Synthesis of the C1′-C11′ segment of leucascandrolide A

Full text of DOI:10.1021/jo005787q

3242
J . Org. Chem. 2001, 66, 3242-3245
was able to remove the side chain from the macrolide
Syn th esis of th e C1′-C11′ Segm en t of
and determine that the latter is essential for cytotoxic
activity, whereas the former contributes mainly to the
antifungal properties of leucascandrolide A. Leucascan-
drolide B lacks significant cytotoxic and antifungal
activity.
Leucascandrolide A has an attractive range of cytotoxic
and antifungal activities, but further investigations of its
pharmacological and clinical potential are limited due to
the lack of success in retrieving additional natural
compound from sponge.2 As part of our program on the
synthesis of bioactive marine natural products,4 we are
interested in developing a total synthesis approach that
can address the supply problem and provide crucial
insights into structure-activity relationships. We now
report the preparation of the C1′-C11′ side chain segment
of the natural product.5,6
Leu ca sca n d r olid e A
Peter Wipf* and Thomas H. Graham
Department of Chemistry, University of Pittsburgh,
Pittsburgh, Pennsylvania 15260
pwipf+@pitt.edu
Received December 29, 2000
In tr od u ction
Leucascandrolide A is a doubly bridged 18-membered
macrolide of a new structural type isolated in 1996 from
a calcareous sponge Leucascandra caveolata collected
along the east coast of New Caledonia, Coral Sea.1 It is
quite likely that the actual biosynthetic origin of this
compound as well as of leucascandrolide B is microbial.2
While leucascandrolides were isolated in significant
quantities (70 mg from 240 g, 0.03% yield, for leucas-
candrolide A) from largely necrotic and therefore possibly
extensively colonized sponge, a subsequent expedition
collecting intact sponge did not provide any trace of
product.2 The structure of leucascandrolide was deter-
mined by HRMS and MS-MS as well as by 2D NMR,
and its absolute configuration was derived from degrada-
tion and Mosher ester studies.1 The functional group and
ring array of leucascandrolide A is vaguely reminiscent
of the mycalolide class of macrolides.3
Resu lts a n d Discu ssion
In consideration of the sensitivity of the two (Z)-alkenes
in the oxazole side chain of leucascandrolide A, we
decided to use an alkyne as a surrogate of the C9′-C10′
alkene and install the alkene moiety via Lindlar hydro-
genation followed by a Still-Wittig reaction for the chain
extension at the C2′-C3′ (Z)-alkene. A segment condensa-
tion of the readily available acid 3 and amino alcohol 4
would provide the precursor for the oxazole segment
(Figure 1).
N-Acylation of propargylamine (5) with methylchloro-
formate followed by deprotonation with lithium hexa-
methyldisilazide and carboxylation of the anion with solid
carbon dioxide provide alkynoate 3 in 55% overall yield
(Scheme 1). After activation with PyBrOP,7 condensation
of acid 3 with amino alcohol 4 led to the unstable diamide
2 in 82% yield. The silyl ether 4 was obtained in a
surprisingly selective monosilylation from known ami-
nodiol 6.8
Among other side reactions, hydroxy amide 2 readily
rearranged to an amino ester derivative upon storage and
was therefore immediately used for the next step. In
preparation for a modified Robinson-Gabriel synthesis
of the oxazole,9 the primary alcohol function in 2 was
(4) (a) Wipf, P. Chem. Rev. 1995, 95, 2115. (b) Wipf, P.; Lim, S.
Chimia 1996, 50, 157. (c) Wipf, P.; Xu, W. J . Org. Chem. 1996, 61,
6556. (d) Wipf, P.; Venkatraman, S. J . Org. Chem. 1996, 61, 6517. (e)
Wipf, P.; Fritch, P. C. J . Am. Chem. Soc. 1996, 118, 12358. (f) Wipf, P.
In Alkaloids: Chemical and biological perspectives; Pelletier, S. W.,
Ed.; Pergamon: New York, 1998; pp 187-228. (g) Wipf, P.; Yokokawa,
F. Tetrahedron Lett. 1998, 39, 2223. (h) Wipf, P.; Uto, Y. J . Org. Chem.
2000, 65, 1037. (i) Wipf, P.; Reeves, J . T.; Balachandran, R.; Guiliano,
K. A.; Hamel, E.; Day, B. W. J . Am. Chem. Soc. 2000, 122, 9391. (j)
Wipf, P.; Miller, C. P.; Grant, C. M. Tetrahedron 2000, 56, 9143.
(5) For the preparation of the C1-C13 fragment of leucascandrolide
A, see: Crimmins, M. T.; Carroll, C. A.; King, B. W. Org. Lett. 2000,
2, 597.
(6) After the completion of our work, the first total synthesis of
leucascandrolide A was disclosed: Hornberger, K. R.; Hamblett, C. L.;
Leighton, J . L. J . Am. Chem. Soc. 2000, 122, 12894.
(7) (a) Frerot, E.; Coste, J .; Pantaloni, A.; Dufour, M.-N.; J ouin, P.
Tetrahedron 1991, 47, 259. (b) Andrus, M. B.; Lepore, S. D.; Turner,
T. M. J . Am. Chem. Soc. 1997, 119, 12159.
Leucascandrolide B differs from A by a smaller (i.e.,
16- vs 18-membered), not uniformly 1,3-dioxygenated
lactone ring, the lack of an oxazole in the side chain, and
more extensive methyl branching. The biological profile
of leucascandrolide A is quite powerful. The compound
showed strong cytotoxic activity in vitro on KB and P388
cells (IC50’s of 50 and 250 ng/mL), as well as very strong
inhibition of Candida albicans (inhibition diameter of 26/
40, 23/20, and 20/10 mm/µg per disk). The latter is
interesting in view of the growing concern among AIDS
patients about this pathogenic yeast. The Pietra group
(1) D’Ambrosio, M.; Guerriero, A.; Debitus, C.; Pietra, F. Helv. Chim.
Acta 1996, 79, 51.
(8) (a) Houston, D. M.; Dolence, E. K.; Keller, B. T.; Patel-Thombre,
U.; Borchardt, R. T. J . Med. Chem. 1985, 28, 467. (b) Herbrandson, H.
F.; Wood, R. H. J . Med. Chem. 1969, 12, 620.
(9) (a) Wipf, P.; Miller, C. P. J . Org. Chem. 1993, 58, 3604. (b) Wipf,
P.; Lim, S. J . Am. Chem. Soc. 1995, 117, 558. (c) Wipf, P.; Lim, S.
Chimia 1996, 50, 157.
(2) (a) D′Ambrosio, M.; Tato, M.; Pocsfalvi, G.; Debitus, C.; Pietra,
F. Helv. Chim. Acta 1999, 82, 347. (b) D’Ambrosio, M.; Tato, M.;
Pocsfalvi, G.; Debitus, C.; Pietra, F. Helv. Chim. Acta 1999, 82, 1135.
(3) See, for example: Matsunaga, S.; Liu, P.; Celatka, C. A.; Panek,
J . S.; Fusetani, N. J . Am. Chem. Soc. 1999, 121, 5605-5606.
10.1021/jo005787q CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/12/2001

Notes
J . Org. Chem., Vol. 66, No. 9, 2001 3243
Sch em e 2
F igu r e 1. Retrosynthetic approach.
Sch em e 1
strategy are the preparation of the oxazole moiety by a
mild three-stage process involving oxidation-cyclodehy-
dration-dehydrohalogenation of the unstable alcohol 2
and the semihydrogenation of the alkyne moiety at the
stage of the relatively stable oxazole 8 to give the novel
cis-alkenyl oxazole derivative that is a unique structural
feature of leucascandrolide A. Further progress toward
the macrolide portion of this natural product will be
reported in due course.
oxidized with Dess-Martin reagent (Scheme 2).10 Cyclo-
dehydration of the resulting aldehyde with triphenylphos-
phine in the presence of 2,6-di-tert-butyl-4-methylpyri-
dine provided the intermediate bromooxazoline 7,9b,c
which readily eliminated hydrogen bromide upon treat-
ment with DBU to give the chemically relatively stable
oxazole 8 in 32% overall yield. At this point, the alkyne
was converted to the (Z)-alkene under Lindlar condi-
tions,11 and primary alcohol 9 was obtained in 60% yield
after removal of the silyl ether with TBAF in THF. A
second Dess-Martin oxidation followed by a Still-Wittig
condensation12 provided the desired leucascandrolide
side-chain methyl ester 1 as a single stereoisomer in 62%
yield in addition to 16% of recovered aldehyde. All
Exp er im en ta l Section
Gen er a l Meth od s. All reactions with moisture-sensi-
tive compounds were conducted in oven-dried glassware
under an atmosphere of dry nitrogen. Solvents were dried
by distillation from sodium benzophenone (THF) or from
CaH2 (CH2Cl2). Starting materials that were commer-
cially available were used without purification. Melting
points are uncorrected and were determined using crys-
tallized samples. NMR spectra were obtained at 300
MHz/75 MHz (1H/13C) in CDCl3 unless otherwise noted.
2-Aminopentane-1,5-diol (6) was prepared from L-glutam-
ic acid according to literature procedures.8
1
spectroscopic data, in particular, the H and 13C NMR
resonances of 1, were in close agreement with the
corresponding shifts reported for the natural product.1
Con clu sion s
P r op -2-yn ylca r ba m ic Acid Meth yl Ester . A solu-
tion of propargylamine (1.0 g, 18 mmol) and triethy-
lamine (3.5 mL, 25 mmol) in CH2Cl2 (30 mL) was cooled
to 0 °C and treated dropwise with methyl chloroformate
(1.50 mL, 19.5 mmol). The reaction mixture was allowed
to warm to room temperature, stirred overnight, and
quenched with 6 N HCl (3 mL) and water (20 mL). The
aqueous layer was extracted with ethyl acetate, and the
combined organic layers were washed with brine, dried
An efficient convergent pathway provided the C1′-C11′
side chain of the antifungal antitumor agent leucascan-
drolide A in nine steps and in 3.2% overall yield for the
longest linear sequence. Highlights of the synthetic
(10) (a) Dess, D. B.; Martin, J . C. J . Am. Chem. Soc. 1991, 113, 7277.
(b) Ireland, R. E.; Liu, L. J . Org. Chem. 1993, 58, 2899.
(11) Marvell, E. N.; Li, T. Synthesis 1973, 457.
(12) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405.

3244 J . Org. Chem., Vol. 66, No. 9, 2001
Notes
(Na2SO4), and filtered. The solution was evaporated onto
SiO2 and purified by chromatography on SiO2 (ethyl
acetate/hexanes, 3:7 to 6:4) to yield 1.36 g (67%) of prop-
2-ynylcarbamic acid methyl ester as a colorless oil: Rf
0.5 (ethyl acetate/hexanes, 2:3); IR (neat) 3296, 2955,
aqueous NaHCO3. The organic layer was diluted with
CH2Cl2 (8 mL), sequentially washed with 0.1 N HCl,
water, and brine, and dried (Na2SO4). The solvent was
removed under vacuum to yield an orange viscous oil.
Purification by chromatography on SiO2 (ethyl acetate)
gave 163 mg (82%) of 2 as a slightly yellow oil: Rf 0.5
(ethyl acetate). This compound proved to be very unstable
and was therefore used immediately after preparation.
1
2123, 1712, 1530, 1256 cm-1; H NMR δ 5.3-5.1 (b, 1
H), 3.92 (bs, 2 H), 3.64 (s, 3 H), 2.21 (s, 1 H); 13C NMR δ
156.8, 80.0, 71.5, 52.5, 30.9; MS(EI) m/z (rel intensity)
113 (M+, 49), 98 (100), 82 (18); HRMS m/z calcd for
C5H7N1O2 113.0477, found 113.0479.
[3-[4-[3-(t er t -Bu t yld im e t h ylsila n yloxy)p r op yl]-
oxa zol-2-yl]p r op -2-yn yl]ca r ba m ic Acid Meth yl Es-
ter (8). To a solution of 2 (75 mg, 0.20 mmol) in CH2Cl2
(5 mL) was added Dess-Martin periodinane (171 mg,
0.404 mmol). The reaction mixture was stirred for 1 h
and purified by chromatography on SiO2 (ethyl acetate/
hexanes, 3:2). The resulting clear oil was immediately
dissolved in CH2Cl2 (10 mL) and treated with triph-
enylphosphine (165 mg, 0.629 mmol), 2,6-di-tert-butyl-
4-methylpyridine (332 mg, 1.617 mmol), and 1,2-dibromo-
1,1,2,2-tetrachloroethane (204 mg, 0.626 mmol). The
reaction mixture was stirred for 10 h, treated with DBU
(266 µL, 1.78 mmol), and stirred for an additional 6 h.
Purification by chromatography on SiO2 (ethyl acetate/
hexanes, 3:7) gave 23 mg (32%) of 8 as a slightly yellow
oil: Rf 0.5 (ethyl acetate/hexanes, 2:3); IR (neat) 2954,
2929, 2857, 2250, 1729, 1587, 1534, 1472, 1255, 1102,
4-Meth oxyca r bon yla m in obu t-2-yn oic Acid (3). A
solution of lithium hexamethyldisilazide (1.06 M, 11.5
mL, 12.2 mmol) in THF (120 mL) was cooled to -78 °C
and treated dropwise with a solution of prop-2-ynylcar-
bamic acid methyl ester (649 mg, 5.74 mmol) in THF (10
mL). The reaction mixture was stirred for 1 h at -78 °C.
Subsequently, carbon dioxide (from dry ice) was bubbled
through the solution for 2 h. The reaction mixture was
quenched at -78 °C by aqueous, saturated NaHCO3 (5
mL). After being warmed to room temperature, the
solution was acidified with 6 N HCl and extracted with
ethyl acetate. The combined organic layers were concen-
trated, and the residue was dissolved in aqueous NaH-
CO3 and washed with CH2Cl2. The aqueous layer was
acidified by slow addition of concentrated HCl and
extracted with ethyl acetate. The combined organic
extracts were dried (Na2SO4) and concentrated to yield
770 mg (85%) of 3 as a slightly yellow oil that crystallized
to a white solid upon standing: Rf 0.4 (ethyl acetate/
hexanes/TFA, 40:60:1): mp 95-96 °C; IR (neat) 3337,
1
837, 777 cm-1; H NMR δ 7.33 (s, 1 H), 5.11 (bs, 1 H),
4.22 (d, 2 H, J ) 5.5 Hz), 3.70 (bs, 3 H), 3.62 (t, 2 H, J )
6.1 Hz), 2.57 (t, 2 H, J ) 7.6 Hz), 1.82 (tt, 2 H, J ) 7.2,
6.6 Hz), 0.87 (s, 9 H), 0.02 (s, 6 H); 13C NMR δ 156.7,
145.8, 142.1, 135.4, 87.9, 71.7, 62.2, 52.8, 31.5, 31.3, 26.1,
22.7, 18.5, -5.1; MS(EI) m/z (rel intensity) 337 ([M -
CH3]+, 68), 295 (100), 263 (45), 238 (31), 98 (14), 89 (11),
75 (19), 73 (15), 59 (12); HRMS m/z calcd for C13H19N2O4-
Si (M - C(CH3)3) 295.1114, found 295.1116.
1
2959, 2244, 1708, 1532, 1255 cm-1; H NMR (CD3OD) δ
4.04 (s, 2 H), 3.68 (s, 3 H); 13C NMR (CD3OD) δ 159.3,
156.1, 85.2, 75.8, 53.0, 31.2; MS(EI) m/z (rel intensity),
157 (M+, 54), 139 (38), 113 (7), 98 (100), 82 (15), 81(15),
59 (20); HRMS m/z calcd for C6H7N1O4 157.0375, found
157.0380.
[3-[4-(3-Hyd r oxyp r op yl)oxa zol-2-yl]a llyl]ca r ba m -
ic Acid Meth yl Ester (9). To a solution of 8 (90 mg,
0.26 mmol) in ethyl acetate (30 mL) were added quinoline
(50 µL, 0.42 mmol) and Lindlar’s catalyst (90 mg). The
reaction mixture was stirred for 3 h at room temperature
under hydrogen (1 atm) and filtered through Celite, and
the resulting yellow-orange residue was dissolved in THF
(20 mL). Tetrabutylammonium fluoride (150 mg, 0.57
mmol) was added, and the reaction mixture was stirred
at room temperature for 14 h. The solvent was removed
under vacuum, and purification of the resulting red, oily
residue by chromatography on SiO2 (ethyl acetate) gave
9 as a slightly yellow oil (36.4 mg, 60%): Rf 0.2 (EtOAc);
2-Am in o-5-(ter t-bu tyld im eth ylsila n yloxy)p en ta n -
1-ol (4). To a suspension of freshly washed (hexanes)
sodium hydride (60% dispersion in mineral oil, 41 mg,
1.03 mmol) in THF (40 mL) was added a solution of
2-amino-1,5-pentanediol (122 mg, 1.03 mmol) in hot THF
(10 mL). The reaction mixture was stirred for 8 h at room
temperature and then treated dropwise with tert-bu-
tyldimethylsilyl chloride (1 M in THF, 1 mL, 1 mmol).
After 20 min, the reaction mixture was evaporated onto
SiO2, and chromatography on SiO2 (CH2Cl2/methanol/
NH4OH, 95:5:0 to 90:10:0 to 90:10:1) yielded 160 mg
(68%) of 4 as a pale yellow oil: Rf ) 0.3-0.4 (CH2Cl2/
MeOH, 9:1, 3-fold developed); IR (neat) 3345, 2929, 2858,
IR (neat) 3327, 2925, 2851, 1704, 1523, 1264, 1055 cm-1
;
1H NMR δ 7.37 (s, 1 H), 6.29 (d, 1 H, J ) 11.8 Hz), 6.08
(dt, 1 H, J ) 11.7, 6.4 Hz), 5.50 (bs, 1 H), 4.35-4.25 (m,
2 H), 3.72 (t, 2 H, J ) 6.1 Hz), 3.68 (s, 3 H), 2.66 (t, 2 H,
J ) 7.1 Hz), 2.25 (bs, 1 H), 1.90 (tt, 2 H, J ) 6.8, 6.4 Hz);
13C NMR δ 160.2, 157.4, 141.7, 136.7, 134.0, 116.7, 62.3,
52.4, 39.7, 31.4, 23.0; MS(EI) m/z (rel intensity) 240 (M+,
100), 222 (7), 208 (40), 195 (29), 181 (23), 163 (25), 151
(29), 136 (31), 47 (125), 81 (57), 66 (45), 54 (33); HRMS
m/z calcd for C11H16N2O4 240.1110, found 240.1119.
1635, 1521, 1471, 1388, 1361, 1255, 1098, 836, 776 cm-1
;
1H NMR δ 4.05-3.95 (b, 4 H), 3.69 (t, 1 H, J ) 9.4 Hz),
3.63 (t, 3 H, J ) 5.7 Hz), 3.45 (t, 1 H, J ) 9.4), 3.07 (bs,
1 H), 1.65-1.49 (m, 4 H), 0.88 (s, 9 H), 0.05 (s, 6 H); 13
C
NMR δ 65.0, 63.1, 53.4, 29.3, 26.2, 18.6, -5.1; MS(EI)
m/z (rel intensity) 202 ([M - CH2OH]+, 27), 176 (4), 159
(34), 141 (10), 129 (7), 101(12), 84 (47), 75 (84), 70 (100),
56 (20); HRMS m/z calcd for C10H24NOSi (M - CH2OH)
202.1627, found 202.1629.
[3-[4-(3-Oxopr opyl)oxazol-2-yl]allyl]car bam ic Acid
Meth yl Ester . To a solution of 9 (7 mg, 0.03 mmol) in
CH2Cl2 (10 mL) was added Dess-Martin periodinane (24
mg, 0.057 mmol). The reaction mixture was stirred for 1
h, concentrated, and purified by chromatography on SiO2
(ethyl acetate/hexanes, 4:1-1:0) to yield 4 mg (58%) of
[3-[4-(3-oxopropyl)oxazol-2-yl]allyl]carbamic acid methyl
ester as a clear, colorless oil: Rf 0.4, ethyl acetate/
[3-[4-(ter t-Bu tyld im eth ylsila n yloxy)-1-h yd r oxym -
et h ylb u t ylca r b a m oyl]p r op -2-yn yl]ca r b a m ic Acid
Meth yl Ester (2). To a solution of 4 (149 mg, 0.639
mmol) and 3 (84 mg, 0.54 mmol) in CH2Cl2 (2 mL) was
added diisopropylamine (186 µL, 1.07 mmol). The reac-
tion mixture was cooled to -10 °C, treated with PyBrOP
(350 mg, 0.751 mmol), and allowed to warm to room
temperature. After 6 h, the reaction was quenched with

Notes
J . Org. Chem., Vol. 66, No. 9, 2001 3245
hexanes, 4:1). This compound proved to be unstable and
was therefore used immediately after preparation.
5-[2-(3-Meth oxyca r bon yla m in op r op en yl)oxa zol-4-
yl]p en t-2-en oic Acid Meth yl Ester (1). A solution of
18-crown-6 (104 mg, 0.393 mmol) and bis(2,2,2-trifluo-
roethyl)(methoxycarbonylmethyl) phosphonate (20 µL,
0.10 mmol) in THF (2 mL) was cooled to -78 °C and
treated with a solution of potassium hexamethyldisilazide
(16 mg, 0.080 mmol) in THF (1 mL). The reaction mixture
was stirred for 5 min, and then a solution of [3-[4-(3-
oxopropyl)oxazol-2-yl]allyl]carbamic acid methyl ester
(18.3 mg, 0.0770 mmol) in THF (5 mL) was added
dropwise with stirring. After 3 h at -78 °C, the mixture
was quenched with saturated aqueous NH4Cl. Upon
warming to room temperature, the aqueous layer was
diluted with water and extracted with CH2Cl2, and the
combined organic layers were dried (Na2SO4) and filtered.
The solvent was removed under vacuum to give a slightly
yellow oil that was purified by chromatography on SiO2
(ethyl acetate/CH2Cl2, 1:4 to 2:3) to yield 3 mg (16%) of
recovered aldehyde and 14 mg (62%) of 1 as a clear,
colorless oil: Rf 0.4 (ethyl acetate/CH2Cl2, 1:4); IR (neat)
3348, 3136, 2993, 2951, 2923, 2852, 1721, 1521, 1252,
1198, 1003 cm-1; 1H NMR δ 7.37 (s, 1 H); 6.23-6.31 (m,
2 H), 6.10 (dt, 1 H, J ) 11.6, 6.4 Hz), 5.82 (dt, 1 H, J )
11.5, 1.6 Hz), 5.57 (bs, 1 H), 4.35-4.25 (m, 2 H), 3.71 (s,
3 H), 3.68 (s, 3 H), 3.01 (ddt, 2 H, J ) 1.5, 7.3, 7.3 Hz),
1
2.70 (t, 2 H, J ) 7.2 Hz); H NMR (C5D5N) δ 8.44 (bs, 1
H), 7.61 (s, 1 H); 6.38 (d, 1 H, J ) 11.9 Hz), 6.32-6.22
(m, 2 H), 5.92 (d, 1 H, J ) 11.5 Hz), 4.80 (bt, 2 H, J ) 5.4
Hz), 3.74 (s, 3 H), 3.63 (s, 3 H), 3.14 (dt, 2 H, J ) 7.3, 7.3
Hz), 2.69 (t, 2 H, J ) 7.4 Hz); 13C NMR δ 166.9, 160.2,
157.4, 149.1, 141.4, 136.4, 134.1, 120.4, 116.9, 52.4, 51.3,
39.5, 27.7, 25.8; 13C NMR (C5D5N) δ 166.6, 160.5, 158.1,
141.7, 138.6, 134.5, 120.4, 115.5, 51.9, 51.0, 40.8, 28.0,
25.9; MS(EI) m/z (rel intensity) 294 (M+, 100), 263 (28),
233 (63), 206 (40), 195 (48), 160 (31), 114 (41), 91 (41);
HRMS m/z calcd for C14H18N2O5 294.1216, found 294.1220.
Ack n ow led gm en t. This work was supported by the
National Institutes of Health (GM 55433).
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O005787Q