New azaphilones from Chaetomium globosum isolated from the built environment

Tetrahedron Letters 54 (2013) 568–572

Contents lists available at SciVerse ScienceDirect

Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

New azaphilones from Chaetomium globosum isolated from the built environment David R. McMullin a, Mark W. Sumarah a, , Barbara A. Blackwell b, J. David Miller a,⇑ a b

Ottawa Carleton Institute of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, 960 Carling Avenue, Ottawa, Ontario, Canada K1A 0C6

a r t i c l e

i n f o

Article history: Received 5 October 2012 Revised 19 November 2012 Accepted 21 November 2012 Available online 29 November 2012 Keywords: Chaetomium globosum Azaphilone Chaetomugilin Chaetoviridin Isochromophilone

a b s t r a c t A strain of Chaetomium globosum (DAOM 240359) was isolated from an indoor air sample in Ottawa, Ontario, Canada. When fermented in liquid culture, this strain produced a number of known metabolites including chaetoglobosins A (6), C, and F (7), chaetomugilin D (5), chaetoviridin A (4), and three new nitrogenous azaphilones; 40 -epi-N-2-hydroxyethyl-azachaetoviridin A (1), N-2-butyric-azochaetoviridin E (2), and isochromophilone XIII (3). The structures were elucidated by spectroscopic analysis including; HRMS, 1D and 2D NMR, UV, and ORD. Compounds 2–7 were antimicrobial when tested using quantitative growth inhibition assays. Ó 2012 Elsevier Ltd. All rights reserved.

Introduction Chaetomium globosum and related species have been implicated with ovine illthrift through the production of toxic metabolites in pasture. Some of these compounds are very toxic to rumen bacteria and there has been some interest in its use as a biological control for various plant diseases.1,2 C. globosum is commonly found on damp building materials in North America and Europe.3,4 At exposures that can occur in the built environment, toxins from other fungi result in the expression of various chemokines that are markers for inflammation and asthma.5,6 As such, we have been studying its allergens7 and metabolites.8 C. globosum strains isolated from moldy building materials from a number of locations across Canada primarily produced chaetoglobosins A, C, and F, chaetomugilin D, and chaetoviridin A in varying amounts. This was the first report of the latter two metabolites derived from North American strains of C. globosum.8 Chaetoglobosins are antifungal,9 antibacterial, and cytotoxic.10 Chaetomugilins are potently cytotoxic11 and chaetoviridin A is potently antifungal.12 C. globosum DAOM 240359 was grown in liquid culture and the ethyl acetate extracts of the aqueous media were fractionated by silica gel column chromatography and semi-preparative HPLC. This strain had previously been shown to produce chaetoglobosins A ⇑ Corresponding author. E-mail address: [email protected] (J.D. Miller). Current address: Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada, 1391 Sandford Street, London, Ontario, Canada N5V 4T3.  

0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.11.084

(6), C, and F (7), chaetoviridin A (4), chaetomugilin D (5)8 as well as other unidentified compounds in minor amounts. Three of those metabolites are reported herein. Results and discussion Compound 1 was isolated as an optically active dark red gum with the molecular formula C25H30NO6Cl determined by the HRESI-MS in positive mode at m/z 476.1847 [M+H]+. A 3:1 isotopic peak ratio for [M+H]+:[M+H+2]+ was observed indicating the presence of a single chlorine in the molecule. The odd molecular weight in addition to the interpretation of the 13C and 1H NMR spectra suggest the presence of a single nitrogen atom and 11 units of unsaturation attributed to 3 rings and 8 double bonds. The 1H spectrum indicated the presence of four aliphatic methyl groups at d 0.91 (t, J = 7.5), d 1.03 (d, 7.0), d 1.09 (d, J = 6.7), and d 1.11 (d, J = 6.4), a downfield tertiary methyl at d 1.67, a multiplet methylene at d 1.47, and a multiplet methine at d 2.33. Two methines were observed at d 3.51 (p, J = 6.4/7.0) and d 3.95 (p, J = 6.4/7.0) suggesting they were vicinal. Two additional substituted methylenes that were not observed in the 1H spectrum of chaetoviridin A (4) appeared at d 3.75 (t, J = 5.1) and d 4.08 (m). Two proton signals at d 6.44 (d, J = 15.4) and d 6.49 (dd, J = 6.2, 15.4) illustrate a trans-olefinic group due to their chemical shifts and large coupling constants. Two unsaturated proton singlets at d 6.99 and d 7.92 are similar to those of chaetoviridin A (4) but chemical shift differences suggested the nitrogen at position 2 instead of an oxygen.

569

D. R. McMullin et al. / Tetrahedron Letters 54 (2013) 568–572

The 13C spectrum displayed 25 carbon signals of which 10 were quaternary which is indicative of a highly conjugated, planar structure. These signals were a result of a chlorine bearing carbon at d 95.3, three carbonyls at d 171.0, d 193.4, and d 201.1, an oxygenated carbon at d 87.7, and five conjugated sp2 carbons at d 109.0, d 116.6, d 145.7, d 151.2, and d 165.5. These data are similar to that of chaetoviridin A (3) reported by Takahashi et al. (1990)13 but again chemical shift differences suggests that the lone nitrogen atom occurs at position 2. Analysis of COSY, HSQC, and HMBC identified the 3-methyl-1pentenyl and 2-butanol-3-yl moieties as in chaetoviridin A (4). The HMBC correlation between H-9 and C-3 attaches the 3-methyl-1-pentyl chain to C-3. HMBC correlations to C-30 from H-40 , H-40 -Me, and H-50 attach the 2-butanol-3-yl substituent to the conjugated carbonyl. The COSY spectrum showed that the two substituted methylenes (d 3.75 and d 4.08) not present in chaetoviridin A (3) are vicinal and an HMBC correlation from H-1 to C-100 indicates that the nitrogen occupies position 2 and bears a hydroxy ethyl group analogous to isochromophilone VI.14 The 1 H and 13C assignments of 1 are reported in Tables 1 and 2. The structure of 1 was established as a N-2-hydroxy ethyl derivative of chaetoviridin A. The optical rotation of 1, [a]D 40 (c 0.01, MeOH), has the opposite sign compared to chaetoviridin A (4) isolated here, [a]D 80 (c 0.01, MeOH), and reported by Takahashi et al. (1990),13 ([a]D 98, c 0.05), suggesting a change in stereochemistry. The stereochemistry of the C-7 and C-11 methyl moieties were both determined to be of the (S) configuration by Takahashi et al. (1990)13 and later confirmed by X-ray crystallography.15 Comparison of our 1H, 13C (Tables 1 and 2) and NOE data for compounds 1 and 4 demonstrates the same (S) configuration at these two stereocenters. A strong NOE correlation between H-9/ H-11 and a weak correlation between H-9/H-11-Me in both compounds 1 and 4 also confirms the stereochemistry at C-11 as (S), consistent with reported chaetovirdins,13 chaetomugilins,11,16 and isochromophilones.14 Germain et al. (2011)17 who examined the stereoselective synthesis of sclerotiorin compounds at the C-7 position showed that a nitrogen bearing a non-chiral moiety does not affect the sign of the optical rotation. Together, these data suggest that the difference in stereochemistry accounting for the opposite sign of the optical rotation between 1 and 4 is attributed to the 2butanol-3-yl moiety. A difference was observed between the H-50 chemical shifts of 4 (d 3.69) and 1 (d 3.95). Comparison of chemical shifts for positions C-40 , C-50 , and C-60 of compound 1 to that of

Table 2 13 C [CD3CN, 100 MHz] spectral data for compounds 1–4

a

Position

1

2a

3

4

1 3 4 4a 5 6 7 8 8a 9 10 11 12 13 7-Me 11-Me 10 20 30 40 50 60 40 -Me 100 200 300 400

143.2 151.2 113.0 145.7 95.3 193.4 87.7 165.5 116.6 121.1 150.5 40.0 29.6 12.0 30.0 19.5 171.0 109.0 201.1 52.5 69.7 20.6 12.5 57.2 60.5

140.3 151.1 111.9 148.2 99.4 182.0 89.9 164.3 113.8 121.2 151.2 40.6 30.2 12.2 26.4 19.7 170.1 125.7 192.1 139.1 147.5 15.3 10.7 55.1 26.6 31.4 176.3

142.7 150.7 111.6 147.7 98.8 187.7 84.2 197.8 116.2 121.0 149.9 40.0 29.8 12.0 29.4 19.5 56.8 60.7

152.0 158.2 105.9 141.4 108.7 184.3 88.3 162.2 111.3 120.8 148.2 39.5 29.6 11.9 25.7 19.3 169.1 126.7 201.8 51.8 70.8 21.4 12.9

Chemical shift data in CD3OD.

chaetoviridin A, 40 -epi-chaetoviridin A, and 50 -epi-chaetoviridin A as reported by Borges et al. (2011)15 suggests the R configuration for position C-50 . NOE correlations were observed between H-40 / H-60 and H-40 -Me/H-50 but no NOE was observed between H-40 and H-50 that is consistent with data for 40 -epi-chaetoviridin A indicating that the stereochemistry at C-40 and C50 is (R) and (R) respectively for (1). Furthermore, invoking a hydrogen bond stabilization between the C-30 carbonyl and 50 hydroxyl confirms the observed NOEs and the coupling constant between H-40 and H-50 of 7.0 Hz. A similar argument was utilized by Takahashi et al. (1990)13 to establish the stereochemistry of C-40 and C-50 as (S) and (R), respectively, of chaetoviridin A. The single point stereochemical difference at position C-40 accounts for the opposite sign of the optical rotation compared to chaetoviridin A. Thus, the

Table 1 1 H (CD3CN, 400 MHz) NMR data for compounds 1–4a

a b

Position

1

2b

3

4

1 4 9 10 11 12 13 7-Me 11-Me 10 20 40 50 60 40 -Me 100 200 300

7.92 s 6.99 s 6.44 d (15.4) 6.49 dd (6.2, 15.5) 2.33 m 1.47 m 0.91 t (7.5) 1.67 s 1.09 d (6.7)

7.89 s 7.02 s 6.64 d (15.3) 6.51 dd (7.6, 15.5) 2.38 m 1.50 m 0.95 t (7.5) 1.65 s 1.14 d (7.0)

7.87 s 6.94 s 6.43 d (15.5) 6.44 dd (6.1, 15.5) 2.32 m 1.46 m 0.91 t (7.4) 1.43 s 1.1 d (6.7) 4.06 t (5.2) 3.76 t (5.2)

8.59 s 6.66 s 6.25 d (15.4) 6.65 dd (7.1, 15.6) 2.29 m 1.43 m 0.89 t (7.4) 1.63 s 1.08 (7.0)

3.51 p (6.4, 7.0) 3.95 p (6.4, 7.0) 1.11 d (6.4) 1.03 d (7.0) 4.08 m 3.75 t (5.1)

J in Hz. Chemical shift data in CD3OD.

6.60 q (6.8) 1.87 d (6.7) 1.86 s 4.12 t (7.6) 2.00 m 2.36 t (5.6)

3.48 3.69 1.06 1.04

p (7.0) p (6.4) (6.40) d (7.0)

570

D. R. McMullin et al. / Tetrahedron Letters 54 (2013) 568–572

structure of 1 was established as 40 -epi-N-2-hydroxyethyl-azachaetoviridin A. Compound 2 was isolated as an optically active orange gum with the molecular formula C27H30NO6Cl determined from the HRESI-MS in positive mode at m/z 500.1867 [M+H]+. A similar 3:1 isotopic peak ratio was observed and the odd molecular weight suggests a single chlorine and nitrogen in the molecule. The 13 units of unsaturation were attributed to 10 double bonds and 3 rings indicating two additional double bonds compared to 1 plus the addition of 2 carbon atoms. The 1H spectrum displayed many of the features of compound 1 including two aliphatic methyl groups at d 0.95 (t, J = 7.5), d 1.14 (d, J = 7.0), a tertiary methyl at d 1.65, a multiplet methylene at d 1.50, and a multiplet methine at d 2.38. The proton singlets at d 7.02, d 7.89 as well as the two proton signals at d 6.51 (dd, J = 7.6, 15.5) and d 6.64 (d, J = 15.5) that are similar to compounds 1 and 4 indicate the same chlorinated, conjugated lactone structure with a 9, 10 trans double bond. In contrast to 1, two downfield methyl groups at d 1.86 and d 1.87 (d, J = 6.7) as well as a quartet methine at d 6.60 (q, J = 6.8) identify a 2-butene-3-yl moiety as seen in chaetoviridin E.18 Finally, three vicinal methylene groups were observed at d 4.12, d 2.00, and d 2.36 suggesting a different chain from the N-2 position compared to 1. The 13C spectrum of 2 was similar to that of 1 (Table 2) however two additional resonances were observed including an acid function at d 176.3 and methylene at d 31.4. A new C@C was observed at d 139.1 and d 147.5, concomitant with the absence of the C-40 and C-50 methines of 1. The C-200 of 1 at d 60.5 has been replaced by a methylene at d 26.6 indicating the loss of the hydroxy group. Analysis of COSY, HSQC and HMBC revealed the same core structure as 1 but with differences in the N-2 and C-30 conjugated carbonyl side chains. COSY correlations from H-200 to H-100 and H-300 as well as HMBC correlations from H-200 and H-300 to the carboxylic acid C-400 establish the N-2 side chain as gamma butyric acid. HMBC correlations between C-40 -Me singlet (d 1.86) and 13C resonances at d 192.1, d 139.1, and d 147.5 as well as between the methyl doublet at d 1.87 to d 139.1 and d 147.5 establish a 2-butene-3-yl function at C-30 . This represents a beta oxidation at C-40 , C-50 over 1 as seen in chaetoviridin E.18 NOE correlations between H-9/H-11, H-9/H-11-Me and H-10/ H-12 establish the stereochemistry at the C-11 position as (S) as in 1 and 4. Absence of an NOE between H-40 -Me and H-50 as well as similar chemical shifts indicates the configuration of the double bond as (E) as in chaetoviridin E. The 40 and 60 methyl moieties are too close in chemical shift to observe the confirming NOE. Additional NOE correlations between H-1/H-100 and H-200 /H-9 indicate interactions between the planar pyrano quinone core and the flexible side chain off the nitrogen, which was also observed in 1. The optical rotation of 2, [a]D 80 (c 0.01, MeOH), is of the same sign as chaetoviridin E, [a]D 385 (c 0.02, CHCl3)18 establishing the structure of 2 as N-2-butyric-azochaetoviridin E. Compound 3 was isolated as an optically active orange gum with the molecular formula C18H22NO4Cl determined by HRESIMS at m/z 352.1379 [M+H]+. Again, the 3:1 isotopic ratio of [M+H]+:[M+H+2]+ was observed in the mass spectrum and the odd molecular weight suggested the presence a single chlorine and nitrogen in the molecule. The 8 units of unsaturation calculated from the molecular formula were a result of 6 double bonds and 2 rings indicating a planar, isochromophilone structure.14 The 1H spectrum of 3 showed characteristic chemical shifts of the pyrano quinone structure as in 4 and 5 (Table 1). Two vicinal substituted methylene signals at d 3.76 (t, J = 5.2) and d 4.06 (t, J = 5.2) were observed as in 4 identifying the same N-2-hydroxy ethyl side chain. The 13C spectrum also showed pyrano quinone characteristic chemical shifts as in 1 and 2. The same carbon chemical shifts for the N-2-hydroxy ethyl side chain were also observed

as in 1 (Table 2). Interpretation of COSY, HSQC and HMBC spectra of 3 identified the same 3-methyl-1-pentyl moiety as in 1, 2, 4, and 5. No other aliphatic moieties were observed and the C-8 chemical shift of d 197.8 indicates a ketone as opposed to the fused lactone ring in 1, 2, 4, and 5. The stereochemistry at C-7 and C-11 were determined to be of the (S) configuration when comparing the optical rotation, [a]D 90 (c 0.2, MeOH), to that of compounds isolated here and to the literature.14 The negative orientation of the optical rotation is also consistent with Germain et al. (2011)17 who examined the stereoselective synthesis of sclerotiorin compounds at the C-7 position. Similar natural products where both C-11 and C-7 are (S) had a negative optical rotation however when C-7 was (R) the sign switched to positive with the same magnitude. This demonstrated that a nitrogen bearing, non-chiral moiety does not affect the sign of the optical rotation. The structures of chaetoviridin A (4), chaetomugilin D (5), and chaetoglobosins A (6), C, and F (7) were determined by high resolution mass spectrometry, NMR spectroscopy, and comparison to the literature. They were reported from DAOM 240359 by McMullin et al. (2012).8 NMR data of chaetoviridin A (4) are provided here to allow comparison to the new structures. Chaetomugilin D was originally characterized by Yasuhide et al. (2008)16 however other manuscripts addressing this metabolite convolute the literature. Qin et al. (2009)19 incorrectly report the stereochemistry of C-11 as (R). Our physical and spectroscopic data agree with that of Yasuhide et al (2008)16 and Takahashi et al. (1990)13 who originally published the structures of chaetomugilin D (5) and chaetoviridin A (4). An LC-UV-HRMS spectrum of the crude extract is provided as supplemental Figure 1. Aside from the ethyl acetate extracts, all chaetoglobosins and azaphilones reported herein were present in freeze dried media (data not shown). In vitro, 200 lM of compounds 2, 3, 5, 6, and 7, respectively, resulted in significant reduction in the growth of the Gram positive bacterium and the Gram negative bacterium (please see Table 3). Additionally, 20 lM concentrations of compounds 2, 5, 6, and 7 reduced the growth of both species (ANOVA, Tukey’s Test, P < 0.05). 200 lM of 2 and 5, respectively, gave similar results compared to the same concentration of the positive control, chloramphenicol whereas the rest were less antibiotic. 200 lM of compounds 2, 6, and 7 inhibited Saccharomyces cerevisiae. Compound 5 showed antifungal activity at 2 mM. Structurally diverse azaphilones have been shown to be inhibitors of acetyl-CoA cholesterol acyltransferases and here we demonstrate their antimicrobial properties. Few studies have reported both chaetoglobosins and azaphilones from C. globosum. We found that a number of metabolites reported for this species were probably based on misidentifications of the isolates.8 The C. globosum strains we investigated were morphologically correct, had appropriate ITS sequences and are deposited in the Agriculture Canada culture collection for future study.8 All azaphilones from this strain had a chlorine atom at C-5 and conserved stereochemistry at the C-7 and C-11 positions that is consistent with the literature. Additionally, all azaphilones reported here had the same 3-methyl-1-pentyl chain attached to C3 similar to the known chaetomugilins and chaetoviridins. Various chains branching from the C-3 position are possible including 3-methyl-4-hydroxy-1-pentyl chains observed in some chaetomugilins11 and the longer 3,5-dimethyl-1,3-heptadienyl residue observed in isochromophilones,14 luteusins,20 and sclerotiorin.20 The 3-methyl-4-hydroxy-1-pentyl chain appears prevalent in azaphilones produced by the genus Chaetomium. The latter appears to be more common in Penicillium azaphilones.20 Compound (1) was isolated as a nitrogenous derivative of chaetoviridin A (4) with an addition of an N-2-hydroxy ethyl at chain and the stereochemistry at the C-40 position is (R) instead of (S). Compound (6) had the

571

D. R. McMullin et al. / Tetrahedron Letters 54 (2013) 568–572

Cl O

13

3

4a

N

8a

O

8

1'

2'

O

11 10

7

Cl

9

4

5 6

12

2"

1

OH

1"

O

4'

O

5'

6'

11 10

N

8a

O

8

1'

2'

12

COOH

2"

1

1"

4"

3"

3'

O

HO

13

3

4a

7

3'

9

4

5 6

4'

O

6'

5'

1

2 Cl O

Cl O

9

4

5 4a

3

8a

N

6

8

HO

12

10

7 1

2' 1'

O

O

13

11

O

OH

O

O

HO 4

3

O

Cl O

H

O

H H O

O HO

OH

N OO H

O

O

N H 6

5

Figure 1. Compounds (1–6) isolated from C. globosum strain DAOM 240359.

Table 3 Inhibition of B. subtilis, P. putida and S. cerevisiae by compounds 2–7 at 20 and 200 lMa Compound

(2)

Assay P. putida B. subtilis S cerevisiae

20 lM + + 

(3) 200 lM + + +

20 lM   

(4) 200 lM + + 

20 lM   

(5) 200 lM + + 

20 lM + + 

(6) 200 lM + + 

20 lM + + 

(7) 200 lM + + +

20 lM + + 

200 lM + + +

+—Inhibition; —no inhibition. a ANOVA, Tukey’s test, (P < 0.05).

same N-2-hydroxy ethyl group that is also observed in isochromophilone VI14 and the cytotoxic chaetoglobin B.21 Compound (2) was isolated as a nitrogenated derivative of chaetoviridin E with a gamma-aminobutyric acid (GABA) at position 2 as observed in isochromophilone IX.22 These are the first reported azaphilones with the 3-methyl-1-pentyl and an N-2 side chain. (E)-5-chloro-9-(3-hydroxy-2-methylbutanoyl)-2-(2-hydroxyethyl)-6a-methyl-3-(3-methylpent-1-en-1-yl)-furo-[2,3-h]isoquinoline-6,8(2H,6aH)-dione (1): (4.1 mg); optically active red gum; [a]D 40 (c 0.01, MeOH); UV (MeOH)/nm kmax (log e) 267 (4.22), 435 (3.98), 515 (4.01); HRMS m/z 476.1847 [M+H]+ (calculated for C25H31NO6Cl, 476.1840). 4-((S)-5-chloro-6a-methyl-9-((E)-2-methylbut-2-enoyl)-3-((S,E)3-methylpent-1-en-1-yl)-6,8-dioxo-6,6a-dihydrofuro[2,3-h] isoquinolin-2(8H)-yl)butanoic acid (2): (6.1 mg); optically active orange gum; [a]D 80 (c 0.01, MeOH); UV (MeOH)/nm kmax (log e) 238 (2.66), 294 (2.48), 348 (2.54), 390 (1.77), 472 (0.41); HRMS m/z 500.1867 [M+H]+ (calculated for C27H31NO6Cl: 500.1840). (E)-5-chloro-7-hydroxy-2-(2-hydroxyethyl)-7-methyl-3-(3-methylpent-1-en-1-yl)-isoquinoline-6,8-(2H,7H)-dione (3): (9.8 mg);

optically active orange gum; [a]D 90 (c 0.2, MeOH); UV (MeOH)/nm kmax (log e) 204 (2.66), 277 (2.48), 379 (2.54), 463 (1.77); HRMS m/z 352.1379 [M+H]+ (calculated for C18H23NO4Cl: 352.1316). Acknowledgments This research was funded by a NSERC IRC to JDM and an OGS to DRM. We would like to thank Dan Sørensen for HR-LC-MS analysis. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012. 11.084. References and notes 1. Brewer, D.; Taylor, A. Can. J. Microbiol. 1978, 24, 1078–1081. 2. Kaewchai, S.; Soytong, K.; Hyde, K. Fungal Divers. 2009, 38, 25–50.

572

D. R. McMullin et al. / Tetrahedron Letters 54 (2013) 568–572

3. Andersen, B.; Frisvad, J.; Søndergaard, I.; Rasmussen, I.; Larsen, L. Appl. Environ. Microbiol. 2011, 77, 4180–4188. 4. Miller, J.; Rand, T.; McGregor, H.; Solomon, J.; Yang, C. Recognition, Evaluation and Control of Indoor Mold. American Industrial Hygiene Association, Fairfax, VA. 2008, pp 43–51. 5. Miller, J.; Sun, M.; Gilyan, A.; Roy, J.; Rand, T. Chem. Biol. Interact. 2010, 183, 113–124. 6. Rand, T.; DiPenta, J.; Robbins, C.; Miller, J. Chem. Biol. Interact. 2011, 190, 139– 147. 7. Provost, N.; Shi, C.; She, Y.; Cyr, T.; Miller, J. Med. Mycol. 2012. on line early Sept 18. 8. McMullin, D.; Sumarah, M.; Miller, J. Mycot. Res. 2012. http://dx.doi.org/ 10.1007/s12550-012-0144-9. 9. Amemiya, Y.; Kondo, A.; Hirano, K.; Hirukawa, T.; Kato, T. Tech. Bull Faculty Hortic.-Chiba Univ. 1994, 48, 13–18. 10. Scherlach, K.; Boettger, D.; Remme, N.; Hertweck, C. J. Nat. Prod. 2010, 27, 869– 886. 11. Muroga, Y.; Yamada, T.; Numata, A.; Tanaka, R. Tetrahedron 2009, 65, 7580– 7586.

12. Park, J.; Choi, G.; Jang, K.; Lim, H.; Kim, H.; Cho, K.; Kim, J. FEMS Microbiol. Lett. 2005, 252, 309–313. 13. Takahashi, M.; Koyama, K.; Natori, S. Chem. Pharm. Bull. 1990, 38, 625–628. 14. Arai, N.; Shiomi, K.; Tomoda, H.; Tabata, N.; Yang, D.; Masuma, R.; Kawakubo, T.; Omura, S. J. Antibiot. 1995, 48, 702–969. 15. Borges, W.; Mancilla, G.; Guimarães, D.; Durán-Patrón, R.; Collado, I.; Pupo, M. J. Nat. Prod. 2011, 74, 1182–1187. 16. Yasuhide, M.; Yamada, T.; Numata, A.; Tanaka, T. J. Antibiot. 2008, 61, 615–622. 17. Germain, A.; Bruggemeyer, D.; Zhu, J.; Genet, C.; O’Brein, P.; Porco, J. J. Org. Chem. 2011, 76, 2577–2584. 18. Phonkerd, N.; Kanokmedhakul, S.; Kanokmedhakul, K.; Soytong, K.; Prabpai, S.; Kongsearee, P. Tetrahedron 2008, 64, 9636–9645. 19. Qin, J.; Zhang, Y.; Gao, J.; Bai, M.; Yang, S.; Laatsch, H.; Zhang, A. Bioorg. Med. Chem. Lett. 2009, 15, 1572–1574. 20. Osmanova, N.; Shultze, W.; Ayoub, N. Phytochem. Rev. 2010, 9, 315–342. 21. Ming, G.; Yun, Z.; Ding, G.; Saparpakorn, P.; Chun, S.; Hannongbua, S.; Tan, R. Chem. Commun. (Cambridge, UK) 2008, 45, 5978–5980. 22. Michael, A.; Grace, E.; Kotiw, M.; Barrow, R. Aust. J. Chem. 2003, 56, 13–15.