An iterative acetylene-epoxide coupling strategy for the total synthesis of aspinolide A

Article abstract of DOI:10.1002/hlca.201000161

The total synthesis of aspinolide A (1) was successfully achieved by an iterative acetylene-epoxide coupling strategy and a Yamaguchi lactonization as the key steps.

Full text of DOI:10.1002/hlca.201000161

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Helvetica Chimica Acta – Vol. 94 (2011)
An Iterative AcetyleneꢀEpoxide Coupling Strategy for the Total Synthesis of
Aspinolide A
by Gowravaram Sabitha*, Teega Rammohan Reddy, Chitti Srinivas, and Jhillu Singh Yadav
Organic Division I, Indian Institute of Chemical Technology, Hyderabad 500007, India
(phone: þ 91-40-27191629; fax: þ 91-40-27160512; e-mail: gowravaramsr@yahoo.com)
The total synthesis of aspinolide A (1) was successfully achieved by an iterative acetyleneꢀepoxide
coupling strategy and a Yamaguchi lactonization as the key steps.
Introduction. – Aspinolides A – C (1 – 3, Fig.), new 10-membered macrolides, were
isolated by Zeeck and Fuchser [1] in 1997 from cultures of Aspergillus ochraceus. Their
structures were established by detailed spectroscopic analysis. Other representative 10-
membered natural lactones such as nonenolide (4) [2a][2b], modiolide A (5) [2c],
putaminoxin (6) [2d], and herbarumin III (7) [2e] are shown in the Figure. The
molecules containing medium-sized ring systems attracted the attention of synthetic
organic chemists recently since they form the core of many natural products. The first
total synthesis of aspinolide A (1) has been recently reported [3], which relied on a
RCM (ring-closing metathesis) reaction for the macrolactonization.
Figure. 10-Membered natural macrolides
As a part of our current interest in naturally occurring pharmacologically active
macrolides [2b][2d][2e], we report herein the second synthesis of aspinolide A through
an iterative acetyleneꢀepoxide opening and Yamaguchi lactonization. Our retrosyn-
thetic analysis is outlined in Scheme 1. The macrolide 1 could be obtained by
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Yamaguchi lactonization of 8, which, in turn, could be made from 9. The diol 9 could be
prepared by iterative coupling of a triple-bond compound obtainable from 10 and (2R)-
2-methyloxirane (12).
Scheme 1. Retrosynthetic Analysis of Aspinolide A (1)
Results and Discussion. – The known epoxy alcohol 10, prepared from homo-
propargyl alcohol (¼ but-3-yn-1-ol), was converted to the corresponding epoxy
chloride 11 in 87% yield by using TPP (¼ 5,10,15,20-tetraphenylporphyrin) and
NaHCO3 in dry CCl4 and refluxing for 4 h (Scheme 2). Accordingly, compound 11 was
subjected to LiNH2 in liquid NH3 to furnish the desired terminal acetylene [4], which
was coupled in situ with 5 equiv. of (2R)-2-methyloxirane (12) to get the chiral
propargyl alcohol 9 in a one-pot reaction in 80% yield. The triple bond in 9 was reduced
with LiAlH4 in THF providing (E)-allyl alcohol 13. The differentiation of the two
secondary OH groups of 13 relied on the discovery that the propargyl or the allyl
alcohol function reacts selectively [5] with (tert-butyl)chlorodimethylsilane (tBuMe2-
SiCl). Accordingly, diol 13 was converted to alcohol 14 followed by removal of the 4-
methoxybenzyl protecting group by using DDQ (¼4,5-dichloro-3,6-dioxocyclohexa-
1,4-diene-1,2-dicarbonitrile), in CH2Cl2/H2O 19 :1, which resulted in diol 15 in 91%
yield. Selective oxidation of the primary-alcohol group in diol 15 to an aldehyde
function was achieved with BAIB/TEMPO (¼ [bis(acetyloxy)iodo]benzene/2,2,6,6-
tetramethylpiperidin-1-yloxy) [6] in CH2Cl2 at room temperature, which, without
isolation, was converted to the corresponding acid 8 by Pinnick oxidation [7] (83%
yield over the two steps), followed by Yamaguchi lactonization (2,4,6-trichlorobenzoyl
chloride in refluxing toluene) to provide macrolactone 16 (70% yield). Finally, removal
t
of the BuMe2Si group with Bu4NF provided the target molecule 1 in 90% yield. The
1H- and 13C-NMR data and optical-rotation value of synthetic 1 were in good accord
with those of the natural product [1].
Conclusion. – An iterative acetyleneꢀepoxide coupling and Yamaguchiꢂs protocols
were successfully applied leading to the second synthesis of aspinolide A (1).
T. R. M. R. and C. S. thank UGC, New Delhi, for the award of fellowships.

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Scheme 2
a) TPP, NaHCO3, reflux, CCl4, 4 h; 87%. b) Li, liq. NH3, Fe(NO3)3 (cat.), dry THF, 8 h; 80%. c) LiAlH4,
t
dry THF, 4 h; 85%. d) BuMe2SiCl, 1H-imidazole, dry CH2Cl2, 4 h; 86%. e) DDQ, CH2Cl2/H2O, 1 h ;
91%. f) 1. BAIB/TEMPO, CH2Cl2, 2 h; 2. NaClO2, NaH2PO4, DMSO, 1 h; 83%. g) 2,4,6-Trichlor-
obenzoyl chloride, Et3N, DMAP (¼ N,N-dimethylpyridin-4-amine), toluene, reflux, 12 h; 70%. h)
Bu4NF, dry THF, 6 h; 90%.
Experimental Part
General. Reactions were conducted under N2 in anh. solvents such as CH2Cl2, THF, and AcOEt.
Yields refer to chromatographically and spectroscopically (1H- and 13C-NMR) homogeneous material.
Air-sensitive reagents were transferred by syringe or double-ended needle. Column chromatography
(CC): silica gel (60 – 120 mesh. Acme Chemical Co., India). TLC: Merck 60 F-254 silica gel plates and
light petroleum ether (b.p. 60 – 808) for reation monitoring; detection by UV light. Optical rotations:
Jasco-DIP-370 polarimeter; at 258. IR Spectra: Perkin-Elmer Infrared-683 spectrometer; ˜n in cmꢀ1. 1H-
and 13C-NMR Spectra: Varian-FT-200 (Gemini) and Bruker-UXNMR-FT-300 (Avance) spectrometers;
in CDCl3; d in ppm rel. to Me4Si as internal standard, J in Hz. ESI-MS: LC-MSD (Agilent Technologies)
spectrometers at 70 eV; in m/z (rel. %). HR-ESI-MS: QSTAR-XL hybrid ms/ms system (Applied
Biosystems/MDS Sciex, Foster City, USA), equipped with an ESI source (IICT, Hyderabad); in m/z.
(2S,3R)-2-(Chloromethyl)-3-{4-[(4-methoxyphenyl)methoxy]butyl}oxirane (11). To a stirred soln. of
alcohol 10 (3.2 g, 12.01 mmol) in dry CCl4 (50 ml), TPP (3.78 g, 14.41 mmol) and NaHCO3 (1.51 g,
18.02 mmol) were added. The mixture was heated to vigorous reflux for 4 h, and the resulting solid was
filtered and washed with Et2O. Concentration and purification by CC afforded 11 (2.97 g, 87%). Viscous
liquid. [a]2D5 ¼ þ5.86 (c ¼ 3.4, CHCl3). IR (neat): 3444, 2934, 2856, 1775, 1684, 1609, 1512, 1459, 1362,
1
1301, 1248, 1174, 1094, 1036. H-NMR (500 MHz): 7.12 (d, J ¼ 8.8, 2 H); 6.82 (d, J ¼ 8.8, 2 H); 4.39 (s,
2 H); 3.79 (s, 3 H); 3.58 (dd, J ¼ 4.2, 11.7, 1 H); 3.36 – 3.42 (m, 3 H); 2.92 (ddd, J ¼ 1.9, 5.8, 10.7, 1 H); 2.79

Helvetica Chimica Acta – Vol. 94 (2011)
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(ddd, J ¼ 1.9, 5.8, 10.7, 1 H); 1.48 – 1.67 (m, 6 H). 13C-NMR (75 MHz): 159.2; 130.5; 129.2; 113.6; 72.5;
69.6; 58.9; 57.1; 55.2; 44.6; 31.1; 29.3; 22.5. ESI-MS: 307 ([M þ Na]þ). HR-ESI-MS: 307.1809 ([M þ Na]þ,
C15H21ClNaOþ3 ; calc. 307.1077).
(2R,6R)-10-[(4-Methoxyphenyl)methoxy]dec-4-yne-2,6-diol (9). To freshly dist. NH3 (50 ml), in a
two-necked round-bottomed flask fitted with a cold finger condenser, was added a cat. amount of
Fe(NO3)3, followed by the portion-wise addition of Li metal (0.48 g, 68.82 mmol) at ꢀ 338. The resulting
gray suspension was stirred for 30 min. Then 11 (2.8 g, 9.83 mmol) in dry THF (20 ml) was added within
20 min, and the mixture was stirred for 2 h at ꢀ 338. Then the addition of a cat. amount of BF3 · Et2O
followed by (2R)-2-methyloxirane (12; 3.44 ml, 49.16 mmol) was carried out successively, the mixture
stirred for 5 h at ꢀ 338 and quenched by the addition of solid NH4Cl (10 g), and NH3 then allowed to
evaporate. The mixture was diluted with H2O (10 ml) and AcOEt (50 ml) and filtered over a small pad of
Celite. The filtrate was extracted with AcOEt (3 ꢁ 50 ml), the combined org. layer dried (Na2SO4) and
concentrated, and the residue purified by CC: 9 (2.41 g, 80%). Pale yellow viscous liquid. [a]2D5 ¼ ꢀ5.4
(c ¼ 2.2, CHCl3). IR (neat): 3383, 2928, 2860, 1774, 1611, 1572, 1457, 1371, 1300, 1248, 1173, 1089, 1033.
1H-NMR (500 MHz): 7.19 (d, J ¼ 8.3, 2 H); 6.81 (d, J ¼ 8.3, 2 H); 4.38 (s, 2 H); 4.25 – 4.30 (m, 1 H); 3.76
(s, 3 H); 3.85 – 3.91 (m, 1 H); 3.41 (t, J ¼ 6.2, 2 H); 2.35 (ddq, J ¼ 2.1, 4.2, 16.6, 1 H); 2.24 (ddq, J ¼ 2.1, 4.2,
16.6, 1 H); 1.57 – 1.67 (m, 4 H); 1.44 – 1.51 (m, 2 H); 1.20 (d, J ¼ 6.24, 3 H). 13C-NMR (75 MHz): 159.1;
130.5; 129.2; 113.7; 83.7; 81.6; 72.5; 69.8; 66.2; 62.3; 55.2; 37.6; 29.2; 29.1; 22.3; 21.9. ESI-MS: 324 ([M þ
NH4]þ), 307 ([M þ H]þ). HR-ESI-MS: 329.1725 ([M þ Na]þ, C18H26NaOþ4 ; calc. 329.1729).
(2R,4E,6R)-10-[(4-Methoxyphenyl)methoxy]dec-4-ene-2,6-diol (13). To a stirred suspension of
LiAlH4 (0.409 g, 10.77 mmol) in dry THF (30 ml) at 08 was added dropwise a soln. of 9 (2.2 g, 7.18 mmol)
in dry THF (20 ml). The mixture was allowed to warm to r.t. and stirred for 4 h. After cooling to 08, the
mixture was diluted with Et2O and quenched by dropwise addition of sat. aq. Na2SO4 soln. (10 ml). The
solid material was filtered and washed thoroughly several times with hot AcOEt. The combined org.
layer dried (Na2SO4), the solvent evaporated, and the residue purified by CC: 13 (1.88 g, 85%). Viscous
liquid. [a]2D5 ¼ ꢀ3.4 (c ¼ 2.6, CHCl3). IR (neat): 3397, 2925, 2853, 1776, 1611, 1512, 1459, 1374, 1300, 1248,
1170, 1082, 1034. 1H-NMR (300 MHz): 7.19 (d, J ¼ 8.5, 2 H); 6.85 (d, J ¼ 8.5, 2 H); 5.47 – 5.68 (m, 2 H);
4.39 (s, 2 H); 4.0 – 4.14 (m, 2 H); 3.79 (s, 3 H); 3.40 (t, J ¼ 6.2, 2 H); 2.0 – 2.25 (m, 2 H); 1.36 – 1.64 (m,
6 H); 1.17 (d, J ¼ 6.3, 3 H). 13C-NMR (75 MHz): 159.1; 136.3; 130.5; 129.2; 127.2; 113.6; 72.4; 69.8; 67.0;
55.1; 41.7; 36.8; 29.9; 29.4; 22.6; 22.08. ESI-MS: 331 ([M þ Na]þ). HR-ESI-MS: 331.1880 ([M þ Na]þ,
C18H28NaOþ4 ; calc. 331.1885).
(2R,4E,6R)-6-{[(tert-Butyl)dimethylsilyl]oxy}-10-[(4-methoxyphenyl)methoxy]dec-4-en-2-ol (14).
To a stirred soln. of 13 (1.6 g, 4.75 mmol) and 1H-imidazole (0.667 g, 9.51 mmol) in dry CH2Cl2
t
(20 ml) was added BuMe2SiCl (1.07 g, 7.13 mmol), portion-wise at 08. The mixture was stirred at 08
for 4 h and then quenched with sat. aq. NH4Cl soln. and extracted with CH2Cl2 (3 ꢁ 40 ml). The aq. layer
was extracted with additional CH2Cl2 (2 ꢁ 30 ml), the combined org. layer washed with H2O (30 ml) and
brine (30 ml), dried (Na2SO4), and concentrated, and the residue purified by CC: 14 (1.84 g, 86%).
Colorless liquid. [a]2D5 ¼ ꢀ3.3 (c ¼ 2.4, CHCl3). IR (neat): 3415, 2927, 2853, 1776, 1612, 1512, 1459, 1372,
1300, 1247, 1173, 1084, 1036, 972. 1H-NMR (300 MHz): 7.20 (d, J ¼ 9.1, 2 H); 6.82 (d, J ¼ 9.1, 2 H); 5.40 –
5.66 (m, 2 H); 4.39 (s, 2 H); 4.00 – 4.09 (m, 1 H); 3.80 (s, 3 H); 3.74 – 3.83 (m, 1 H); 3.38 (t, J ¼ 6.8, 2 H);
2.06 – 2.25 (m, 2 H); 1.54 – 1.65 (m, 2 H); 1.36 – 1.53 (m, 4 H); 1.17 (d, J ¼ 6.04, 3 H); 0.88 (s, 9 H); 0.03 (s,
3 H); 0.01 (s, 3 H). 13C-NMR (500 MHz): 159.0; 137.2; 130.7; 129.2; 125.7; 113.7; 73.2; 72.5; 70.0; 67.2;
55.2; 42.1; 38.1; 29.7; 25.9; 22.7; 22.0; 18.2; ꢀ 4.3; ꢀ 4.8. ESI-MS: 445 ([M þ Na]þ). HR-ESI-MS:
445.2748 ([M þ Na]þ, C24H42NaO4Siþ; calc. 445.2750).
(5R,6E,9R)-5-{[(tert-Butyl)dimethylsilyl]oxy}dec-6-ene-1,9-diol (15). To a soln. of 14 (1.6 g,
3.55 mmol) in CH2Cl2/H2O 19 :1 (30 ml), DDQ (1.208 g, 5.32 mmol) was added, and the soln. was
stirred for 1 h at r.t. The mixture was filtered, the filtrate washed with 5% NaHCO3 soln. (30 ml) and
brine (30 ml), dried (Na2SO4), and concentrated, and the residue purified by CC: 15 (1.06 g, 91%).
[a]2D5 ¼ ꢀ3.1 (c ¼ 1.4, CHCl3). IR (neat): 3358, 2928, 2854, 1777, 1613, 1513, 1546, 1262, 1155, 1080, 1040,
973. 1H-NMR (500 MHz): 5.45 – 5.66 (m, 2 H); 4.01 – 4.12 (m, 1 H); 3.74 – 3.86 (m, 1 H); 3.62 (t, J ¼ 6.8,
2 H); 2.06 – 2.28 (m, 2 H); 1.46 – 1.61 (m, 4 H); 1.33 – 1.42 (m, 2 H); 1.19 (d, J ¼ 6.04, 3 H); 0.89 (s, 9 H);
0.04 (s, 3 H); 0.01 (s, 3 H). 13C-NMR (75 MHz): 137.2; 125.9; 73.2; 67.2; 62.8; 42.1; 37.9; 32.6; 25.9; 22.7;

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21.4; 18.2; ꢀ 4.3; ꢀ 4.7. ESI-MS: 325 ([M þ Na]þ). HR-ESI-MS: 325.2175 ([M þ Na]þ, C16H34NaO3Siþ;
calc. 325.5272).
(5R,6E,9R)-5-{[(tert-Butyl)dimethylsilyl]oxy}-9-hydroxydec-6-enoic Acid (8). BAIB (0.847g,
2.66 mmol) was added to a soln. of 15 (0.8 g, 2.42 mmol) and TEMPO (0.038 g 0.242 mmol) in CH2Cl2
(1 ml). The mixture was stirred until 15 was no longer detectable (TLC), and then it was diluted with
CH2Cl2 (20 ml). The mixture was washed with sat. aq. Na2S2O3 soln. (20 ml) and extracted with CH2Cl2
(4 ꢁ 20 ml). The combined org. extract was washed with aq. NaHCO3 soln. (30 ml) and brine (30 ml),
dried (Na2SO4), and concentrated. The unstable crude aldehyde was immediately used for the next
reaction.
A soln. of NaClO2 (0.33 g, 3.65 mmol) in H2O (2 ml) was added dropwise within 5 min at r.t. to a
stirred soln. of the above crude aldehyde (0.8 g, 2.43 mmol) in DMSO (5 ml) and NaH2PO4 (0.759 g,
4.869 mmol) in H2O (5 ml). The mixture was left overnight at r.t., and then 5% aq. NaHCO3 soln. was
added. The aq. phase was extracted (3 ꢁ 30 ml) with CH2Cl2 (3 ꢁ 30 ml), the extract washed with brine
(30 ml), dried (Na2SO4), and concentrated, and the residue purified by CC: 8 (0.692 g; 83% yield over
the two steps). Yellowish liquid. [a]2D5 ¼ þ1.2 (c ¼ 2.5, CHCl3). IR (neat): 3424, 2926, 2856, 1774, 1724,
1
1377, 1246, 1170, 1059, 972, 838. H-NMR (400 MHz): 5.45 – 5.61 (m, 2 H); 4.06 – 4.13 (m, 1 H); 3.75 –
3.83 (m, 1 H); 2.94 (s, 1 H); 2.33 (t, J ¼ 6.8, 2 H); 2.07 – 2.25 (m, 2 H); 1.59 – 1.73 (m, 2 H); 1.46 – 1.56
(m, 2 H); 1.18 (d, J ¼ 6.8, 3 H); 0.88 (s, 9 H); 0.04 (s, 3 H); 0.01 (s, 3 H). 13C-NMR (50 MHz): 179.2;
136.8; 126.1; 72.9; 67.4; 41.9; 37.5; 33.9; 25.9; 22.7; 20.5; 18.2; ꢀ 4.3; ꢀ 4.8. ESI-MS: 339 ([M þ Na]þ).
HR-ESI-MS: 339.1978 ([M þ Na]þ, C16H32NaO4Siþ; calc. 339.1968).
(6R,7E,10R)-6-{[(tert-Butyl)dimethylsilyl]oxy}-3,4,5,6,9,10-hexahydro-10-methyl-2H-oxecin-2-one
(16). To a soln. of 8 (0.4 g, 1.16 mmol) and Et3N (0.245 mg, 1.74 mmol) in THF (3 ml), 2,4,6-
trichlorobenzoyl chloride (0.272 ml, 1.74 mmol) was added at r.t. The soln. was stirred at r.t. for 3 h,
diluted with toluene (6 ml), and added into a refluxing soln. of DMAP (0.709 g, 5.80 mmol) and toluene
(50 ml). The mixture was refluxed for 6 h and then cooled to r.t. Sat. aq. NaHCO3 soln. was added, the aq.
layer further extracted with AcOEt (3 ꢁ 20 ml), the combined org. layer washed with H2O (20 ml) and
brine (20 ml), dried (Na2SO4), and concentrated and the crude product purified by CC: 16 (0.265 g,
70%). Colorless oil. [a]2D5 ¼ ꢀ3.4 (c ¼ 0.7, CHCl3). IR (neat): 3446, 2923, 2853, 1733, 1641, 1462, 1365,
1254, 1220, 1186, 1159, 1062, 972, 942. 1H-NMR (300 MHz): 5.37 (ddd, J ¼ 15.3, 10.0, 4.3, 1 H); 5.27 (dd,
J ¼ 15.5, 9.0, 1 H); 5.09 – 5.20 (m, 1 H); 3.90 – 4.01 (m, 1 H); 2.29 – 2.49 (m, 2 H); 1.71 – 2.07 (m, 5 H);
1.43 – 1.65 (m, 1 H); 1.3 (d, J ¼ 6.4, 3 H); 0.86 (s, 9 H); 0.03 (s, 3 H); 0.02 (s, 3 H). 13C-NMR (75 MHz):
175.7; 138.5; 129.5; 74.6; 71.7; 42.2; 39.9; 35.7; 29.7; 25.9; 22.2; 18.1; ꢀ 4.3; ꢀ 4.7. ESI-MS: 321 ([M þ
Na]þ). HR-ESI-MS: 321.1876 ([M þ Na]þ, C16H30NaO3Siþ; calc. 321.1862).
(6R,7E,10R)-3,4,5,6,9,10-Hexahydro-6-hydroxy-10-methyl-2H-oxecin-2-one (¼Aspinolide A; 1). To
a soln. of 16 (0.15 g, 0.459 mmol) in THF (10 ml) was added 1m Bu4NF in THF (0.689 ml, 0.689 mmol) at
08. The mixture was stirred for 6 h and then diluted with H2O and extracted with AcOEt (3 ꢁ 10 ml). The
org. layer was washed with H2O (20 ml) and brine (20 ml), dried (Na2SO4), and concentrated. The crude
product was purified by CC: 1 (0.087 g, 90%). Colorless oil. [a]2D5 ¼ ꢀ41.8 (c ¼ 0.6, MeOH). IR (neat):
1
3442, 2926, 2855, 1728, 1448, 1270, 1187, 1054, 977. H-NMR (500 MHz): 5.51 (ddd, J ¼ 15.8, 10.6, 4.5,
1 H); 5.30 (dd, J ¼ 15.8, 9.8, 1 H); 5.09 – 5.20 (m, 1 H); 3.98 (ddd, J ¼ 3.0, 3.7, 9.8, 1 H); 2.26 – 2.45 (m,
2 H); 1.98 – 2.06 (m, 2 H); 1.86 – 1.93 (m, 2 H); 1.42 – 1.70 (m, 2 H); 1.32 (d, J ¼ 6.8, 3 H). 13C-NMR
(75 MHz): 176.6; 137.1; 131.8; 74.1; 71.7; 42.1; 38.7; 35.6; 22.3; 19.8. ESI-MS: 207 ([M þ Na]þ). HR-ESI-
MS: 207.099 ([M þ Na]þ, C10H16NaO3þ ; calc. 207.0997).
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Received April 21, 2010