A short enantioselective synthesis of a component of cryptophycin A and arenastatin A

Article abstract of DOI:10.1016/S0957-4166(98)00119-0

Synthesis of 1, a component of cryptophycin A 2 and arenastatin A 3, was achieved by applying palladium-catalyzed reductive ring opening of optically active alkenyl oxirane 13 for the construction of the vicinal stereogenic centers.

Full text of DOI:10.1016/S0957-4166(98)00119-0

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 1351–1357
A short enantioselective synthesis of a component of
cryptophycin A and arenastatin A
Masaaki Furuyama and Isao Shimizu ∗
Department of Applied Chemistry, School of Science and Engineering, Waseda University, Okubo 3-4-1, Shinjuku-ku, Tokyo
169, Japan
Received 22 January 1998; accepted 16 March 1998
Abstract
Synthesis of 1, a component of cryptophycin A 2 and arenastatin A 3, was achieved by applying palladium-
catalyzed reductive ring opening of optically active alkenyl oxirane13 for the construction of the vicinal stereogenic
centers. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Methyl (5S,6R)-(E,E)-5-hydroxy-6-methyl-8-phenyl-2,7-octadienoate 1a1,2 is a synthetic component
of depsipeptides, cryptophycins 23,4 and arenastatins 3.5,6 Due to their important biological activities,7–10
development of practical synthesis of 2 and 3 including their synthetic analogs is of current interest.11–14
One of the problems in the synthesis of these compounds is the preparation of the hydroxy ester 1 by
controlling both the absolute and the relative stereochemistry of the methyl group at C6 and the hydroxy
group at C5.
∗
Corresponding author. E-mail: shimizui@mn.waseda.ac.jp
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00119-0

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2. Results and discussion
We have reported that the stereoselective synthesis of acyclic compounds which have a methyl
and a hydroxy group at vicinal carbons can be carried out by a palladium-catalyzed reductive ring
opening reaction of optically active alkenyl oxiranes using formic acid as a hydrogen source.15 syn-
Stereochemistry is constructed starting with E-alkenyloxiranes (E)-4, since the hydride derived from
formic acid attacks via π-allylpalladium complex 6 from the palladium side as shown in Scheme 1. On
the contrary, anti-stereochemistry of the methyl and the hydroxy groups can be expected to be generated
if the π–σ–π interconversion takes place to form 8 prior to the hydride attack. In this paper we have
established a facile preparation of 1 having anti-stereochemistry from the oxirane 13 by retention in the
hydride attack via π–σ–π interconversion (6-→7-→8 in Scheme 1).
Scheme 1.
2-p-Methoxyphenyl-1,3-dioxorane 10 was reduced with DIBAH, and subsequent Swern oxida-
tion gave 3-p-methoxyphenyl-1-propanal,16 which was converted to the allylic alcohol 11 by an
Emmons–Horner type Wittig reaction and DIBAH reduction. The Sharpless asymmetric epoxidation
of 11 with (+)-DET gave the chiral epoxy alcohol with 99% ee in 95% yield, subsequent Swern
oxidation gave the aldehyde 12 in 80% yield. The Wittig reaction using [Ph3P(Ph)CH2]Br under salt
free conditions17 gave the (Z)-alkenyloxirane 13 in 67% yield without observation of the (E)-isomer.
The alkenyloxirane 13 was subjected to Pd-catalyzed transfer hydrogenolysis with HCOOH to give
the homoallyl alcohol 14 selectively in 91% yield. Deprotection of the p-methoxy benzyl ether using
AlCl3 gave the diol 15, from which the synthesis of 1b has been reported.1 Thus, the hydroxy groups of
the diol 15 were protected as the TBS ethers in 94% yield. The silyl ether of the primary alcohol was
deprotected selectively using AcOH at room temperature in 85% yield.2 The alcohol 16 was oxidized
to the corresponding aldehyde followed by the Emmons–Horner type Wittig reaction with trimethyl
phosphonoacetate to give the ester 1b in 72% overall yield (see Scheme 2), which was identified from
the physical and spectral data reported in the literature.1
In conclusion, we have established a convenient route for the synthesis of the ester 1b, a key
intermediate for the synthesis of cryptophycins and arenastatins. The palladium-catalyzed reaction

M. Furuyama, I. Shimizu / Tetrahedron: Asymmetry 9 (1998) 1351–1357
1353
Scheme 2.
described in this paper provides a useful method for construction of chiral building blocks which have
anti-stereochemistry of methyl and hydroxy groups at the vicinal carbons.
We thank the Materials Characterization Central Laboratory, Waseda University, for NMR and HRMS
measurement and elemental analyses. This research was carried out as a part of the research program in
the Materials Research Laboratory for Bioscience and Photonics, Waseda University.
3. Experimental section
1H NMR and 13C NMR spectra were recorded on a JEOL JNM-EX270 in CDCl3. IR spectra were
recorded on a Perkin–Elmer 1640. Optical rotations were measured in CHCl3 with a JASCO DIP-1000.
Mass spectra were recorded on a JEOL JMS-SX102A. Anhydrous reactions were carried out under Ar
using freshly dried solvents.
3.1. (2E)-5-(p-Methoxybenzyl)oxy-2-methyl-2-penten-1-ol 11
To a solution of 10 (2.01 g, 10.3 mmol) in Et2O (40 ml) at 0°C was added DIBAH (0.95 M in
hexane, 16.3 ml). The mixture was stirred for 1 h. The reaction mixture was quenched with Rochelle
salt aqueous solution (0.5 M, 35 ml) at r.t., then the resulting mixture was stirred for 3 h. The reaction
mixture was extracted with Et2O. The extract was washed with brine, dried over MgSO4, filtered, and
concentrated in vacuo. The residue was chromatographed on silica gel (hexane:AcOEt=3:1) to give 3-(p-
methoxybenzyl)oxy-1-propanol 17 (2.00 g, 99%). 1H NMR δ 7.25 (d, 8.6, 2H), 6.87 (d, 8.4, 2H), 4.45 (s,
2H), 3.80 (s, 3H), 3.76 (dt, 5.6/5.8, 1H), 3.63 (dt, 5.6/5.8, 2H), 2.45 (broad, 1H), 1.89–1.80 (tt, 5.6/5.8,
2H); 13C NMR δ 159.2, 130.1, 129.2, 113.8, 72.8, 69.0, 61.8, 55.2, 32.0; IR ν 3422, 2950, 2858, 1613,
1514, 1248, 1121, 1084, 1034, 873, 820 cm−1; HREIMS m/z calcd for C11H16O3: 196.1100, found:
196.1120.

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M. Furuyama, I. Shimizu / Tetrahedron: Asymmetry 9 (1998) 1351–1357
To a solution of DMSO (2.50 ml) in CH2Cl2 (30 ml) were added (COCl)2 (1.33 ml) and the alcohol
17 (1.98 g, 10.1 mmol) in CH2Cl2 (15 ml) sequentially at −78°C. After the mixture was stirred for 1 h,
Et3N (8.43 ml) was added and the mixture was stirred for 1 h at r.t. The reaction mixture was quenched
with sat. NH4Cl, and the aqueous layer was extracted with CH2Cl2. The extract was washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo to give a crude aldehyde, which was used in the
next step without purification.
To a suspension of [1-(ethoxycarbonyl)ethyl]triphenylphosphonium bromide (5.36 g) in THF (30 ml)
was added n-BuLi (1.66 M in hexane, 6.8 ml) at 0°C. After the mixture was stirred for 10 min, the
crude aldehyde obtained above in THF (15 ml) was added and the mixture was stirred for 2 h. The
reaction mixture was quenched with H2O, and the aqueous layer was extracted with CH2Cl2. The
extract was washed with brine, dried over MgSO4, filtered, and concentrated in vacuo. The residue
was chromatographed on silica gel (hexane:AcOEt=9:1) to give ethyl (2E)-5-(p-methoxybenzyl)oxy-
2-methyl-2-heptenoate 18 (2.08 g, 74% from 10). 1H NMR δ 7.26 (d, 8.58, 2H), 6.88 (d, 8.58, 2H), 6.77
(ddd, 1.3/5.9/8.6, 1H), 4.46 (s, 2H), 4.19 (q, 7.26, 2H), 3.81 (s, 3H), 3.53 (dd, 6.6/6.9, 2H), 2.48 (ddd,
0.99/6.9/11.9, 2H), 1.84 (d, 0.99, 1H), 1.29 (t, 7.3, 3H); 13C NMR δ 168.0, 159.2, 138.3, 130.3, 129.4,
129.2, 113.8, 72.7, 68.3, 60.4, 55.3, 29.4, 14.3, 12.5; IR ν 2956, 2934, 2904, 2858, 1709, 1513, 1248,
1097, 1036, 821 cm−1; HREIMS m/z calcd for C16H22O4: 278.1996, found: 278.1519.
To a solution of 18 (2.05 g, 7.39 mmol) in Et2O (30 ml) at 0°C was added DIBAH (0.95 M in hexane,
19.4 ml). After the mixture was stirred for 1 h, the reaction mixture was quenched with Rochelle salt
aqueous solution (0.5 M, 40 ml) at r.t. After the mixture was stirred for 3 h the solution was extracted
with Et2O, washed with brine, dried over MgSO4, filtered, and concentrated in vacuo. The residue was
1
chromatographed on silica gel (hexane:AcOEt=5:1) to give 11 (1.69 g, 97%). H NMR δ 7.25 (d, 8.6,
2H), 6.86 (d, 8.3, 2H), 5.41 (t, 6.9, 1H), 4.43 (s, 2H), 3.96 (s, 2H), 3.79 (s, 3H), 3.45 (t, 6.9, 2H), 2.34 (dt,
6.9, 2H), 2.03 (broad, 1H), 1.65 (s, 3H); 13C NMR δ 159.0, 136.7, 130.4, 129.2, 121.8, 113.7, 72.4, 69.3,
68.4, 55.2, 28.2, 13.7; IR ν 3405, 2934, 2910, 2858, 1612, 1513, 1248, 1091, 1035, 820 cm−1; HREIMS
m/z calcd for C14H20O3: 236.1413, found: 236.1379.
3.2. (2S,3S)-2,3-Epoxy-5-(p-methoxybenzyl)oxy-2-methyl-1-pentanal 12
To a mixture of 4Å–MS in CH2Cl2 (18 ml) at −25°C was added Ti(OiPr)4 (0.211 ml), L-(+)-DET
(0.145 ml) and 11 (1.68 g, 7.10 mmol). After the mixture was stirred for 30 min, TBHP was added
and the mixture was stirred for 4.5 h. The resulting mixture was quenched by a small amount of Me2S
and excess sat. NaF aqueous solution. After being stirred for 12 h, the solvent was filtered and extracted
with CH2Cl2, washed with brine, dried over MgSO4, filtered, and concentrated in vacuo. The residue was
chromatographed on silica gel (hexane:AcOEt=7:3) to give (2S,3S)-2,3-epoxy-5-(p-methoxybenzyl)oxy-
2-methyl-1-pentanol 19 (1.69 g, 95%). 1H NMR δ 7.26 (d, 8.6, 2H), 6.88 (d, 8.6, 2H), 4.46 (s, 2H), 3.80
(s, 3H), 3.68–3.55 (m, 4H), 3.18–3.14 (dd, 5.3/6.9, 1H), 1.91–1.82 (m, 2H), 1.28 (s, 3H); 13C NMR δ
159.0, 130.3, 129.2, 113.8, 72.7, 67.1, 65.3, 60.8, 57.9, 55.2, 29.0, 14.3; IR ν 3447, 2932, 2860, 2360,
2342, 1612, 1513, 1248, 1094, 1034, 820 cm−1; [α]D25=−44 (c 1.34, CHCl3); HREIMS m/z calcd for
C14H20O4: 252.1362, found: 252.1384. The enantiomeric excess of 19 (99% e.e.) was calculated from
the 19F NMR of its (+)-MTPA ester.
To a solution of DMSO (1.65 ml) in CH2Cl2 (20 ml) were added sequentially (COCl)2 (0.872 ml) and
the alcohol 19 (1.67 g, 6.64 mmol) in CH2Cl2 (12 ml) at −78°C. After the mixture was stirred for 1 h,
Et3N (5.55 ml) was added and the mixture was stirred for 1 h at r.t. The reaction mixture was quenched
with sat. NH4Cl, and the aqueous layer was extracted with CH2Cl2. The extract was washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo. The residue was chromatographed on silica gel

M. Furuyama, I. Shimizu / Tetrahedron: Asymmetry 9 (1998) 1351–1357
1355
1
(hexane:AcOEt=5:1) to give 12 (1.32 g, 80%). H NMR δ 8.85 (s, 1H), 7.23 (d, 8.9, 2H), 6.88 (d, 8.6,
2H), 4.46 (s, 2H), 3.80 (s, 3H), 3.63 (t, 5.9, 2H), 3.30 (t, 5.9, 1H), 1.97–1.87 (m, 2H), 1.40 (s, 3H); 13C
NMR δ 199.7, 179.1, 130.2, 129.2, 113.9, 72.9, 66.5, 62.1, 57.9, 55.3, 28.8, 10.1; IR ν 2934, 2859, 1728,
1612, 1514, 1248, 1095, 1034, 821 cm−1; [α]D28=37 (c 1.04, CHCl3).
3.3. (1Z)-(3S,4S)-3,4-Epoxy-6-(p-methoxybenzyl)oxy-3-methyl-1-phenyl-1-heptene 13
To a solution of [Ph3P(Ph)CH2]Br (3.38 g) in THF (12 ml) was added NaN(TMS)2 (1.0 M in THF, 6.24
ml). After the mixture was stirred for 1 h, HMPA (8 ml) was added dropwise at 0°C and the resulting
mixture was stirred for 1 h. The aldehyde 12 (1.30 g, 5.19 mmol) in THF was added at −100°C and
the mixture was stirred for 10 h at r.t. The reaction mixture was quenched with sat. NH4Cl, and the
aqueous layer was extracted with Et2O. The extract was washed with brine, dried over MgSO4, filtered,
and concentrated in vacuo. The residue was chromatographed on silica gel (hexane:AcOEt=9:1) to give
13 (1.12 g, 67%). 1H NMR δ 7.37–7.18 (m, 7H), 6.86 (d, 8.6, 2H), 6.44 (d, 11.9, 1H), 5.86 (d, 11.9, 1H),
4.42 (s, 2H), 3.80 (s, 3H), 3.53 (dd, 6.3/6.9, 2H), 2.89 (dd, 4.3/7.3, 1H), 2.04–1.93 (m, 1H), 1.80–1.70
(m, 1H), 1.46 (s, 3H); 13C NMR δ 159.3, 132.3, 130.5, 129.2, 128.8, 128.2, 128.0, 127.4, 113.9, 96.2,
72.7, 67.2, 62.6, 58.9, 55.3, 29.4, 17.6; IR ν 2961, 2932, 2858, 1612, 1512, 1248, 1095, 1034, 696 cm−1
[α]D27=84.2 (c 0.67, CHCl3).
;
3.4. (1E)-(3R,4S)-4-Hydroxy-6-(p-methoxybenzyl)oxy-3-methyl-1-phenyl-1-heptene 14
To a mixture of Pd2(dba)3CHCl3 (0.178 g) and n-Bu3P (0.0855 ml) in dioxane (6 ml) was added a
mixture of HCOOH (0.661 ml) and Et3N (0.957 ml) in dioxane (3 ml) at r.t. The mixture was stirred
for 15 min. The oxirane 13 (1.11 g, 3.43 mmol) in dioxane (6 ml) was added and the mixture was
stirred for 10 h. The reaction mixture was quenched with H2O and the aqueous layer was extracted with
Et2O and the combined extract was washed with sat. NH4Cl and brine, dried over MgSO4, filtered, and
concentrated in vacuo. The residue was chromatographed on silica gel (hexane:AcOEt=9:1) to give 14
(1.02 g, 91%). 1H NMR δ 7.37–7.15 (m, 7H), 6.87 (d, 8.6, 2H), 6.41 (d, 15.8, 1H), 6.21 (dd, 7.9/15.8,
1H), 4.44 (s, 2H), 3.79 (s, 3H), 3.77–3.58 (m, 3H), 2.87 (broad, 1H), 2.42–2.35 (m, 1H), 1.79–1.70 (m,
2H), 1.13 (d, 6.9, 3H); 13C NMR δ 159.3, 137.4, 132.1, 130.7, 130.1, 129.3, 128.5, 127.1, 126.1, 113.8,
74.7, 73.0, 69.0, 55.3, 43.5, 33.8, 16.4; IR ν 3462, 3003, 2960, 2932, 2867, 1612, 1513, 1248, 1089,
1035, 752, 694 cm−1; [α]D34=49.2 (c 0.67, CHCl3)
3.5. (1E)-(3R,4S)-3-Methyl-1-phenyl-1-heptene-4,6-diol 15
To a solution of the alcohol 14 (496 mg, 1.52 mmol) in CH2Cl2 (8 ml) was added AlCl3 (223 mg)
at r.t. After stirring for 30 min AlCl3 (81.2 mg) was added and the mixture was stirred for 30 min. The
reaction mixture was quenched with sat. NaHCO3 and Rochelle salt aqueous solution (0.5 M, 10 ml).
After stirred for 10 h, the solution was extracted with CH2Cl2, washed with brine, dried over MgSO4,
filtered and concentrated in vacuo. The residue was chromatographed on silica gel (hexane:AcOEt=4:1)
1
to give the alcohol 15 (235 mg, 75%). H NMR δ 7.35–7.14 (m, 5H), 6.43 (d, 16.2, 1H), 6.09 (dd,
8.6/15.8, 1H), 3.87–3.76 (m, 2H), 3.75–3.66 (m, 1H), 2.50–2.29 (broad, 1H, m, 2H), 1.80–1.59 (m, 1H),
1.08 (d, 6.6, 3H); 13C NMR δ 137.0, 131.8, 131.5, 128.6, 127.4, 126.2, 75.5, 61.8, 44.1, 35.5, 16.5; IR ν
3356, 2961, 2878, 1493, 1448, 1053, 970, 750, 694 cm−1; [α]D26=57.0 (c 1.47, CHCl3).

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M. Furuyama, I. Shimizu / Tetrahedron: Asymmetry 9 (1998) 1351–1357
3.6. (1E)-(3R,4S)-4-(tert-Butyldimethylsilyl)oxy-3-methyl-1-phenyl-1-hepten-6-ol 16
To a solution of the alcohol 15 (230 mg, 1.12 mmol) and imidazole (455 mg, 6.69 mmol) in DMF (5 ml)
was added TBDMSCl (504 mg) and DMAP (cat.) at r.t. After the mixture was stirred for 10 h, the reaction
mixture was quenched with H2O, and the aqueous layer was extracted with Et2O. The extract was washed
with brine, dried over MgSO4, filtered and concentrated in vacuo. The residue was chromatographed
on silica gel (hexane:AcOEt=9:1) to give (1E)-(3R,4S)-4,6-bis(tert-butyldimethylsilyl)oxy-3-methyl-1-
1
phenyl-1-heptene 20 (455 mg, 94%). H NMR δ 7.35–7.15 (m, 5H), 6.35 (d, 16.17, 1H), 6.17 (dd,
7.6/15.8, 1H), 3.86–3.80 (dt, 4.0/5.9, 1H), 3.70–3.60 (m, 2H), 2.50–2.43 (m, 1H), 1.67–1.60 (dd, 6.6/12.9,
2H), 1.10 (d, 6.9, 3H), 0.91 (s, 9H), 0.89 (s, 9H), 0.07 (s, 6H), 0.03 (s, 6H); 13C NMR δ 137.8, 132.8,
130.0, 128.4, 126.8, 126.0, 72.6, 60.1, 42.7, 36.7, 25.9, 18.3, 18.1, 15.5, −4.5, −5.3; IR ν 2955, 2942,
2928, 2856, 1472, 1466, 1256, 1098, 836, 775, 692 cm−1; [α]D32=48.2 (c 1.04, CHCl3); HREIMS m/z
calcd for C25H46O2Si2: 435.3116, found: 435.3082.
To a solution of 20 (450 mg, 1.04 mmol) in THF (2 ml) and H2O (1 ml) was added AcOH (1 ml) at
r.t. After the mixture was stirred for 3 days the reaction mixture was quenched with NaHCO3, and the
aqueous layer was extracted with Et2O. The extract was washed with brine, dried over MgSO4, filtered
and concentrated in vacuo. The residue was chromatographed on silica gel (hexane:AcOEt=9:1) to give
16 (281 mg, 85%). 1H NMR δ 7.36–7.17 (m, 5H), 6.39 (d, 16.2, 1H), 6.13 (dd, 7.9/16.2, 1H), 3.91–3.86
(dd, 5.6/10.9, 1H), 3.79–3.70 (m, 2H), 2.59–2.51 (m, 1H), 1.76–1.70 (m, 2H), 1.10 (d, 6.6, 3H), 0.91 (s,
9H), 0.01 (d, 0.66, 6H); 13C NMR δ 132.6, 130.0, 128.5, 127.0, 126.0, 74.6, 60.5, 42.7, 35.0, 25.9, 18.0,
14.8, −4.5, −4.6; IR ν 3410, 2957, 2943, 2929, 2856, 1733, 1472, 1257, 1094, 1058, 1031, 836, 758,
693 cm−1; [α]D21=150.8 (c 0.17, CHCl3)
3.7. (5S,6R)-(E,E)-5-(tert-Butyldimethylsilyl)oxy-6-methyl-8-phenyl-octa-2,7-dienoate 1b
To a solution of DMSO (0.206 ml) in CH2Cl2 (2 ml) were added sequentially (COCl)2 (0.109 ml) and
the alcohol 16 (265 mg, 0.829 mmol) in CH2Cl2 (2 ml) at −78°C. After the mixture was stirred for 1 h,
Et3N (0.693 ml) was added and the mixture was stirred for 1 h at r.t. The reaction mixture was quenched
with sat. NH4Cl, and the aqueous layer was extracted with CH2Cl2. The extract was washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo to give a crude aldehyde, which was used in the
next step without purification.
To a solution of NaH (66.3 mg) in THF (2 ml) was added trimethyl phosphonoacetate (0.268 ml)
at 0°C. After the mixture was stirred for 10 min, the crude aldehyde obtained above in THF (2 ml)
was added and the mixture was stirred for 4 h at r.t. The reaction mixture was quenched with H2O,
and the aqueous layer was extracted with Et2O. The extract was washed with brine, dried over MgSO4,
filtered, and concentrated in vacuo. The residue was chromatographed on silica gel (hexane:AcOEt=9:1)
to give 1b (222 mg, 2 steps 72%). 1H NMR δ 7.36–7.17 (m, 5H), 7.01–6.90 (m, 1H), 6.37 (d, 15.8, 1H),
6.20–6.11 (m, 1H), 5.84 (d, 15.8, 1H), 3.78–3.73 (m, 1H), 3.72 (s, 3H), 2.48–2.40 (m, 1H), 2.38–2.33
(m, 2H), 1.10 (d, 6.6, 3H), 0.90 (s, 9H), 0.05 (d, 0.25, 6H); 13C NMR δ 166.8, 146.4, 137.6, 131.9, 130.5,
128.5, 127.0, 126.0, 122.9, 75.0, 51.4, 42.8, 37.6, 25.9, 18.1, 16.2, −4.4, −4.5; IR ν 2954, 2929, 2856,
1727, 1658, 1436, 1322, 1258, 1169, 1097, 970, 837, 775, 748, 693 cm−1; [α]D27=66.1 (c 0.80, CHCl3);
HRFABMS m/z calcd for C22H34O3Si: 374.2278, found: 374.2253.
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