Formal synthesis of Leucascandrolide A

Article abstract of DOI:10.1002/anie.200351817

Asymmetric allylation reactions of stannane-derived allyl diazaborolanes, and the use of the Terashima reagent for the highly selective asymmetric hydride reduction of a β-alkoxy ketone are key features of a highly convergent, stereocontrolled formal synt

Full text of DOI:10.1002/anie.200351817

Communications
Macrolide Synthesis
Formal Synthesis of Leucascandrolide A**
David R. Williams,* Scott V. Plummer, and
Samarjit Patnaik
Extracts from the calcareous sponge Leucascandra caveolata
were collected from waters along the northeastern coast of
New Caledonia by Pietra and co-workers, and yielded the
novel marine macrolide leucascandrolide A (1).[1] The struc-
ture of 1 was elucidated by extensive 2D NMR experiments
and analysis of Mosher ester derivatives, which revealed an
18-membered macrolide containing two bridging trisubsti-
tuted tetrahydropyran rings and an ester side chain with a Z-
a,b-unsaturated oxazole. The natural product displayed
significant in vitro cytotoxicity against both KB oral epider-
moid carcinoma and P388 leukemia cell lines (IC50 = 0.05 and
0.25 mgmLꢀ1, respectively), as well as antifungal activity
against the pathogenic yeast Candida albicans. Attempts to
isolate additional quantities of the natural product through
subsequent harvesting of sponge samples proved unfruitful,
which led to the suggestion that 1 may be derived from the
[*] Prof. D. R. Williams, S. V. Plummer, S. Patnaik
Department of Chemistry, Indiana University
800 East Kirkwood Avenue, Bloomington, IN, 47405 (USA)
Fax: (+1)812-855-8300
E-mail: williamd@indiana.edu
[**] Generous financial support for this research was provided by the
NIH (GM-42897).
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DOI: 10.1002/anie.200351817
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metabolism of microbial species harbored within the
sponge.[2] The biological activity and complex molecular
architecture of 1 have provided the impetus for several studies
toward its synthesis.[3] Recently, total[4] and formal syntheses[5]
of leucascandrolide A have been reported by several groups.
Herein we describe the culmination of our efforts, which have
led to a convergent, highly stereocontrolled formal synthesis
of 1 in the preparation of the macrolactone 2.
Our retrosynthetic analysis of the leucascandrolide A
macrolactone 2 inspired an asymmetric allylation strategy
based on the nonracemic aldehyde 3 and the complex
optically active allyl stannane 4 (Scheme 1). This efficient
ꢀ
C9 C10 bond construction would directly incorporate all the
carbon atoms necessary for macrocycle formation, and the
major stereochemical features associated with the macrolide
2. Late-stage incorporation of the C18–C23 side chain
through an alkenyl zinc addition, and subsequent formation
of the allylic alcohol at C17 through an asymmetric hydride
reduction would precede the final macrolactonization step.
The preparation of the C1–C9 aldehyde 3 commenced
with the conversion of the known epoxide 5 (readily available
from (+ )-epichlorohydrin)[6] into the allyl silane 6 through
the copper-catalyzed addition of the Grignard reagent
Scheme 2. Reagents and conditions: a) Mg0, THF, (2-bromoallyl)tri-
methylsilane; then 5, CuI, ꢀ508C; ꢀ50!ꢀ108C, 2 h; 79%; b) TBSCl,
imidazole, DMF; 100%; c) NBS, propylene oxide, CH2Cl2/DMF (2:3),
ꢀ788C; d) Bu3SnLi, CuBr·DMS, THF, ꢀ788!ꢀ408C; 77% (2 steps);
e) the (S,S)-1,2-diamino-1,2-diphenylethane bis(sulfonamide), BBr3,
CH2Cl2, 08C, 1 h; then 7, room temperature, 10 h; then 8, ꢀ788C,
1.5 h; 100%, d.r. 11:1; f) TsCl, Et3N, DMAP, CH2Cl2; 100%; g) HF·pyr,
CH3CN; 99%; h) NaH, PhH, 908C; 75%; i) MeI, CaCO3, CH3CN/H2O
(9:1), 16 h; 100%. DMAP=4-(dimethylamino)pyridine, DMF=N,N-
dimethylformamide, DMS=dimethyl sulfide, NBS=N-bromosuccini-
mide, pyr=pyridine, TBS=tert-butyldimethylsilyl, Ts=p-toluenesul-
fonyl.
prepared
from
2-bromo-3-trimethylsilylpropene
(Scheme 2).[7] Subsequent protection of the resulting homo-
allylic alcohol gave the TBS ether 6. Treatment of 6 with
freshly recrystallized NBS at ꢀ788C led to the immediate
formation of the labile corresponding allylic bromide, which
was displaced directly with a tributylstannylcuprate to give
the allyl stannane 7. Asymmetric allylation was effected
following the tin-to-boron transmetalation of the allylstan-
nane 7 by using the boron bromide reagent developed by
Corey et al., which is derived from a (S,S)-1,2-diamino-1,2-
corresponding tosylate, removal of the silyl protecting group
at C7, and internal backside displacement to yield an 11:1
ratio of diastereomers, which were separated by flash
chromatography. Mild hydrolysis of the dithiane was pro-
moted by methylation of the sulfur atoms,[11] which provided
the aldehyde 3 in excellent overall yield.
[8]
diphenylethane bis(sulfonamide)and boron tribromide.
Nucleophilic addition to the aldehyde 8[9] provided the
S homoallylic alcohol 9 as the major component of a mixture
of diastereomers epimeric at C3 (100%, d.r. 11:1).[10] Ring
closure of 9 to afford the 2,6-cis-tetrahydropyranyl moiety of
3 was carried out by conversion of the alcohol at C3 into the
Preparation of the novel allyl stannane 4 began with the
copper(i)-catalyzed addition of the Grignard reagent derived
from (2S)-1-bromo-2,3-dimethyl-3-butene 10[12] to the non-
racemic epoxide 11,[13] which led directly to the alcohol 12 in
80% yield (Scheme 3). Protection of the secondary
alcohol in 12 as its TBS ether and subsequent oxidative
cleavage of the terminal olefin by ozonolysis gave the
methyl ketone 13 in excellent yield. Conversion of 13
into the corresponding enol triflate through kinetic
deprotonation at low temperature and sulfonate for-
mation by use of the Comins reagent[14] was followed
by nickel-catalyzed cross-coupling with (trimethylsi-
lylmethyl)magnesium chloride to provide the allyl
silane 14.[15] As we have noted previously,[10] the
transmetalation of allyl silanes to allyl boranes tends
to be ineffective. Thus, conversion of 14 into the allyl
stannane, as described for 7, led to the C10–C17 allyl
stannane 4 in 77% yield after flash silica-gel chroma-
tography.
Our convergent strategy toward 2 was to examine
the selective introduction of asymmetry at C9. This
feature was of particular interest for our studies of
asymmetric allylation owing to the presence of adja-
cent asymmetry at C12 in the allyl component 4, and
Scheme 1. Retrosynthetic analysis of the leucascandrolide A macrolactone 2.
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the anti-Felkin product 18 in high yield (95%). To ensure
formation of the 2,6-trans-tetrahydropyran, tosylation of the
alcohol 18 was followed by selective removal of the TBS
groups and treatment with sodium hydride. The resulting
internal backside displacement led to the exclusive formation
of the desired six-membered ring, and Dess–Martin oxida-
tion[19] of the primary alcohol yielded the aldehyde 19.
To achieve our final objective we required efficient
incorporation of the C18–C23 carbon chain and flexibility
for the development of stereogenicity at C17.[20] To this end,
the hydrozirconation of 4-methyl-1-pentyne with the
Schwartz reagent was followed by transmetalation with
dimethylzinc as adapted from the reports of Wipf et al.
Scheme 3. Reagents and conditions: a) Mg0, THF, 10; then
CuBr·DMS, THF, ꢀ788C; then 11, THF, ꢀ788C!RT; 80%;
b) TBSCl, imidazole, DMF; 96%; c) O3, CH2Cl2/MeOH (1:1),
ꢀ788C; then DMS, ꢀ788C!RT; 98%; d) KHMDS, THF,
ꢀ788C; then Comins reagent, THF; 87%; e) [Ni(acac)2],
TMSCH2MgCl, THF, room temperature; 75%; f) NBS, propyl-
ene oxide, CH2Cl2/DMF (3:2), ꢀ78!ꢀ108C; g) Bu3SnLi,
CuBr·DMS, THF, ꢀ788!ꢀ408C; 77% (2 steps). acac=acetyl-
acetone, Comins reagent=2-[N,N-bis(trifluoromethylsulfonyl)-
amino]-5-chloropyridine, HMDS=hexamethyldisilazide.
the possible formation of diastereotopic transition
states in the reaction of the intermediate chiral allyl
borane with the aldehyde 3.[16] The homoallylic
alcohol (9R)-16 (95%, d.r. 8.5:1)was prepared by
transmetalation of optically pure 4 with the R,R
bromoborane 15 to yield an intermediate allylic
borane for low-temperature condensation with the
tetrahydropyranyl aldehyde
3 (Scheme 4). Facial
selectivity of the nucleophilic addition was dominated
by the chiral auxiliary, and the diastereomers were
readily separated by flash chromatography. Methyl-
ation of the homoallylic alcohol at C9 in 16 was
followed by oxidative cleavage (OsO4, NMO; NaIO4)
to provide the corresponding diketone 17. L-Selec-
tride promoted selective reduction at C5 of the
tetrahydropyranone 17, which led to the correspond-
ing axial alcohol in 84% yield (d.r. > 95:5), and this
alcohol was protected as its TBDPS ether. Attention
was then focused on the asymmetric reduction of the
ketone at C11. The use of the Terashima reagent[17] for
selective aluminum hydride reductions of acyclic b,b’-
dialkoxy ketones has been reported recently from
these laboratories.[18] In this case, the Terashima
reduction of the ketone at C11 in the presence of
the ligand (ꢀ)-N-methylephedrine resulted in an
effective reagent-based hydride addition with excel-
lent diastereofacial selectivity (d.r. > 95:5)to provide
Scheme 4. Reagents and conditions: a) the (R,R)-1,2-diamino-1,2-diphenyl-
ethane bis(sulfonamide), BBr3, CH2Cl2, 08C, 1 h; then 4, room temperature,
10 h; then 3, ꢀ788C, 1.5 h; 96%, d.r. 8.5:1; b) Me3OBF4, proton sponge, 4-Š
molecular sieves, CH2Cl2; 96%; c) OsO4, NMO, acetone/H2O (2:1), 16 h;
d) NaIO4, THF/phosphate buffer (pH 7; 1:1), 16 h; 80% (2 steps); e) L-Selec-
tride, THF, ꢀ788C, 1.5 h; 84%; f) TBDPSCl, imidazole, DMF, 40 h; 73%;
g) LiAlH4 (2.0 equiv), (ꢀ)-N-methylephedrine (2.0 equiv), N-ethylaniline
(4.0 equiv), Et2O, ꢀ788C, 2 h; 95%; h) Ts2O, pyridine, CH2Cl2; 100%; i) HF·pyr,
pyridine, THF; 84%; j) NaH, PhH, 608C, 16 h; 73%; k) Dess–Martin periodi-
nane, NaHCO3, CH2Cl2; 95%. NMO=N-methylmorpholine N-oxide, L-Selec-
tride=lithium tri-sec-butylborohydride, TBDPS=tert-butyldiphenylsilyl.
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the efficient, convergent construction of complex molecules.
Our studies have also provided unprecedented results for the
use of the Terashima hydride reduction in the stereoselective
formation of saturated, acyclic alcohols. Overall, we have
carried out an efficient synthesis of the leucascandrolide A
macrolactone 2 with a high level of stereoselectivity. Further
studies are in progress in our laboratory.
Received: May 6, 2003 [Z51817]
Keywords: allylation · antitumor agents · asymmetric synthesis ·
.
macrolide · natural products
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Scheme 5. Reagents and conditions: a) 4-methyl-1-pentyne, CH2Cl2,
Cp2Zr(H)Cl, room temperature; then Me2Zn, ꢀ788C; then 19, ꢀ788!
08C, 1 h; 87%; b) Dess–Martin periodinane, NaHCO3, CH2Cl2; 75%;
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d) Ac2O, pyridine, DMAP, CH2Cl2; 97%; e) DDQ, CH2Cl2/phosphate
buffer (pH 7)/tBuOH (40:10:1), 1.5 h; quant.; f) Dess–Martin periodi-
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aqueous tBuOH, 08C, 45 min; 56% (2 steps); h) K2CO3, MeOH, 16 h;
i) 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, benzene; 63%
(2 steps); j) TBAF, THF; 67%. DDQ=2,3-dichloro-5,6-dicyano-1,4-ben-
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2-lithio-1,3-dithiane
with
(R)-epichlorohydrin
(97% ee,
(Scheme 5).[21] Reactions of the resulting alkenyl zinc species
with the aldehyde 19 resulted in the efficient formation of a
mixture of diastereomeric allylic alcohols (1:1 ratio), which
was oxidized directly to the enone 20 (75% yield over two
steps). Interestingly, the asymmetric hydride reduction of the
a,b-unsaturated ketone 20 under Terashima conditions gave
rise to conjugate reduction as a major reaction pathway. This
result is atypical as the Terashima conditions are generally
utilized for the production of chiral allylic alcohols. However,
Corey–Bakshi–Shibata (CBS)borohydride reduction [22] of 20
with the CBS reagent (S)-2-methyloxazaborolidine in the
presence of borane–tetrahydrofuran complex gave an 89%
yield of a 5:1 mixture of separable diastereomers, epimeric at
C17, in favor of the R alcohol 21. Acetylation to 22 was
followed by oxidative deprotection of the alcohol at C1. The
seco-acid 23 was obtained by oxidation of the resulting
primary alcohol to the carboxylic acid and subsequent basic
methanolysis of the acetate at C17. The crude product was
subjected to the Yonemitsu-modified Yamaguchi[23] protocol
to give the macrolide in good yield (63% over two steps).
Finally, deprotection of the alcohol at C5 by treatment with
fluoride provided the leucascandrolide A macrolactone 2,
whose physical and spectroscopic data were identical in all
respects to those previously reported.[4a] Leighton and co-
workers have also described the conversion of 2 into
leucascandrolide A, and thus, our efforts constitute a formal
synthesis of the natural product 1.
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In summary, our investigations into asymmetric allylation
methodology have extended this fundamental technique to
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found [Ni(acac)2] to be an efficient catalyst for the conversion of
the enol triflate into the allyl silane 14.
[16] A complete study of matched and mismatched elements of
stereochemistry will be detailed in further reports devoted to the
asymmetric-allylation process.
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macrocyclization with inversion of configuration at C17. For
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