Structural and stereochemical revision of isocyanide and isothiocyanate amphilectenes from the Caribbean marine sponge Cribochalina sp.

Tetrahedron 61 (2005) 8049–8053

Structural and stereochemical revision of isocyanide and isothiocyanate amphilectenes from the Caribbean marine sponge Cribochalina sp. Maria Letizia Ciavatta,a,* Margherita Gavagnin,a Emiliano Manzo,a Raffaella Puliti,a Carlo Andrea Mattia,b Lelio Mazzarella,c Guido Cimino,a Jamie S. Simpsond and Mary J. Garsone a

Istituto di Chimica Biomolecolare, C.N.R., 80078 Pozzuoli, Italy Dipartimento di Scienze Farmaceutiche, Universita` di Salerno, via Ponte Don Melillo, I-84084 Fisciano, Salerno, Italy c Dipartimento di Chimica, Universita` “Federico II”, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy d Department of Medicinal Chemistry, Victorian College of Pharmacy, Monash University, 381 Royal Parade, Parkville Vic. 3052, Australia e School of Molecular and Microbial Sciences, The University of Queensland, Brisbane Qld 4072, Australia b

Received 24 March 2005; revised 9 May 2005; accepted 26 May 2005 Available online 17 June 2005

Abstract—The absolute stereochemistry of amphilectene metabolites from Cribochalina sp. has been revised by a detailed NMR spectroscopic study of the Mosher ester derivatives of a related alcohol. The relative stereochemistry of the previously described amphilectenes has been reinvestigated and reassigned on the basis of the X-ray structural analysis carried out on one of them. The structure of a new amphilectene metabolite, which is an isothiocyanato analogue is also presented. q 2005 Elsevier Ltd. All rights reserved.

1. Introduction Cytotoxic terpenes containing isocyano or isothiocyanato substituents are characteristic metabolites from marine sponges and associated molluscs.1 Diterpene representatives of this bioactive suite of metabolites can be classified as oxygenated decalins of the kalihinol or kalihinene type,2 usually associated with sponges of the genus Acanthella, or as non-oxygenated tricyclic or tetracyclic products with an amphilectene or cycloamphilectene ring system3–12 that are characteristic of haplosclerid sponges such as Amphimedon terpenensis, Ciocalypta sp. or Halichondria sp. In both the ring systems, structure and stereochemical assignment by NMR spectroscopy is frequently complicated by poor signal resolution in the proton spectra, even when data are acquired at high field, thus X-ray analysis remains a basic structural tool for this group of metabolites. Amphilectene and cycloamphilectene structures, whose relative stereochemistry has been secured by X-ray, include (1)–(3);3,5 Konig et al. have provided a comprehensive 2D NMR spectroscopic analysis of diterpenes from the tropical marine sponge Keywords: Diterpenes; NMR; Mosher analysis; X-ray; Stereochemistry; Sponges. * Corresponding author. Tel.: C39 0818675243; fax: C39 0818041770; e-mail: [email protected] 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.05.102

Cymbastela hooperi.8 Moreover, the relative stereochemistry of some of these metabolites has been secured by means of X-ray studies.9 In 1999, we described a series of amphilectene metabolites, compounds 4–7, and a related bifloradiene (8), from a marine sponge Cribochalina sp. collected off the Caribbean coast of Mexico.10 We investigated the absolute stereochemistry of compound 4 by modified Mosher analysis of the alcohol derivative 9. However, our study was hampered by the presence of many overlapping signals in the proton NMR spectra of these compounds in CDCl3, even when data were acquired at 500 MHz. Now, we have examined these derivatives in d5-pyridine, a solvent in which the proton NMR signals are better dispersed. Our results allow us to report a revised absolute stereochemistry for isocyanide (4) that matches that deduced for the cycloamphilectene 7,20-diisocyanoadociane (1) by total synthesis,13 and by X-ray analysis on its p-bromobenzamide derivative.14 Since our initial report, we have also isolated the minor compound 10, the isothiocyanate derivative of 4, as well as succeeded in obtaining single crystals of compound 7 suitable for an X-ray diffraction study, which fully clarified its relative stereochemistry and, in particular, firmly established the uncommon 8,13 cis ring-junction. This cis junction is then assured also for the biosynthetically-related metabolites (5)–(8).

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2. Results and discussion An aliquot of the diethyl ether soluble portion (1 g) of the acetone extract of Cribochalina sp. was purified by silicagel chromatography (light petroleum ether/diethyl ether gradient) according to our literature conditions.10 Along with already reported compounds 4–8, a novel minor related metabolite (10) was isolated from the less polar fraction.

First of all, we focused our attention on the main compound 4 to re-investigate its absolute stereochemistry proposed to be opposite to the related cycloamphilectenes (e.g., 1) of known absolute configuration.13,14 Alcohol 9, re-obtained by reductive ozonolysis from compound 4,10 was dissolved in d5-pyridine and submitted to a careful NMR spectroscopic analysis leading to a complete proton and carbon

assignment (see Table 1 and Section 3), which confirmed the axial orientation of the hydroxy group at C11 (H-11, d 4.16, JW1/2 7.5 Hz). According to the known procedure,15 treatment of compound 9 with (R)- and (S)-MTPA chlorides in dry methylene chloride and DMAP afforded the corresponding (S)-MTPA (9a) and (R)- MTPA (9b) esters. The two Mosher derivatives were dissolved in d5-pyridine and analysed by 1H NMR spectroscopy. The NMR signals

were significantly better dispersed than those recorded in CDCl3,10 thus allowing the full assignment, as reported in Table 1 and Section 3. The chemical shift analysis of (S)-MTPA (9a) and (R)MTPA (9b) esters clearly indicated an R absolute configuration at C11, opposite to that previously

Table 1. 1H NMR assignmentsa,b in Pyr-d5 of alcohol 9 and its Mosher’esters Position

Compound 9 dH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 a b

2.02 2.14/0.83 1.03 1.08 1.85/0.77 1.86/1.43 1.32 — — — 4.16 1.33 1.72 1.82/1.25 — 1.43 1.38 0.80 0.94

9a (S-ester)

m (J, Hz) m m m m m m m

br s bt 10.5 m s s d 6.2 d 6.6

dH 1.59 2.24/0.83 0.93 1.04 1.83/0.69 1.88/1.40 1.20 — 1.39 2.12/1.96 5.56 1.54 1.28 1.86/1.30 — 1.30 1.36 0.79 0.88

Dd

9b (R-ester)

m (J, Hz) m m m m m m m m m br s t 10.9 m m s s d 6.2 d 6.2

Bruker AMX 600 MHz, Pyr-d5; d values are reported referred to Pyr-d5 (d 8.71, 7.56, 7.19). Assignments determined by 1H–1H COSY, HSQC, HMBC experiments.

dH 1.68 2.26/0.85 0.98 1.06 1.84/0.70 1.85/1.36 1.15 — 1.39 2.05/1.92 5.57 1.55 1.33 1.89/1.30 — 1.30 1.35 0.80 0.85

m (J, Hz) m m m m m m m — m m br s t 1.05 m s s d 6.5 d 6.6

K0.09 K0.02/K0.02 K0.05 K0.02 K0.01/K0.01 C0.03/C0.04 C0.05 0.00 C0.07/C0.04 K0.01 K0.01 K0.05 K0.03/0.00 0.00 K0.01 K0.01 C0.03

M. L. Ciavatta et al. / Tetrahedron 61 (2005) 8049–8053

suggested,10 so the absolute stereochemistry of compound 4 must be revised as indicated in the given structure. Next, we also reconsidered the relative stereochemistry of amphilectenes 5–7, previously suggested on the basis of NMR spectroscopic analysis. Compounds 5–7 are characterised by the same cyclic framework that exhibits the isocyanide function linked to carbon C7 bearing a methyl group. Even though, the spin systems of 5–7 were confidently assigned by NMR spectroscopic analysis, the relative stereochemistry was suggested in the first paper by analogy with the related main metabolite, compound 4, as well as by a series of NOE experiments.10 Indeed, re-analysis of these spectra led us to reconsider the previous stereochemical assignment and to further investigate this aspect by X-ray diffraction analysis. We succeeded in growing good single crystals of compound 7, which is the most abundant of these amphilectenes, from n-hexane. The X-ray structure has removed all the ambiguities regarding its relative stereochemistry. The structure was solved using SIR9716 and refined by SHELXL package17 to a conventional discrepancy R factorZ0.044 on 2466 observed reflections and 202 variables. A perspective view of the final X-ray model of 7 is shown in Figure 1.

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The crystal study revealed the cis ring-junction along the C8 and C13 bond (B/C cis-fusion), which is rather unusual in the amphilectene and cycloamphilectene classes. In particular, a cis fusion has been found in only two isomeric cycloamphilectenes from a Palauan sponge Halichondria sp.6 and more recently11 in N-formyl-7-amino-11-cycloamphilectene, a metabolite from Vanuatu sponge Axinella sp. This is the first case of cis ring junction in amphilectene diterpenoids. Indeed, the previously found cis junction in 7-isocyanoneoamphilecta-11,15-diene7 regards a different kind of tricyclic neoamphilectene skeleton with a spirocyclic rearrangement. Apart from the exocyclic methylene group at C11, the tricyclic system of 7 presents an isobutenyl substitution at C1 and a secondary methyl group at C3, both in equatorial orientations. Moreover, a methyl group and a isocyanide functionality (b and a oriented, respectively), are present at the C7 carbon. Intramolecular geometry agrees well with the generally accepted values for correlated molecules.3,6,7,9,11,14,18 In particular, the carbon–carbon single ˚ and the longest bond bonds are in the range 1.498–1.567 A ˚ corresponds to B, C ring cis distance C8–C13Z1.567(3) A fusion. The geometry of isocyanide group is: C7–NZ ˚ , N–C21Z1.150(4) A ˚ , C7–N–C21Z177.7(5)8. 1.464(3) A The ring A is in a chair conformation, puckering parameters19 are: qZ4.0(2)8 and fZK4(3)8, with C2 and ˚ , respectively, and in C13 displaced 0.603(2) and 0.675(2) A the opposite direction with respect to the best plane through the remaining ring carbons. The other rings adopt twist-boat conformations, slightly distorted toward boat forms, with qZ86.2(2)8 and fZ26.0(2)8 for the B ring and qZ86.5(1)8 and fZ20.6(1)8 for the C ring. In the molecule, the shortest non-1,4 intramolecular distances are N–C4Z3.052(3) and ˚ . The molecular packing is governed C14–C20Z3.362(4) A only by van der Waals interactions and the shortest contacts involve the nitrile carbon: C21/C19(1⁄2 Cx, 1⁄2 Ky, 1Kz)Z ˚. 3.656(4) and C21/C18(1Kx, yK1⁄2 , 1⁄2 Kz)Z3.717(4) A On the basis of these results, the structure of amphilectenes 5–7 must to be revised as now reported. The presence of the 8,13 cis-junction in these compounds is in agreement with the structure of co-occurring metabolite 8,20 which could be considered a possible putative precursor of amphilectenes 5–7. The relative stereochemistry of 8 was erroneously reported in our previous paper.10

Figure 1. Perspective view of the final X-ray model of 7 with the atomic labelling for non-H atoms. Displacement ellipsoids are drawn at the 30% probability level.

In the absence of atoms with a strong anomalous scattering contribution, reliable evidence of the absolute stereochemistry could not be obtained. The enantiomer depicted is based on our reassigned absolute stereochemistry (NMR and Mosher analyses) here reported of compound 4, and agrees with the literature data.13,14 On this basis, the compound 7 is defined as (1S,3S,4R,7S,8R,12S, 13S)-7isocyanoamphilecta-11(20),15-diene. It is the 8-epimer of 7-isocyanoamphilecta-11(20),15-diene (11), a metabolite from the tropical marine sponge C. hooperi, whose structure was assigned by 2D NMR spectroscopy8 and confirmed by X-ray analysis.9

Finally, we have taken into consideration the novel compound 10, which was obtained as a colourless oil (1 mg, Rf 0.75, light petroleum ether/diethyl ether 9:1). Its HRESIMS spectrum displayed a sodiated-molecular peak at m/z 379.2164 (MCNa)C, consistent with the molecular formula C22H32N2S. Both proton and carbon NMR spectroscopic data of 10 strongly resembled those of diterpene 4 suggesting a closely related structure. In particular, analysis of 13C NMR spectrum clearly indicated that 10 differed from 4 only in the presence of an isothiocyanate function in the place of the isocyanate one at C-21 (d 126.0 in 10, d 156.2 in 4). All resonances of compound 10 were assigned as reported in Table 2 by NMR spectroscopic analysis and comparison with compound 4. In conclusion, we have presented here the stereochemical

8052 Table 2. 1H and

M. L. Ciavatta et al. / Tetrahedron 61 (2005) 8049–8053 13

C NMR dataa,b for compounds 10 and 4c

Position

Compound 10 dH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

m (J, Hz)

1.92 2.18/0.90 1.10 1.13 2.00/0.86 1.53 1.40

m m m m m m m

2.30/1.32 2.32

m m

1.85 1.05 2.05/1.28

m m m

1.43 1.45 0.93 0.99 4.86/4.61

br s br s d 6.7 d 6.0 s

Compound 4 dC 33.0 41.0 35.5 42.6 29.7 29.9 40.9 67.0 39.6 33.5 150.1 46.0 55.5 46.7 60.6 29.4 31.9 19.8 15.7 106.8 156.3 126.0

dH

m CH CH2 CH CH CH2 CH2 CH C CH2 CH2 C CH CH CH2 C CH3 CH3 CH3 CH3 CH2 C C

m (J, Hz)

1.99 2.28/0.93 1.10 1.15 1.99/0.86 1.53/1.43 1.39

m m m m m m m

2.30/1.32 2.32

m m

1.88 1.08 2.05/1.28

bt 10.5 m d 15.0/m

1.43 1.45 0.93 0.97 4.86/4.64

br s br s d 6.0 d 6.2 s

dC 33.1 40.8 35.4 42.5 29.6 29.8 40.7 66.9e 39.5 33.4 149.7 45.9 55.4 45.7 56.5 29.8 31.8 19.7 15.6 106.2 156.2 154.3

m CH CH2 CH CH CH2 CH2 CH C CH2 CH2 C CH CH CH2 C CH3 CH3 CH3 CH3 CH2 C C

a

Bruker AMX 600 MHz, CDCl3; d values are reported referred to CHCl3 (d 7.26) and CDCl3 (d 77.0). Assignments determined by 1H–1H COSY, HSQC, HMBC experiments. c NMR data of compound 4 are reported from Ref. 10. d Erroneously reported in Ref. 10. b

reassignment of isocyanide and isothiocyanate amphilectene diterpenes from Caribbean sponge Cribochalina sp. The structures of these compounds were incorrectly reported in our previous paper.10 The revised absolute stereochemistry of the main isocyanide 4 matches that previously demonstrated for the related compound 1 whereas an uncommon 8,13-cis junction has been now defined for amphilectenes 5–7. This stereochemical feature is unprecedented among amphilectenes with a regular tricyclic skeleton. Isocyanide amphilectenes are typical metabolites of haplosclerid sponges of genera Amphimedon, Ciocalypta or Halichondria.1 Interestingly, compound 4 has been recently isolated also from nudibranch Phyllidiella pustulosa, which most likely obtains this metabolite from an haplosclerid sponge prey.21

3. Experimental 3.1. General experimental procedures Precoated TLC plates Merck (Darmstadt, Germany). Si gel 60 F254 were used for analytical TLC and Merck Kieselgel 60 powder was used for preparative column chromatography. Optical rotations were measured on a Jasco DIP 370 digital polarimeter; IR spectra were measured on a Biorad FTS 155 FTIR spectrophotometer; 1D and 2D NMR spectra were recorded on a Bruker AMX 400 (400.13 MHz), and on a Bruker AMX 600 equipped with a TXI CryoProbe; 13C NMR were recorded on a Bruker AMX 300 (75.47 MHz); HRESIMS was carried out on a Micromass Q-TOF micro.

3.2. Biological material See Ref. 10. 3.3. Extraction and isolation of diterpenes The remaining part of the ether soluble extract of the sponge (1 g) was chromatographed on a Si-gel column (light petroleum ether and increasing amounts of diethyl ether) giving fractions containing all the known compounds 4–8 as reported in the previous paper.6 One fraction was submitted to HPLC giving the new compound 10 (eluent n-hexane/ ethyl acetate 99:1, flow 1 ml/min, column Kromasil, 5m). 3.4. Single crystal X-ray analysis of compound 7 Single crystals were grown as colourless rectangular prisms by slow evaporation from n-hexane at room temperature. A sample of size 0.40!0.20!0.06 mm3 was selected for data collection on a Bruker KappaCCD diffractometer using ˚ ). graphite-monochromated Mo Ka radiation (lZ0.71069 A ˚ , bZ10.2668(5) A ˚, Crystal data. C21H31N, aZ9.0224(8) A ˚ , VZ1886.6(2) A ˚ 3, orthorhombic system, cZ20.367(1) A space group P21 2121 , ZZ4, M w Z297.47, Dc Z 1.047 g cmK3, mZ0.059 mmK1. Accurate cell parameters were obtained by least-squares refinement of the setting angles of 374 reflections at medium q (4.3!q!16.6). A total of 14380 reflections (K12%h%11, K13%k%10, K22%l% 27), 4652 of, which were independent (RintZ0.0207), were measured at room temperature. Data were collected up to qZ28.988 (96.5% of completeness) using COLLECT package.22 The structure was solved by direct methods using SIR9711 and refined by full-matrix least-squares calculations on F2 using

M. L. Ciavatta et al. / Tetrahedron 61 (2005) 8049–8053

SHELXL.12 All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were included at ideal positions, with isotropic thermal parameters set 1.2 times the Ueq of the parent atom, and not refined except for the torsion angles of methyl groups. At convergence, the final discrepancy index R was 0.0441 based on 2466 observed reflections [IO2s(I)] and 202 variable parameters. The overall Rw value was 0.1173 with wZ1/[s2(F2o)C (0.0261P)2C0.0153P] where PZ(F2oC2F2c )/3; SZ1.626; (D/s)max!0.001. No residual electron density was outside ˚ K3. The anomalous dispersion the range K0.26 to 0.20 e A effect is small and no reliable evidence of the absolute configuration could be obtained, indeed the final Flack parameterZK2(4), using 1902 Friedel opposite reflections, is not significant. All the crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC number 267942. 3.4.1. Compound 9. Compound 4 was submitted to a reductive ozonolysis10 to obtain alcohol 9. The structure of the alcohol was secured by comparison with chemical features of the previously reported compound 9. Complete assignments of 9 was made in deuterated pyridine. 13C NMR was recorded on a 300 MHz Bruker apparatus (75.47 MHz), resonances are given in ppm: 156.2 (C-21, C), 154.4 (C-22, C), 67.0 (C-8, C), 63.0 (C-11, CH), 57.0 (C-15, C), 45.5 (C-12 and C-13, CH), 45.3 (C-14, CH2), 43.6 (C-2, CH2), 42.5 (C-4, CH), 40.6 (C-7, CH), 36.6 (C-3, CH), 33.3 (C-1, CH), 31.1 (C-17, CH3), 30.4, 30.3, 30.2, 29.9, (C-5, CH2; C-6, CH2; C-9, CH2; C-10, CH2, all these resonances could be interchangeable), 28.7 (C-16, CH3), 20.1 (C-18, CH3), 15.8 (C-19, CH3). 3.4.2. Compound 9a(S-ester). 13C NMR (75.47 MHz, C5D5N): 166.0*, 130.5*, 129.3*, 128.2* (* all resonances belonging to MTPA residue), 71.7 (C-11, CH), 66.3 (C-8, C), 55.6 (C-15, C), 46.5 (C-13, CH), 44.5 (C-14, CH2), 44.1 (C-12, CH), 42.3 (C-4, CH), 40.7 (C-2, CH), 40.6 (C-7, CH), 36.1 (C-3, CH), 33.6 (C-1, CH), 32.3 (C-17, CH3), 30.3 (C-9, CH2), 30.1 (C-5, CH2), 29.9 (C-6, CH2), 28.1 (C-16, CH3), 26.0 (C-10, CH3), 19.8 (C-18, CH3), 15.6 (C-19, CH3). 3.4.3. Compound 9b (R-ester). 13C NMR (75.47 MHz, C5D5N): 164.5*, 130.5*, 129.3*, 128.1* (* all resonances belonging to MTPA residue), 71.9 (C-11, CH), 66.3 (C-8, C), 56.0 (C-15, C), 46.6 (C-13, CH), 44.6 (C-14, CH2), 43.8 (C-12, CH), 42.2 (C-4, CH), 41.0 (C-2, CH2), 40.5 (C-7, CH), 36.3 (C-3, CH), 33.9 (C-1, CH), 32.1 (C-17, CH3), 30.1 (C-9 and C-5, CH2), 29.9 (C-6, CH2), 28.6 (C-16, CH3), 25.8 (C-10, CH2), 19.8 (C-18, CH3), 15.6 (C-19, CH3). 3.4.4. Compound 10. 1 mg: pale yellow oil; [a]D K52 (c 0.1, CHCl3); IR (liquid film) nmax: 2955, 2923, 2867, 2124, 2090, 1736, 1646, 1456, 895 cmK1; HRESIMS: found 379.2164 (379.2184 calculated for C22H32N2NaS). Acknowledgements We thank ICB Mass Service and ICB NMR Service (Mrs. D. Melck is kindly acknowledged), and Mr. R. Turco for graphical work. We are grateful to the ‘Centro Regionale di Competenza Nuove Tecnologie per le Attivita`

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Produttive’ (CRdC-NTAP) of Campania Region for the use of Bruker Kappa CCD diffractometer. M.J.G. and J.S.S. thank the Australia Research Council and the Research School of Chemistry, the Australian National University for financial support. This work has been partially supported by Regione Campania Project: ‘Valorizzazione di risorse naturali non tradizionali della Campania: studio chimico di invertebrati marini.’.

References and notes 1. Garson, M. J.; Simpson, J. S. Nat. Prod. Rep. 2004, 21, 164–179. 2. (a) Chang, C. W. J.; Patra, A.; Roll, D. M.; Scheuer, P. J.; Matsumoto, G. M.; Clardy, J. J. Am. Chem. Soc. 1984, 106, 4644–4646. (b) Patra, A.; Chang, C. W. J.; Scheuer, P. J.; van Duyne, G. D.; Matsumoto, G. M.; Clardy, J. J. Am. Chem. Soc. 1984, 106, 7981–7983. (c) Chang, C. W. J.; Patra, A.; Baker, J. A.; Scheuer, P. J. J. Am. Chem. Soc. 1987, 109, 6119–6123. 3. Baker, J. T.; Wells, R. J.; Oberha¨nsli, W. E.; Hawes, G. B. J. Am. Chem. Soc. 1976, 98, 4010–4012. 4. Wratten, S. J.; Faulkner, D. J.; Hirotsu, K.; Clardy, J. Tetrahedron Lett. 1978, 45, 4345–4348. 5. Kazlauskas, R.; Murphy, P. T.; Wells, R. J.; Blount, J. F. Tetrahedron Lett. 1980, 21, 315–318. 6. Molinski, T. F.; Faulkner, D. J.; Van Duyne, G. D.; Clardy, J. J. Org. Chem. 1987, 52, 3334–3337. 7. Sharma, H. A.; Tanaka, J.; Higa, T.; Lithgow, A.; Bernardinelli, G.; Jefford, C. W. Tetrahedron Lett. 1992, 33, 1593–1596. 8. Ko¨nig, G. M.; Wright, A. D.; Angerhofer, C. K. J. Org. Chem. 1996, 61, 3259–3267. 9. Linden, A.; Ko¨nig, G. M.; Wright, A. D. Acta Crystallogr., Sect. C 1996, 52, 2601–2607. 10. Ciavatta, M. L.; Fontana, A.; Puliti, R.; Scognamiglio, G.; Cimino, G. Tetrahedron 1999, 55, 12629–12636. 11. Ciasullo, L.; Cutignano, A.; Casapullo, A.; Puliti, R.; Mattia, C. A.; Debitus, C.; Riccio, R.; Gomez-Paloma, L. J. Nat. Prod. 2002, 65, 1210–1212. 12. Mitome, H.; Shirato, N.; Miyaoka, H.; Yamada, Y.; van Soest, R. W. M. J. Nat. Prod. 2004, 67, 833–837. 13. Corey, E. J.; Magriotis, P. A. J. Am. Chem. Soc. 1987, 109, 287–289. 14. Fookes, C. J. R.; Garson, M. J.; MacLeod, J. K.; Skelton, B. W.; White, A. H. J. Chem. Soc., Perkin Trans. 1 1988, 1003–1010. 15. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092–4097. 16. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. 17. Sheldrick, G. M. SHELXL97; University of Go¨ttingen: Germany, 1997. 18. Schwarz, O.; Brun, R.; Bats, J. W.; Schmalz, H.-G. Tetrahedron Lett. 2002, 43, 1009–1013. 19. Cremer, D.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 1354–1358. 20. Hirota, H.; Tomono, Y.; Fusetani, N. Tetrahedron 1996, 52, 2359–2368. 21. Manzo, E.; Ciavatta, M. L.; Gavagnin, M.; Mollo, E.; Guo, Y.-W.; Cimino, G. J. Nat. Prod. 2004, 67, 1701–1704. 22. Hoft, R. W. W. COLLECT. Bruker Nonius BV, Delft, The Netherlands, 2000–2002.