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. Author manuscript; available in PMC: 2019 Sep 18.
Published in final edited form as: J Am Chem Soc. 2019 May 16;141(21):8431–8435. doi: 10.1021/jacs.9b03576

Calyciphylline B-Type Alkaloids: Total Syntheses of (−)-Daphlongamine H and (−)-Isodaphlongamine H

Cedric L Hugelshofer 1,*, Vignesh Palani 1, Richmond Sarpong 1,*
PMCID: PMC6750003  NIHMSID: NIHMS1048566  PMID: 31074980

Abstract

The first total synthesis of the complex hexacylic Daphniphyllum alkaloid (−)-daphlongamine H has been accomplished. Key to the success of the strategy are a complexity-building Mannich reaction, efficient cyclizations, and a highly diastereoselective hydrogenation to assemble multigram quantities of the tricyclic core bearing four contiguous stereocenters. Following construction of the hydro-indene substructure by means of a Pauson–Khand reaction, endgame redox manipulations delivered the natural product. Importantly, the synthetic studies have also given access to (−)-isodaphlongamine H and led to a revision of the reported structure of deoxyisocalyciphylline B.

Daphniphyllum alkaloids are a large family of structurally distinct natural products with a wide breadth of pharmacological potential.1 Following the pioneering synthetic studies by Heathcock and co-workers on a subset of these alkaloids,2 the sheer complexity and varied biological activities of these natural products have continued to entice synthetic chemists to build other congeners.1e Specifically, calyciphylline A-type and daphmanidin A-type alkaloids have received the most attention from synthetic chemists in the past decade (Figure 1a), with the groups of Carreira,3 Li,4 Smith,5 Fukuyama,6 Zhai,7 and Dixon8 having elegantly solved the formidable synthetic challenges associated with preparing these dauntingly complex molecules.

Figure 1.

Figure 1.

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(A) Daphniphyllum alkaloids; (B) our retrosynthetic analysis of the calyciphylline B-type subfamily.

The calyciphylline B-type alkaloids are a structurally distinct subfamily among the Daphniphyllum alkaloids, featuring a unique hexacyclic framework (rings A–F) with a central piperidine moiety decorated with seven contiguous stereocenters (Figure 1b). Following the isolation of calyciphylline B (l),9 the related congeners deoxycalyciphylline B (2), deoxyisocalyciphylline B (3),10 and daphlongamine H (4)11 have been discovered in recent years.12 The corresponding methanolysis products derived from 2 and 3 have also been reported,13 suggesting a likely labile δ-lactone moiety that may open during isolation or structural elucidation. In addition, the basic tertiary amine resident in all calyciphylline B-type compounds adds to the challenge of their handling and purification.14 Notably, of all calyciphylline B-type alkaloids isolated from nature to date, only the structure of deoxycalyciphylline B (2) has been unambiguously confirmed by X-ray crystallographic analysis.10

The closely related structures of 2–4, differing only in their C5/C6 configurations, suggests a common biosynthetic pathway. As has been proposed,10 precursor A (Figure 1b) containing a tetrasubstituted olefin (ΔC5–C6) and appended propionic acid side chain was hypothesized to undergo nonselective hydroacyloxylation to give diastereomeric lactone products. In this respect, synthetic and computational studies by Hanessian and co-workers have provided valuable insight.15 Their efforts culminated in an elegant total synthesis of isodaphlongamine H (5)16 starting from three chiral building blocks. Importantly, Hanessian’s studies introduced isodaphlongamine H (5) as possibly being the “missing” diastereomeric congener of the calyciphylline B-type alkaloid quartet that has yet to be isolated from natural sources.

Given the limited synthetic studies focusing on the calyciphylline B-type subfamily,17 and their fascinating architectures, we embarked on a synthesis of these compounds. Herein, we detail studies which have culminated in total syntheses of (−)-daphlongamine H (4) and (−)-isodaphlongamine H (5).

A pivotal feature of our retrosynthesis was a recognition that all six rings in 1–5 could arise from an acyclic precursor.18 Careful orchestration of subsequent cyclizations would forge the highly fused framework in a diastereoselective manner from a precursor bearing a single chiral center. Thus, the calyciphylline B-type framework was traced back to enyne 6 as a substrate for a late-stage Pauson-Khand reaction (Figure 1b). In turn, the tricyclic substructure of 6 would be forged from acyclic precursor 7 that contains the retron for a simplifying Mannich reaction.

Our studies commenced with the synthesis of α-quaternary β-amino ester 7 using the key complexity-building Mannich reaction. While Ellman’s pioneering studies have revealed that adding ester enolates to N-tert-butanesulfinyl imines result in good diastereoselectivities at the β-amino stereocenter,19 the selectivity outcome at the α-center remains difficult to predict when fully substituted, unsymmetrical enolates are employed. Following an initial survey of imines and enolates,20 allylated valerolactone 821 and sulfinyl imine 922 were identified as suitable precursors (Scheme 1). Thus, reaction of 9 with the lithium enolate derived from 8 led to an intriguing Mannichretro–Mannich equilibration of the β-amino lactones (SS-10 and SR-10), and optimization studies eventually yielded an ~1:1 mixture of C8 epimers in 82% combined yield on multigram scale.20 Following chromatographic separation, undesired β-amino lactone SS-10 was recycled to furnish 8 and 9.23 Treatment of SR-10 with HCl in methanol effected cleavage of the sulfinyl group with concomitant methanolysis of the lactone. The intermediate ammonium salt was N-alkylated to furnish vinyl bromide 11 which, after silylation of the hydroxy group and acetylation of the secondary amine, gave amide 12.

Scheme 1. Gram Scale Synthesis of Tricyclic Core 16a.

Scheme 1.

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aReagents and conditions: (a) LDA, 8, THF, −78 °C, then 9 (44%, 53% brsm SS-10; 38%, 45% brsm RS-10); (b) NaH, THF, −78 to 23 °C (37% 8; 94% 9); (c) 4 M HCl in 1,4-dioxane, MeOH, 50 °C; (d) 2,3-dibromopropene, iPr2NEt, CH3CN, 80 °C, (77%, 2 steps, + 11% from second cycle); (e) TBSCl, imidazole, CH2Cl2, 0 to 23 °C (89%); (f) Ac2O, PhH, 90 °C (96%); (g) Hoveyda-Grubbs II catalyst, CH2CL2, 40 °C (82%); (h) LiHMDS, THF, −78 °C (90%); (i) Pd(OAc)2, PPh3, NEt3, MeCN, 50 °C (72%); (j) 50 atm H2 [lr(cod)(PCy3)(py)]PF6, CH2Cl2 23 °C; (k) 1 atm H2, Pd/C, NaHCO3, MeOH, 23 °C (58%, 2 steps, 4:1 d.r.); (l) Bu3SnH, Et3B, O2, PhH, 23 °C (91%); (m) 1 atm H2/Ar (1:1), Pd(OH)2, CH2Cl2 23 °C (87%, ≥ 20:1 d.r.). bTBS omitted for clarity.

This acyclic precursor was then subjected to a stepwise tricyclization sequence that commenced with a ring-closing methathesis. Treatment of the resulting intermediate with LiHMDS smoothly induced a Dieckmann condensation and gave bromo bicycle 13. For the synthesis of the tricyclic substructure, we initially conducted an intramolecular Heck coupling to furnish diene 14. Although this cyclization proved efficient, the subsequent hydrogenation to access tricycle 16 was unexpectedly troublesome. Ultimately, a two-step protocol was identified using Crabtree’s catalyst (50 atm of H2), followed by a hetereogeneous hydrogenation that gave 16 (4:1 d.r.).24 This sequence proved impractical on larger scales because of extended reaction times and difficult separation of the diastereomers. To circumvent challenges associated with the hydrogenation of 14, we sought to prepare a related tricycle, saturated at C3-C4, that would present a sterically more biased concave face. To this end, we discovered that 13 underwent efficient organoborane-initiated,25 reductive radical ring closure to give a tricyclic core (91% yield) containing an exocyclic olefin that could be reduced with excellent diastereoselectivity (≥20:1 d.r.). Notably, under the optimized hydrogenation conditions, rapid initial isomerization of the exo-olefin to enamide 15 took place.20 This improved sequence gave access to multigram quantities of tricycle 16 whose structure was confirmed by single crystal X-ray analysis.

Synthetic efforts then continued toward construction of the E and F rings by alkylation of 16 to furnish alkene 17 (Scheme 2). Next, reduction of the δ-lactam carbonyl group to give enaminone 18 following elimination was investigated. A survey of various reduction methods revealed Dixon’s combination of Vaska’s complex and TMDS8,26 to be uniquely effective. While careful control of the reaction conditions and workup procedure was required, the successful reduction of the lactam amide as a part of a 1,3-dicarbonyl system in the presence of a very sensitive terminal olefin group highlights the remarkable chemoselectivity of these conditions. Interestingly, attempts to carry out the analogous reduction of tricycle 16 (prior to alkylation) were unsuccessful. Activation of enaminone 18 with TMSOTf, followed by addition of ethynylmagnesium bromide, gave a silyl enol ether (≥20:1 d.r.) that was hydrolyzed upon workup to furnish C6-epimeric enynes 19 and 20 (3.6:1 ratio) in 79% combined yield. 7 With an eye toward the synthesis of deoxycalyciphylline B (2) and the reported structure of deoxyisocalyciphylline B (3), which feature the S-C6 configuration, enyne 20 was subjected to a Pauson-Khand reaction, followed by silylation of the primary hydroxy group to give enone 21. Single-crystal X-ray analysis of 21 confirmed the trans-disposed enyne moiety in 20 being unsuitable for the syntheses of 2 and 3, since 21 was generated with undesired C10 stereochemistry. Attempts to epimerize 10-Hβ in enone 21 (and derivatives thereof) were unsuccessful, leading us to conclude that the calyciphylline B-type alkaloids with S-C6 configuration were not easily accessible from enyne 20.

Scheme 2. Synthesis of Daphlongamine H (4) and Isodaphlongamine H (5)a.

Scheme 2.

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aReagents and conditions: (a) NaH, 4-iodo-butene, DMF, 0 °C (62%, 72% brsm); (b) TMDS, [lrCl(CO)(PPh3)2], PhMe, 23 °C (73%, + 13% from second cycle); (c) TMSOTf, CH2Cl2, −78 °C, then HCCMgBr, THF, −78 to 0 °C, then aq. 6 M HCl, 0 to 23 °C (62% 19; 17% 20); (d) Co2(CO)8, CH2Cl2, 23 °C, then TMANO·2H2O, 0 to 23 °C; (e) TBSCl, imidazole, CH2Cl2, 23 °C (69%, 2 steps); (f) LaCl3·2LiCl, THF, 23 °C, then MeLi, −25 °C (94%, ≥ 20:1 d.r.); (g) Co2(CO)8, CH2Cl2, 23 °C, then MeCN, TMANO·2H2O, −78 to 23 °C (59%); (h) NaCNBH3, BF3·Et2O, THF, 23 to 66 °C; (i) CrO3, H2SO4, H2O, acetone, 0 °C (68%, 2 steps); (j) TfOH, MeNO2, 23 °C (70%); (k) TFAA, CH2Cl2, −78 °C, then SOCl2, −78 to 0 °C (71%); (l) aq. H2O2, TFAA, CH2Cl2, 0 to 23 °C (35%); (m) LiAlH4, THF, −78 to 66 °C; (n) NaCNBH3, BF3·Et2O, THF, 23 to 66 °C; (o)CrO3, H2SO4, H2O, acetone, 0 °C; (p) cyanuric chloride, NEt3, MeCN, 23 °C (13%, 4 steps). bTBS omitted for clarity.

At this juncture, we returned to enyne 19 and focused on the final synthetic stages toward daphlongamines 4 and 5. Treatment of 19 with excess methyllithium furnished the corresponding tertiary alcohol in 94% yield (≥20:1 d.r.).28 In line with Hanessian and co-workers’ studies,15,16 we were unsuccessful in altering this stereochemical outcome using other methyl nucleophiles. The enyne diol generated in this manner then underwent Pauson-Khand reaction29 to give pentacyclic enone 22 bearing the desired 10-Hα orientation.30 With all eight stereogenic centers in place, the final redox adjustments were pursued. Pleasingly, treatment of 22 with excess NaCNBH3 in the presence of a Lewis acid31 resulted in an efficient one-step deoxygenation of the enone moiety to furnish the corresponding cyclopentene. Finally, Jones oxidation forged the cis-lactone and thus completed the synthesis of isodaphlongamine H (5). The spectroscopic data for 5 were in full agreement with those reported by Hanessian and co-workers.16

Given the proposed biosynthetic pathway to the calyciphylline B-type alkaloids,10,16 we envisioned that subjecting isodaphlongamine H (5) to acidic conditions might induce a sequence involving elimination–hydroacyloxylation (cf. A, Figure 1b) to deliver a mixture of 2–5. In practice, treatment of 5 with various acids (AcOH, TFA, HClO4, pTsOH, H2SO4, 23 to 80 °C) returned either unreacted starting material or led to nonspecific decomposition. The only productive conversion, upon treating 5 with excess TfOH in nitromethane, was the formation of enone 26. Mechanistically, we hypothesize that this formal dehydration results from a Prins-like cyclization of acylium intermediate 25.

Finally, for the synthesis of daphlongamine H (4), the formal stereochemical inversion of the tertiary alcohol in pentacyclic enone 22 was pursued. Subjecting 22 to TFAA and then SOCl2 smoothly protected the primary and eliminated the tertiary hydroxy groups, respectively. The resulting exocyclic alkene was treated with trifluoroperacetic acid32 to afford epoxide 23.33 Opening of the epoxide at the terminal position was carried out with LiAlH4, and the resulting triol was subjected to the established deoxygenation conditions. After Jones oxidation of the thus obtained amino diol, the resulting, highly polar trans-seco acid 24 did not undergo facile lactonization, unlike in the case of isodaphlongamine (5), which possesses the cis-lactone. Eventually we identified cyanuric chloride as a viable reagent34 for the final bond formation and were able to isolate daphlongamine H (4) possessing the highly strained and sensitive trans-lactone.35

Interestingly, initial NMR spectra for our synthetic daphlongamine H (4) did not match the reported spectroscopic data of 4,11 but surprisingly were in good agreement with those reported for deoxyisocalyciphylline B (3).10 Eventually, it was discovered that, by recording NMR spectra of our synthetic daphlongamine H (4) using old CDCl3 (putatively containing residual acid), we were able to obtain spectroscopic data that was in better agreement with the reported values for 4. Furthermore, interpretation of both our and the reported 2D NMR data of deoxyisocalyciphylline B (3) revealed that the S-C6 configuration in 3 had been misassigned.20 We therefore conclude that isolated deoxyisocalyciphylline B (revised structure: 3′) and daphlongamine H (4) are effectively the same natural product. It appears that the structure of the former had been misassigned, while the reported spectroscopic data of the latter correspond to its putative ammonium salt. The structural revision of 3 as 4 via total synthesis warrants further study of the proposed biosynthetic pathway for all members of the calyciphylline B-type alkaloids.10,15,16,20

In conclusion, we have developed the first total synthesis of the complex Daphniphyllum alkaloid (–)-daphlongamine H (4). Furthermore, our synthetic approach has provided access to (−)-isodaphlongamine H (5) and led to revision of the reported structure of deoxyisocalyciphylline B (3). Key features of the synthesis include a Mannich reaction using an Ellman sulfinimine derivative, rapidly building target-relevant complexity in the opening step. Finally, a series of efficient ring-forming events using an acyclic precursor forged the complex, hexacyclic framework of the calyciphylline B-type alkaloids.

Supplementary Material

CH6_131p
CH8_097p
CH8_155p
Supporting Info

ACKNOWLEDGMENTS

C.L.H. is grateful for a postdoctoral scholarship from the Swiss National Science Foundation. V.P. acknowledges TRDRP for a predoctoral fellowship. Financial support for this research was provided to R.S. by the National Institutes of Health (NIGMS R35 GM130345). We thank Dr. Hasan Celik (UC Berkeley) for assistance with NMR experiments, Dr. Nicholas Settineri (UC Berkeley) for single-crystal X-ray diffraction studies, and Prof. Jian-Min Yue (SIMM) for sharing NMR data of 3’ in CD3OD. We are grateful to Prof. Stephen Hanessian (U Montréal) for insightful discussions.

Footnotes

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b03576.

Experimental procedures, analytical data (1H and 13CNMR, HRMS, IR, [α]D), and optimization tables (PDF)

Crystallographic data for 16 (CIF)

Crystallographic data for 21 (CIF)

Crystallographic data for S27 (CIF)

The authors declare no competing financial interest.

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Associated Data

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

Supplementary Materials

CH6_131p
NIHMS1048566-supplement-CH6_131p.pdf (147.9KB, pdf)
CH8_097p
NIHMS1048566-supplement-CH8_097p.pdf (594.6KB, pdf)
CH8_155p
NIHMS1048566-supplement-CH8_155p.pdf (155.4KB, pdf)
Supporting Info
NIHMS1048566-supplement-Supporting_Info.pdf (6.8MB, pdf)

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