A concise total synthesis of (-)-maoecrystal Z

Article abstract of DOI:10.1021/ja2073356

The first total synthesis of (-)-maoecrystal Z is described. The key steps of the synthesis include a diastereoselective Ti^III-mediated reductive epoxide coupling reaction and a diastereoselective Sm ^II-mediated reductive cascade cyclization reaction. These transformations enabled the preparation of (-)-maoecrystal Z in only 12 steps from (-)-γ-cyclogeraniol.

Full text of DOI:10.1021/ja2073356

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A Concise Total Synthesis of (À)-Maoecrystal Z
Jacob Y. Cha, John T. S. Yeoman, and Sarah E. Reisman*
The Warren and Katharine Schlinger Laboratory for Chemistry and Chemical Engineering, Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
b
ABSTRACT: The first total synthesis of (À)-maoecrystal Z
is described. The key steps of the synthesis include a
diastereoselective TiIII-mediated reductive epoxide coupling
reaction and a diastereoselective SmII-mediated reductive
cascade cyclization reaction. These transformations enabled
the preparation of (À)-maoecrystal Z in only 12 steps from
(À)-γ-cyclogeraniol.
aoecrystal Z (6) is an unusual rearranged 6,7-seco-ent-
kauranoid natural product that was isolated in 2006 as a
M
Figure 1
minor constituent from the Chinese medicinal herb Isodon
eriocalyx.1 Its compact tetracyclic ring system comprises six
vicinal stereogenic centers, two of which are all-carbon quatern-
ary centers. Maoecrystal Z is closely related to several additional
6,7-seco-ent-kauranoid natural products, including trichorabdals
A (1)2 and B (2),3 shikodonin (3),4 longirabdolactone (4),5 and
effusin (5),6 as well as the rearranged ent-kauranoid maoecrystal V
(7)7 (Figure 1). Collectively, these compounds share a common
central spiro-fused lactone. Compounds 1À3 exhibit in vivo
antitumor activity against Ehrlich ascites carcinoma in mice,4,8
while 6 and 7 display in vitro cytotoxicity toward A2780 ovarian
and HeLa cancer cell lines, respectively.1,7
Scheme 1. (a) Retrosynthetic Analysis for Maoecrystal Z (6);
(b) Methoxide-Mediated Retro-Dieckmann/Aldol Reaction
of Trichorabdal B (2)
Although the structures and biological activities of 1À5 have
been known for decades, there are few reports of synthetic
studies leading toward the 6,7-seco-ent-kauranoid natural prod-
ucts. In 1986, Mander and co-workers reported a 33-step total
synthesis of 15-desoxyeffusin from 3,5-dimethoxybenzoic acid;9
12 years later, the same group completed a 29-step semisynthesis
of 4 from giberellic acid.10 In contrast, since its isolation in 2004,
several groups have published approaches to 7,11 with Yang and
co-workers reporting the first total synthesis in 2010.12 Our
interest was drawn to maoecrystal Z, with the objective of
developing a synthetic route to the central spirolactone core that
may also provide access to the trichorabdals. In this communica-
tion, we report the first total synthesis of (À)-maoecrystal Z (6).
Retrosynthetically, it was envisioned that maoecrystal Z should
be accessible from diol 9 and that the central C ring could be
formed through an intramolecular aldol reaction (Scheme 1a).
This synthetic plan was guided in part by a 1981 communication
by Fujita and co-workers in which treatment of trichorabdal B (2)
with dilute sodium hydroxide in methanol furnished methyl ester
8, an oxidized congener of maoecrystal Z (Scheme 1b).3 Tetracycle
8 presumably results from a methoxide-promoted retro-Dieck-
mann reaction followed by aldol ring closure of the transiently
generated enolate. This finding suggests that the intramolecular
aldol reaction to form the C ring of 6 should occur under mild
conditions. In the context of our synthetic plan, we hypothesized
that it might be possible to construct both the six-membered
D ring and five-membered C ring simultaneously from dialde-
hyde 11 through a SmII-mediated cascade cyclization reaction.
Elegant studies by Procter and colleagues have recently demon-
strated that SmI2 can promote reductive cascade cyclizations of
Received: August 4, 2011
Published: August 30, 2011
r
2011 American Chemical Society
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Scheme 2. Preparation of Spirolactone 12
Scheme 3. Synthesis and SmII-Mediated Reductive
Cyclization of Aldehyde 23
dialdehydes for rapid construction of polycyclic motifs.13 It
was envisioned that selective ketyl generation could be effected
at the more accessible side-chain C11 aldehyde of 11. Intramo-
lecular addition to the unsaturated lactone followed by a second
one-electron reduction would generate the C7ÀC8 enolate,
which could undergo aldol ring closure with the proximal
aldehyde in accord with the precedent of the Fujita system.3
Because of the increased steric encumbrance imposed by the
adjacent quaternary centers, samarium ketyl formation at the
C6 aldehyde was expected to be kinetically disfavored. Approach
of the C11 ketyl to the enoate face opposite the C6 aldehyde
was anticipated to provide the correct configuration at C9; on
the other hand, the stereochemical outcome at the two newly
formed carbinol stereogenic centers was difficult to predict a
priori. Notably, if diastereoselective, this transformation would
serve to build two rings and set four stereogenic centers in a
single step.
Dialdehyde 11 was expected to be derived from spirolactone
12 through sequential alkylation and desaturation (Scheme 1a).
In a second key step, spirolactone 12 was anticipated to be
accessible by the TiIII-mediated reductive coupling of enantioen-
riched epoxide 13 and acrylic acid derivative 14.14 Epoxide 13
can be prepared in a short sequence from the known compound
(À)-γ-cyclogeraniol15 (15) (see Scheme 2).
respectively, supporting the intermediacy of radical 16.18 The
diastereoselectivity in this reaction is proposed to derive from
approach of the acrylate syn to the C5 proton of 16, minimizing
nonbonding interactions with the adjacent siloxy and axial methyl
substituents.
The first stage of our synthesis focused on the preparation
spirolactone 12, which required the diastereoselective formation
of the C10 all-carbon quaternary center. Starting with (À)-γ-
cyclogeraniol (15), protection of the primary alcohol as the tert-
butyldimethylsilyl (TBS) ether and epoxidation with m-chloro-
peroxybenzoic acid (m-CPBA) provided epoxide 13 as a 3:1
mixture of diastereomers (Scheme 2). We were pleased to find
that use of methyl acrylate and Gans€auer's modified conditions16
for reductive epoxide couplings [Cp2TiCl2 (0.5 equiv), Zn0 (2.0
As described in the retrosynthetic analysis, we ultimately plan-
ned to utilize a SmII-mediated cascade cyclization reaction to
simultaneously generate the C and D rings of 6; however, a
stepwise route was initially pursued in order to study the dia-
stereoselectivity and efficiency of the first cyclization step. To this
end, pent-4-enoic acid (18) was converted to pseudoephedrine
amide 19, which was alkylated with tert-butyl(2-iodoethoxy)-
dimethylsilane following Myers’s protocol19 to furnish amide
20 in 92% isolated yield and >20:1 dr (Scheme 3). Reductive
cleavage of amide 20 followed by treatment of the resulting alcohol
with iodine and triphenylphospine provided enantioenriched
alkyl iodide 21. After considerable optimization, we were pleased
to find that subjection of a mixture of lactone 12 and iodide 21 to
lithium hexamethyldisilazide (LHMDS) at 0 °C followed by
warming to room temperature furnished the alkylation product
22 in 63% yield as an inconsequential 1:1 mixture of diastereo-
mers. Selenation/oxidation of 22 provided the unsaturated
lactone, and chemoselective ozonolytic cleavage of the terminal
alkene delivered aldehyde 23.
equiv), 2,4,6-collidine HCl (2.5 equiv); not shown in Scheme 2]
3
furnished spirolactone 12 as a single diastereomer,17 albeit in a
modest 28% yield. The major byproduct of the reaction was
allylic alcohol 17, potentially resulting from Lewis acid-mediated
rearrangement of epoxide 13. A screen of reaction parameters
determined that use of 2,2,2-trifluoroethylacrylate, a more elec-
trophilic coupling partner, and the portionwise addition of
Cp2TiCl2 (1.6 equiv) to a suspension of Zn0 (1.5 equiv) in the
presence 2,4,6-collidine HCl (3.0 equiv) in tetrahydrofuran
3
(THF) provided lactone 12 in 74% yield. Under these condi-
tions, the formation of 17 was minimal. As expected, separation
of the anti and syn diastereomers of 13 and independent
subjection of each to the TiIII-mediated coupling conditions
provided lactone 12 as a single diastereomer in 75 and 68% yield,
With access to aldehyde 23, we were poised to study the
SmII-mediated reductive cyclization reaction (Scheme 3).20
Initial reactions conducted with SmI2 in THF at 0 °C resulted
in rapid decomposition of the starting material, although trace
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Scheme 4. Reductive Cascade Cyclization of Dialdehyde 11
Scheme 5. Completion of the Synthesis of (À)-Maoecrystal
Z (6)
and acetylation of the C11 carbinol were problematic, often
providing complex mixtures.26 In no case were we able to protect
the C6 carbinol selectively, acetylate the C11 carbinol, and then
reveal the C6 carbinol selectively in serviceable yields.
After considerable efforts to optimize conditions for selective
C11 monoacetylation, we elected to pursue instead the mono-
deacetylation of acetyl-maoecrystal Z (31). To this end, treat-
ment of 9 with acetic anhydride and catalytic TMSOTf provided
diacetate 30 in 74% yield (Scheme 5). Ozonolytic cleavage of the
alkene and treatment of the aldehyde with Eschenmoser’s salt27
and triethylamine gave enal 31. Exposure of 31 to sodium hydroxide
in aqueous methanol delivered maoecrystal Z (6) in 38% yield.
Characterization data obtained for synthetic 6 were fully con-
sistent with the data for the natural compound reported by Han
and co-workers.1 The modest yield of the final step was due to
competitive bis-deacetylation to the corresponding diol as well as
monodeactylation of the C11 acetate.28 Notably, this synthesis
proceeded in just 12 synthetic steps from (À)-γ-cyclogeraniol (15).
In summary, the first total synthesis of (À)-maoecrystal Z has
been described. The key steps include a highly diastereoselective
TiIII-mediated reductive epoxide coupling and a SmII-mediated
reductive cascade cyclization. Collectively, these transformations
illustrate the utility of single-electron chemistry for the prepara-
tion of congested polycyclic systems bearing vicinal stereogenic
centers. Efforts to employ readily accessible spirolactone 12 in
the syntheses of additional seco-ent-kauranoid natural products,
such as trichorabdals A and B, are the subject of ongoing research
in our laboratory.
quantities of the desired product were observed. In order to
modulate the reactivity of SmI2, several additives were evaluated.
This screen revealed that treatment of 23 with SmI2 in the
presence of LiCl21 and t-BuOH at 0 °C provided the desired
tricycle 26 in 45% yield as a single diastereomer.22
The stereochemical outcome of this transformation can be
rationalized as the result of reaction through the ketyl conforma-
tion shown as 25. It is hypothesized that in the disfavored con-
formation 24, the samarium ketyl experiences a destabilizing
nonbonding interaction with the methylene of the spiro-fused
cyclohexane. In conformation 25, the potentially destabi-
lizing 1,3-diaxial interaction is alleviated by the fact that C7 is
sp2-hybridized.
Encouraged by the high diastereoselectivity observed in this
transformation, we turned our attention to the preparation of the
key dialdehyde double-cyclization substrate (Scheme 4). To this
end, spirolactone 12 was alkylated with iodide ent-2123 and
elaborated to enoate 28 by a selenation/selenoxide elimination
sequence analogous to that described above. Double deprotec-
tion of the silyl ethers was accomplished smoothly with fluoro-
silicic acid, and the corresponding diol was oxidized to dialde-
hyde 11 with DessÀMartin periodinane.24 In the event, exposure
of dialdehyde 11 to SmI2 and LiCl in the presence of t-BuOH at
À78 °C provided tetracyclic diol 9, albeit in low yield. In this
case, use of LiBr as an additive provided improved results,
delivering diol 9 in 54% yield. The stereochemistry of the
product was assigned by 1D and 2D NMR methods. Notably,
both the C6 and C11 carbinols possessed the correct relative
stereochemistry for advancement to 6. The major byproduct of
the reaction appeared to be that resulting from monocyclization
to form the D ring followed by protonation of the enolate.
Completion of the synthesis required acetylation of the C11
carbinol and installation of the enal. Unfortunately, treatment of
diol 9 with acetic anhydride and 4-dimethylaminopyridine
(DMAP) resulted in rapid monoacetylation of the C6 carbinol
(29, R1 = Ac, R2 = H). Use of excess reagent and longer reaction
times provided the corresponding diacetate along with an iso-
meric compound that appeared to result from skeletal rearrange-
ment.25 Attempts at sequential monoprotection of the C6 carbinol
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, char-
b
acterization and spectral data for all compounds, and crystal-
lographic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
reisman@caltech.edu
’ ACKNOWLEDGMENT
We thank Dr. Michael Day and Mr. Larry Henling for X-ray
crystallographic structure determination and Dr. David Vander-
Velde for assistance with NMR structure determination. We
thank Prof. Brian Stoltz and the Caltech Center for Catalysis and
Chemical Synthesis for access to analytical equipment and Dr. Scott
Virgil for insightful discussions. The Bruker KAPPA APEXII
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COMMUNICATION
X-ray diffractometer was purchased through an award to the
California Institute of Technology by the NSF CRIF program
(CHE-0639094). Financial support from the California Institute
of Technology is gratefully acknowledged.
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assigned by extensive characterization using 1D and 2D NMR techni-
ques (1H-1H COSY, HMQC, HMBC, and NOESY) and three-bond
1
1HÀ H coupling constant analysis (see the SI).
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