Supported synthesis of oxoapratoxin A

Article abstract of DOI:10.1021/jo900583j

(Chemical Equation Presented) A new synthesis of an oxazoline analogue of apratoxin A has been performed using a solid support. The efficient synthesis of the polyketide part on gram scale and the serine vinylogue is described. The use of chlorotrityl res

Full text of DOI:10.1021/jo900583j

Supported Synthesis of Oxoapratoxin A
Arnaud Gilles, Jean Martinez, and Florine Cavelier*
IBMM, UMR-CNRS 5247, UniVersite´s Montpellier 1 & 2, place Euge`ne Bataillon,
34095 Montpellier cedex 5, France
florine@uniV-montp2.fr
ReceiVed March 17, 2009
A new synthesis of an oxazoline analogue of apratoxin A has been performed using a solid support. The
efficient synthesis of the polyketide part on gram scale and the serine vinylogue is described. The use of
chlorotrityl resin allowed the construction of two linear precursors corresponding to two different
cyclization sites. This study describes a facile synthesis of analogues for future SAR studies of this
potent antitumor compound.
Introduction
Apratoxins A, B, C1,2 and the more recently discovered E
and D3,4 forms are cyclodepsipeptides of marine origin that
present antitumoral activity. These molecules possess a common
structure, including a tetrapeptide and a polyketide part, bridged
by a thiazoline ring on one side and by an ester bond on the
other side (Figure 1).
The mode of action of the highly cytotoxic apratoxin A still
remains to be fully elucidated.5 However, it has attracted much
attention, and as such the total synthesis of apratoxin A became
a competitive challenge. The more accessible oxazoline analogue
FIGURE 1. Apratoxins.
of apratoxin A exhibits excellent antitumor activity9 and thus
became our synthetic target. Three total syntheses of apratoxin
A,6-8 one total synthesis of its oxazoline analogue,9 and one
synthesis of its polyketide part10 have been achieved to date,
(1) Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J.; Corbett, T. H.
J. Am. Chem. Soc. 2001, 123, 5418–5423.
(2) Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J. Bioorg. Med.
Chem. 2002, 10, 1973–1978.
(3) Luesch, H.; Matthew, S.; Schupp, P. J. J. Nat. Prod. 2008, 71, 1113–
1116.
(4) Gutie´rrez, M.; Suyama, T. L.; Engene, N.; Wingerd, J. S.; Matainaho,
T.; Gerwick, W. H. J. Nat. Prod. 2008, 71, 1099–1103.
(5) Luesch, H.; Chanda, S. K.; Raya, R. M.; DeJesus, P. D.; Orth, A. P.;
Walker, J. R.; Izpisua Belmonte, J. C.; Schultz, P. G. Nat. Chem. Biol. 2006, 2,
158–167.
(6) Chen, J.; Forsyth, C. J. J. Am. Chem. Soc. 2003, 125, 8734–8735.
(7) Doi, T.; Numajiri, Y.; Munakata, A.; Takahashi, T. Org. Lett. 2006, 8,
531–534.
(8) Ma, D.; Zou, B.; Cai, G.; Hu, X.; Liu, J. O. Chem.sEur. J. 2006, 12,
7615–7626.
(9) Zou, B.; Wei, J.; Cai, G.; Ma, D. Org. Lett. 2003, 5, 3503–3506.
(10) Xu, Z.; Chen, Z.; Ye, T. Tetrahedron: asymmetry 2004, 15, 355–363.
4298 J. Org. Chem. 2009, 74, 4298–4304
10.1021/jo900583j CCC: $40.75 2009 American Chemical Society
Published on Web 05/08/2009

Supported Synthesis of Oxoapratoxin A
SCHEME 1. Retrosynthesis
all involving formation of the heterocycle before macrocycliza-
tion and the same macrocyclization site, between N-Me-Ile and
Pro, with a high risk of epimerization at the N-Me-Ile residue.
Although it is well-known that such coupling reaction generates
epimerization at the CR center of the C-terminal amino acid,
especially when N-methylated amino acid residues are in-
volved,11 the risk of epimerization during macrocyclization was
not mentioned in the described syntheses. Herein, we report a
more versatile total synthesis of the oxazoline analogue of
apratoxin A involving the efficient use of chemistry on solid
support,12 providing a simpler method for further structure-
activity relationship studies. This strategy allowed us to
investigate three different cyclization sites where no epimer-
ization could occur. As outlined in the retrosynthetic scheme
(Scheme 1), the 25-membered macrocycle was closed by
lactamization at two different sites or by lactonization. We saw
here the opportunity to use supported peptide synthesis meth-
odology to supply a convergent and more convenient synthesis
of apratoxin A. Thus, the linear precursors were obtained from
three building blocks further coupled together.
somers (dr ) 95:5)15,16 in 69% yield. Reaction of the diol 2
with thionyl chloride in pyridine at 0 °C afforded a diastere-
oisomeric mixture of the cyclic sulfite 3 in quantitative yield.
Then the sulfite was oxidized into its corresponding sulfate 4
using RuCl3 as oxidative agent and NaIO4 as co-oxidant in a
heterogeneous solvent mixture (CCl4, ACN, H2O).17 The sulfate
4 was then subjected to regioselective substitution, which was
the first key step in the synthesis. The less sterically hindered
position was preferred for substitution by the allyl group using
the corresponding Grignard reagent, with complete inversion
of configuration. After sulfate hydrolysis and purification, the
desired allylic compound 5 was obtained in 90% yield. Silylation
of the resulting secondary alcohol with TESCl and Et3N
proceeded smoothly to give 6, followed by oxidative cleavage
of the terminal double bond with ozone to yield the correspond-
ing aldehyde 7. The second crucial step was the formation of
the last two stereogenic centers. Unfortunately, various attempts
to obtain the condensation product from the aldehyde using an
anti-Evans aldol type of reaction18 were unsuccessful and led
to the complete degradation of the substrate. A different strategy
using Brown’s crotylation reaction with diisopinocampheylbo-
rane yielded the homoallylic alcohol 8 in 93% yield as two
separable diastereoisomers (dr ) 9:1).19 After protection of 6
as a silyl ether using TBDMS triflate and imidazole in DCM to
give 9, ozonolysis of the allylic function afforded the aldehyde
10 in 88% yield, which was oxidized with NaClO2 quantitatively
to give the corresponding acid 11.20 Selective deprotection of
the TES-protected alcohol under mild acidic conditions (AcOH,
DCM, MeOH, H2O) led to the desired polyketide 12 in 88%
yield. The polyketide moiety was obtained from the pivalalde-
Results and Discussion
The first challenge in this synthesis was the preparation of
the polyketide moiety, requiring introduction of four stereo-
chemical centers, two alcohol functions, and one carboxylic acid
function.
The synthesis was achieved in 12 steps starting from
commercially available pivalaldehyde, as outlined in Scheme
2. List’s aldolization between pivalaldehyde and acetone using
proline as chiral organocatalyst gave the corresponding hydrox-
yketone 1,13 which was reduced by NaBH4 under Prasad reaction
conditions (Et3BOMe, MeOH, THF, -78 °C).14 The pure syn
1,3-diol 2 was isolated after separation of the two diastereoi-
(15) Relative configurations were assigned by 13C NMR of their correspond-
ing acetonide following Rychnovsky’s method (ref 16).
(16) Rychnovsky, S. D.; Rogers, B.; Yang, G. J. Org. Chem. 1993, 58, 3511–
3515.
(17) Byun, H. S.; He, L.; Bittman, R. Tetrahedron 2000, 56, 7051–7091.
(18) Walker, M. A.; Heathcock, C. H. J. Org. Chem. 1991, 56, 5747–5750.
(19) Brown, H. C.; Bhat, K. S. J. Am. Chem. Soc. 1986, 108, 293–294.
(20) Bal, B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981, 37,
2091–2096.
(11) Teixido, M.; Albericio, F.; Giralt, E. J. Pept. Res. 2005, 65, 153–166.
(12) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–54.
(13) List, B.; Lerner, R. A.; Barbas, C. F. J. Am. Chem. Soc. 2000, 122,
2395–2396.
(14) Chen, K. M.; Gunderson, K. G.; Hardtmann, G. E.; Prasad, K.; Repic,
O.; Shapiro, M. J. Chem. Lett. 1987, 1923–1926.
J. Org. Chem. Vol. 74, No. 11, 2009 4299

Gilles et al.
SCHEME 2. Polyketide Synthesis
persilylated intermediate.23 Selective hydrolysis of the silylated
ester with potassium carbonate completed the synthesis of 15,
which was obtained in 80% yield.
SCHEME 3. Vinylic Homologation of Serine
With those two building blocks and the four amino acid
derivatives in hand, it was then possible to perform the precursor
elongation using the appropriate resin support, i.e., the chlori-
nated chlorotrityl resin. This method allowed us to direct the
synthesis toward different linear precursors. The cyclization site
depends on which carboxylic acid was first anchored to the
polymeric support.
When the peptide was first synthesized on the resin, followed
by the vinylogue moiety and then the polyketide part, the
macrocycle should close via the ester bond, but the lactonization
reaction between polyketide and peptide moieties, carried under
various conditions, failed to yield the expected compound, as
already reported by Forsyth.24 Thus, we decided to focus our
efforts on the two lactamization ring closures involving precur-
sors 21 and 24.
For the linear precursor 21 (Scheme 4), the vinylic analogue
of serine was anchored first on the support, followed by removal
of the Fmoc protecting group and coupling of the polyketide
12 in the presence of HATU or BOP as coupling reagent, leading
to 16.25 In the case of precursor 24 (Scheme 5), the polyketide
moiety was attached first.
hyde in 19% overall yield in 12 steps. This synthesis has been
carried out on gram scale.
The synthesis of the vinylic anologue of serine 16 started
from the corresponding commercially available protected serine
(Scheme 3). Conversion of the N-Fmoc-protected serine into
its Weinreb amide proceeded efficiently using BOP as the
coupling reagent, followed by treatment with LiAlH4 to yield
the corresponding aldehyde 13, which was used without further
purification to avoid epimerization at its CR chiral center.21
Wittig condensation of the aldehyde 13 with the preformed
phosphorane in DCM furnished the (E)-vinylic analogue (or
vinylogue) of protected serine 14 with 67% yield in two steps.22
The last step of this building block synthesis was deprotection
of the carboxylic acid. To be consistent with our protection
strategy, the t-Bu ether of the serine vinylogue was also replaced
by its corresponding silylated alcohol. Displacement of the two
t-Bu groups with TBSOTf under basic conditions gave the
For both supported elongations, esterification of the supported
fragment was performed under Yamaguchi conditions,26 using
5 equiv of proline mixed anhydride (obtained from proline and
2,4,6-trichloro-benzoyl chloride) and DMAP to give 17 and 23.
All successive coupling reactions on both linear intermediates
(23) Franck, X.; Figadere, B.; Cave, A. Tetrahedron Lett. 1995, 36, 711–
714.
(24) Chen, J.; Forsyth, C. J. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 12067–
12072.
(25) For a recent review on peptide coupling reagents, see: Han, S. Y.; Kim,
Y. A. Tetrahedron 2004, 2447–2467.
(26) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989–1993.
(21) Fehrentz, J. A.; Castro, B. Synthesis 1983, 676–678.
(22) Hagihara, M.; Anthony, N. J.; Stout, T. J.; Clardy, J.; Schreiber, S. L.
J. Am. Chem. Soc. 1992, 114, 6568–6570.
4300 J. Org. Chem. Vol. 74, No. 11, 2009

Supported Synthesis of Oxoapratoxin A
SCHEME 4. First Linear Precursor Elongation
SCHEME 5. Second Linear Precursor Elongation
with N-methyl amino acids required the use of HATU, since
the use of BOP gave deletion peptides resulting from the lower
reactivity of N-methylated amino acids.11 After proline amino
group deprotection, the linear precursors elongation was achieved
by three coupling-deprotection sequences using N-Fmoc-N-
Me-Ile (intermediate 18), N-Fmoc-N-Me-Ala (intermediate 19),
and N-Fmoc-Tyr(O-Me) (intermediate 20) for the linear precur-
sor 21 and with one more coupling followed by Fmoc
deprotection of the serine analogue 15 for the linear precursor
24 (supported intermediates are not represented here). The two
linear precursors were then obtained after cleavage from the
solid support under mild acidic conditions (acetic acid in DCM
with TFE). Macrocyclization was attempted from these two
linear precursors. Unexpectedly, it appeared that the TBS
protecting group of the secondary alcohol had to be removed
prior to macrolactamization, otherwise the reaction did not occur.
This deprotection reaction was carried out by simply dissolving
the linear precursor in a 0.1% TFA in acetonitrile solution
followed by evaporation under reduce pressure. This operation
was repeated until complete alcohol deprotection. Under these
conditions, both primary and secondary TBDMS protected
alcohols were released.
The deprotected linear precursors 22 and 25 were then
subjected to macrolactamization using HATU as the coupling
reagent to yield the macrocyclic compound 26 in 40% to 45%
yield after purification by preparative HPLC (the use of BOP
or DPPA also led to the desired macrocycle).24 DAST was used
successfully as the cyclodehydrating agent for conversion of
the serine moiety into its corresponding oxazoline to yield
oxoapratoxin in quantitative yield.27 Formation of the oxazoline
prior to macrocyclization also led to the oxoapratoxin without
any noticeable improvement.
In conclusion, we described here an efficient synthesis on
solid support of the oxazoline analogue of apratoxin A, including
the synthesis on gram scale of the polyketide moiety. We
performed the macrolactamization from two different linear
precursors without epimerization, using different types of
(27) Lafargue, P.; Guenot, P.; Lellouche, J.-P. Heterocycles 1995, 41, 947–
958.
J. Org. Chem. Vol. 74, No. 11, 2009 4301

Gilles et al.
SCHEME 6. Lactamization and Oxazoline Formation
25.5 (CH3), 34.8 (C), 38.7 (CH2), 69.3 (CH), 81.0 (CH). anti isomer:
20
1
[R]
) -38.2 (c 1.57, CHCl3). Mp: 86 °C. H NMR (CDCl3,
D
300 MHz) δ (ppm): 0.93 (s, 9H), 1.31 (d, J ) 6.4 Hz, 3H),
1.55-1.65 (m, 2H), 2.0-2.1 (broad m, 2H), 3.60 (m, 1H), 4.19
(m, 1H). 13C NMR (CDCl3, 75 MHz) δ (ppm): 23.2 (CH3), 25.7
(CH3), 34.6 (C), 38.9 (CH2), 65.5 (CH), 75.7 (CH). [R]20D and NMR
values are consistent with those given in the literature.28,29
(4S,6S)-4-tert-Butyl-6-methyl-2-oxo-1,3,2-dioxathiane (3). To
a solution of diol 2 (995 mg, 6.8 mmol) in pyridine (30 mL) at 0
°C was added dropwise thionyl chloride (2.5 mL, 34 mmol). After
being stirred for 30 min at 0 °C, the mixture was diluted with water,
and the aqueous layer extracted 3 times with DCM. The organic
layer was washed 2 times with KHSO4 (1 M), water, and saturated
NaHCO3 and dried over Na2SO4. After evaporation, the pure
(4S,6S)-4-tert-butyl-6-methyl-2-oxo-1,3,2-dioxathiane (cyclic sulfite)
was obtained as white crystals in 96% yield (1.26 g). The two
diastereoisomers were isolated by silica gel chromatography for
characterization. Rf (hexane/AcOEt 8:2) 0.23 and 0.37. First
diastereoisomer (slow elution) (Rf 0.23): [R]20D ) -27 (c 1, CHCl3).
1
Mp: 70.5 °C. H NMR (CDCl3, 300 MHz) δ (ppm): 0.88 (s, 9H),
1.25 (d, J ) 6.3 Hz, 3H), 1.60-1.85 (m, 2H), 4.57 (dd, J ) 11.6
Hz, 2.4 Hz, 1H), 4.94-5.07 (m, 1H). 13C NMR (CDCl3, 75 MHz)
δ (ppm): 22.8 (CH3), 26.6 (CH3), 35.1 (CH2), 35.2 (C), 66.7 (CH),
76.3 (CH). Second diastereoisomer (fast elution) (Rf 0.37): [R]20
D
) -21.2 (c 0.99, CHCl3). Mp: 71 °C. 1H NMR (CDCl3, 300 MHz)
δ (ppm): 0.84 (s, 9H), 1.27 (d, J ) 6.3 Hz, 3H), 1.41-1.60 (m,
2H), 3.97 (dd, J ) 11.1 Hz, 2.7 Hz, 1H), 4.38-4.50 (m, 1H). 13C
NMR (CDCl3, 75 MHz) δ (ppm): 21.4 (CH3), 25.3 (CH3), 31.8
(CH2), 34.3 (C), 74.0 (CH), 84.6 (CH).
coupling reagents. This versatile construction of oxoapratoxin
could allow the facile synthesis of analogues for further SAR
studies, incorporating new amino acids and a modified or
simplified polyketide part.
(4S,6S)-4-tert-Butyl-6-methyl-2-dioxo-1,3,2-dioxathiane (4). To
a solution of the cyclic sulfite 3 (1.485 g, 7.72 mmol) in a 150 mL
H2O/ACN/CCl4 mixture (2:1:1 respectively) was added at room
temperature RuCl3 (80 mg, 0.39 mmol) followed by NaIO4 (2.5 g,
11.58 mmol). After being stirred for 2 h, the mixture was diluted
in diethylether (250 mL), washed with distilled water (twice),
saturated NaHCO3, and brine. The organic layer was dried over
sodium sulfate and concentrated under reduced pressure. The pure
(4S,6S)-4-tert-butyl-6-methyl-2-dioxo-1,3,2-dioxathiane 4 was ob-
tained in quantitative yield (1.61 g) as white crystals. Rf ) 0.15
(hexane/AcOEt 8:2). [R]20D ) -4.0 (c 1.01, CHCl3). Mp: 112 °C.
1H NMR (CDCl3, 300 MHz) δ (ppm): 1.00 (s, 9H), 1.47 (d, J )
6.3 Hz, 3H), 1.85 (t, J ) 8.6 Hz, 1H), 1.85 (t, J ) 5.5 Hz, 1H),
4.52 (dd, J ) 8.6 Hz, 5.5 Hz, 1H), 4.92 (m, 1H). 13C NMR (CDCl3,
75 MHz) δ (ppm): 20.8 (CH3), 25.2 (CH3), 31.6 (CH2), 34.3 (C),
81.1 (CH), 91.8 (CH).
Experimental Section
(S)-4-Hydroxy-5,5-dimethylhexan-2-one (1). Pivalaldehyde (2.5
g, 29.0 mmol) and D-proline (1.34 g, 11.6 mmol) were dissolved
in a solution of DMSO/acetone 4:1 (125 mL). After 4 days, half-
saturated ammonium chloride solution was added and the mixture
was extracted 3 times with ethyl acetate. The combined organic
layers were washed 2 times with water and 1 time with brine, dried
over sodium sulfate and concentrated under reduced pressure. The
residue was purified by column chromatography on silica gel
(hexane/AcOEt 7:3, Rf ) 0.25) to give the hydroxyketone (2.72 g,
1
69%) as a pale yellow oil. [R]20 ) -71.8 (c 0.85, CHCl3). H
D
NMR (CDCl3, 300 MHz) δ (ppm): 0.89 (s, 9H), 2.18 (s, 3H), 2.47
(dd, J ) 17.3 Hz, 10.2 Hz, 1H), 2.61 (dd, J ) 17.3 Hz, 2.1 Hz,
1H), 2.89 (m, 1H), 3.70 (ddd, J ) 10.2 Hz, 3.4 Hz, 2.1 Hz, 1H).
13C NMR (CDCl3, 75 MHz) δ (ppm): 25.6 (CH3), 30.8 (CH3), 34.1
(C), 45.0 (CH2), 74.8 (CH), 210.4 (C).
(3S,5S)-2,2,5-Trimethyloct-7-en-3-ol (5). To a solution of
(4S,6S)-4-tert-butyl-6-methyl-2-dioxo-1,3,2-dioxathiane 4 (1.64 g,
7.87 mmol) and CuI (1.8 g, 9.45 mmol) in anhydrous THF (20
mL) at -25 °C under argon was added dropwise allylmagnesium
chloride (15.8 mL, 2M/THF). The reaction mixture was stirred for
5 h at -25 °C and allowed to warm to room temperature, and THF
was removed under vacuum. To a solution of the residue in
diethylether (75 mL) was added a 20% H2SO4 solution (30 mL) at
room temperature. After being stirred overnight, the layers were
separated, and the aqueous layer was extracted twice by diethylether.
The combined organic layers were dried over K2CO3 and Na2SO4,
filtered and concentrated under reduced pressure. Purification on
silica gel afforded the unsaturated alcohol 5 as a colorless oil (1.21
(2S,4S)-5,5-Dimethylhexan-2,4-diol (2). To a solution of 5,5-
dimethyl-4-hydroxyhexan-2-one 1 (2.37 g, 16.2 mmol) in an
anhydrous mixture of THF/MeOH 4:1 at -78 °C was added
MeOBEt2 (18 mL, 1 M/THF, 18 mmol). After 20 min under stirring,
NaBH4 (684 mg, 18.0 mmol) was added in small portions. The
reaction mixture was stirred for 4 h at -78° and was quenched by
addition of a 30% H2O2 solution (7.5 mL). The reaction mixture
was then stirred overnight at room temperature. The solvent was
removed under reduced pressure, and the residue was dissolved in
ethyl acetate. The organic layer was washed 3 times with water,
dried over Na2SO4 and concentrated under reduce pressure. The
residue was azeotroped 3 times with methanol and purified by
column chromatography on silica gel (hexane/AcOEt 1:1, Rf )
0.28). The (2S,4S)-5,5-dimethylhexan-2,4-diol syn isomer was
isolated as a white solid in 65% yield (1.56 g). The anti isomer
g, 90%). Rf ) 0.24 (hexane/AcOEt 9:1). [R]20 ) -42.9 (c 1.61,
D
1
CHCl3). H NMR (CDCl3, 300 MHz) δ (ppm): 0.88 (s, 9H), 0.94
(d, J ) 6.6 Hz, 3H), 1.19 (ddd, J ) 14.3 Hz, 10.2 Hz, 4.0 Hz, 1H),
1.41 (ddd, J ) 14.3 Hz, 9.2 Hz, 1.8 Hz, 1H), 1.79 (m, 1H), 1.85
(m, 1H), 2.20 (m, 1H), 3.30 (dd, J ) 10.2 Hz, 1.8 Hz, 1H),
4.96-5.05 (m, 2H), 5.78 (m, 1H). 13C NMR (CDCl3, 75 MHz) δ
was also isolated (94 mg) (hexane/AcOEt 1:1, Rf ) 0.37) (syn/anti
1
95:5). syn isomer: [R]20 ) -5.1 (c 0.99, CHCl3). Mp: 43 °C. H
D
NMR (CDCl3, 300 MHz) δ (ppm): 0.86 (s, 9H), 1.18 (d, J ) 6.2
Hz, 3H), 1.38 (dt, J ) 14.3 Hz, 10.2 Hz, 1H), 1.58 (d broad, J )
14.3 Hz, 1H), 3.20 (m, 2H), 3.47 (dd, J ) 10.2 Hz, 1.0 Hz, 1H),
3.97 (m, 1H). 13C NMR (CDCl3, 75 MHz) δ (ppm): 24.1 (CH3),
(28) Bachki, A.; Foubelo, F.; Yus, M. Tetrahedron: Asymmetry. 1995, 6,
1907–1910.
(29) Chopade, P. R.; Davis, T. A.; Prasad, E.; Flowers, R. A. Org. Lett.
2004, 6, 2685–2688.
4302 J. Org. Chem. Vol. 74, No. 11, 2009

Supported Synthesis of Oxoapratoxin A
(ppm): 20.9 (CH3), 25.6 (CH3), 29.8 (CH), 35.0 (C), 38.4 (CH2),
39.8 (CH2), 77.5 (CH), 116.0 (CH2), 137.1 (CH).
(3R,4S,6S,8S)-4-tert-Butyldimethylsilyloxy-3,6,9,9-tetramethyl-
8-triethylsilyloxy Dec-1-ene (9). To a solution of alcohol 6 (980
mg, 2.86 mmol) in 40 mL of anhydrous DCM was added, at -25
°C, 2,6-lutidine (1 mL, 8.58 mmol) followed by TBDMSOTf (1.31
mL, 5.70 mmol). After being stirred for 2 h at this temperature,
the reaction was quenched by addition of a saturated ammonium
chloride solution. The aqueous layer was extracted 3 times with
DCM, and the combined organic layers were washed with brine,
dried over Na2SO4, and concentrated under reduced pressure. The
residue was purified by silica gel chromatography (hexane/AcOEt
98:2, Rf ) 0.59) to afford the protected homoallylic alcohol as a
colorless oil (1.28 g, 98%). [R]20D ) -22 (c 1.09, CHCl3). 1H NMR
(CDCl3, 300 MHz) δ (ppm): 0.06 (s, 3H), 0.08 (s, 3H), 0.62 (q, J
) 7.9 Hz, 6H), 0.75 (s, 9H), 0.81 (s, 9H), 0.88 (t, J ) 7.7 Hz, 9H),
0.93 (d, J ) 8.9 Hz, 3H), 0.97 (d, J ) 7.9, 3H), 1.13-1.30 (m,
2H), 1.42 (ddd, J ) 14.1 Hz, 8.2 Hz, 4.5 Hz, 1H), 1.51 (ddd, J )
14.1 Hz, 9.7 Hz, 5.2 Hz, 1H), 1.66 (m, 1H), 2.36 (m, 1H), 3.32
(dd, J ) 6.1 Hz, 3.5 Hz, 1H), 3.75 (dt, J ) 6.1 Hz, 3.4 Hz, 1H),
4.95-5.04 (m, 2H), 5.73 (ddd, J ) 17.8 Hz, 9.8 Hz, 7.0 Hz, 1H).
13C NMR (CDCl3, 75 MHz) δ (ppm): -3.8 (CH3), -3.3 (CH3),
6.4 (CH2), 7.9 (CH3), 14.0 (CH3), 18.9 (C), 21.3 (CH), 26.4 (CH3),
26.7 (CH3), 26.9 (CH3), 36.7 (C), 39.7 (CH), 44.0 (CH2), 44.6 (CH),
73.7 (CH), 79.2 (CH), 115.1 (CH2), 141.8 (CH).
((3S,5S)-2,2,5-trimethyloct-7-en-3-yloxy)triethylsilane (6). To
a solution of (3S,5S)-2,2,5-trimethyloct-7-en-3-ol 4 (300 mg, 1.77
mmol) and 1-H-imidazole (241 mg, 3.54 mmol) in anhydrous DMF
(6 mL) at room temperature under argon was added TESCl dropwise
(445 µL, 2.66 mmol). The solution was stirred overnight, diluted
with AcOEt, and washed 3 times with distilled water and brine.
The organic layer was dried over Na2SO4 and concentrated under
reduced pressure. The residue was chromatographed (hexane, Rf
) 0.52) to yield 442 mg (88%) of pure ((3S,5S)-2,2,5-trimethyloct-
7-en-3-yloxy)triethylsilane 6 as a colorless oil. [R]20 ) -27.1 (c
D
1
1.07, CHCl3). H NMR (CDCl3, 300 MHz) δ (ppm): 0.62 (q, J )
7.9 Hz, 6H), 0.84 (s, 9H), 0.90 (d, J ) 6.5 Hz, 3H), 0.97 (t, J )
7.9 Hz, 9H), 1.21 (ddd, J ) 14.1 Hz, 8.5 Hz, 3.6 Hz, 1H), 1.38
(ddd, J ) 14.1 Hz, 9.5 Hz, 2.1 Hz, 1H), 1.60-1.80 (m, 2H), 2.19
(m, 1H), 3.36 (dd, J ) 8.5 Hz, 2.1 Hz), 4.98-5.05 (m, 2H), 5.78
(m, 1H). 13C NMR (CDCl3, 75 MHz) δ (ppm): 5.8 (CH2), 7.2 (CH3),
20.9 (CH3), 29.6 (CH3), 35.6 (C), 40.1 (CH2), 40.4 (CH2), 78.7
(CH), 115.9 (CH2), 137.1 (CH).
(3R,5S)-3,6,6-Trimethyl-5-(triethylsilyloxy)heptanal (7). ((3S,5S)-
2,2,5-Trimethyloct-7-en-3-yloxy)triethylsilane 6 (1.307 g, 4.56
mmol) was dissolved in 60 mL of DCM/MeOH 5:1 mixture under
argon and cooled to -76 °C. Ozone was then bubbled into the
mixture until the appearance of a persistant azure blue color. Excess
of ozone was then removed by bubbling nitrogen, and the
intermediate ozonide was neutralized by addition of PPh3 (3 g, 11.4
mmol). After 20 min at -76 °C, the mixture was allowed to reach
room temperature and concentrated under reduced pressure. The
residue was purified by silica gel chromatography (hexane/DCM
(2R,3R,5S,7S)-3-(tert-Butyldimethylsilyloxy)-2,5,8,8-tetramethyl-
7(triethylsilyloxy)nonanal (10). The protected homoallylic alcohol
9 (1.257 g, 2.75 mmol) was dissolved in a DCM/MeOH 5:1 mixture
(60 mL) under argon, and the mixture was cooled to -76 °C. Ozone
was then bubbled into the mixture until the appearance of a
persistant azure blue color. Excess of ozone was then removed by
bubbling nitrogen, and the intermediate ozonide was neutralized
by addition of PPh3 (1.8 g, 6.86 mmol). After 20 min at -76 °C,
the mixture was allowed to reach room temperature and concen-
trated under reduced pressure. The residue was purified by silica
gel chromatography (hexane/DCM 95:5, Rf ) 0.23) to afford 1.14 g
8:2, Rf ) 0.2) to afford 1.278 g (97%) of the pure aldehyde 7 as a
1
colorless oil. [R]20 ) -14.4 (c 1.11, CHCl3). H NMR (CDCl3,
D
300 MHz) δ (ppm): 0.60 (q, J ) 7.8, 6H), 0.83 (s, 9H), 0.95 (t, J
) 7.8 Hz, 9H), 0.98 (d, J ) 8.4 Hz, 3H), 1.38 (ddd, J ) 14.2 Hz,
8.2 Hz, 3.6 Hz, 1H), 1.40 (ddd, J ) 14.2 Hz, 9.5 Hz, 2.5 Hz, 1H),
2.03-2.18 (m, 2H), 2.47 (broad d, J ) 13.7 Hz, 1H), 3.28 (dd, J
) 8.2 Hz, 2.5 Hz, 1H), 9.76 (m, 1H). 13C NMR (CDCl3, 75 MHz)
δ (ppm): 5.5 (CH2), 7.1 (CH3), 21.5 (CH3), 25.6 (CH), 26.1 (CH3),
35.6 (C), 40.7 (CH2), 50.1 (CH2), 78.6 (CH), 202.6 (CH).
(90%) of the pure aldehyde 10 as a colorless oil. [R]20 ) -16.7
D
1
(c 1.14, CHCl3). H NMR (CDCl3, 300 MHz) δ (ppm): 0.09 (s,
3H), 0.10 (s, 3H), 0.61 (q, J ) 7.9, 6H), 0.83 (s, 9H), 0.89 (s, 9H),
0.94 (d, J ) 7.7, 3H), 0.96 (t, J ) 7.9 Hz, 9H), 1.14 (d, J ) 7.0
Hz, 3H), 1.13-1.30 (m, 3H), 1.42 (ddd, J ) 14.4 Hz, 7.8 Hz, 3.6
Hz, 1H), 2.56 (qdd, J ) 7.1 Hz, 3.6 Hz, 1.3 Hz, 1H), 3.30 (dd, J
) 6.2 Hz, 3.6 Hz, 1H), 4.19 (dt, J ) 8.8 Hz, 3.6 Hz, 1H), 9.71 (d,
J ) 1.4 Hz). 13C NMR (CDCl3, 75 MHz) δ (ppm): -4.8 (CH3),
-3.8 (CH3), 5.6 (CH2), 6.1 (CH3), 8.8 (CH3), 17.0 (C), 20.5 (CH),
25.8 (CH3), 26.2 (CH3), 26.7 (CH3), 36.7 (C), 41.0 (CH2), 43.1
(CH2), 52.5 (CH), 70.0 (CH), 78.4 (CH), 203.7 (CH).
(2S,3S,5S,7S)-3-(tert-Butyldimethylsilyloxy)-2,5,8,8-tetramethyl-
7-(triethylsilyloxy)nonanoic Acid (11). To a solution of aldehyde
10 (900 mg, 1.96 mmol) in tert-butanol (6.5 mL), water (3.5 mL)
and 2-methyl-2-butene (720 µL, 6.77 mmol) at 0 °C was added a
solution of NaClO2 (540 mg, 5.97 mmol) in a phosphate buffer
(pH ) 4.5) (6.3 mL). After being stirred for 5 min at 0 °C the
solution was allowed to reach room temperature and was stirred
for another 30 min, then the reaction was diluted with brine. The
aqueous layer was extracted 3 times with AcOEt, and the combined
organic layers were washed with brine, dried over Na2SO4, filtered
and concentrated under reduced pressure to afford the pure
(3S,4R,6S,8S)-3,6,9,9-Tetramethyl-8-(triethylsilyloxy)dec-1-
en-4-ol (8). To an excess of t-butene was added potassium tert-
butylate (8.38 mmol, 1 M/THF) at -78 °C. After 10 min of stirring
at -78 °C, n-BuLi (8.38 mmol, 1.6 M/hexane) was added dropwise.
The temperature was then allowed to raise to -45 °C, and the
mixture was stirred for 30 min. The mixture was then cooled to
-78 °C, and a solution of diisopinocamphenylborane (2.65 g, 8.38
mmol) in a minimum of THF was added dropwise. After stirring
for 45 min at -78 °C, BF3 ·Et2O (1.38 mL, 10.89 mmol) was
quickly added, immediately followed by a solution of aldehyde 7
(1.2 g, 4.19 mmol, 1 equiv) in THF (5 mL). The mixture was stirred
at -78 °C for 5 h and quenched by addition of 3 N NaOH (25
mL) and 30% H2O2 (25 mL). After 1 h of stirring at room
temperature, the mixture was diluted with AcOEt and brine. The
aqueous layer was extracted 3 times with AcOEt, and the combined
organic layers were washed with brine, dried over Na2SO4 and
concentrated under reduced pressure. Silica gel chromatography
(hexane/AcOEt 93:7, Rf ) 0.38) afforded 1.34 g (93%) of the
homoallylic alcohol 8 as a colorless oil (9:1 dr, anti as the major
carboxylic acid 11 as a colorless thick oil (930 mg, quantitative).
1
[R]20 ) -46.1 (c 11.7, CHCl3). H NMR (CDCl3, 300 MHz) δ
D
(ppm): 0.10 (s, 3H), 0.11 (s, 3H), 0.61 (q, J ) 7.9 Hz, 6H), 0.84
(s, 9H), 0.90 (s, 9H), 0.93 (d, J ) 6.1, 3H), 0.96 (t, J ) 7.9 Hz,
9H), 1.17 (d, J ) 7.1 Hz, 3H), 1.12-1.30 (m, 1H), 1.43 (ddd, J )
14.4 Hz, 7.8 Hz, 3.5 Hz, 1H), 1.65 (m, 2H), 2.72 (qd, J ) 7.1 Hz,
4.1 Hz, 1H), 3.31 (dd, J ) 6.1 Hz, 3.5 Hz, 1H), 4.16 (m, 1H), 10.9
(s, broad, 1H). 13C NMR (CDCl3, 75 MHz) δ (ppm): -5.0 (CH3),
-4.1 (CH3), 5.7 (CH2), 7.1 (CH3), 10.6 (CH3), 18.0 (C), 20.5 (CH),
25.8 (CH3), 26.2 (CH3), 26.7 (CH3), 35.9 (C), 40.0 (CH2), 43.1
(CH2), 45.8 (CH), 70.8 (CH), 78.5 (CH), 179.5 (C).
product, isolated from the syn isomer). [R]20 ) -18.4 (c 1.14,
D
1
CHCl3). H NMR (CDCl3, 300 MHz) δ (ppm): 0.62 (q, J ) 7.0
Hz, 6H), 0.84 (s, 9H), 0.93 (d, J ) 6.6 Hz, 3H), 0.96 (t, J ) 7.7
Hz, 9H), 1.03 (d, J ) 6.8 Hz, 3H), 1.00-1.50 (m, 5H), 1.81 (m,
1H), 2.15 (hex, broad, J ) 7.0 Hz, 1H), 3.32 (dd, J ) 7.5 Hz, 2.5
Hz, 1H), 3.49 (ddd, J ) 10.4 Hz, 8.1 Hz, 2.1 Hz, 1H), 5.08-5.14
(m, 2H), 5.75 (ddd, J ) 17.9 Hz, 11.9 Hz, 8.9 Hz, 1H). 13C NMR
(CDCl3, 75 MHz) δ (ppm): 5.7 (CH2), 7.2 (CH3), 16.3 (CH3), 20.8
(CH3), 26.2 (CH3), 26.6 (CH), 35.7 (C), 41.1 (CH2), 42.2 (CH2),
45.1 (CH), 72.0 (CH), 78.6 (CH), 116.3 CH2), 140.5 (CH).
(2R,3S,5R,7S)-3-(tert-Butyldimethylsilyloxy)-7-hydroxy-2,5,8,8-
tetramethylnonanoic Acid (12). The acid 11 (1.5 g, 3.16 mmol)
J. Org. Chem. Vol. 74, No. 11, 2009 4303

Gilles et al.
was dissolved in 125 mL of a mixture of acetic acid, DCM, water
and methanol in a 3:1:2:1 ratio, respectively, and stirred for 3 h.
Then, the reaction was diluted with AcOEt and water, and the
aqueous layer extracted 3 times with AcOEt. The combined organic
layers were washed 2 times with brine, dried over MgSO4 and
finally concentrated under reduced pressure. The residue was
purified by silica gel chromatography (cyclohexane/AcOEt 3:1, Rf
) 0.25) to afford the pure carboxylic acid 12 (1 g, 88%) as white
(m, 1H), 5.25 (d, J ) 11.1 Hz, 1H), 6.19 (dd, J ) 9.0 Hz, 0.9 Hz,
1H), 6.41 (m, 1H), 6.53 (m, 1H), 6.82 (d, J ) 8.4 Hz, 2H), 7.16
(d, J ) 8.4 Hz, 2H). HRMS FAB+: 842.5278 (calcd 842.5201).
Oxazoline Analogue of Apratoxin A (Oxoapratoxin A). To a
solution of macrocycle 26 (100 mg, 0.12 mmol) in 2 mL of DCM
at -80 °C was added DAST (15.2 µL, 1 equiv). After 1 h, an
additional 1 equiv of DAST was added, and the mixture was stirred
for 1 h. The reaction was quenched at -80 °C by addition of a 4
M solution of NH4OH, diluted with chloroform and washed with
brine. The organic layer was dried over Na2SO4 and concentrated
under reduced pressure to give oxoapratoxin in 58% yield (HPLC
evaluation) and 35% yield after preparative HPLC purification (35
mg of isolated compound). tR: 11.20 min. [R]20D ) -116.7 (c 0.30,
CHCl3). 1H NMR (CDCl3, 300 MHz) δ (ppm): 0.87 (s, 9H),
0.90-1.01 (m, 10H), 1.06 (d, J ) 6.9 Hz, 3H), 1.11 (m, 1H), 1.23
(d, J ) 6.7 Hz, 3H), 1.25 (m, 1H), 1.29 (m, 1H), 1.43 (m, 1H),
1.74-1.86 (m, 3H), 1.91 (d, J ) 0.9 Hz, 3H), 2.05 (m, 1H), 2.13
(m, 1H), 2.25 (m, 1H), 2.33 (m, 1H), 2.42 (m, 1H), 2.74 (s, 3H),
2.81 (s, 3H), 2.86 (m, 1H), 3.00-3.25 (m, 2H), 3.30 (q, J ) 6.6
Hz, 1H), 3.56-3.72 (m, 3H), 3.78 (s, 3H), 4.19 (broad m, 2H),
4.40 (m, 1H), 4.71 (m, 1H), 4.73-4.81 (m, 1H), 4.96 (dd, J )
12.4 Hz, 1.77 Hz, 1H), 5.05 (td, J ) 10.8 Hz, 4.8 Hz, 1H), 5.24
(d, J ) 11.5 Hz, 1H), 5.98 (d, J ) 9.5 Hz, 1H), 6.18 (dd, J ) 9.0
Hz, 0.9 Hz, 1H), 6.80 (d, J ) 8.6 Hz, 2H), 7.16 (d, J ) 8.6 Hz,
2H). HRMS (FAB+): 824.5148 (calcd 824.5174).
crystals. [R]20 ) -39.3 (c 1.12, CHCl3). Mp: 70.5 °C. HRMS
D
1
(ESI): 361.2769 (calcd for C19H40O4Si+H+: 361.2774). H NMR
(CDCl3, 300 MHz) δ (ppm): 0.09 (s, 3H), 0.11 (s, 3H), 0.61 (q, J
) 7.9 Hz, 6H), 0.87(s, 9H), 0.89 (s, 9H), 0.95 (d, J ) 6.6, 3H),
1.17 (d, J ) 7.2 Hz, 3H), 1.25-1.33 (m, 1H), 1.66 (ddd, J ) 12.0
Hz, 8.7 Hz, 3.0 Hz, 1H), 1.79 (m, 1H), 2.72 (qd, J ) 7.2 Hz, 4.5
Hz, 1H), 3.26 (dd, J ) 9.3 Hz, 2.7 Hz, 1H), 4.11 (ddd, J ) 7.5 Hz,
4.2 Hz, 3.0 Hz, 1H), 11.0 (broad s, 1H). 13C NMR (CDCl3, 75
MHz) δ (ppm): -4.4 (CH3), -4.7 (CH3), 11.2 (CH3), 18.0 (C),
21.1 (CH), 25.6 (CH3), 25.8 (CH3), 26.3 (CH3), 34.9 (C), 39.6
(CH2), 39.7 (CH2), 45.9 (CH), 71.4 (CH), 77.3 (CH), 179.2 (C).
Macrolactamization of Linear Precursors 22 and 25 (26). To
a solution of the linear precursor 22 or 25 in DCM/DMF (9:1) at
a concentration of 8 × 10-4 M was added DIEA (5 equiv) followed
by HATU (2 equiv). The mixture was stirred overnight, and the
solvent was removed under reduce pressure. The residue was then
taken up in AcOEt and washed successively with a saturated
solution of citric acid, saturated solution of NaHCO3 and brine.
The organic layer was dried over Na2SO4 and concentrated under
reduced pressure to give the macrocyclic compound 26 as a white
foam with a 70% yield (HPLC evaluation) and 45% yield after
preparative HPLC purification (isolated compound). tR: 12.03 min.
1H NMR (CDCl3, 300 MHz) δ (ppm): 0.86 (s, 9H), 0.95-1.06 (m,
10H), 1.09 (d, J ) 6.9 Hz, 3H), 1.13 (m, 1H), 1.18 (d, J ) 10.1
Hz, 3H), 1.20-1.37 (m, 3H), 1.70-2.15 (m, 8H), 2.20-2.35 (m,
2H), 2.59 (s, 3H), 2.65 (s, 3H), 2.75 (m, 1H), 3.06-3.18 (m, 2H),
3.27 (m, 1H), 3.56-3.72 (m, 3H), 3.78 (s, 3H), 4.08 (m, 1H), 4.15
(m, 1H), 4.32 (m, 1H), 4.40 (m, 1H), 4.71 (m, 1H), 4.73-4.89
(dd, J ) 12.3 Hz, 3.0 Hz, 1H), 4.96 (d, J ) 12.9 Hz, 1H), 5.02-5.18
Acknowledgment. We thank La Ligue contre le Cancer for
financial support.
Supporting Information Available: Experimental proce-
1
dures for all other compounds, H and 13C NMR spectra and
optical rotation for compounds 13-15, HPLC and MS spectra
for supported intermediates and linear precursors. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO900583J
4304 J. Org. Chem. Vol. 74, No. 11, 2009