Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2014 Jul 31.
Published in final edited form as: J Am Chem Soc. 2013 Jul 18;135(30):10886–10889. doi: 10.1021/ja405547f

Enantioselective Total Synthesis of (−)-Citrinadin A and Revision of its Stereochemical Structure

Zhiguo Bian 1, Christopher C Marvin 1,†, Stephen F Martin 1,*
PMCID: PMC3748604  NIHMSID: NIHMS505235  PMID: 23837457

Abstract

The first enantioselective total synthesis of (−)-citrinadin A has been accomplished in 20 steps from commercially available materials via an approach that minimizes refunctionalization and protection/deprotection operations. The cornerstone of this synthesis features an asymmetric vinylogous Mannich addition of a dienolate to a chiral pyridinium salt to set the initial chiral center. A sequence of substrate-controlled reactions, including a highly stereoselective epoxidation/ring opening sequence and an oxidative rearrangement of an indole to furnish a spirooxindole, are then used to establish the remaining stereocenters in the pentacyclic core of (−)-citrinadin A. The successful synthesis of citrinadin A led to a revision of the stereochemical structure of the core substructure of the citrinadins.

Citrinadin A (1), citrinadin B (2), and PF1270 A-C (3–5) are members of a small family of novel spirooxindole alkaloids that exhibit potentially useful biological activities (Figure 1).1,2 Citrinadins A and B, which were isolated by Kobayashi from a culture broth of Penicillium citrinum, are active against murine leukemia L1210 and human epidermoid carcinoma KB cells.1 The absolute and relative stereochemistry of citrinadin A (1) was assigned based upon a combination of 1D and 2D NMR experiments, including ROESY, and CD studies. The related alkaloids PF1270 A-C, which were isolated from Penicillium waksmanii strain PF1270 by Kushida,2 show submicromolar affinities for the human H3 histamine receptor, with 3 being the most active (Ki = 0.07 μM, EC50 = 0.12 μM). The structure and relative stereochemistry of 3–5 were assigned based upon crystallographic analysis of 3.

Figure 1.

Figure 1

Open in a new tab

Citrinadin A, B and PF1270A-C

The alkaloids 1–5 share a number of structural features, but there are also some significant differences. For example, both citrinadins and the PF1270s possess a pentacyclic core structure comprising a spirooxindole motif with two contiguous stereogenic centers (BC ring), a densely functionalized quinolizidine (DE rings), and an α,β-epoxycarbonyl moiety on the A ring. The most notable difference in the assigned structures of citrinadins A and B and PF1270A–C is the relative stereochemistry of the α,β-epoxy ketone and the pentacyclic core. The complex molecular architecture of these alkaloids coupled with their important biological activities have inspired several synthetic investigations, especially toward the citrinadins, but none of these alkaloids have yet been prepared by total synthesis.3 In this communication, we report the first enantioselective total synthesis of citrinadin A (1), and in the accompanying communication Wood and coworkers report the first total synthesis of citrinadin B (2). These investigations have also led to a revision in the structures of 1 and 2, and the reassignment of the absolute stereochemistry of the pentacyclic core to correspond to that of PF1270 A–C. In light of this discovery, the structures in the schemes that follow will depict what we now know to be the correct stereochemical structure of citrinadin A (1*).4

We have been interested in the synthesis of indole and oxindole alkaloids for a number of years,5 and in 2007, we developed a method for the enantioselective synthesis of the spiro oxindole ring system (ABC ring) present in citrinadin A (1*) via a process in which (−)-8-phenylmenthol was utilized as chiral auxiliary to promote the diastereoselective, oxidative rearrangement of an indole to generate an oxindole.3a However, further investigations exploring the feasibility of elaborating such intermediates toward the citrinadins by introducing the requisite D and E rings were not successful. Consequent to these findings, we formulated a new plan that is outlined in retrosynthetic format in Scheme 1. We envisioned that the spirocenter in 1* could be introduced by a late-stage, stereoselective oxidative rearrangement of 6, which would be assembled via a Fisher indole synthesis of the ketal 7. Introduction of the trans amino-alcohol moiety in 7 would then be achieved via substrate-controlled epoxidation/ring opening, whereas the methyl and hydroxyl groups in 7 would be accessible from 8 by a diastereoselective Michael addition and reduction of the carbonyl group. The sole stereocenter in 8, which was destined to control the creation of all of the remaining chirality in the pentacyclic core, would then be established by a diastereoselective, vinylogous Mannich reaction6 of the dienolate derived from 10 to the chiral pyridinium salt 11 to give 9.7

Scheme 1.

Scheme 1

Open in a new tab

Retrosynthetic Analysis of Citrinadin A

The total synthesis of citrinadin A thus commenced with the preparation of 10 from commercially available 2,2-dimethylcyclohexane-1,3-dione (12) in 64% overall yield for the four steps (Scheme 2). With 10 in hand, the stage was set for the key, diastereoselective, vinylogous Mannich reaction involving the chiral pyridinium salt 14 to give 15. Although we are not aware of any examples of the addition of dienolates to chiral acyl pyridinium salts, we were cognizant of the seminal work of Comins and Sahn, who added the zinc enolate of acetone to 14.7b After evaluating different vinylogous enolates, solvents, and reaction temperatures, we discovered that the addition of the zinc dienolate 13 to the pyridinium salt 14, which was generated in situ by the reaction of 3-TIPS-4-methoxypyridine and the chloroformate derivative of (+)-trans-2-(α-cumyl)cyclohexanol [(+)-TCC], provided the adduct 15 in 66% yield with a dr of 92/8. The absolute stereochemistry at the newly created stereocenter at C(16) was assigned at this point by analogy with the findings of Comins.7a Base-induced cleavage of the chiral auxiliary and spontaneous cyclization afforded the tricyclic intermediate 16 in 84% ee together with about 70% of recovered (+)-TCC. Gratifyingly, the optical purity of 16 was readily improved to 98% ee upon recrystallization.

Scheme 2.

Scheme 2

Open in a new tab

Vinylogous Mannich Reaction

Having established a reliable procedure to access 16 with high enantioselectivity, we turned our attention to the preparation of aminoalcohol 7 (Scheme 3). Protiodesilylation of 16 employing excess TBAF and microwave heating afforded enone 8. The stereoselective 1,4-addition of a methyl group to 8 proved to be problematic, presumably owing to the relatively planar nature of the tricyclic ring system of 8. Indeed, conjugate additions of different methyl nucleophiles under a variety of conditions were examined, and we eventually discovered that the copper-mediated addition of (chloromethyl)dimethylphenylsilylmethyl magnesium chloride8 provided a mixture of 1,4-addition products. This mixture was directly reduced with high stereoselectivity using L-Selectride to give the desired isomer 17 in 71% yield over the two steps; 19% of the C(12) epimer of 17 was also isolated. Heating 17 with TBAF in a microwave oven furnished the unsaturated lactam 18.9 Epoxidation of 18 with peroxytrifluoroacetic acid in the presence of sodium carbonate gave a single epoxide 19.10 Although the epoxidation could be performed in unbuffered media, the ketal moiety was cleaved. The diastereoselectivity of this epoxidation was apparently directed by steric effects associated with the adjacent quaternary center at C(19) in which the axial methyl group blocked the top face of the alkene. Treatment of 19 with aqueous methylamine in sealed tube furnished the amino-alcohol 7.11,12

Scheme 3.

Scheme 3

Open in a new tab

Preparation of Tricyclic Aminoalcohol 7

The next stage of the synthesis involved creation of the pentacyclic core of citrinadin A (Scheme 4). In the event, when 7 was heated with o-bromophenyl hydrazine hydrochloride in aqueous acid,13 the desired indole 20 was obtained. Although reduction of the lactam moiety using only alane provided the desired product 6, the procedure with alane followed by NaBCNH3 gave superior yields of 6.14 The moment was now at hand to test the feasibility of creating the critical spirocenter at C(3) by the substrate-controlled oxidative rearrangement of 6. Although oxidative rearrangements of indoles to give spirooxindoles have been well documented,15, inducing such a transformation on 6 proved to be more challenging than anticipated. Indeed, all attempts utilizing a variety of standard oxidants including tert-BuOCl, OsO4 and NBS failed to give 22. We eventually discovered that Davis' oxaziridine, which had been used by Williams to prepare spirooxindole alkaloids,16 was effective. The indole 6 was first treated with pyridinium p-toluenesulfonate (PPTS) to protect the amino groups from oxidation, and an excess of Davis' oxaziridine was added to afford a moderately stable epoxide, which was tentatively assigned the structure 21.16b When 21 was treated with acetic acid, the anticipated semi-pinacol rearrangement ensued to provide the spirooxindole 22.

Scheme 4.

Scheme 4

Open in a new tab

Preparation of Pentacyclic Core 22 of Citrinadin A

At this juncture, it remained to install the requisite side chains on the A and E rings (Scheme 5). In initial experiments, we examined the possibility of directly converting the aryl bromide moiety into an α,β-unsaturated ketone by a carbonylative cross-coupling reaction in analogy with prior work in our laboratories.17 However, these efforts were to no avail, and we resorted to a stepwise process that commenced with the Sonogashira coupling between 22 and 3-methylbut-1-yne to furnish the alkyne 23.18O-Acylation of the hydroxyl group at C(14) with N,N-dimethyl-l-valine in the presence of EDCI and DMAP provided 24,19 the absolute and relative stereo-chemistry of which were unambiguously proven by X-ray crystallography. The gold-promoted oxidation of 24 using 2-bromopyridine N-oxide according to a method reported by Zhang gave the enone 25.20 Finally, the diastereoselective epoxidation of the enone moiety using a method reported by Enders for the enantioselective synthesis of (S)-epoxides from α,β-unsaturated ketones delivered a separable mixture (5:1) of 1* and 26 in 81% yield.21 The CD spectrum of the synthetic 1* thus obtained as the free base is identical with that reported for (−)-citrinadin A,1a whereas the CD spectrum for 26 is different (see Supporting Information).22 The 1H and 13C NMR data of the free base forms of 1* and 26 are wholly consistent with their assigned structures, and the 1H and 13C NMR data of 1* as its putative bis-hydrochloride salt, which was formed upon standing in CDCl3, are in good agreement with those reported for a bis-salt of (−)-citrinadin A (see Supporting Information). Because we were unable to obtain an authentic sample of (−)-citrinadin A or its bis-salt, a direct comparison with the synthetic sample was not possible. Nevertheless, the CD spectra of synthetic 1* coupled with the crystallographic data for 24 strongly suggest that the correct stereochemical structure of (−)-citrinadin A is represented by 1*, not 1 as originally assigned by Kobayashi.1 This revised structure, in which the stereocenters in the pentacyclic core are opposite those depicted in 1, is in agreement with the findings of Wood and coworkers, who completed the first total synthesis of (+)-citrinadin B.23

Scheme 5.

Scheme 5

Open in a new tab

Endgame: Completing the Total Synthesis of 1*

In summary, we completed the first total synthesis of (−)-citrinadin A and revised its stereochemical structure to be that depicted in 1*. The synthesis, which requires only 20 steps from commercially available starting material, features a highly diastereoselective vinylogous Mannich reaction of a dienolate with a chiral pyridinium salt to establish the first stereogenic center. The chirality at this critical center was then used to control the introduction of the remaining stereocenters in the pentacyclic core by substrate control. Further applications of this strategy to the syntheses of citrinadin B (2) and the related alkaloids PF1270 A–C (3–5) are in progress, and the results of these investigations will be reported in due course.

Supplementary Material

1_si_002
2_si_001

ACKNOWLEDGMENT

We thank the National Institutes of Health and the Robert A. Welch Foundation for generous support of this research. We are also grateful to Dr. Vincent Lynch (The University of Texas, Austin) for X-ray crystallography and Dr. John Wood (Colorado State University) for helpful discussions regarding the structure of the citrinadins and suggestions for introducing the epoxy ketone moiety. We also thank Dr. James Sahn (The University of Texas, Austin) for helpful suggestions and Professor Daniel Comins (North Carolina State University) for a generous gift of (−)-TCC that was used in initial synthetic studies. We also thank Dr. Eun Jeong Cho (The University of Texas, Austin) for assistance with obtaining the CD spectra.

Footnotes

Supporting Information.

Complete experimental procedures, full characterization of new compounds, X-ray crystallographic data for 24, comparison of CD spectra of 1*and 26 with those published for (−)-citrinadin A, and a comparison of 1H and 13C NMR data for 1*, both as its free base and bis-salt forms, with those published for a bis-salt of (−)-citrinadin A. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES

  • (1).(a) Tsuda M, Kasai Y, Komatsu K, Sone T, Tanaka M, Mikami Y, Kobayashi J. Org. Lett. 2004;6:3087–3089. doi: 10.1021/ol048900y. [DOI] [PubMed] [Google Scholar]; (b) Mugishima T, Tsuda M, Kasai Y, Ishiyama H, Fukushi E, Kawabata J, Watanabe M, Akao K, Kobayashi J. J. Org. Chem. 2005;70:9430–9435. doi: 10.1021/jo051499o. [DOI] [PubMed] [Google Scholar]
  • (2).Kushida N, Watanabe N, Okuda T, Yokoyama F, Gyobu Y, Yaguchi T. J. Antibiotics. 2007;60:667–673. doi: 10.1038/ja.2007.85. [DOI] [PubMed] [Google Scholar]
  • (3).(a) Pettersson M, Knueppel D, Martin SF. Org. Lett. 2007;9:4623–4626. doi: 10.1021/ol702132v. [DOI] [PubMed] [Google Scholar]; (b) McIver AL, Deiters A. Org. Lett. 2010;12:1288–1291. doi: 10.1021/ol100177u. [DOI] [PubMed] [Google Scholar]; (c) Chandler BD, Roland JT, Li Y, Sorensen EJ. Org. Lett. 2010;12:2746–2749. doi: 10.1021/ol100845z. [DOI] [PMC free article] [PubMed] [Google Scholar]; (d) Guerrero CA, Sorensen EJ. Org. Lett. 2011;13:5164–5167. doi: 10.1021/ol2020362. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • (4).The originally proposed structures of citrinadins A and B are those given by 1 and 2, respectively, whereas the corresponding revised structure for citrinadin A is given by 1*.
  • (5).For some examples see: Martin SF, Rüeger H, Williamson SA, Grzejszczak S. J. Am. Chem. Soc. 1987;109:6124–6131. Martin SF, Mortimore M. Tetrahedron Lett. 1990;31:4557–4560. Martin SF, Benage B, Geraci LS, Hunter JE, Mortimore M. J. Am. Chem. Soc. 1991;113:6161–6171. Martin SF, Clark CW, Corbett JW. J. Org. Chem. 1995;60:3236–3242. Martin SF, Humphrey JM, Ali A, Hillier MC. J. Am. Chem. Soc. 1999;121:866–867. Ito M, Clark CW, Mortimore M, Goh JB, Martin SF. J. Am. Chem. Soc. 2001;123:8003–8010. doi: 10.1021/ja010935v. Liras S, Lynch CL, Fryer AM, Vu BT, Martin SF. J. Am. Chem. Soc. 2001;123:5918–5924. doi: 10.1021/ja010577w. Humphrey JM, Liao Y, Ali A, Rein T, Wong Y-L, Chen H-J, Courtney AK, Martin SF. J. Am. Chem. Soc. 2002;124:8584–8592. doi: 10.1021/ja0202964. Neipp CE, Martin SF. J. Org. Chem. 2003;68:8867–8878. doi: 10.1021/jo0349936. Deiters A, Chen K, Eary CT, Martin SF. J. Am. Chem. Soc. 2003;125:4541–4550. doi: 10.1021/ja0296024. Deiters A, Pettersson M, Martin SF. J. Org. Chem. 2006;71:6547–6561. doi: 10.1021/jo061032t. Miller KA, Martin SF. Org. Lett. 2007;9:1113–1116. doi: 10.1021/ol0700761. Cheng B, Sunderhaus JD, Martin SF. Org. Lett. 2010;12:3622–3625. doi: 10.1021/ol101356u. Fu T-H, McElroy WT, Shamszad M, Martin SF. Org. Lett. 2012;14:3834–3837. doi: 10.1021/ol301424h.
  • (6).For reviews of the vinylogous Mannich reaction, see: Martin SF. Acc. Chem. Res. 2002;35:895–904. doi: 10.1021/ar950230w. Casiraghi G, Battistini L, Curti C, Rassu G, Zanardi F. Chem. Rev. 2011;111:3076–3154. doi: 10.1021/cr100304n.
  • (7).(a) Comins DL, Joseph SP, Goehring RR. J. Am. Chem. Soc. 1994;116:4719–4728. [Google Scholar]; (b) Comins DL, Sahn JJ. Org. Lett. 2005;7:5227–5228. doi: 10.1021/ol052068v. [DOI] [PubMed] [Google Scholar]
  • (8).Comins DL, Al-awar RS. J. Org. Chem. 1995;60:711–716. [Google Scholar]
  • (9).Heitzman CL, Lambert WT, Mertz E, Shotwell JB, Tinsley JM, Va P, Roush WR. Org. Lett. 2005;7:2405–2408. doi: 10.1021/ol0506821. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • (10).Fehr C. Angew. Chem. Int. Ed. 1998;37:2407–2409. doi: 10.1002/(SICI)1521-3773(19980918)37:17<2407::AID-ANIE2407>3.0.CO;2-W. [DOI] [PubMed] [Google Scholar]
  • (11).Sarabia F, Chammaa S, Garcia-Castro M, Martin-Galvez F. Chem. Commun. 2009:5763–5765. doi: 10.1039/b912070j. [DOI] [PubMed] [Google Scholar]
  • (12).The relative and absolute stereochemistry shown in 7 was later confirmed by X-ray crystallographic analysis of 24.
  • (13).For some examples see: Danishefsky S, Chackalamannil S, Harrison P, Silvestri M. J. Am. Chem. Soc. 1985;107:2474–2484. Chen C-Y, Senanayake CH, Bill TJ, Larsen RD, Verhoeven TR, Reider PJ. J. Org. Chem. 1994;59:3738–3741. Blair JB, Kurrasch-Orbaugh D, Marona-Lewicka D, Cumbay MG, Watts VJ, Barker EL, Nichols DE. J. Med. Chem. 2000;43:4701–4710. doi: 10.1021/jm000339w. Roberson CW, Woerpel KA. J. Am. Chem. Soc. 2002;124:11342–11348. doi: 10.1021/ja012152f.
  • (14).(a) Martin SF, Benage B, Geraci LS, Hunter JE, Mortimore M. J. Am. Chem. Soc. 1991;113:6161–6171. [Google Scholar]; (b) Cushing TD, Sanz-Cervera JF, Williams RM. J. Am. Chem. Soc. 1996;118:557–579. [Google Scholar]
  • (15).For a review, see: Marti C, Carreira EM. Eur. J. Org. Chem. 2003:2209–2219. See also: Peterson AC, Cook JM. J. Org. Chem. 1995;60:120–129. Cushing TD, Sanz-Cervera JF, Williams RM. J. Am. Chem. Soc. 1996;118:557–579. Wearing XZ, Cook JM. Org. Lett. 2002;4:4237–4240. doi: 10.1021/ol020170b.
  • (16).(a) Greshock TJ, Grubbs AW, Tsukamoto S, Williams RM. Angew. Chem. Int. Ed. 2007;46:2262–2265. doi: 10.1002/anie.200604378. [DOI] [PubMed] [Google Scholar]; (b) Greshock TJ, Grubbs AW, Williams RM. Tetrahedron. 2007;63:6124–6130. doi: 10.1016/j.tet.2007.03.016. [DOI] [PMC free article] [PubMed] [Google Scholar]; (c) Greshock TJ, Grubbs AW, Jiao P, Wicklow DT, Gloer JB, Williams RM. Angew. Chem. Int. Ed. 2008;47:3573–3577. doi: 10.1002/anie.200800106. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • (17).(a) O'Keefe BM, Simmons N, Martin SF. Org. Lett. 2008;10:5301–5304. doi: 10.1021/ol802202j. [DOI] [PMC free article] [PubMed] [Google Scholar]; (b) O'Keefe BM, Simmons N, Martin SF. Tetrahedron. 2011;67:4344–4351. doi: 10.1016/j.tet.2011.03.074. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • (18).(a) Sonogashira K, Tohda Y, Hagihara N. Tetrahedron Lett. 1975:4467–4470. [Google Scholar]; (b) Sonogashira K. In: Comp. Org. Synth. Trost BM, Fleming I, editors. Vol. 3. Pergamon; Oxford: 1991. pp. 521–549. [Google Scholar]
  • (19).Neises B, Steglich W. Angew. Chem. Int. Ed. 1978;17:522–524. [Google Scholar]
  • (20).Lu B, Li C, Zhang L. J. Am. Chem. Soc. 2010;132:14070–14072. doi: 10.1021/ja1072614. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • (21).Enders D, Zhu J, Raabe G. Angew. Chem. Int. Ed. 1996;35:1725–1728. [Google Scholar]
  • (22).We have also prepared the enantiomers of 1* and 26, but their CD spectra spectra are not consistent with the CD spectrum reported by Kobayashi for (−)-citrinadin A in reference 1a.
  • (23).See accompanying manuscript in this issue.

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

1_si_002
NIHMS505235-supplement-1_si_002.cif (27.9KB, cif)
2_si_001
NIHMS505235-supplement-2_si_001.pdf (6MB, pdf)

RESOURCES