Selective Synthesis of (+)-Dysoline

Article abstract of DOI:10.1021/acs.orglett.8b03777

Dysoline, a novel chromone alkaloid isolated from Dysoxylum binectariferum, was reported to have selective cytotoxicity for HT1080 fibrosarcoma cells (IC₅₀ of 0.21 μM). Given the scarcity of natural material, a concise synthesis of (+)-dysoline was developed, allowing for further biological evaluation. An enantioselective nucleophile-catalyzed aldol lactonization formed the piperidine ring with control of relative and absolute stereochemistry. Construction of the C6-chromone core with complete regioselectivity was achieved using a Danheiser benzannulation.

Full text of DOI:10.1021/acs.orglett.8b03777

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Selective Synthesis of (+)-Dysoline
Aaron Coffin and Joseph M. Ready*
Department of Biochemistry, Division of Chemistry, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, Texas
75390-0938, United States
S
* Supporting Information
ABSTRACT: Dysoline, a novel chromone alkaloid isolated
from Dysoxylum binectariferum, was reported to have selective
cytotoxicity for HT1080 fibrosarcoma cells (IC50 of 0.21 μM).
Given the scarcity of natural material, a concise synthesis of
(+)-dysoline was developed, allowing for further biological
evaluation. An enantioselective nucleophile-catalyzed aldol
lactonization formed the piperidine ring with control of relative and absolute stereochemistry. Construction of the C6-
chromone core with complete regioselectivity was achieved using a Danheiser benzannulation.
hromone and flavonoid alkaloids represent important
rearrangement with regioselectivity dictated by the substitution
Cscaffolds in medicinal chemistry. Secondary metabolites of the starting trimethylphloroglucinol.11
A Danheiser-type benzannulation appeared capable of
with these ring systems are found in a variety of plant species,
many of which provide sources for traditional medicines.1
Interest in chromone and flavonoid alkaloids has led to the
identification and development of several members of this
family that demonstrate an array of medicinal properties.2 C8-
Substituted chromone alkaloids in particular have been well-
documented as potent cyclin-dependent kinase (Cdk)
inhibitors.3 Most notably rohitukine (2), first reported in
1983, provided the starting point for development of
flavopiridol (4) and Riviciclib (5), both of which act through
inhibition of the Cdk9/T1 complex (IC50 = 0.02 μM)3,4,5d and
were given orphan drug status for treatment of myeloid
leukemia.5 In contrast, fewer reports have described biological
activity associated with C6-substituted chromone and
flavonoid alkaloids.6
providing regioselective access to the C6-substituted chromone
core of dysoline (Scheme 1B).12 Additionally, the late-stage
disconnection would provide a divergent synthesis, allowing
for facile generation of analogues. This strategy identified ynol
ether 7 as a key intermediate. Finally, a Wynberg−Romo
nucleophile-catalyzed aldol lactonization (NCAL) was targeted
for construction of the piperidine ring and establishment of the
relative and absolute stereochemistry of the natural product.13
The cis substitution pattern on the piperidine ring would
translate into the cis relationship of the chromone ring and
hydroxyl substituents. In this way, we hoped to avoid
difficulties encountered in a prior synthesis of rohitukine (2),
in which the alcohol stereocenter had to be corrected through
a poorly selective oxidation/reduction sequence.5a
To access the requisite acid/aldehyde 9 for use in the
NCAL, γ-aminobutyric acid (GABA) was alkylated and
protected to give the allylic amine 11 (Scheme 2). Subsequent
ozonolysis generated the acid/aldehyde 9 in good yield. While
different enals could be employed in the reductive amination,
superior results were observed with trans cinnamaldehyde.
Subjection of 9a to the original NCAL conditions reported by
Romo (Mukiyama’s reagent, triethylamine in acetonitrile)
gratifyingly produced the desired bicyclic β-lactone 8a, albeit in
a moderate 28% yield (see Supporting Information). This yield
could be improved by using the more soluble pyridinium salt,
2-bromo ethyl pyridinium tetrafluoroborate 12.14 A slight
increase in yield was also observed using a catalytic amount of
quinuclidine. Despite all attempts, different activating agents,
bases, or the addition of Lewis acids,15 the highest yield
achievable in our hands was 51%. In addition, employing Cbz
or Ts nitrogen protecting groups resulted in lower yield under
these conditions (see Supporting Information).
Dysoline (1) is a C6-substituted chromone alkaloid that was
isolated from Dysoxylum binectariferum in 2013.7 Dysoline was
reported to show cytotoxicity toward HT1080 fibrosarcoma
cells (IC50 = 0.21 μM) while displaying little effect on a panel
of six other human cancer cell lines (IC50 values > 10 μM).
Additionally, dysoline was found to inhibit the production of
TNF-α and IL-6 cytokines. In contrast, dysoline’s C8-isomer
rohitukine (2) arrests the growth of HCT116 (colon cancer)
and HL60 (leukemia) cell lines.5d,8 While possessing a similar
structure, dysoline (1) demonstrated no activity against
HCT116 and did not display substantial inhibition toward
the Cdk family or several other kinases tested.7 Given this
reported biological activity and the scarcity of natural product,
dysoline presents an attractive target for synthesis.
The regioselective synthesis of C6-substituted chromones
remains challenging. Friedel−Crafts alkylation of the dihy-
droxy-chromone aryl ring results in a mixture of regioisomers
with varying levels of selectivity.9,10 Classical methods for the
synthesis of C8-substituted chromone and flavonoid alkaloids
involve construction of the chromone motif via a Claisen
condensation or Vilsmeier−Haack or Baker-Venkataramanen
Received: November 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03777
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Letter
Schwartz reagent provided aldehyde 18 in preparation for a
Corey−Fuchs homologation.17,18 Thus, aldehyde 18 was
homologated to vinyl dibromide 19, which was cleanly
converted to terminal alkyne 20 with n-BuLi.19 Unfortunately,
attempts to oxidize this alkyne with LiOOt-Bu failed to yield
the desired silyl ynol ether 7a. Rather, unreacted alkyne was
recovered intact. While oxidation of the des-hydroxy piperidine
14 proceeded smoothly from either the alkyne or directly from
the vinyl dibromide, this reactivity could not be translated into
our more elaborated substrate. Deprotonation of alkyne 20
with n-BuLi followed by quenching with CH3OD resulted in
complete deuteration of the alkyne, confirming formation and
stability of the lithium acetylide. Use of an alternative oxidant
(MeZnOOt-Bu) also failed to show desired reactivity.
Potential interference from the MOM group led us to explore
alternative protected alcohols to no avail.20
Scheme 1. (A) Chrome Alkaloids Isolated from D.
binectariferum and (B) Retrosynthesis of Dysoline (1)
Unable to oxidize the terminal alkyne 20, we explored an
alternative preparation of the desired ynol ether leveraging
chemistry developed by Kowalski.21 Synthesis of the required
substituted piperidine ethyl ester 22 was easily achieved
through ethanolysis of the β-lactone followed by MOM
protection. Subjection of ester 22 to the lithium anion of
dibromomethane resulted in the desired dibromo ketone 23.
Enolate formation followed by lithium halogen exchange and
alpha elimination initiated alkyl migration, leading to the
desired ynolate, which could be trapped with TIPSOTf to
furnish ynol ether 7a in moderate yield. This sequence could
also be done in one pot directly from ethyl ester 22, although a
slight decrease in overall yield was observed. By contrast, the
monosubstituted piperidine 16 underwent homologation in
substantially higher yield, indicating that steric hindrance by
the OMOM could be retarding the reaction. Nonetheless, the
Kowalski homologation provided the desired ynol ether 7a
without racemization, as indicated by complete retention of ee
of dibromo ketone 23.
Scheme 2. NCAL Reaction for the Preparation of Bicyclic β-
Lactone 8a
We next turned our attention to accessing the requisite
ketene. Accordingly, we targeted pyrone diazoketone 10
(Scheme 4) in anticipation of a Wolf rearrangement.22
Preparation of the precursor pyrone carboxylic acid 26 was
achieved in two steps utilizing a Claisen condensation followed
by acid-promoted cyclization. Hydrolysis then furnished the
desired carboxylic acid 26. Conversion of pyrone carboxylic
acid 26 to diazoketone 10 was achieved using slightly modified
conditions established by Arndt and Eistert.23 While only a
moderate yield was observed for this transformation, the major
byproduct was identified as pyrone ethyl ester 25, which could
be recycled into the sequence.
The propensity of pyrone diazo ketone 10 to undergo a
Wolf rearrangement was explored by subjecting it to both
thermal and photochemical conditions in the presence of
ethanol. As expected, conversion of the diazo ketone to the
homologated ethyl ester proceeded cleanly under both
conditions. It is particularly noteworthy that the rate of
conversion could be controlled through choice of light source.
This element of control proved advantageous in the Danheiser
benzannulation reaction as a means to minimize ketene
dimerization.
With optimized positive results for the racemic NCAL
reaction, we turned to establishing the best conditions to
obtain enantioenriched product. Different cinchona alkaloid
catalysts were evaluated for this reaction, and the dimer
(DHQD)2Phal provided the highest yield and enantioselectiv-
ity. Advantageously, both enantiomers of the β-lactone are
accessible depending on which pseudoenantiomer of cinchona
alkaloid was enlisted (Scheme 2).
With the necessary fragments in hand, we focused on the key
annulation step to construct the aryl ring of dysoline (1).
Irradiation of a solution of diazo ketone 10 and ynol ether 7a
with blue LED light (450 nm) formed the desired chromone
core (Scheme 5). Further optimization revealed that increasing
the temperature of this reaction improved the yield. The major
Initial efforts for conversion of β-lactone 8a to the desired
ynol ether for use in the benzannulation focused on alkyne
oxidation with LiOOt-Bu in accordance with the method of
Julia et al. (Scheme 3).16 Opening of the lactone with
Weinreb’s amine followed by MOM protection of the resulting
alcohol gave Weinreb amide 17. Subsequent reduction using
B
DOI: 10.1021/acs.orglett.8b03777
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Scheme 3. Homologation and Preparation of Ynol Ether 7a: (A) Parent Piperdine As a Test Substrate and (B) Preparation of
Desired Ynol Ether 7a
Scheme 4. Preparation of Diaketone and Homologation to
Ethyl Ester
byproduct observed in this reaction was the dimethyl pyrone,
presumably resulting from decarboxylation of the homologated
carboxylic acid. Formation of this product was decreased,
although not eradicated completely, with the use of molecular
sieves.
Deprotection of the resulting chromone followed by
methylation of the piperidine nitrogen furnished synthetic
dysoline (1). Mass data and 13C NMR of the synthetic product
matched that of the isolation report, while small variation of
the proton NMR was observed for the signals adjacent to the
piperidine nitrogen.24 The deviation is likely attributed to
differences in protonation status of the synthetic sample
compared to the natural sample. Further evidence of structure
was obtained by X-ray crystallography of optically active,
synthetic material with (−)-CSA. This crystal structure allowed
for assignment of absolute (1′R, 2′S) as well as the relative
stereochemistry of the natural product.
We next sought to confirm the reported biological activity of
dysoline (1). Testing of racemic and nonracemic samples for
cytotoxicity toward HT1080 and HCT116 cell lines
unfortunately showed none of the reported activity. Addition-
Scheme 5. Benzannulation and Final Steps for the Synthesis of Dysoline (1)
C
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Organic Letters
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(3) Nguyen, T. B.; Lozach, O.; Surpateanu, G.; Wang, Q.; Retailleau,
ally, synthetic dysoline did not inhibit IL-6 cytokine response,
contrary to the reported 83% inhibition at 0.1 μM.7
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With this result, we considered the possibility that the
reported activity could arise from the oxidation of the
piperidine nitrogen of dysoline (1). Although dysoline N-
oxide (30) was not reported by the isolation group, rohitukine
N-oxide (3) was isolated from the same source. We therefore
considered N-oxidation as a potential explanation for the
discrepancy in biological activity. Thus, treatment of the
natural product with m-CPBA resulted in clean conversion to a
single diastereomer of the N-oxide. Testing of this product
failed to show any cytotoxicity toward HT1080 cells.
In conclusion, we developed a concise enantio-, diastereo-,
and regioselective synthesis of dysoline (1). Despite the lack of
biological activity, the chromone scaffold of dysoline (1) holds
promise as a validated scaffold for drug discovery. In addition,
the regioselective and divergent nature of this synthetic
method should provide value as a means of accessing
additional C6-chromone, flavone, or xanthone alkaloids.
(6) Lee, H.-H.; Shin, J.-S.; Lee, W.-S.; Ryu, B.; Jang, D. S.; Lee, K.-T.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03777.
■
S
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Experimental methods, characterization data, and NMR
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Accession Codes
CCDC 1876035 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
(13) (a) Cortez, G. S.; Tennyson, R. L.; Romo, D. J. Am. Chem. Soc.
2001, 123, 7945. (b) Liu, G.; Shirley, M. E.; Romo, D. J. Org. Chem.
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: joseph.ready@utsouthwestern.edu
ORCID
(14) Oh, S. H.; Cortez, G. S.; Romo, D. J. Org. Chem. 2005, 70,
2835.
Joseph M. Ready: 0000-0003-1305-9581
Notes
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126, 5352. (b) Leverett, C. A.; Purohit, V. C.; Romo, D. Angew.
Chem., Int. Ed. 2010, 49, 9479.
(16) Julia, M.; Saint-Jalmes, V. P.; Verpeaux, J.-N. Synlett 1993,
1993, 233.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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2000, 122, 11995. (b) Zhao, Y.; Snieckus, V. Org. Lett. 2014, 16, 390.
(18) DIBAL and LiAlH4 showed conversion to the desired aldehyde;
however, isolation proved difficult under standard workup conditions,
presumably due to increased chelation of the MOM protecting group.
(19) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769.
(20) The corresponding Si(Et3) protected alcohol was attempted in
the path with no success. See Supporting Information for more
details.
(21) (a) Kowalski, C. J.; Fields, K. W. J. Am. Chem. Soc. 1982, 104,
321. (b) Kowalski, C. J.; Lal, G. S. J. Am. Chem. Soc. 1988, 110, 3693.
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Am. Chem. Soc. 1990, 112, 3093.
We thank Noelle Williams, Anthony Yuan, and Deepak
Nijhawan (UT Southwestern) for assistance testing the
biological activity of synthetic dysoline. X-ray crystallography
was performed by Dr. V. Lynch (University of Texas at
Austin). Financial support from CPRIT (RP180457) and the
Welch Foundation (I-1612) is acknowledged.
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D
DOI: 10.1021/acs.orglett.8b03777
Org. Lett. XXXX, XXX, XXX−XXX