Decarboxylative Grob-type fragmentations in the synthesis of trisubstituted Z olefins: Application to peloruside A, discodermolide, and Epothilone D

Full text of DOI:10.1002/anie.200901740

Communications
DOI: 10.1002/anie.200901740
Olefin Synthesis
Decarboxylative Grob-Type Fragmentations in the Synthesis of
Trisubstituted Z Olefins: Application to PelorusideA, Discodermolide,
and EpothiloneD**
Kathrin Prantz and Johann Mulzer*
Dedicated to Professor Albert Eschenmoser
The stereocontrolled synthesis of methyl branched trisubsti-
tuted Z olefins has been pursued intensively during the past
few years, as this structural motif is found in numerous
polyketides with promising anticancer activity, such as
epothilone D (1), discodermolide (2), and peloruside A (3).
Scheme 1. Grob-type hydroxide-induced fragmentation of lactone 4.
Ms=methanesulfonyl.
of a hydroxide ion leads to the tetrahedral intermediate 5,
which undergoes fragmentation to form olefin 6 stereo-
unambiguously. In the preparation of lactone 4, three
stereogenic centers, one of which is quaternary, must be
generated with the relative configurations indicated. For
stereoelectronic reasons, clean fragmentation can be expected
when the hydroxide ion attacks axially and the lactone adopts
a chair conformation with the OMs substituent in an
equatorial position.[5] This conformation may be facilitated
by introducing a bulky residue R2 cis to the OMs group.
To prove the feasibility of the approach a simplified
racemic test system was developed (Scheme 2). Lactone 8 was
As a result of these extensive efforts, a variety of approaches
have emerged based on carbonyl olefination, olefin meta-
thesis, alkyne functionalization, allylic rearrangements, and
cross-coupling reactions.[1] As most of these protocols make
use of expensive and/or toxic reagents, we wondered why E2
elimination reactions, and in particular Grob fragmentations
with their simple operability and virtually “green” conditions,
have been neglected in the construction of acyclic olefins.[2]
After all, Grob fragmentations have been rediscovered for
the generation of cycloolefins.[3]
Herein we report a novel hydroxide-induced decarbox-
ylative Grob-type fragmentation and its application to the
Scheme 2. Model study. a) LDA, HMPA, then PhCHO, THF, ꢀ788C;
synthesis of compounds 1–3.[4] Our strategy was centered
1m KOH, then HCl, 08C, 64% (2 steps); b) [Pd(PPh3)4], allyl acetate,
K2CO3, BnEt3NCl, EtOAc/H2O, 98%, d.r. 4:1; c) NaBH4, MeOH, 08C,
around mesyloxy lactones such as 4 (Scheme 1). The addition
97%. d) MsCl, Et3N, Et2O, ꢀ108C; e) KOH, MeOH, 08C, >95% (2
steps). Bn=benzyl, HMPA=hexamethylphosphoramide, LDA=
lithium diisopropylamide.
[*] Mag. K. Prantz, Prof. J. Mulzer
Institute of Organic Chemistry, University of Vienna
Wꢀhringerstrasse 38, 1090 Vienna (Austria)
Fax: (+43)1-4277-52189
E-mail: johann.mulzer@univie.ac.at
Homepage: http://www.univie.ac.at/rg_mulzer/
prepared, and the quaternary center was introduced by a
biphasic Trost–Tsuji allylation.[6] After reduction of the
b-carbonyl group, diastereomer 10 was mesylated to give 11,
which was treated with sodium hydroxide in methanol at 08C
to give Z olefin 12 as the sole product in quantitative yield.
[**] K.P. thanks the Ernst Schering foundation for a doctoral fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901740.
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Encouraged by this result, we applied our method to the
synthesis of 1 (Schemes 3 and 4). To obtain a compound with
uniform chirality at C15, lactone 26 had to be prepared in a
diastereo- and enantioselective manner. We started with the
quaternary center,[7] which then served to control the
formation of the remaining centers by means of a stereo-
chemical relay. Hence, malonate 13 was hydrolyzed with pig
liver esterase (PLE) to provide the monocarboxylic acid 14
with 95% ee,[8] which was readily converted into silyl ether 15,
a general building block whose allylic appendage can be
modified in many ways. Thus, for the synthesis of 1, chain
elongation of 15 to give 16 was accomplished in high yields
either by cross-metathesis with 18 or by modified Julia
olefination with 19.[9,10] From 16, aldehyde 17 was available in
three simple steps with 83% overall yield.
Next, aldehyde 17 was activated with freshly prepared
.
MgBr2 Et2O and then added to the lithium enolate of ketone
20. Adduct 21 was obtained as single 13R diastereomer in
91% yield, presumably via the Felkin–Anh transition state 22.
Even with stronger Lewis acids, a chelated transition state
could not be induced. Substrate-controlled syn reduction with
catecholborane yielded the dihydroxy ester 23, whose sapo-
nification and lactonization led to lactone 24.[11] Clean
inversion at C13 was achieved by means of an oxidation–
reduction sequence to give 25. Subsequent mesylation and
fragmentation delivered the C7–C21 fragment 27 of epothil-
one D (16 steps, 27% overall yield). Compound 25 was
identical in every respect with an authentic sample,[12b] which
served as an intermediate in several approaches to 1.[12a–c]
To extend the scope of the fragmentation protocol, we
investigated lactone 24 (Scheme 5). Because of the axial
configuration of the 13-OH group,[13] a Grob-type fragmen-
Scheme 3. Conversion of malonate 14 into aldehyde 17. a) PLE, 0.05m
KH2PO4, 90%; b) ClC(O)OMe, Et3N, THF, 08C; NaBH4, MeOH, 08C,
75%; c) TESCl, py, RT, quant; d) Grubbs–Hoveyda cat., 18, CH2Cl2,
reflux, 93% or 1. O3, PPh3, PPTS, CH2Cl2, ꢀ788C, quant.; 2. LiHMDS,
19, THF, ꢀ788C to ꢀ308C, 96%; e) PPTS, MeOH, RT, 92%; f) PtO2,
H2, EtOAc, 99%; g) DMP, CH2Cl2, 08C, 91%. BT=2-benzothiazolyl,
DMP=Dess–Martin periodinane, HMDS=hexamethyldisilazane,
PPTS=pyridinium p-toluenesulfonate, py=pyridine, TBS=tert-butyldi-
methylsilyl, TES=triethylsilyl.
Scheme 5. Fragmentation of lactone 24. a) MsCl, Et3N, Et2O, 08C;
b) LiOH, THF, 08C, 38% (28) and 52% (29); c) DMF, reflux, 85%.
tation in a chair conformation should not be possible. On the
other hand, the boat conformation 30b might be suitable
stereoelectronically to undergo fragmentation although the
species is energetically unfavorable. Nevertheless, olefin 28
(38% yield) was obtained under the usual conditions, along
with b-lactone 29 (52% yield). This result may be rationalized
in terms of a ring opening of 24 to form carboxylate 31, which
undergoes both fragmentation to give E olefin 28 and SN2
cyclization to form the b-lactone 29.[14] Thermolysis of 29 also
gave 28, so that, overall, olefin 28 is obtained in E geometry
exclusively and acceptable yield. This result underlines the
versatility of the method, as both the Z and the E olefins are
available from lactone 24 by analogous routes.
Scheme 4. Synthesis of the C7–C21 fragment 27 of epothilone D.
a) LiHMDS, THF, ꢀ788C, then 17, MgBr2·Et2O, 91%; b) catecholbor-
ane, THF, ꢀ108C, 88%; c) LiOH, THF, 08C; d) EDC·HCl, DMAP,
CH2Cl2, 94% (2 steps); e) DMP, NaHCO3, CH2Cl2, 94%; f) NaBH4,
MeOH, ꢀ788C, 93%; g) MsCl, Et3N, Et2O, 08C; h) LiOH, THF, 08C,
81%; i) TBSOTf, 2,6-lutidine, CH2Cl2, RT, quant. DMAP=4-(dimethyl-
amino)pyridine, EDC=1-ethyl-3-(3-dimethylaminopropyl)carbodiimide,
OTf=trifluoromethanesulfonate.
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An additional degree of freedom lies in the configuration
of center C15 (Scheme 6). Thus, after the reduction of ketone
21 to anti-diol 32, lactones 33 and 34 were available.[15]
Fragmentation of 33 via the chair transition state cleanly led
Scheme 7. Synthesis of 47, the precursor for the fragmentation to give
discodermolide. a) Cy2BCl,Et3N, 39, then 38, Et2O, ꢀ788C to 08C,
76%; b) Me4NBH(OAc)3, CH3CN/AcOH, ꢀ308C, 94% (b.r.s.m.);
c) dimethoxypropane, CSA, CH2Cl2, RT, 86%; d) 35% HF·py, CH3CN,
RT, 97%; e) IBX, EtOAc, reflux, 86%; f) 2-methyl-2-butene, NaClO2,
NaH2PO4, tert-butyl alcohol/H2O, RT, quant.; g) CSA, CH2Cl2, RT, 83%.
b.r.s.m.=based on recovered starting material, CSA=camphorsulfonic
acid, Cy=cyclohexyl, IBX=2-iodoxybenzoic acid, PMB=para-methoxy-
benzyl.
Scheme 6. Fragmentation of lactones 33 and 34. a) Me4NBH(OAc)3,
CH3CN/AcOH, ꢀ308C, 87%; b) LiOH, THF, 08C; c) EDC·HCl, DMAP,
CH2Cl2, 85% (2 steps); d) DMP, NaHCO3, CH2Cl2, 94%; e) NaBH4,
MeOH, ꢀ788C, 90%; f) MsCl, Et3N, Et2O, 08C; g) LiOH, THF, 08C,
64% (35), 36% (36), 52% (37).
to the E olefin 35, whereas 34 gave b-lactone 36 and the
Z olefin 37, presumably via the carboxylate. Thus, four
diastereomers of the northern fragment of 1 are available
from intermediate 21 using the same fragmentation approach.
For approaching 2, the required quaternary center was
introduced via known aldehyde 40 (Scheme 7).[16] An anti-
selective aldol addition with ketone 41 was used to construct
lactone 44 stereoselectively.[17] To intersect Smithꢀs C9–C21
discodermolide intermediate 51 (Scheme 8),[18] lactone 42 was
fragmented to the Z olefin 44 and then transformed into the
Oppolzer sultam 46.[19] Asymmetric methylation followed by
reduction gave aldehyde 47, which was used in a syn-selective
Evans aldol addition. The remaining two stereocenters were
installed by a Roush crotylation.[20] Oxidative cleavage of the
terminal olefin, via the epoxide, led to the free 19,21-diol,
which was protected as the PMP acetal 51, whose analytical
data were in full agreement with those reported.[18]
For the synthesis of the peloruside A fragment 58
(Scheme 9), enantiomerically pure monocarboxylic acid 52
was converted to aldehyde 53.[21] Evans aldolization with
oxazolidinone 54 gave lactone 55,[22] whose fragmentation led
to the Z olefin 57 stereoselectively. For further manipulation,
the protecting groups were changed to give fragment 58.[23]
In conclusion we have shown that decarboxylative Grob
fragmentations are a versatile tool for the stereoselective
construction of chiral trisubstituted Z olefins. In contrast to
Scheme 8. Conversion to fragment 51 of discodermolide. a) MsCl,
DMAP, py, CH2Cl2, RT; b) LiOH, THF, RT, 88% (2 steps); c) TBSOTf,
2,6-lutidine, CH2Cl2, RT, quant.; d) mCPBA, NaOAc, CH2Cl2, ꢀ208C,
92%; e) HIO4·2H2O, THF/Et2O, 08C, 90%; f) 2-methyl-2-butene,
NaClO2, NaH2PO4, tert-butyl alcohol/H2O, RT, quant.; g) (1R)-cam-
phor-2,10-sultam, DIC, DMAP, CH2Cl2, RT, 96%; h) NaHMDS, MeI,
THF, ꢀ788C, 89%; i) DIBAL-H, CH2Cl2, ꢀ1008C, 94%; j) Bu2BOTf,
Et3N, 48, then 47, CH2Cl2, ꢀ788C to 08C, 65% (99% b.r.s.m.); k)
TBSOTf, 2,6-lutidine, CH2Cl2, RT, quant.; l) LiBH4, Et2O, MeOH, 08C,
86%; m) IBX, DMSO, RT; n) (R,R)-diisopropyl tartrate (E)-crotylboro-
nate, toluene, ꢀ78!08C, 87%; o) [VO(acac)2], tBuOOH, CH2Cl2, 08C,
87% (2 steps); p) HIO4·2H2O, Et2O/THF, 08C, 51% b.r.s.m.; q) anis-
aldehyde dimethyl acetal, CSA, CH2Cl2, RT, 86%. acac=acetylaceto-
nate, DIBAL-H=diisobutylaluminum hydride, DIC=N,N’-diisopropyl-
carbodiimide, mCPBA=m-chloroperbenzoic acid, PMP=p-methoxy-
phenyl.
conventional syntheses of such systems, which put the
olefination step at the end of the sequence, we start with
the formation of the olefin moiety from chiral aldehydes such
as 17, 38, and 53, and use their stereogenic information for the
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Scheme 9. Synthesis of the C15–C19 fragment of peloruside A.
a) ClC(O)OMe, Et3N, THF, 08C; NaBH4, MeOH, 08C, 83%; b) IBX,
DMSO, RT, 80%; c) Bu2BOTf, Et3N, 56, then 55, CH2Cl2, ꢀ78!08C,
85%; d) LiBH4, Et2O, MeOH, 08C, 80%; e) K2CO3, MeOH, RT, 1n
HCl, quant.; f) MsCl, DMAP, CH2Cl2, RT, 99%; g) LiOH, dioxane, RT,
83%; h) BnBr, Ag2O, TBAI, quant.; i) TFA, CH2Cl2, RT, 91%. TBAI=
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connectivity and compatibility with aldol reactions, high
stereocontrol, mild conditions, and simple reagents. Most
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intermediates do not require purification. Studies on the
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Received: March 31, 2009
Published online: June 2, 2009
Keywords: aldol reaction · antitumor agents · lactones ·
.
natural products · quaternary stereocenters
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