Total Synthesis and Biological Evaluation of Cell Adhesion Inhibitors

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Cite This: Org. Lett. 2018, 20, 7003−7006

Total Synthesis and Biological Evaluation of Cell Adhesion Inhibitors Peribysin A and B: Structural Revision of Peribysin B Hanuman P. Kalmode,† Kishor L. Handore,†,∥ Raveena Rajput,‡,∥ Samir R. Shaikh,§,∥ Rajesh G. Gonnade,§,∥ Kiran A. Kulkarni,*,‡,∥ and D. Srinivasa Reddy*,†,∥ Organic Chemistry Division, ‡Biochemical Science Division, and §Center for Materials Characterization, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India ∥ Academy of Scientific and Innovative Research (AcSIR), 110025 New Delhi, India Downloaded via KAOHSIUNG MEDICAL UNIV on November 16, 2018 at 13:59:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Total synthesis of potent cell-adhesion inhibitors peribysins A and B has been accomplished for the first time in racemic form. A Diels−Alder/aldol sequence to build the skeleton and decoration of the desired functionalities of the targeted natural products using highly stereoselective operations are the highlights. The structures of synthesized peribysins were fully characterized using spectral data and single-crystal X-ray analysis. Through this total synthesis, the initially proposed structure of peribysin B has been revised. Furthermore, the cell-adhesion inhibition potential of the scaffold (two peribysins + three analogues) was confirmed using anti-adhesion assay.

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oxidation,8 chemoselective epoxidation,9 and stereoselective reduction. Compound 3 could be prepared from 4 using functional group interconversions, which in turn could be prepared using a Diels−Alder/aldol sequence developed by our group.10 The second target peribysin B (2) could be prepared from peribysin A (1) through a cyclization step to form the fused THF ring followed by dihydroxylation of the exocyclic double bond.11 As per the plan, our synthesis commenced with a Diels−Alder reaction between tiglic aldehyde and diene 5, prepared using a known procedure, in the presence of a Lewis acid (BF3·Et2O) at −78 °C to rt for 16 h to give a crude Diels−Alder adduct, which on intramolecular aldol condensation by 15% aq KOH in MeOH afforded cis-decalin in 46% yield and ∼95% dr for two steps (Scheme 2). This transformation established the desired stereochemical pattern of the peribysin family of natural products. The Diels−Alder reaction produced the endo adduct having the aldol partners (i.e., aldehyde and ketone) in close proximity, allowing for subsequent intramolecular aldol reaction to give dienone 4. The isolated double bond present in 4 was chemoselectively reduced with the help of Wilkinson’s catalyst12 under H2 balloon pressure to give product 6 in 86% yield. The diene 7 prepared from 6 through a Wittig olefination13 was subjected to allylic oxidation conditions using SeO2−TBHP14 in

amada and co-workers isolated new eremophilane type sesquiterpenoids called peribysins (A−I) from a strain of Periconia byssoides OUPS-N133 which was originally separated from the sea hare Aplysia kurodai.1 The absolute and relative stereochemistry of peribysins was elucidated on the basis of NMR techniques, CD, and some chemical transformations (Figure 1). All of the peribysins have a cis-decalin framework, except peribysin E, which has a cis-hydrindane system. All peribysins possess cell-adhesion inhibitory potential, which is significantly better than the standard (herbimycin A).2,3 Celladhesion inhibitors are useful in developing drugs for the treatment of cancer,4a inflammation,4b and sickle cell disease.4c,d Because of their biological properties and interesting structural features, peribysins have become attractive targets for synthetic chemists, including our group. Elegant syntheses of peribysin E have been reported by Danishefsky5 and Sha,6 followed by our group.7 Enantiospecific synthesis of both enantiomers of peribysin E necessitated its structural revision. Our synthesis helped to access (±)-peribysin E and its analogues in a short sequence. However, there are no reports on synthetic efforts toward other peribysins, in spite of their high anti-cell-adhesive potential. Here, we present the first total synthesis of more potent natural products from this family (peribysin A and B) and confirmed their anti-cell-adhesion property. The retrosynthetic analysis of peribysin A (1) and B (2) is described in Scheme 1. We envisioned our first target peribysin A to be obtained from the intermediate 3 through allylic © 2018 American Chemical Society

Received: September 5, 2018 Published: October 26, 2018 7003

DOI: 10.1021/acs.orglett.8b02838 Org. Lett. 2018, 20, 7003−7006

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Organic Letters

Scheme 2. Total Synthesis of Peribysin A and Its ORTEP

Figure 1. Structures of selected peribysins and background.

Scheme 1. Retrosynthetic Analysis of Peribysins A and B

CH2Cl2. Here, we observed the formation of desired allylic alcohol 3 as a major product and its overoxidized product (aldehyde) as a minor product. Fortuitously, no allylic oxidation on the ring was observed under these conditions. We did not make attempts to purify individual compounds; instead, the crude material obtained after workup was subjected to Luche reduction conditions (NaBH4, CeCl3·7H2O) that resulted in allylic alcohol 3 in 67% yield.15 To achieve the second allylic oxidation, primary alcohol was protected as an acetate, and the desired transformation took place when we used Chandrasekaran’s protocol (pyridinium dichromate−tert-butyl hydroperoxide, PDC−TBHP)8 and obtained the required product 8 in moderate yields. The endocyclic double bond present in compound 8 was chemoselectively epoxidized (H2O2−NaOH

in MeOH) in a highly stereoselective manner to afford 9 in 83% yield as a mixture with its lactol form (9′). Finally, the mixture of 9 and 9′ on reduction16 using NaBH4 in MeOH afforded peribysin A (1) along with its minor diastereomer 10 in very good yields. The spectral data (1H and 13C) of synthetic 7004

DOI: 10.1021/acs.orglett.8b02838 Org. Lett. 2018, 20, 7003−7006

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Organic Letters Scheme 3. Total Synthesis and Structural Revision of Peribysin B and Its ORTEP

peribysin A (1) were in complete agreement with those reported1b for the isolated natural product. Further, the stereochemistry of peribysin A (1) was unambiguously determined with the help of single-crystal X-ray analysis (ORTEP is shown in Scheme 2). In an alternate route, peribysin A was also synthesized from a known intermediate,17 11, in six steps (Scheme 2). The enone (11) was converted to vinyl iodide (12) by utilizing I2 and pyridine in good yield. Installation of the side chain was achieved by a Suzuki−Miyaura cross-coupling reaction18 of vinyl iodide (12) and substituted vinyl boronic ester19 (13) to afford dienone (14) in 88% yield. It is worth mentioning that a similar transformation was carried out by Danishefsky’s group during the synthesis of peribysin E.5a The TBS group of dienone (14) was deprotected using TBAF to afford allylic alcohol (15) in 82% yield, and it was converted to the target compound by following the steps described above. The second route has a slight advantage over the first route by considering the yields obtained (7% vs 3%). Toward the total synthesis of peribysin B (2), we have chosen peribysin A (1) itself as the starting point which was converted to its tosylate through a selective reaction of primary alcohol, which immediately underwent in situ intramolecular cyclization to afford the tetrahydrofuran intermediate 16 in good yield. In the final step, Upjohn dihydroxylation on 16 resulted in the formation of target peribysin B in 62% yield as the sole product in a highly stereoselective manner. All of the spectral data of synthetic peribysin B (17) were in full agreement with the data reported for the natural product during isolation.1b Although we are in agreement with the stereochemical assignments by Yamada’s group, considering the presence of several stereo-

Figure 2. Fluorescent (left) and brightfield (right) images of adherent HL60 cells on HUVEC monolayer: (a) control, (b) 9, (c) 10, (d) 16, (e) peribysin A (1), (f) peribysin B (17), (g) percentage inhibition of compounds relative to the untreated HUVEC cells.

centers (seven) and having access to sufficient quantities of the product, we recrystallized it and subjected it to single-crystal Xray analysis to further confirm all stereocenters without any 7005

DOI: 10.1021/acs.orglett.8b02838 Org. Lett. 2018, 20, 7003−7006

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ACKNOWLEDGMENTS We thank Prof. Takeshi Yamada (Osaka University of Pharmaceutical Sciences) for providing NMR spectral details and helpful discussions. We acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, for their financial support through the CSIR-Sickle Cell Anaemia Mission Mode Project (HCP0008). H.P.K. thanks SERB for the award of a National Postdoctoral Fellowship (PDF/2016/ 001638), and K.L.H. thanks CSIR, New Delhi, for the award of a Senior Research Fellowship.

ambiguity. To our surprise, one of the quaternary chiral centers (OH attached) was found to be of different stereochemistry. It was further supported by NOE correlation (Scheme 3) of peribysin B (17). Yamada et al.1b have reported the observation of NOE between −C6H and −C13H, which was absent in revised peribysin B (17). Thus, there is a need to revise the structure of peribysin B. Accordingly, we have revised the structure. For further details of the 2D spectrum (such as COSY, NOESY, HSQC, and HMBC), refer to the Supporting Information (SI). Having access to the natural products and their analogues, next we explored the anti-cell-adhesion property of synthesized peribysins including three additional compounds 9, 10, and 16 using cell-adhesion assay. Details of the assay are available in the SI. As shown in Figure 2, all of these compounds inhibited the adhesion of HL-60 cells to HUVEC at the tested concentrations (100 μM). However, peribysin A (1) and peribysin B (17) showed relatively higher inhibitory potency, suggesting that both of the natural products are better than their three analogues. In conclusion, we have accomplished the first total synthesis of potent anti-cell-adhesion natural products peribysins A (1) and B (17) in racemic form. During this process, the structure of peribysin B has been revised. Highlights of the synthesis are the Diels−Alder/aldol sequence and stereoselective functionalization. The developed strategy could be easily applied to prepare related natural products and analogues towards lead optimization. Having access to both the peribysins, we have performed biological assays to confirm their cell-adhesion potential. Going forward on this project, in particular, the utility of the present scaffold in discovering drugs for the treatment of sickle cell disease will be the subject of future publications.

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DEDICATION Dedicated with respect to Professor Goverdhan Mehta (University of Hyderabad) on the occasion of his 75th birthday. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02838. Comparison of 1H, 13C NMR data of natural and synthetic peribysins A and B; experimental procedures, analytical data, biological assay details, 1H, 13C NMR spectra of all compounds, 2D-NMR spectra of 17 (PDF) Accession Codes

CCDC 1848977 and 1848990 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hanuman P. Kalmode: 0000-0002-1577-5103 Rajesh G. Gonnade: 0000-0002-2841-0197 D. Srinivasa Reddy: 0000-0003-3270-315X Notes

The authors declare no competing financial interest. 7006

DOI: 10.1021/acs.orglett.8b02838 Org. Lett. 2018, 20, 7003−7006