Convergent Synthesis and Structural Confirmation of Phellodonin and Sarcodonin ε

Abstract

The first synthesis of members of the sarcodonin family, phellodonin and sarcodonin ε, is reported herein. This verifies that the unprecedented and seemingly unstable N,N-dioxide-containing benzodioxazine framework can be constructed in the laboratory, and lends further support to the proposed structures. The key step in the synthesis involves a biomimetic hetero-Diels–Alder reaction between a pyrazine N-oxide and an ortho-quinone.

Phellodonin (1a) is an alkaloid isolated from the fungus Phellodon niger in 2010 (Figure 1A)¹ and is part of a growing family of sarcodonin natural products (Figure 1B).² All the members of this family were reported to have a striking molecular architecture consisting of a tricyclic benzodioxazine core and an unprecedented N,N-dioxide ring junction (see structure 1).³ A series of elegant but inconclusive NMR and molecular modeling studies were used to establish the connectivity and relative stereochemistry of the benzodioxazine ring junction. Following the initial report,^3a,3b two other research groups^3c,3d have published similar structural assignments, although all subsequently disclosed structures were based on the original assignment. From these structures, it was postulated that the sarcodonins are biosynthetically formed via an unusual [4+2] cycloaddition between an ortho-quinone (2) and the C=N double bond of an appropriately oxidized pyrazine (3).

Figure 1.

A. Structures of recently isolated natural products, phellodonin (1a) and sarcodonin ε (1b). B. The originally published structure for the sarcodonin family (1) and our recently proposed structure revision (1′), both arising from a hypothetical biosynthesis via a formal [4+2] cycloaddition of ortho-quinone 2 and pyrazine derivative 3.

Our laboratory took an interest in the striking molecular architecture of the sarcodonins as well as its intriguing bioactivity, and undertook studies towards the total synthesis of this natural product family.⁴ However, when we attempted to mimic the proposed biosynthetic [4+2] cycloaddition, we obtained not the proposed tricyclic benzodioxazine (with the core structure shown in 1) but a bicyclic benzodioxanone (with the core of 1′) instead, a result confirmed by X-ray crystallography.⁴ Because of the unprecedented nature of the structure of 1 and the spectroscopic similarity of the crystalline benzodioxanone to the natural sarcodonins, we proposed a significant structural revision of the sarcodonin family members from the original structure 1 to the structure 1′, although no ultimate conclusions were drawn.

Even with this reassignment, several key structural issues were left unresolved in 1′: (1) the E/Z configuration of the 2α oxime, (2) whether the methyl group found in some members of the natural product family is located on the hydroxamic acid at 1β or on the 1α oxime, (3) the location of the biaryl substituent at C1 or C6 of the benzodioxanone, and (4) the relative configuration of the 2β aminal stereocenter. To solve these questions as well as to unambiguously identify the core structure of the sarcodonin family, it was realized that only determination by X-ray crystallography would be effective. Recent X-ray analysis conducted by Fujimoto and co-workers have now indicated that a benzodioxazine similar to 1 forms the core structure of the sarcodonins, and that the unprecedented N,N-dioxide moiety is a stable chemical entity.² Furthermore, they have isolated a new sarcodonin natural product, sarcodonin ε (1b). Herein, we report the first synthesis of phellodonin (1a) and sarcodonin ε (1b), and acknowledge Fujimoto’s confirmation of the originally proposed structure³ for this unusual heterocyclic natural product family.

Aiming for the synthesis of phellodonin (1a), our retrosynthesis mimics the proposed biosynthesis of the sarcodonins, with ortho-quinone 2 undergoing a [4+2] cycloaddition with a suitably oxidized pyrazine 3 to give the desired benzodioxanone core (see Figure 1). To pursue this approach, however, a pyrazine bis-N-oxide had to be prepared as the dienophile, which we had previously been unable to achieve.⁴ After further investigation, a synthetic route was devised, and therefore a diketopiperazine of L-isoleucine (4) was converted into bis-chloropyrazine 6 in three steps using known conditions (Scheme 1).⁵ The first N-oxidation of 6 was quite facile; however, the resulting mono-N-oxide 7 was highly deactivated, and vigorous oxidizing conditions were necessary to achieve the second N-oxidation to give 8 in sufficient quantities.⁶ After this difficult second oxidation, preparation of the dienophile 10 was completed without serious incident: nucleophilic aromatic substitution of the heteroaryl chlorides proceeded smoothly, after which careful control of deprotection conditions permitted the selective removal of one 2-(trimethylsilyl)ethyl (TMSE) group and generation of methoxy compound 10.

Scheme 1.

Preparation of pyrazine dienophile 10.

Reagents and conditions: (a) POCl₃, neat, 100 °C, 10 h, 70% 5 + 4% 6; (b) m-CPBA (3.0 equiv), CH₂Cl₂, 40 °C, 4 h; (c) POCl₃, neat, 100 °C, 2 h, 61% from 5; (d) H₂O₂ (6.0 equiv), TFA, 50 °C, 3 h, 70% 7 + 1% 8; (e) H₂O₂ (6.0 equiv), TFA, 50 °C, 3 h, 25% (52% brsm) from 7; (f) 2-TMS-ethanol (3.3 equiv), NaO^tBu (3.5 equiv), THF, 23 °C, 12 h, 63%; (g) TBAF (1.0 equiv), then MeI (5.0 equiv), THF, 23 °C, 2 h, 99%.

Preparation of the other Diels–Alder partner, the ortho-quinone moiety, was achieved via a combination of our previous studies,³ literature precedent⁷ and our recently reported conditions⁸ for quinone arylation with boronic acids (Scheme 2). To this end, 2,5-dichloro-1,4-benzoquinone (11) was first arylated using the reported conditions for silver-mediated arylation of quinones,⁸ giving arylquinone 13. Chloride displacement, then selective bromination followed by a Suzuki cross-coupling furnished 17. The complete terphenyl skeleton now in place, the methoxy groups were replaced with hydroxyl groups, the central quinone was reduced,^7a and the resulting tetraol was fully acetylated to give terphenyl 18. Finally, oxidative deprotection of the catechol^7b and oxidation using previously reported conditions^7c gave the desired ortho-quinone 20.

Scheme 2.

Preparation of ortho-quinone heterodiene 20.

Reagents and conditions: (a) Boronic acid 12, AgNO₃ (20 mol %), K₂S₂O₈ (3 equiv), α,α,α-trifluorotoluene, H₂O, 23 °C, 10 h, 73%; (b) NaOMe, MeOH, 23 °C, 1 h, 47%; (c) N-bromosuccinimide (3.0 equiv), MeCN, 70 °C, 3 h, 40–60%; (d) boronic acid 16 (1.5 equiv), Pd(dppf)Cl₂ (2 mol %), CsF (2 equiv), toluene, 70 °C, 2 h, 88%; (e) aq NaOH, MeOH, 80 °C, 2 h; (f) Na₂S₂O₄ (7.0 equiv), MeOH:H₂O (6:1), 23 °C, 1 h; (g) AcCl (30 equiv), Et₃N (27 equiv), DMAP (0.6 equiv), dioxane, 23 °C, 1 h, 69% over three steps; (h) Pb(OAc)₄ (2.0 equiv), benzene, 80 °C, 12 h; (i) TFA:MeOH:H₂O (1:1:1), 23 °C, 1 h, 51% over two steps; (j) NaIO₄ (1.1 equiv), ⁿBu₄NBr (1.5 mol %), CH₂Cl₂, 23 °C, 1 h, 81%.

With the two coupling partners in hand, the convergent assembly of phellodonin began with an acidic deprotection of the remaining TMSE group on pyrazine 10 (Scheme 3). The in situ-generated enol 21 was then immediately treated with ortho-quinone 20; gratifyingly, desired cycloaddition⁹ adducts 22a and 22b were obtained in 69% yield as a 1:1 mixture that can be chromatographically separated. However, 22a was actually present as a 2:1 mixture of inseparable epimers, most likely containing an epimer at the 1β nitrogen atom (1β-epi-22a); likewise, 22b was generated as a 1:3 mixture of inseparable epimers, most likely containing an epimer at the 1α nitrogen atom (1α-epi-22b). The mixtures 22a:1β-epi-22a and 22b:1α-epi-22b were then deprotected separately: 22a:1β-epi-22a was found to give a deprotection product whose NMR spectroscopic properties matched those reported for phellodonin (1a),¹ and 22b:1α-epi-22b gave sarcodonin ε (1b).² This marks the completion of the first synthesis of members of the sarcodonin natural product family.¹⁰ Two epimeric sarcodonins 1β-epi-1a and 1α-epi-1b were synthesized as well, which are quite possibly natural products themselves.¹¹ To the best of our knowledge, these compounds represent the most highly oxidized diketopiperazine-derived molecules synthesized to date, and this was achieved by a series of oxidative events on diketopiperazine 4.

Scheme 3.

A formal hetero-Diels–Alder reaction for the synthesis of phellodonin (1a) and sarcodonin ε (1b).

Reagents and conditions: (a) TFA, CH₂Cl₂, 23 °C, 30 min, then 20, CDCl₃, 23 °C, 1 h, 69% (of which half consists of a mixture of inseparable epimers 22a:1β-epi-22a and the other half is a mixture of 22b:1α-epi-22b); (b) H₂ (1 atm), Pd-BaSO₄ (10 mol %), MeOH, 23 °C, 30 min, 82% for 1a:1β-epi-1a, 84% for 1b:1α-epi-1b.

The regioselectivity of the pyrazine-quinone formal [4+2] cycloaddition merits additional discussion. As originally observed in our model studies with simple pyrazines,⁴ the ortho-quinone underwent reaction exclusively with the enol double bond, giving rise to a benzoquinone structure whose identity was confirmed by X-ray crystallography. In the case of pyrazine N-oxide 21, however, the ortho-quinone instead undergoes reaction with the nitrone C=N double bond. It is unclear whether the reaction proceeds as a concerted [4+2], or whether alternative reaction pathways (such as an initial [3+2] dipolar cycloaddition, followed by subsequent rearrangement to the [4+2] product) are taking place. However, since epimeric side products are obtained, it is likely that the [4+2] union occurs in stepwise fashion.

The unusual and seemingly unstable N,N-dioxide-containing benzodioxazine framework has prompted a detailed analysis in structural revision,⁴ which has subsequently been revisited and reanalyzed by X-ray crystallography.² A successful chemical synthesis now supports the conclusions from the X-ray analysis,² simultaneously confirming the elusive structure¹² of the sarcodonin family, as well as demonstrating the synthetic feasibility and chemical stability of this family of natural products. In summary, a convergent, biologically inspired synthesis of phellodonin (1a) and sarcodonin ε (1b) has been accomplished, involving a key formal hetero-Diels–Alder reaction. Combining the strengths of X-ray crystallography and chemical synthesis allows the unambiguous determination of even highly hetero-substituted compounds and verifies that this unprecedented framework can be constructed in a laboratory setting.