Total synthesis of (±)-cortistatin J from furan

Article abstract of DOI:10.1021/ja206138d

A concise, diastereoselective total synthesis of (±)-cortistatin J has been completed in 20 steps from furan. Key steps include an intramolecular [4 + 3] cyclization of a disubstituted furan with a (Z)-2-(trialkylsilyloxy)-2- enal to construct the tetracyclic core and a (Z)-vinylsilane/iminium ion cyclization to form the A ring.

Full text of DOI:10.1021/ja206138d

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Total Synthesis of (()-Cortistatin J from Furan
Mark G. Nilson and Raymond L. Funk*
Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States
S Supporting Information
b
Scheme 1. [4 + 3] Cyclizations of (Z)-2-(Silyloxy)-2-enals
ABSTRACT: A concise, diastereoselective total synthesis
of (()-cortistatin J has been completed in 20 steps from
furan. Key steps include an intramolecular [4 + 3] cycliza-
tion of a disubstituted furan with a (Z)-2-(trialkylsilyloxy)-
2-enal to construct the tetracyclic core and a (Z)-vinyl-
silane/iminium ion cyclization to form the A ring.
cyclizations8 with a variety of dienes, in this case furan to yield
cycloadducts 5.9 During the course of this study we made several
key findings: (1) endo adducts are uniformly preferred over exo
adducts, (2) the stereoselectivity is better with smaller silyl
substituents (e.g., TES vs TBS) and (3) the endo/exo ratio can
be significantly improved by the choice of Lewis acid. These results
encouraged us to advance a retrosynthetic analysis of cortistatin J,
outlined in Scheme 2. Thus, the dimethylaminocyclohexene A ring
could be stereoselectively constructed by intramolecular addition of
(Z)-vinylsilane 6 onto the equatorially oriented exo-iminium ion
following the Overman protocol.10 The (Z)-vinylsilane moiety of 6
could be installed by a Sonagashira coupling of triflate 7 with
(trimethylsilyl)acetylene followed by semireduction with DIBAL-
H. Oxidation of the alcohol functionality would provide the
aldehyde precursor to the iminium ion. Dienol triflate 7 could be
obtained via triflation of the dienolate derived from enone 8. In
he cortistatins are a family of novel steroidal alkaloids
T
recently isolated by Kobayashi and co-workers from the
marine sponge Corticium simplex collected off Flores Island,
Indonesia.1 The unprecedented reorganization of the steroidal
framework in addition to the unusual amino and isoquinoline
substituents contribute to the appeal of these natural products as
targets for total synthesis. However, their biological properties
are even more significant. Cortistatins AÀD1a and JÀL1c in-
hibited the proliferation of human umbilical vein endothelial cells
(HUVECs) with high selectivity. Cortistatins J (1) and A (2)
(Figure 1) are the most active and selective of the natural
Scheme 2. Retrosynthetic Analysis of Cortistatin J (1)
Figure 1. Structures of cortistatin J (1) and A (2).
products, having shown cytostatic and antiproliferative activity
(IC50 = 8 and 1.8 nM for 1 and 2, respectively) against HUVECs
with a selective index of 300À1100 for 1 and greater than 3000
for 2 in comparison with that of normal human dermal fibroblast
or several tumor cell lines. The anti-angiogenic properties of 2
were also evaluated, and it was found that the migration and
tubular formation of HUVECs induced by VEGF or bFGF could
be inhibited at 2 nM concentration. Thus, the cortistatins
represent exciting new leads for anticancer drug discovery efforts
based upon the inhibition of angiogenesis.2 Accordingly, these
natural products have attracted significant attention from the
synthetic community, resulting in numerous approaches3 and
several total4 and formal5 syntheses.
We recognized that the cortistatins represented the ideal
targets to apply our methodology for the [4 + 3] cyclization of
(Z)-2-(trialkylsilyloxy)-2-enals,6 which are readily prepared via
retrocycloaddition of 5-(trialkylsilyloxy)-1,3-dioxins.7 Two rep-
resentative and relevant examples are shown in Scheme 1. Thus,
thermally promoted retrocycloaddition of dioxins 3 provided
(Z)-enals 4, which underwent Lewis acid catalyzed [4 + 3]
Received: July 1, 2011
Published: July 16, 2011
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2011 American Chemical Society
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turn, enone 8 would be provided by oxidation of R-hydroxy ketone
9 to an R-hydroxy enone and reductive removal of the superfluous
hydroxyl functionality. Stille coupling of the enol triflate moiety of 9
with 7-(trimethylstannyl)isoquinoline would install the isoquino-
line functionality. In the key step, it was hoped the tetracyclic core 9
could be constructed by a diastereoselective, endo [4 + 3] cycliza-
tion between the disubstituted furan and (Z)-2-(triethylsilyloxy)-2-
enal moiety in 10, which could be obtained by stereoselective
retrocycloaddition of 5-(triethylsilyloxy)-1,3-dioxin 11.
Scheme 4. Completion of the Total Synthesis of Cortistatin J
The preparation of the key dioxin 11 began with utilization
of Kim’s protocol for the conjugate addition of organo-
aluminates to R,β-unsaturated ketones.11 Thus, TIPS-protected
2-furylethanol12 12 was metalated with n-butyllithium and then
converted to the lithium trimethylaluminate, which underwent
trimethylsilyl triflate-promoted conjugate addition to 2-methyl-
cyclopenten-1-one to afford the TMS enol ether 13 (Scheme 3).
Scheme 3. Tetracyclic Core Synthesis by [4 + 3] Cyclization
Stille coupling of the enol triflate moiety of 9 with 7-(trimethyl-
stannyl)isoquinoline 1815 proceeded uneventfully to give iso-
quinoline 19. Stereoselective and global alkene reduction was
achieved with diimide16 and provided tetrahydrofuran 20 in
excellent yield. A three-step oxidation/deoxygenation sequence
(Swern oxidation of R-hydroxy ketone 20, R-hydroxy enone
triflation and Pd-catalyzed reduction of triflate 21) gave enone 8.
The 9(11),10(19)-diene unit was introduced as a dienol triflate
via generation of the dienolate of enone 8 with LiHMDS and
sulfonylation with PhNTf2. Removal of the TIPS protecting
group with aqueous HCl yielded alcohol 7. The (Z)-vinyl-
silane was incorporated in one step,17 utilizing the known
potassium trifluoroborate reagent 2218 and Molander’s con-
ditions for SuzukiÀMiyaura coupling.19 ParikhÀDoering
oxidation of the alcohol smoothly provided key aldehyde
23. To our delight, we found that heating aldehyde 23 in the
presence of excess dimethylamine hydrochloride in aceto-
nitrile at 60 °C overnight provided cortistatin J as a single
diastereomer in excellent yield. The cyclization presumably
proceeds through iminium ion conformer 6 (Scheme 2), with
the dimethyliminium ion adopting a pseudoequatorial posi-
tion to avoid an incipient 1,3-diaxial interaction with the
ethylene bridge of the tetrahydrofuran.
The enolate derivative of 13, generated upon treatment of with
methyllithium, was diastereoselectivly alkylated with methyl
iodoacetate to provide ketone 14. A simple three-step sequence
(preparation of the enol triflate 15, ester reduction and
iodination) yielded iodide 16, which was used to alkylate the
azaenolate of the dimethyl hydrazone13 of 2,2-dimethyl-1,3-
dioxan-5-one. The alkylated hydrazone was hydrolyzed during
workup to yield dioxanone 17. Silylation of the kinetic enolate of
dioxanone 17 followed by thermally promoted retrocycloaddi-
tion of the resultant silyloxydioxin gave the requisite (Z)-
2-(triethylsilyloxy)-2-enal 10. After much experimentation,14 it
was found that subjection of silyloxyenal 10 to catalytic triflic acid
in dichloromethane at À78 °C for 1 h effected the desired [4 + 3]
cyclization with TES cleavage in the same pot to provide the
tetracyclic core 9 as a single diastereomer, whose structural
assignment was based on extensive NOE experiments.
In summary, we have completed a concise total synthesis
of (()-cortistatin J in 20 steps from furan. The tetracyclic core
was assembled by an intramolecular, diastereoselective [4 + 3]
cyclization of a disubstituted furan and a (Z)-2-(triethylsilyl-
oxy)-2-enal, obtained by retrocycloaddition of a 5-(triethylsilyl-
oxy)-1,3-dioxin, and represents the first application of our
methodology. The total synthesis was completed with the Over-
man-type (Z)-vinylsilane/iminium ion cyclization to construct
With gram quantities of the tetracyclic core 9 in hand, we
turned our attention to installation of the isoquinoline function-
ality at this point in the synthesis since its presence was not
expected to complicate subsequent steps. (Scheme 4). Thus,
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the A ring. The preparation of other cortistatin congeners, in
particular cortistatins A and K via the pivotal enone 8, are now
under investigation.
Harmata and co-workers have also examined the TIPS congener of this
compound, see: (b) Harmata, M.; Sharma, U. Org. Lett. 2000, 2, 2703–
2705.
(7) Aungst, R. A., Jr.; Funk, R. L. Org. Lett. 2001, 3, 3553–3555.
(8) For reviews on [4 + 3] cycloadditions, see: (a) Harmata, M.
Chem. Commun. 2010, 46, 8904–8922. (b) Harmata, M. Chem. Commun.
2010, 46, 8886–8903. (c) Harmata, M. Adv. Synth. Catal. 2006,
348, 2297–2306. (d) Rigby, J. H.; Pigge, F. C. Org. React. 1997, 51, 351–
478.
(9) A possible explanation for the preferred endo adduct and
improved diastereoselectivity with smaller silyl substituents is provided
by DFT analysis (B3LYP/6-31G* level). The reaction of furan with
2-(trimethylsilyloxy)acrolein was calculated to proceed via a stepwise
mechanism involving conjugate addition of the complexed aldehyde,
closure of the silyl enol ether onto the resulting furonium ion, and rate-
limiting silyl transfer, see: (a) Sꢀaez, J. A.; Arnꢀo, M.; Domingo, L. R.
Tetrahedron 2005, 61, 7538–7545. (b) A more direct, concerted parth-
way involves a [4 + 3] cycloaddition of a complexed aldehyde to afford
the same silyl oxocarbenium ion intermediate followed by silyl transfer
and decomplexation, see ref 8. An alternative pathway involving a
DielsÀAlder cycloaddition followed by an R-ketol rearrangement has
been advanced, see: (c) Davies, H. M. L.; Dai, X. J. Am. Chem. Soc. 2004,
126, 2692–2693.
(10) For the only examples of exocyclic iminium ion/vinylsilane
cyclizations, see: (a) Overman, L. E.; Burk, R. M. Tetrahedron Lett. 1984,
25, 5739–5742. For a review of the electrophilic substitution of allyl- and
vinylsilanes, see: (b) Fleming, I.; Dunoguꢀes, J.; Smithers, R. Org. React.
1989, 37, 57–575.
(11) Kim, S.; Park, J. H. Synlett 1995, 163–164.
(12) Prepared via alkylation of 2-lithiofuran with (2-iodoethoxy)-
triisopropylsilane, see Supporting Information (SI).
(13) Prepared via condensation of 2,2-dimethyl-1,3-dioxan-5-one
with dimethylhydrazine, see SI.
’ ASSOCIATED CONTENT
S
Supporting Information. Spectroscopic data and experi-
b
mental details for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
rlf@chem.psu.edu
’ ACKNOWLEDGMENT
We appreciate the financial support provided by the National
Institutes of Health (GM28553). We thank David J. Roach for
model system studies.
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3
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