; Schaumann, K.; Stohr, R.; Wiese, J.; Imhoff, J. F.; Perovic-Ottstadt, S.;
Boreiko, O.; Muller, W. E. G. Tetrahedron 2005, 61, 7252–7265.
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2007, 266–282. (c) Reingruber, R.; Brase, S. Chem. Commun. 2008, 105–107.
(9) Hartmann, C. E.; Gross, P. J.; Nieger, M.; Brase, S. Org. Biomol.
€
Chem. 2009, 7, 5059-5062.
(10) For enantioselective addition to ketimines, see: (a) Lauzon, C.;
Charette, A. B. Org. Lett. 2006, 8, 2743–2745. (b) Suto, Y.; Kanai, M.;
Shibasaki, M. J. Am. Chem. Soc. 2007, 129, 500–501. (c) Fu, P.; Snapper, M.
L.; Hoveyda, A. H. J. Am. Chem. Soc. 2008, 130, 5530–5541.
€
(5) For a review on mycotoxins, see: Brase, S.; Encinas, A.; Keck, J.;
Nising, C. F. Chem. Rev. 2009, 109, 3903–3990.
DOI: 10.1021/jo902026s
r
Published on Web 12/03/2009
J. Org. Chem. 2010, 75, 229–232 229
2009 American Chemical Society

Gross et al.
JOCNote
SCHEME 1. Retrosynthetic Analysis for Methoxyfumimycin (2)
SCHEME 2. Synthesis of Phenol 6 via Allylation and Dakin
Oxidation
SCHEME 3. Sequence To Yield Ketimine 14
mCPBA in a Dakin oxidation. Thus the new hydroxy group
was obtained protected as formate ester. We planned to
allylate the free phenol group of 8 under mild conditions to
furnish 10, followed by cleaving the ester function in basic
methanol. Various conditions were tested for the allylation
of 8, but due to the instability of the formate ester, the best
conditions (allyl iodide, BaO, DMF) generated an unaccep-
table yield of 6.11
Therefore we changed the reaction order: Allylation of
vanillin (7) gave 9 in quantitative yield. Oxidation of the
aldehyde was accomplished without epoxidation of the
olefinic side chain, using acidic hydrogen peroxide, and the
best results were obtained following a procedure12 employ-
ing boric acid as an additive.
Claisen rearrangement of allyl ether 6 smoothly provided
11 (Scheme 3),13 but subsequent acylation was not success-
ful, presumably because of steric hindrance. Friedel-Crafts
acylation of the unhindered phenol 6 with ethyl oxalyl
chloride gave ester 5 as a single regioisomer.
Protection of the phenol with a tert-butylsilyl group (TBS)
and condensation of the keto function with hydroxylamine
furnished oxime 13 as 1:1 mixture of E and Z diastereomers.
To enable a 1,2-addition to the NdC function, the electron-
withdrawing group N-diphenylphosphinoyl (dpp) was in-
stalled.14 This reaction starts with an N-O-P formation,
presumably followed by a radical fragmentation/combina-
tion mechanism.15 Conversion of the E/Z mixture of 13 led
to a single diastereomer 14,16 which was identified by crystal
structure as the Z-product.17
The best results for the 1,2-addition to imine 14 were
achieved with methyl magnesium bromide in a nonpolar
solvent, establishing the fully substituted center of methoxy-
fumimycin (Scheme 4). As a next step, we intended to induce
a Claisen rearrangement at high temperature. Surprisingly,
(11) Green, I. R.; De Koning, C. B.; Hugo, V. I. South Afr. J. Chem. 1999,
52, 112–119.
(12) Roy, A.; Reddy, K. R.; Mohanta, P. K.; Ila, H.; Junjappa, H. Synth.
Commun. 1999, 29, 3781–3791.
(13) (a) Yadav, J. S.; Reddy, B. V. S.; Swamy, T. Tetrahedron Lett. 2003,
44, 4861–4864. (b) Wolska, I.; Poplawski, J.; Lozowicka, B. Pol. J. Chem.
1998, 72, 2331–2341.
(14) (a) Steinig, A. G.; Spero, D. M. Org. Prep. Proced. Int. 2000, 32, 205–
234. (b) Ferraris, D. Tetrahedron 2007, 63, 9581–9597. (c) Riant, O.;
Hannedouche, J. Org. Biomol. Chem. 2007, 5, 873–888.
(15) Lin, X.; Artman, G. D.; Stien, D.; Weinreb, S. M. Tetrahedron 2001,
57, 8779–8791.
(16) It is reported that N-diphenylphosphinoyl ketimines exist in a very
fast equilibrium between E- and Z-isomers: Masumoto, S.; Usuada, H.;
Suzuki, M.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2003, 125, 5634–
5635.
(17) Crystallographic data (excluding structure factors) for the structures
reported in this work have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC 743652 (14)
and no. CCDC 743653 (18). Copies of the data can be obtained free of charge
on application to The Director, CCDC, 12 Union Road, Cambridge DB2
1EZ, UK (fax: int.code þ (1223)336-033; e-mail: deposit@ccdc.cam.ac.uk).
230 J. Org. Chem. Vol. 75, No. 1, 2010

Gross et al.
JOCNote
SCHEME 4. Final Assembly of Methoxyfumimycin 2
Final deprotection of the tert-butyl ester 21 to the acid was
simply achieved by treatment with TFA, furnishing methox-
yfumimycin (2) in quantitative yield. All 1H and 13C NMR
data of 2 match quite well with the respective data of
fumimycin (1).
We tried to deprotect the methyl ether in 2 to the free
phenol group at various stages of the synthesis, but the
isolation and further conversion of the catechol products
were difficult, presumably because of their instability.
Further studies to complete the (asymmetric) synthesis10 of
fumimycin as well as the elucidation of the pharmacophore
are currently in progress in our laboratories and will be
reported in due course.
In summary, we have developed a concise route to methoxy-
fumimycin (2) that involves generation of a hydroxyl group
via Dakin oxidation. Further carbon atoms for the scaffold
were introduced by allylation and Friedel-Crafts acylation.
A key step for the formation of the stereogenic fully sub-
stituted carbon is a 1,2-addition to a ketimine. Tandem
Claisen rearrangement-lactonization and subsequent iso-
merization established the olefinic side chain. Amidation and
acid deprotection finally accomplished the first synthesis of
(()-methoxyfumimycin.
Experimental Section
E)-3-(3-Carboxyacrylamido)-5-hydroxy-6-methoxy-3-methyl-
3H-benzofuran-2-one (Methoxyfumimycin) (2). A solution of
21 (2.9 mg, 8.2 μmol) in CH2Cl2 (0.60 mL) was cooled to 0 °C
and treated with trifluoroacetic acid (0.30 mL). After 1.5 h of
stirring, the reaction was allowed to reach rt. After 1 h the
solvent was removed under reduced pressure without warming
to afford the acid 2 as a brown film (2.5 mg, quant.). 1H NMR
(500 MHz, CD3OD) δ 1.67 (s, 3H), 1.91 (d, J = 5.8 Hz, 3H), 3.89
(s, 3H), 6.40 (d, J = 15.6 Hz, 1H), 6.59 (d, J = 15.4 Hz, 1H), 6.73
(s, 1H), 6.74 (m, 1H), 6.99 (d, J = 15.6 Hz, 1H); 13C NMR (125
MHz, CD3OD) δ 19.8, 23.4, 57.1, 59.6, 95.0, 118.2, 122.1, 123.1,
132.8, 134.7, 135.6, 143.0, 147.30, 149.8, 165.1, 168.1, 178.1; IR
(KBr) ν-1 3302, 1806, 1665, 1439, 1153, 1032, 924 cm-1; MS
(EI), m/z (%) 347 (72) [Mþ], 281 (24), 205 (34); HRMS (EI) calcd
for C17H17NO7 347.1005, found 347.1002.
Ethyl 2-[5-Allyloxy-2-(tert-butyldimethylsilanyloxy)-4-meth-
oxyphenyl]-2-diphenylphosphinoylaminopropionate (15). A solu-
tion of 14 (10.67 g, 17.97 mmol, 1.00 equiv) in toluene (240 mL)
was cooled to -78 °C. MeMgBr (3 M in Et2O, 13.4 mL, 35.9
mmol, 2.00 equiv) was added dropwise. After 3 h of stirring, sat.
KHSO4 solution (600 mL) was added. The phases were sepa-
rated; the aqueous layer was extracted with EtOAc (3 ꢀ 150
mL). The combined organic extracts were washed with brine
(130 mL), dried over Na2SO4, and concentrated under reduced
pressure. Flash chromatography (cyclohexane:EtOAc 2:1 f
2:3) afforded the amine 15 as highly viscous brown oil (7.11 g,
65%). 1H NMR (400 MHz, acetone-d6) δ 0.24 (s, 3H), 0.33 (s,
3H), 0.94 (s, 9H), 1.14 (t, J = 7.1 Hz, 3H), 1.83 (s, 3H), 3.79 (s,
3H), 4.06 (dq, J = 10.8, 7.1 Hz, 1H), 4.24 (dq, J = 10.8, 7.1 Hz,
1H), 4.38 (mc, 2H), 5.20 (dq, J = 10.5, 1.5 Hz, 1H), 5.39 (dq, J =
17.3, 1.5 Hz, 1H), 6.04 (ddt, J = 17.3, 10.5, 5.3 Hz, 1H), 6.45 (s,
1H), 6.88 (s, 1H), 7.28-7.34 (m, 2H), 7.40-7.56 (m, 4H),
7.62-7.70 (m, 2H), 7.90-7.96 (m, 2H); 13C NMR (100 MHz,
acetone-d6) δ -2.6, 15.3, 20.3, 25.8 (d, JP = 4.1 Hz), 27.6, 57.3,
62.4, 63.1, 72.4, 105.5, 116.5, 118.0, 126.4, 129.9 (d, JP = 12.7
Hz), 130.2 (d, JP = 12.5 Hz), 133.0 (d, JP = 2.6 Hz), 133.4 (d,
JP = 2.8 Hz), 133.5 (JP = 9.5 Hz), 133.7 (JP = 9.5 Hz), 136.1 (d,
JP = 129.4 Hz), 136.5, 137.1 (d, JP = 131.9 Hz), 143.5, 159.1,
151.5, 176.4 (d, JP = 8.7 Hz); IR (KBr) ν-1 3335, 2932, 1735,
1611, 1510, 1391, 1245, 1123, 913, 839 cm-1; MS (EI), m/z (%)
heating of 15 in dry DMF at 120 °C first led to the formation
of the lactone ring. Presumably, the required deprotection of
the TBS group is facilitated by an intramolecular attack of
the nitrogen via a six-membered ring.
Further heating of the lactone 16 at 153 °C finally led to a
rearrangement of the allyl ether to the desired product 17. A
one-pot synthesis involving lactonization-Claisen rearrange-
ment proved to be advantageous, giving 17 in a overall 73%
yield, and we were gratified that this sequence avoided an extra
TBS-deprotection step. Isomerization of the terminal double
bond was achieved under mild conditions by transition metal
catalysis. Treatment of 17 with PdCl2(PhCN)2 in CH2Cl2 at
40 °C18 gave 18 in good yield, but with a poor E:Z ratio of 4:1.
Better results were obtained employing RhCl3 H2O in etha-
3
nol,19 which gave an E:Z ratio of 10:1. The structure of 18 was
validated by crystal structure. The cleavage of the diphenyl-
phosphinoyl group to the ammonium salt 19 proved to be
challenging. In acidic methanol deprotection took place, but
decomposition also was observed. The conversion was opti-
mized by NMR analysis of the reaction mixture (DCl in
CD3OD). Best results were achieved at 65 °C for 3 h.
After various optimizations, direct conversion of 19
with tert-butyl protected acid chloride 20 gave the amide
21 in 50% yield over two steps. This method avoids
isolation of the free amine.
(18) Danishefsky, S. J.; Uang, B. J.; Quallich, G. J. Am. Chem. Soc. 1985,
107, 1285–1293.
(19) Boulin, B.; Arreguy-San Miguel, B.; Delmond, B. Tetrahedron 2000,
56, 3927–3932.
J. Org. Chem. Vol. 75, No. 1, 2010 231

Gross et al.
JOCNote
609 (16) [Mþ], 568 (52) [C30H39NO6PSiþ], 552 (100)
[C29H35NO6SiPþ], 536 (78); HRMS (EI) calcd for C33H44NO6-
SiP 609.2676, found 609.2673.
0 °C. A solution of (E)-tert-butyl-4-chloro-4-oxobut-2-enoate
(0.32 mmol, 1.3 equiv) in CH2Cl2 (1.0 mL) was added dropwise.
After 2 h of stirring, the mixture was diluted with cold water
(10 mL). The phases were separated; the aqueous layer was
extracted with EtOAc (4 ꢀ 15 mL). The combined organic
extracts were washed with brine (20 mL), dried over Na2SO,4
and concentrated under reduced pressure. Flash chromatogra-
phy (cyclohexane:EtOAc 5:2 f 2:1) afforded the amide 21 as a
3-Diphenylphosphinoylamino-5-hydroxy-6-methoxy-3-methyl-
4-[(E)-propenyl]-3H-benzofuran-2-one (18). A mixture of 17
(299 mg, 0.666 mmol, 1.00 equiv) and rhodium(III) chloride
hydrate (12.8 mg, 33.3 μmol, 0.050 equiv) in EtOH (9.0 mL) was
heated to 45 °C. After 4 h, the reaction was allowed to cool to rt
and filtrated through a 3 cm pad of Celite. The fitrate was
concentrated under reduced pressure. Flash chromatography
(cyclohexane:EtOAc 3:2 þ 5% MeOH f 3:2 þ 10% MeOH)
afforded the lactone 18 as light brown solid (E:Z 10:1, 288 mg,
96%; an analytical portion was recrystallized from hexane/
CHCl3 to furnish pure E product). Mp 199-200 °C; 1H NMR
(500 MHz, CD3OD) δ 1.70 (d, J = 1.4 Hz, 3H), 1.82 (dd, J =
6.6, 1.5 Hz, 3H), 3.85 (s, 3H), 6.47 (dq, J = 15.8, 1.5 Hz, 1H),
6.52 (s, 1H), 6.62 (dq, J = 15.8, 6.6 Hz, 1H), 7.31-7.37 (m, 2H),
7.41-7.48 (m, 3H), 7.52-7.60 (m, 3H), 7.68-7.74 (m); 13C
NMR (125 MHz, CD3OD) δ 20.0, 26.4 (d, JP = 9.3 Hz), 57.0,
60.4, 94.8, 118.7, 122.8, 124.2, 129.0 (d, JP = 13.1 Hz), 129.4 (d,
JP = 13.1 Hz), 132.9 (d, JP = 130.6 Hz), 133.0 (d, JP = 2.5 Hz),
133.0 (JP = 10.5 Hz), 133.1 (JP = 10.4 Hz), 133.4, 133.6, 133.8
(d, JP = 126.1 Hz), 142.9, 146.5, 150.0, 181.0; IR (KBr) ν-1
1
light brown solid (49.0 mg, 50%). Mp 203-205 °C; H NMR
(400 MHz, acetone-d6) δ 1.47 (s, 9H), 1.71 (s, 3H), 1.87 (dd, J =
6.6, 1.6 Hz, 3H), 3.90 (s, 3H), 6.52 (d, J = 15.5 Hz, 1H), 6.57 (dq,
J = 15.8, 1.6 Hz, 1H), 6.75 (s, 1H), 6.62 (dq, J = 15.8, 6.7 Hz,
1H), 6.95 (d, J = 15.5 Hz, 1H), 7.59 (b. S, 1H), 8.86 (br s, 1H);
13C NMR (100 MHz, acetone-d6) δ 20.8, 24.6, 29.1, 57.9, 59.8,
82.9, 95.7, 119.1, 122.1, 124.0, 134.2, 134.9, 135.9, 143.2, 147.8,
149.7, 164.2, 165.9, 177.5; IR (KBr) ν-1 3466, 2979, 2850, 2472,
1811, 1626, 1370, 1155, 973 cm-1; MS (EI), m/z (%) 493 (14)
[Mþ], 232 (14); HRMS (EI) calcd for C21H25NO7 403.1631,
found 403.1635.
Acknowledgment. We gratefully acknowledge the ex-
perimental assistance of Canh Nguyen Thang. We thank
the Foundation of German Business (fellowship to P.G.),
3487, 3242, 2975, 2848, 1631, 1469, 1314, 1174, 1038, 924 cm-1
;
€
€
the Landesgraduiertenforderung Baden-Wurttemberg
(fellowships to P.G. and C.H.), and the BMBF for financial
support.
MS (EI), m/z (%) 449 (24) [Mþ], 406 (14), 218 (68), 201 (100);
HRMS (EI) calcd for C25H24NO5P 449.1392, found 449.1400.
(E)-3-(4-tert-Butoxy-4-oxobut-2-enamido)-5-hydroxy-6-methoxy-
3-methyl-3H-benzofuran-2-one (21). A solution of 18 (110 mg,
0.245 mmol) in HCl in MeOH (0.5 M, 2.0 mL) was heated to 65
°C for 4 h. The solvent was removed under reduced pressure,
then the residue was diluted with CH2Cl2 (1.0 mL) and
pyridine (0.20 mL, 0.19 g, 2.5 mmol, 10 equiv) and cooled to
Supporting Information Available: General information, 1H
and 13C NMR spectra for all new compounds, and crystal-
lographic data for 14 and 18. This material is available free of
charge via the Internet at http://pubs.acs.org.
232 J. Org. Chem. Vol. 75, No. 1, 2010