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. Author manuscript; available in PMC: 2012 May 23.
Published in final edited form as: Bioorg Med Chem Lett. 2011 Mar 16;21(8):2327–2329. doi: 10.1016/j.bmcl.2011.02.091

Biomimetic conversion of (−)-fusoxypyridone and (−)-oxysporidinone to (−)-sambutoxin: further evidence for the structure of the tricyclic pyridone alkaloid, (−)-fusoxypyridone

E M Kithsiri Wijeratne 1, A A Leslie Gunatilaka 1,*
PMCID: PMC3359017  NIHMSID: NIHMS283327  PMID: 21419624

Abstract

Biomimetic-type reactions of the tricyclic pyridone alkaloid, (−)-fusoxypyridone [(−)-4,6′-anhydrooxysporidinone] (1), recently encountered in an endophytic strain of Fusarium oxysporum, and (−)-oxysporidinone (2) afforded (−)-sambutoxin (3) and an analogue of 1, identified as (−)-1’(6’)-dehydro-4,6′-anhydrooxysporidinone (4), thus confirming the structure previously proposed for 1 and suggesting that 1 – 3 bear the same relative stereochemistry. Oxidation of 4 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded a hitherto unknown sambutoxin analogue, (−)-4,2′-anhydrosambutoxin (5).

Keywords: Biomimetic reaction; 4-hydroxy-2-pyridone alkaloids; (−)-fusoxypyridone; (−)-oxyporidinone; (−)-4,2’-anhydrosambutoxin

4-Hydroxy-2-pyridones are a small group of fungal-derived alkaloids bearing the central 2-pyridone ring linked to two substituents at C-3 and C-5 positions. The members of this group display a range of biological activities and include (−)-4,6′-anhydrooxysporidinone (1)1 [now named as (−)fusoxypyridone], (−)-oxysporidinone (2),1-3 (−)-6-deoxy-oxysporidinone,1 oxysporidinone dimethylketal,2 6-epi-oxysporidinone,2 (−)-sambutoxin (3),2 N-demethyl-sambutoxin,2 apiosporamide,4 ilicicolin,5 funiculosin,6 liporins A7 and B,8 fischerin (YM 215343),9 tenellin,10 militarinone,11 fusarinones A–C,12 asparidones A and B,13 and fusapyridons A (7) and B (8).14 Among these, the N-methylated members, (−)-oxysporidinone (2), (−)-sambutoxin (3), fusapyridon A (7), and funiculosin are of particular interest as they exhibit antifungal,2 mycotoxic,15 antibacterial,14 and antiviral6 activities, respectively. We have recently encountered the first tricyclic N-methyl-4-hydroxy-2-pyridone alkaloid, (−)-fusoxypyridone (1), in an endophytic strain of Fusarium oxysporum.1 Subsequent to our report two analogs of 1, fusapyridons A (7) and B (8) have been encountered in another endophytic fungal strain Fusarium sp. YG-45.14 The structure of 1 was elucidated by the application of extensive 2D NMR techniques.1

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We have now investigated biomimetic-type reactions of both 1 and (−)-oxysporidinone (2) which resulted in the formation of (−)-sambutoxin (3) and the tricyclic analogue, (−)-1’(6’)-dehydro-4,6′-anhydro-oxysporidinone (4), providing further evidence for the previously proposed structure and stereochemistry of (−)-fusoxypyridone (1) and stereochemical disposition of the methyl groups of the trimethylheptenyl substituent at C-11 of (−)-oxysporidinone (2). (−)-Fusoxypyridone (1) on treatment with p-TsOH (p-toluenesulfonic acid) in toluene at 25 °C yielded two products,16 one of which was identified as (−)-sambutoxin (3) by direct comparison (mp, [α]D, and 1H NMR) with an authentic sample.2 The HRFABMS of the more polar product 4 exhibited its [M+H]+ at m/z 454.2948, consistent with the molecular formula C28H40O4N, suggesting that it is a dehydration product of 1 (C28H42O5N). 1H and 13C NMR spectroscopic data of 4 were similar to those of 11 except for the chemical shifts of protons and carbons in the disubstituted cyclohexanone ring. In the 13C NMR spectrum of 4, when compared with (−)-fusoxypyridone (1), the signals due to C-1’ and C-6’ were shifted from δ 77.2 to 109.8 and from δ 92.0 to 150.5, respectively, indicating that a molecule of H2O has been lost from these positions in 1 to form a double bond. Compound 4 was thus identified as (−)-1’(6’)-dehydro-4,6′-anhydro-oxysporidinone which was further confirmed by detailed analysis of its HMBC data (Figure 1).

Figure 1.

Figure 1

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Selected HMBC correlations for 4

Although the gross structure of (−)-oxysporidinone (2), including the connectivity and the relative stereochemistry of the trisubstituted tetrahydropyran, has been determined by extensive application of 2D NMR techniques,2,3 stereochemistry of the 1,3-dimethyl array at C-14 and C-16 and its absolute configuration have remained undefined. The relative configurations at C-14 and C-16 of some naturally occurring 4-hydroxy-2-pyridone alkaloids have been deduced by comparison with experimental and calculated 13C NMR shifts for anti- and syn-isomers of model compounds having comparable partial structures.15 However, this method is known to suffer from a major disadvantage that it is only predictive and not conclusive.17 Considering the structural relationship between (−)-oxysporidinone (2) and (−)-fusoxypyridone (1), it was of interest to see if a similar biomimetic-type transformation of 2 would lead to (−)-sambutoxin (3). As expected, the treatment of (−)-oxysporidinone (2) with p-TsOH yielded 3 and (−)-1’(6’)-dehydro-4,6′-anhydrooxysporidinone (4) (Scheme 1).16

Scheme 1.

Scheme 1

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Proposed pathway for biomimetic-type reactions of 1 and 2 giving 3 – 5.

Formation of (−)-sambutoxin (3) from both (−)-fusoxypyridone (1) and (−)-oxysporidinone (2) suggests that the polyketide-derived trimethylheptenyl substituent at C-11 of the tetrahydropyran ring of 1 and 2 displays the anti-1,3-dimethyl relationship as in (−)-sambutoxin (3). Since the relative and absolute stereochemistry of the optical antipode of 3 namely, (+)-sambutoxin, has been confirmed by an enantiocontrolled total synthesis,18 the relative stereochemistry of the C-11 trimethylheptenyl substituent of compounds 1 – 4 should be as shown.

Oxidation of 4 with DDQ gave 5 as the only product. HRMS data of this compound suggested the molecular formula C28H37O4N. 1H NMR spectroscopic data of 5, when compared with that of 3, indicated the absence of the six aliphatic protons assigned to the disubstituted cyclohexenone ring; instead it showed the presence of three aromatic protons suggesting that compound 4 on treatment with DDQ had undergone dehydrogenation followed by aromatization to give (−)-4,2’-anhydosambutoxin (5) (Scheme 1). Methylation of (−)-sambutoxin (3) with Me2SO4/K2CO3 gave its new analogue (−)-dimethylsambutoxin (6) as the only product, structure of which was confirmed by analysis of its spectral data (1H and 13C NMR and HRFABMS).

Acknowledgments

This work was supported by a grant from the National Cancer Institute (Grant No. R01CA090265-06A1). We are also thankful to Dr. N. P. D. Nanayakkara (University of Mississippi) for providing samples and copies of spectroscopic data of 2 and 3.

Footnotes

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References and notes

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After 30 min, toluene was removed under reduced pressure and the residue was separated on preparative TLC (silica gel) using hexane/acetone (1: 2.5) as eluant to give (−)-sambutoxin (3) (1.1 mg, 38%) and 4 (1.6 mg, 56%).(−)-1’(6’)-Dehydro-4,6′-anhydrooxysporidinone 4: mp 175–177 °C; [α]D −65.8 (c 0.3, CH3OH); UV (EtOH) λmax (log ε) 336 (3.53), 264 (4.79), 230 (4.79) nm; 1H NMR (CDCl3) δ 7.34 (1H, s, H-6), 5.12 (1H, d, J = 9.1 Hz, H-13), 4.98 (1H, d, J = 8.7 Hz, H-7), 3.62 (3H, s, N-CH3), 3.52 (2H, brs, H2-5’) 3.46 (1H, d, J = 9.9 Hz, H-11), 2.77 (2H, t, J = 6.2 Hz, H2-2’), 2.70 (2H, t, J = 6.2 Hz, H2-3’), 2.43 (1H, m, H-14), 2.01 (1H, dq, J = 13.8, 3.6 Hz, H-8a), 1.90 (1H, m, H-9a), 1.77 (1H, dq, J = 13.2, 2.5 Hz, H-8b), 1.70 (1H, m, H-10), 1.65 (3H, d, J = 1.0 Hz, H3-20), 1.38 (1H, m, H-9b), 1.35 (1H, m, H-17a), 1.32 (1H, m, H-16), 1.17 (1H, m, H-15a), 1.03 (2H, m, H-15b, H-17b), 0.86 (3H, d, J = 6.4 Hz, H3-21), 0.83 (3H, m, H3-18), 0.82 (3H, d, J = 6.4 Hz, H3-22), 0.73 (3H, d, J = 6.6 Hz, H3-19); 13C NMR (CDCl3) δ 204.8 (C, C-4’), 161.9 (C, C-2), 161.2 (C, C-4), 150.4 (C, C-6’), 136.2 (CH, C-13), 132.1 (C, C-12), 126.2 (CH, C-6), 114.6 (C, C-5), 112.1 (C, C-3), 109.8 (C, C-1’), 91.4(CH, C-11), 72.8 (CH, C-7), 44.9 (CH2, C-15), 38.8 (CH3, C-23), 38.7 (CH2, C-5’), 38.4 (CH2, C-3’), 32.9 (CH2, C-9), 31.9 (CH, C-16), 31.8 (CH, C-10), 30.2 (CH2, C-8), 29.5 (CH2, C-14), 29.2 (CH2, C-17), 20.6 (CH3, C-21), 19.5 (CH3, C-22), 17.9 (CH3, C-19), 17.8 (CH2, C-2’), 11.3 (CH3, C-18), 11.2 (CH3, C-20); HRFABMS m/z 454.2948 [M+H]+ (calcd for C28H40O4N, 454.2957).Reaction of (−)-oxysporidinone with p-TsOH: To a solution of (−)-oxysporidinone (2) (5.0 mg) in toluene (0.5 mL) was added p-TsOH (1 crystal) and stirred for 30 min (TLC control). Solvent was then removed under reduced pressure and the residue was separated on preparative TLC (silica gel) using hexane/acetone (1:2.5) as eluant to give (−)-sambutoxin (3) (1.8 mg, 39%) and (−)-1’(6’)-dehydro-4,6′-anhydro-oxysporidinone (4) (2.7 mg, 58%).Oxidation of 4 to (−)-4,2′-anhydosambutoxin 5: To a stirred solution of 4 (0.5 mg) in 1,4-dioxane (0.2 mL) was added DDQ (3.0 mg). After 3 h at 25 °C (TLC control), the reaction mixture was diluted with EtOAc (20 mL), washed with water (3×15 mL), dried (Na2SO4), and evaporated. The residue thus obtained was purified by prep. TLC using hexane/acetone (1:2.5) as eluant to give 5 (0.3 mg, 60%) as a white amorphous powder; [α]D −78.2 (c 0.2, CH3OH); UV (EtOH) λmax (log ε) 338 (3.71), 310 (3.89), 258 (4.32), 211 (4.29) nm; 1H NMR (CDCl3) δ 7.68 (1H, s, H-6), 7.37 (1H, d, J = 8.3 Hz, H-6’), 6.86 (1H, d, J = 2.2 Hz, H-3’), 6.68 (1H, dd, J = 8.3, 2.2 Hz, H-5’), 5.14 (1H, d, J = 9.9 Hz, H-13), 5.02 (1H, dd, J = 11.7, 2.3 Hz, H-7), 3.63 (3H, s, N-CH3), 3.49 (1H, d, J = 9.8 Hz, H-11), 2.45 (1H, m, H-14), 2.16 (1H, m, H-8a), 1.93 (1H, dq, J = 13.2, 3.4 Hz, H-9a), 1.76 (1H, m, H-10), 1.73 (3H, s, H3-20), 1.68 (1H, m, H-8b), 1.40 (1H, m, H-9b), 1.35 (1H, m, H-17a), 1.32 (1H, m, H-16a), 1.17 (1H, m, H-15a), 1.04 (1H, m, H-15b), 1.03 (1H, m, H-17b), 0.86 (3H, d, J = 6.7 Hz, H3-21), 0.84 (3H, m, H3-18), 0.82 (3H, d, J = 6.3 Hz, H3-22), 0.76 (3H, d, J = 6.6 Hz, H3-19); HRFABMS m/z 452.2795 [M+H]+ (calcd for C28H38O4N, 452.2801).Methylation of (−)-sambutoxin 3: To a solution of (−)-sambutoxin (3) (2.0 mg) in acetone (0.6 mL) were added K2CO3 (20 mg) and Me2SO4 (20 μL). After 3 h under reflux (TLC control), the reaction mixture was filtered, the filtrate evaporated under reduced pressure, and the resulting residue was purified on prep TLC using hexane/acetone (1:2) as eluant to give 6 (2.1 mg, 98%) as a white amorphous powder; [α]D −92.4 (c 0.8, CH3OH); UV λmax (log ε) 315 (3.61), 260 (4.14), 215 (4.11) nm; 1H NMR (CDCl3) δ 7.29 (2H, d, J = 8.6 Hz, H-2’ and H-6’), 7.12 (1H, s, H-6), 6.90 (2H, d, J = 8.6 Hz, H-3’ and H-5’), 5.11 (1H, d, J = 9.0 Hz, H-13), 4.96 (1H, d, J = 11.6 Hz, H-7), 3.82 (3H, s, OCH3), 3.49 (3H, s, OCH3), 3.41 (1H, d, J = 9.8 Hz, H-11), 3.38 (3H, s, N-CH3), 2.39 (1H, m, H-14a), 2.37 (1H, m, H-8a), 1.89 (1H, dd, J = 12.9, 3.0 Hz, H-9a), 1.67 (1H, m, H-10), 1.62 (3H, s, H3-20), 1.56 (1H, m, H-8b), 1.38 (1H, m, H-9b), 1.30 (1H, m, H-16), 1.30 (1H, m, H-17a), 1.16 (1H, m, H-15a), 1.02 (2H, m, H-15b, H-17b), 0.84 (3H, d, J = 6.5 Hz, H3-21), 0.81 (3H, d, J = 6.6 Hz, H3-22), 0.80 (3H, m, H3-18), 0.70 (3H, d, J = 6.5 Hz, H3-19); 13C NMR (CDCl3) δ 165.7 (C, C-4), 162.8 (C, C-2), 158.9 (C, C-4’), 136.8 (CH, C-13), 136.3 (CH, C-6), 132.1 (C, C-12), 129.7 (CH, C-2’ and C-6’), 127.0 (C, C-1’), 122.4 (C, C-5), 117.5 (C, C-3), 114.0 (CH, C-3’ and C-5’), 92.2 (CH, C-11), 73.7 (CH, C-7), 44.9 (CH2, C-15), 37.6 (CH3, C-23), 33.5 (CH2, C-9), 32.0 (CH, C-10), 31.8 (CH, C-16), 29.5 (CH2, C-8), 29.3 (CH2, C-17), 29.1 (CH, C-14), 20.6 (CH3, C-21), 19.5 (CH3, C-22), 18.0 (CH3, C-19), 11.6 (CH3, C-20), 11.2 (CH3, C-18); HRFABMS m/z 482.3247 [M+H]+ (calcd for C30H44O4N, 482.3270).
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