Total synthesis of (-)-crinipellin A

Article abstract of DOI:10.1021/ja5054412

The first total synthesis of (-)-crinipellin A is described. The tetraquinane core skeleton of crinipellin A was assembled through the tandem [2 + 3] cycloaddition reaction of an allenyl diazo substrate containing a cyclopentane ring in a single operation. The absolute stereochemistry was confirmed through the total synthesis.

Full text of DOI:10.1021/ja5054412

Communication
pubs.acs.org/JACS
Total Synthesis of (−)-Crinipellin A
Taek Kang, Seog Boem Song, Won-Yeob Kim, Byung Gyu Kim, and Hee-Yoon Lee*
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Korea
S
* Supporting Information
Recently we reported a synthetic methodology for the
ABSTRACT: The first total synthesis of (−)-crinipellin A
is described. The tetraquinane core skeleton of crinipellin
A was assembled through the tandem [2 + 3] cyclo-
addition reaction of an allenyl diazo substrate containing a
cyclopentane ring in a single operation. The absolute
stereochemistry was confirmed through the total synthesis.
synthesis of triquinanes from linear acyclic substrates through
tandem cycloaddition reaction sequence via trimethylene-
methane (TMM) diyl.6 Application of the new synthetic
methodology could be extended to the construction of
tetraquinanes starting from cyclopentanes with proper
appendages (Scheme 1a). A big challenge of the tetraquinane
Scheme 1. Synthetic Analysis of Crinipellins
rinipellins, first isolated in 1979 from basidiomycete
CCrinipellis stipitaria with antibacterial and anticancer
activity, are the only tetraquinane natural products (Figure
1).1,2 The structures of crinipellin A (1) and crinipellin B (4)
Figure 1. Members of crinipellins
along with their analogues (2, 3 and 5) were deduced by NMR
analysis and were confirmed by an X-ray structure determi-
nation of crinipellin B (4).1b Recently, structurally diverse
crinipellins that do not possess the enone moiety (6−9) were
isolated from a different fungal strain, Crinipellis sp. 113 and
showed only moderate anticancer acitivity.1c The structural
complexity in addition to the biological activities of crinipellins
has drawn attention from the synthetic organic chemists.
However, there have been only handful reports of synthetic
efforts of the total synthesis of crinipellins,3 and Piers’ report of
the total synthesis of crinipellin B4 is the sole example of the
total synthesis of crinipellin natural products.
synthesis via TMM diyl cycloaddition reaction is tolerance of
the cycloaddition reaction against the added steric and
electronic effect in the substrates.7 Though challenging, high
reactivity of TMM diyl and seemingly favorable conformational
constraint in the tether prompted us to initiate the total
synthesis of crinipellins.
The eight contiguous stereocenters in a congested
tetraquinane skeleton poses still a formidable challenge to
synthetic chemists and a test ground for synthetic strategies
developed toward the ideal synthesis.5
Received: May 30, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja5054412 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Synthetic analysis revealed that crinipellins could be
synthesized from the tetraquinane 10 that could be obtained
from 12 through the tandem cycloaddition reaction via TMM
diyl 11 (Scheme 1b). The relative stereochemistry at the C-8,
the C-10 and the C-14 stereocenters of 12 appears to be
important as the OTBDPS group has to adopt pseudoequato-
rial position for the TMM diyl cycloaddition reaction. It was
well-documented in the linear triquinane synthesis that the
stereochemistry at that position controlled the relative
stereochemistry of the cycloaddition product as it preferred
the equatorial position in the transition state of the TMM
cycloaddition reaction.8 The substrate for 12 can be assembled
from the intermediate 13 where the allylic alcohol can
introduce the C-8 hydroxy group enantioselectively to control
the relative stereochemistry of 12. The allylic alcohol 13 can be
prepared from a known compound 14 that was available in
enantiomerically pure form.9
of 16 to the corresponding aldehyde and subsequent treatment
with Bestmann−Ohira reagent.12 The epoxyalkyne moiety of
17 set the stage for the introduction of the remaining carbon
atoms necessary for the tetracyclic core of crinipellins with
formation of the allene functionality. The iron catalyzed SN2′-
type reaction13 of the epoxyalkyne 17 with the Grignard
reagent 18 produced allene compound 19 as an inseparable 1:1
mixture of diastereomers after protection of the alcohol of the
product with the silyl protecting group. Finally, the acetal of 19
was removed by acidic hydrolysis14 to unmask the aldehyde and
the aldehyde was treated with p-toluenesulfonehydrazide to
form the hydrazone 20.
The key tandem cycloaddition reaction was initiated by
generating the diazo functionality from 20 through the anion
formation from the hydrazone of 20 under refluxing toluene
solution.15 To our delight, despite of seemingly high steric
congestion during the TMM diyl cycloaddition reaction,
successive cycloaddition reaction of the diazo intermediate via
TMM diyl intermediate 11 produced the tetraquinane 10 in
87% yield with complete stereocontrol via the preferred
conformer 11′ as anticipated in Scheme 1 (Scheme 3).
The total synthesis started with the known enantiomerically
pure compound 149 synthesized from 2-methyl-2-cyclopenten-
1-one (Scheme 2). Wittig olefination of the ketone of 14
Scheme 2. Preparation of the Precursor for the TMM Diyl
a
Cycloaddition Reaction
Scheme 3. Tandem Cycloaddition Reaction to Form
Tetraquinane 10
With the complete carbon framework of crinipellins in hand,
completion of the total synthesis of crinipellin A required
introduction of various oxygen functionalities with stereo-
control. After deprotection of TBDPS group of 10, PCC
oxidation produced ketone 22. α-Hydroxylation of 22 followed
by PCC oxidation gave diketone 23. Since α-hydroxylation of
22 using Davis’ oxaziridine16 gave the β-configuration of the
hydroxyl group at the C-9 position that is the opposite
strereochemistry of crinipellin A, the alcohol was oxidized to
the diketone 23. Since the reduction of the diketone of 23
reduced the C-8 ketone selectively,4a,b we devised an indirect
way for introducing proper stereochemistry at the C-9 position.
Treatment of the diketone 23 with sulfoximine anion produced
24 selectively.17 In line with the chemo- and stereoselectivity
for the reduction of the diketone 23,4b sulfoximine anion
attacked the C-8 ketone selectively from the back side to
produce the tertiary alcohol with β-configuration. The
stereochemistry of this alcohol of 24 was used for the
introduction of the stereochemistry at the C-9 position of
crinipellin A. Hydroxyl group directed reduction of the ketone
a
t
t
(a) Ph3PCH3Br, BuOK, BuOH, Et2O, r.t., 99%; (b) TBAF, THF,
r.t., 99%; (c) (COCl)2, DMSO, THF; TEA, −78 °C to r.t.; (d)
Ph3PCCH3COOEt, THF, reflux, 91% for 2 steps; (e) LAH, Et2O, 0 °C
to r.t., 96%; (f) D-DET, TBHP, Ti(OiPr)4, CH2Cl2, −30 °C, 87%; (g)
(COCl)2, DMSO, CH2Cl2; TEA, −78 °C to r.t.; (h) K2CO3,
Bestmann−Ohira reagent, MeOH, r.t., 87% for 2 steps; (i) Fe(acac)3,
18, THF, Toluene, −15 °C, 94%; (j) TBDPSCl, imidazole, DMAP,
CH2Cl2, r.t., 96%; (k) p-TsOH·H2O, HCHO, THF, H2O, r.t., 93%; (l)
H2NNHTs, MeOH, r.t., 97%.
followed by deprotection of the TBS-ether produced 15. The
alcohol of 15 was oxidized to the corresponding aldehyde using
Swern’s protocol.10 Stereoselective Wittig olefination of the
aldehyde produced the allylic alcohol 13 after LAH reduction of
the ester. Asymmetric Sharpless epoxidation reaction of 13
produced the epoxide 16 along with its diastereomeric product
in 8:1 ratio, since the asymmetric epoxidation of such allylic
alcohols is known to be less selective than other types of allylic
alcohols.11 The epoxyalcohol 16 was converted into the alkyne
17 with one carbon extension through oxidation of the alcohol
18
of 24 using NaBH(OAc)3 produced the diol 25 with
complete stereocontrol. After protection of the alcohol at the
C-9 of 25, allylic oxidation of the olefin using PDC gave enone
B
dx.doi.org/10.1021/ja5054412 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
26.19 At this stage, the sulfoximine group was removed easily to
regenerate the ketone 27 by simply refluxing in toluene.17
Treatment of 27 with hydrogen peroxide under basic condition
followed by treatment with LDA and Eschenmoser’s salt20
introduced an epoxide and a methylene group to afford 28 that
completed installation of all carbons and the proper functional
groups of crinipellin A. The final deprotection reaction of TBS
group was tested under various conditions.21 Only TASF
produced crinipellin A reproducibly with low conversion due to
instability of 28 under basic condition. Nonetheless,
desilylation of 28 using TASF produced natural (−)-crinipellin
A (Scheme 4). All the physical and spectroscopic data for
synthetic (−)-1 were identical to the naturally occurring
(−)-crinipellin A.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (KRF-2008-314-C00198).
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedure and spectral data for compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
leehy@kaist.ac.kr
(21) We tested numerous deprotection conditions of TBS group
including TBAF, TBAF/NH4F, TBAF/AcOH, HF·Pyr., HF/Urea,
HF/TEA, NH4F·HF, SiF4, and TASF.
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dx.doi.org/10.1021/ja5054412 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX