Stereoselective synthesis of marine macrolide Aspergillide D

Article abstract of DOI:10.24820/ark.5550190.p010.939

A formal stereoselective synthesis of the naturally occurring 16-membered macrolide aspergillide D is described. The origins of the chiral centers are ribose, lactic acid and the Sharpless asymmetric epoxidation protocol. The foremost reactions involved a

Full text of DOI:10.24820/ark.5550190.p010.939

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Stereoselective synthesis of marine macrolide Aspergillide D
Anil Talakokkula, Karunakar Baikadi, and A. Venkat Narsaiah*
Organic Synthesis Laboratory, Fluoro-Agrochemicals Department,
CSIR-Indian Institute of Chemical Technology, Hyderabad, Telangana, India
Email: vnakkirala2001@yahoo.com; vnakkirala@csiriict.in
Received 03-25-2019
Abstract
Accepted 05-30-2019
Published on line 06-07-2019
A formal stereoselective synthesis of the naturally occurring 16-membered macrolide aspergillide D is
described. The origins of the chiral centers are ribose, lactic acid and the Sharpless asymmetric epoxidation
protocol. The foremost reactions involved are Yadav's protocol, the Ohira- Bestmann reaction and alkyl-iodide
coupling.
Keywords: Aspergillide D, D-ribose, methyl L-lactate, epoxidation, coupling, macrolide
DOI: https://doi.org/10.24820/ark.5550190.p010.939
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Introduction
Aspergillus is a genus consisting of hundreds of species of fungi found in various climates and causes
aspergillosis diseases. They also produce secondary metabolites like aspergillides, aspergillic acid along with
many other compounds. Aspergillides A-C are 14-membered macrolides, incorporating a trans-tetra
hydropyran ring and aspergillic acid, which is a substituted pyrazine.1,2 Aspergillide D is a 16-membered
macrolide, isolated from the extract of a gorgonian associated marine fungal strain Aspergillus
sp.SCSGAF0076. The structural confirmation by 1H, 13C NMR, NOESY, HRMS-ESI and DEPT spectral analysis was
performed by the Qi group.3 The first asymmetric synthesis of aspergillide D was reported by Mohapatra et
al.4
Figure 1
Results and Discussion
As part of our regular research program for the synthesis of biologically active natural products,5-8 herein we
report the formal stereoselective synthesis of aspergillide D. The retrosynthetic analysis (Scheme 1) reveals
that aspergillide D could be synthesized by intramolecular macrolactonisation of fragment 16. The ester (16)
could be derived by the coupling of alkyne (3) and iodide fragments (12). The alkyne fragment could be
accomplished from D-ribose and the iodide intermediate from methyl L-lactate (4).
Scheme 1
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As depicted in Scheme 2, the synthesis of the alkyne fragment was started from the known alcohol 1,
which was prepared from commercially available D-ribose using well known reports.9-12 The alcohol 1 was
protected as its benzyl ether13 with benzyl bromide in presence of NaH in THF to afford compound 2, which on
further oxidative cleavage of terminal olefin via Jin’s protocol by using OsO4, 2,6-lutidine, and NaIO4 leads to
the corresponding terminal aldehyde,14,15 which was subsequentially subjected to the Ohira-Bestmann
reagent16 to get the terminal alkyne fragment (3) in good yields.
o
Reagents & Conditions: (a) NaH, BnBr, THF 0 C to rt, 16 h; (b) (i) OsO4, NaIO4, 2,6-lutidine, 1,4-dioxane/ H2O
(3:1), 25 oC, 3 h; (ii) Dimethyl(1-diazo-2-oxopropyl)phosphonate, K2CO3, MeOH, rt., 5 h, 68% over two steps.
Scheme 2
The synthesis of the iodide fragment 12, was started from commercially available methyl L-lactate17 (4), by
protection of the secondary alcohol as its silyl ether using TBDPS-Cl in the presence of base. This was followed
by a controlled reduction of the ester to give the desired aldehyde, which was immediately subjected to a
homologation reaction with a two carbon Wittig yilde leading to the α,β-unsaturated ester 5. Reduction of the
o
ester moiety was efficiently carried out using DIBAL-H at -78 C in CH2Cl2 to afford the allylic alcohol in good
yield. Thus obtained double bond was subjected to Sharpless asymmetric epoxidation18-20 using (+)-DIPT as a
o
chiral source in presence of Ti(Oi-Pr)4 and TBHP at -20 C to furnish the desired epoxide 6 in good yields.
Yadav's protocol21,22 was applied here for a base induced elimination reaction to give the chiral propargyl
alcohol 7 in 90% yield overall covering two steps from the epoxy alcohol (6) using CCl4-Ph3P under reflux
o
conditions followed by reaction with n-BuLi at -20 C in THF. The hydroxyl functionality in propargyl alcohol 7
was protected as a methoxymethyl ether with MOM-Cl in the presence of DIPEA to afforded compound 8 in
excellent yield.23
Coupling of (5R,6S)-5-ethynyl-6,9,9-trimethyl-8,8-diphenyl-2,4,7-trioxa-8-siladecane (8) and tert-butyl-(4-
iodobutoxy)-dimethylsilane was carried out in the presence of n-BuLi24,25 to afford compound 9 in 70% yield.
The selective desilylation of TBS in the presence of TBDPS ether with pyridinium p-toluenesulfonate26 in
methanol at room temperature afforded the primary alcohol 10. Saturation of the triple bond was achieved
with Pd/C27 (10%) under a hydrogen atmosphere in EtOAc to yield alcohol 11. The corresponding alcohol was
transformed into the iodide28 in the presence of iodine-TPP at toluene-acetonitrile (3:1) under reflux
conditions to afford compound 12.
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Reagents and Conditions: (a) (i) DIBAL-H, CH2Cl2, -78 oC, 30 min, 85%; (ii) (+)-DIPT, Ti(OiPr)4, TBHP, CH2Cl2, -20
oC, 12 h, 95%; (b) (i) CCl4, TPP, reflux, 5 h, 95%; (ii) n-BuLi, -20 oC, 30 min, THF, 85%; (c) MOMCl, DIPEA, CH2Cl2,
rt., 12 h, 96%; (d) tert-Butyl-(4-iodobutoxy)dimethylsilane, n-Buli, THF:HMPA (2:1), -78 oC, 1.5 h, 70%; (e) PPTS,
MeOH, rt, 1 h, 90%; (f) H2, Pd/C (10%), EtOH, rt, 16 h, 90%, (g) TPP, I2, Imidazole, Toluene / Acetonitrile (3:1),
rt, 30 min, 95%.
Scheme 3
The coupling of iodide 12 and the alkyne compound 3 was carried out in the presence of n-BuLi24,25 in a
o
THF-HMPA (2:1) mixture at -78 C to obtain the desired compound 13 in 73% yield. The concomitant removal
of the triple bond as well as the benzyl group was achieved via a Pd/C (10%) catalyzed hydrogenation in EtOAc
to afford the saturated primary alcohol29 14 in 90% yield.
o
Reagents & Conditions: (a) 3, n-BuLi, THF/HMPA (2:1), -78 C, 90 min, 73%; (b) H2, Pd/C, EtOH, rt, 16 h, 90%;
o
(c) (i) DMP, NaHCO3, CH2Cl2, 0 C to rt, 1 h, (ii) Ph3P=CHCO2Et, toluene, reflux, 2 h, 75% over two steps; (d)
HF•Py, THF, 0 oC to rt, 12 h, 75%.
Scheme 4
Oxidation of the primary alcohol with Dess-martin periodinane in CH2Cl2 gave the corresponding aldehyde
and was immediately reacted with the C2-ylide in a Wittig reaction resulting in the α,β- unsaturated ester30 in
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good yield. Finally, the TBDPS group in compound 15 was deprotected using HF•Pyridine31,32 in THF at room
temperature to afford the secondary alcohol 16 in very good yield. This completed the formal synthesis of the
aspergillide D fragment 16 which was identical in all aspects with the compound reported in the literature
data.4
Conclusions
In summary, we have synthesized the desired intermediate 16, which completes the formal synthesis of
aspergillide D in an efficient and completely stereo controlled manner with the longest linear sequence of 14
steps in good yields. The synthesis was started from commercially available starting materials and the majority
of the required stereo centers were deduced from natural sources such as methyl L-lactate and D-ribose. The
key reactions involved in the synthesis are the Sharpless asymmetric epoxydation, Ohira-Bestmenn reagent,
Yadav's protocol for propargyl alcohols and the alkyl iodide coupling.
Experimental Section
General. All the air and moisture sensitive reactions were carried out under an inert atmosphere (nitrogen or
argon). Oven-dried glass ware was used to perform all the reactions. Freshly distilled anhydrous solvents were
used for air and moisture sensitive reactions. Commercially available reagents were used as such. The
purification of the compounds was carried out via column chromatography using silica gel (60-120 mesh)
1
packed in glass columns. H NMR and 13C NMR were recorded in CDCl3 on a 400 MHz and 500 MHz Brucker
spectrometer, using TMS as an internal standard. IR spectra were recorded on a Perkin-Elmer FT-IR 240-c
spectrophotometer using KBr / Thin Film optics. Mass spectra were recorded on a Finnigan MAT 1020 mass
spectrometer operating at 70 eV. Optical rotation values were recorded on a Horiba sepa 300 polarimeter.
High resolution mass spectra (HRMS) [ESI+] were obtained using either a TOF or a double focusing
spectrometer.
(4S,5R)-4-[(Benzyloxy)methyl]-2,2-dimethyl-5-vinyl-1,3-dioxolane (2). To a stirred solution of NaH (0.46 g,
11.3 mmol) in THF (20 mL) was added [(4S,5R)-2,2-dimethyl-5-vinyl-1,3-dioxolan-4-yl]methanol (1.55 g, 9.5
o
mmol), which was dissolved in THF (10 mL). BnBr was slowly added (1.24 mL, 10.4 mmol) at 0 C, then the
reaction mixture was stirred at room temperature for 16 h. After completion of the reaction (confirmed by
TLC), the reaction mixture was diluted by adding EtOAc (25 mL) and was quenched with saturated NaHCO3.
The reaction mixture was extracted with EtOAc (2×20 mL) and the combined organic layers were dried over
Na2SO4, then evaporated under reduced pressure. The crude product was purified by column chromatography
using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (1:9) mixture to afford, olefin 2 (1.4 g, 90%) as an
25
25
oil. Optical rotation [α]D +5.5 (c = 1, CHCl3). Lit[α]D +1.7 (c = 1, CHCl3)13; IR (neat): 2987, 2929, 2868, 1696,
1374, 1223, 1092, 737, 772, 697 cm-1; 1H NMR (400 MHz, CDCl3): δ 7.37 - 7.27 (m, 5H), 5.86 - 5.76 (m, 1H), 5.35
(dt, 1H, J 17.1, 1.3 Hz), 5.21 (dt, 1H, J 10.3, 0.9 Hz), 4.56 - 4.64 (m, 2H), 4.51 (d, 1H, J 12.0 Hz), 4.42 - 4.36 (m,
1H), 3.47 (d, 1H, J 2.4 Hz), 3.45 (d, 1H, J 1.3 Hz), 1.50 (s, 3H), 1.39 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 137.9,
133.5, 128.3, 127.7, 127.6, 118.1, 108.8, 78.4, 76.9, 73.4, 69.4, 27.8, 27.3; HRMS (ESI): m/z [M + Na]+ calcd for
C15H20NaO3: 271.1059; found: 271.1279.
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(4S,5R)-4-[(Benzyloxy)methyl]-5-ethynyl-2,2-dimethyl-1,3-dioxolane (3). To a stirred solution of olefin 2 (1.25
g, 5.0 mmol) in a dioxane-water (3:1, 12 mL) mixture was added 2,6-lutidine (1.2 mL) and OsO4 (0.025 g, 0.1
mmol), which was dissolved in tert-BuOH, followed by NaIO4 (4.3 g, 20.2 mmol) at room temperature. The
resulting mixture was stirred for 12 h. The completion of reaction was confirmed by TLC, then diluted with
CH2Cl2 (15 mL) and quenched with water (15 mL). The organic layer was separated and the aqueous layer was
extracted with CH2Cl2 (2×25 mL). The combined organic layers were washed with a brine solution, dried over
Na2SO4, and concentrated under vacuum. The crude product was purified by column chromatography using
silica gel (60-120 mesh) by eluting with a EtOAc-hexane (5:5) mixture to afford, (4S,5S)-5-[(benzyloxy)methyl]-
2,2-dimethyl-1,3-dioxolane-4-carbaldehyde (1.0 g, 80%) as a liquid.
To a stirred solution of the above aldehyde (0.98 g, 3.9 mmol) in methanol (15 mL) was added dimethyl-1-
o
diazo-2-oxopropyl phosphonate (0.9 g, 4.7 mmol) at 0 C and then K2CO3 was added (1.35 g, 9.8 mmol) in
o
portions over 10 minutes. The reaction was allowed to stir at 0 C for 2 h and then slowly warm up to r.t. and
stirred for 1 h. After the completion of the reaction (monitored by TLC), the solvent was evaporated under
reduced pressure. The residue was quenched with saturated ammonium chloride and extracted with EtOAc
(2×20 mL), the organic layers were washed with brine, dried over Na2SO4, and evaporated under reduced
pressure. Purification was performed by column chromatography using silica gel (60-120 mesh) by eluting with
25
a EtOAc-hexane (1:9) mixture to afford alkyne 3 (0.72 g, 72%) as an oil. Optical rotation [α]D +26.8 (c = 0.5,
1
CHCl3); IR (neat): 3032, 2923, 2854, 2225, 1220, 1063, 772 cm-1; H NMR (400 MHz, CDCl3): δ 7.38 - 7.27 (m,
5H), 4.83 (dd, 1H, J 5.8, 2.2 Hz), 4.59 (ABq, 2H, J 24.9, 12.1 Hz), 4.37 - 4.32 (m, 1H), 3.76 (dd, 2H, J 6.2, 2.7 Hz),
2.49 (d, 1H, J 2.2 Hz), 1.55 (s, 3H), 1.37 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 137.8, 128.4, 127.9, 127.7, 110.6,
79.4, 76.4, 75.5, 73.6, 69.8, 67.6, 27.6, 25.9; HRMS (ESI): m/z [M + Na]+ calcd. for C15H18O3: 269.11482; found:
269.11794.
Ethyl-(4S,2E)-4-[(t-butyldiphenylsilyl)oxy]pent-2-enoate (5). To a stirred solution of (S)-2-[(tert-butyldi
phenylsilyl)oxy]propanal (4.5 g, 14.4 mmol) in toluene (50 mL) was added Ph3P=CHCO2Et (6.5 g, 18.7 mmol).
The resulting mixture was refluxed for 2 h and the completion of reaction was confirmed by TLC, then
quenched with water and extracted with EtOAc (2×30 mL). The combined organic layers were dried over
Na2SO4, the solvent was evaporated under reduced pressure and the crude was purified by column
chromatography using silica gel (60-120 mesh) by eluting with EtOAc-hexane (1:9) mixture to afford, (4S,2E)-
25
ethyl-4-[(tert-butyldiphenylsilyl)oxy]pent-2-enoate, 5 (5.1 g, 92%) as a liquid. Optical rotation [α]D -38.0 (c =
25
1
1, CHCl3), Lit[α]D -38.0 (c = 1, CHCl3)17; IR (neat): 2925, 2855, 1725, 772, 703, 578 cm-1; H NMR (400 MHz,
CDCl3): δ 7.70 - 7.61 (m, 4H), 7.46 - 7.33 (m, 6H), 6.89 (dd, 1H, J = 15.5, 4.4 Hz), 6.00 (dd, 1H, J 15.5, 1.6 Hz),
4.50 - 4.42 (m, 1H), 4.24 - 4.14 (m, 2H), 1.30 (t, 3H, J 7.0 Hz), 1.13 (d, 3H, J 6.6 Hz), 1.08 (s, 9H); 13C NMR (100
MHz, CDCl3): δ 166.8, 151.5, 135.8, 135.7, 133.9, 133.4, 129.7, 127.6, 127.6, 119.1, 68.6, 60.3, 26.9, 23.3, 19.2,
14.3; HRMS (ESI): m/z [M + Na]+ calcd. for C23H30O3NaSi: 405.18564; found: 405.18602.
{(2S,3R)-3-((S)-1-[(t-Butyldiphenylsilyl)oxy)ethyl]oxiran-2-yl}methanol (6). To a stirred solution of ester 5 (4.5
o
g, 11.8 mmol) in dry CH2Cl2 (60 mL) was added DIBAL-H (15.4 mL, 27.1 mmol) drop wise at -78 C and was
stirred for 30 min at the same temperature. The reaction was then quenched with saturated Rochelle salt and
stirred for a further 6 h. The organic layer was separated and the aqueous layer was extracted with EtOAc
(3×20 mL). The combined organic layers were washed with brine (30 mL) and dried over Na2SO4. Evaporation
of the solvent under reduced pressure led to a crude product that was purified by column chromatography
using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (2:8) mixture to afford, (4S,2E)-4-[(tert-
butyldiphenylsilyl)oxy]pent-2-en-1-ol (3.4 g, 85%) as colorless liquid.
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To a stirred solution of (+)-DIPT (0.58 mL, 2.8 mmol) and molecular sieves (2.0 g, 4 oA) in anhydrous CH2Cl2 at -
o
20 C were added Ti(OiPr)4 (1.13 mL, 3.7 mmol) and TBHP (5.1 mL, 56.4 mmol). After stirring for 20 min at -20
oC the allylic alcohol (3.2 mL, 9.4 mmol) was added, which was dissolved in dry CH2Cl2 (20 mL), then stirred for
o
12 h at -20 C. The completion of reaction was detected by TLC, then quenched with a 20% NaOH solution (35
mL). The reaction mixture was stirred for a further 5 h and the reaction mixture was extracted with CH2Cl2
(2x20 mL). The combined organic layers were washed with brine and dried over Na2SO4. Evaporation of the
solvent under reduced pressure gave a crude product that was purified by column chromatography using silica
gel (60-120 mesh) by eluting with a EtOAc-hexane (2:8) mixture to afford epoxy alcohol 6 (3.2 g, 95%) as a
colorless liquid. Optical rotation [α]D25 -48.6 (c = 1.5, CHCl3). IR (neat) 3589, 2925, 2855, 1697, 1464, 1109, 772
1
cm-1; H NMR (400 MHz, CDCl3): δ 7.72 - 7.64 (m, 4H), 7.47 - 7.35 (m, 6H), 3.75 - 3.60 (m, 2H), 3.46 - 3.37 (m,
1H), 2.90 (dd, 1H, J 5.6, 2.1 Hz), 2.69 - 2.65 (m, 1H), 1.54 - 1.48 (m, 1H), 1.22 (d, 3H, J 6.2 Hz), 1.05 (s, 9H); 13
C
NMR (100 MHz, CDCl3): δ = 135.7, 135.9, 133.9, 129.9, 127.6, 127.6, 68.7, 61.2, 58.8, 57.1, 26.9, 20.8, 19.2;
HRMS (ESI): m/z [M + Na]+ calcd. for C21H28O3NaSi: 379.169; found: 379.171.
(3R,4S)-4-[(t-Butyldiphenylsilyl)oxy]pent-1-yn-3-ol (7). To a stirred solution of epoxy alcohol 6 (3.0 g, 8.4
mmol) in CCl4 (50 mL) was added TPP (2.2 g, 8.4 mmol) and was refluxed for 5 h. After completion of the
o
reaction as indicated by TLC, the reaction mixture was cooled to 0 C, then diluted with hexane (30 mL) and
filtered through a celite bed. The filtrate was evaporated under reduced pressure and the crude product was
purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (1:9)
mixture to afford, tert-butyl [(S)-1-((2R,3R)-3-(chloromethyl)oxiran-2-yl)ethoxy]diphenyl silane (3.0 g, 95%) as
a colorless oil.
To a stirred solution of the above epoxychloride (2.9 g, 7.75 mmol) in anhydrous THF (20 mL) was added n-
o
BuLi (9.30 mL, 23.3 mmol) dropwise at -20 C under a nitrogen atmosphere. The reaction mixture was further
stirred for 30 min. After completion of the reaction (monitored by TLC), the reaction was quenched by adding
saturated NH4Cl, was then diluted with EtOAc (30 mL) and extracted with EtOAc (2×20 mL). The combined
organic layers were washed with brine, dried over Na2SO4, then evaporated under reduced pressure. The
crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-
hexane (1:9) mixture to afford, alkyne compound 7 (2.2 g, 85%) as colorless oil. Optical rotation [α]D25 13.5 (c =
1
1, CHCl3); IR (neat): 3590, 3050, 2926, 2855, 2230, 1464, 1109, 772 cm-1; H NMR (400 MHz, CDCl3): δ 7.73 -
7.67 (m, 4H), 7.48 - 7.37 (m, 6H), 4.30 - 4.25 (m, 1H), 4.03 - 3.95 (m, 1H), 2.48 - 2.42 (m, 2H), 1.15 (d, 3H, J 6.3
Hz), 1.09 (s, 9H); 13C NMR (100 MHz, CDCl3): δ 135.8, 135.7, 133.6, 133.4, 129.9, 129.8, 127.8, 127.6, 81.6,
74.2, 71.9, 66.9, 26.9, 19.3, 17.9; HRMS (ESI): m/z [M+Na]+ calcd. for C21H26O2NaSi: 361.15943; found:
361.16016.
(5R,6S)-5-Ethynyl-6,9,9-trimethyl-8,8-diphenyl-2,4,7-trioxa-8-siladecane (8). To a stirred solution of alkyne 7
(1.5 g, 4.4 mmol) in anhydrous CH2Cl2 (15 mL) was added subsequently DIPEA (3.9 mL, 22.2 mmol) and MOM-
Cl (1.0 mL, 13.3 mmol) at r.t. and was then stirred for 12 h. After completion of the reaction (confirmed by
TLC), the reaction was quenched with cooled water (15 mL) and extracted with EtOAc (2×20 mL). The
combined organic layers were dried over Na2SO4, evaporated under reduced pressure and the crude product
was purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (1:9)
25
mixture to afford methoxymethyl ether 8 (1.52 g, 90%) as a colorless oil. Optical rotation [α]D -49.0 (c = 0.5,
CHCl3); IR (neat): 3048, 2926, 2855, 2230, 1696, 1465, 1109, 1039, 772, 703 cm-1; 1H NMR (400 MHz, CDCl3): δ
7.75 - 7.67 (m, 4H), 7.45 - 7.34 (m, 6H), 4.91 (d, 1H, J 6.6 Hz), 4.60 (d, 1H, J 6.6 Hz), 4.37 - 4.32 (m, 1H), 4.05 -
3.97 (m, 1H), 3.35 (s, 3H), 2.37 (s, 1H), 1.16 (d, 3H, J 6.1 Hz), 1.07 (s, 9H); 13C NMR (100 MHz, CDCl3): δ 135.9,
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135.9, 134.2, 133.7, 129.6, 129.5, 127.5, 94.4, 80.6, 74.5, 71.4, 70.4, 55.6, 26.9, 19.3, 18.6; HRMS (ESI): m/z [M
+ Na]+ calcd. for C23H30O3NaSi: 405.18564; found: 405.18593.
t-Butyl(4-iodobutoxy)dimethylsilane. To a stirred solution of 4-[(tert-butyldimethylsilyl)oxy]butan-1-ol (1.5 g,
7.35 mmol) in a toluene-acetonitrile (3:1, 25 mL) mixture was added TPP (1.7 g, 6.6 mmol), imidazole (0.75 g,
11.0 mmol) and iodine (2.8 g, 11.0 mmol) and was then stirred for 30 min at r.t. After completion of the
o
reaction (confirmed by TLC), the reaction was terminated with a Hypo solution at 0 C and was stirred for 15
min, then extracted with EtOAc (2×25 mL). The combined organic layers were washed with brine, dried over
Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography
using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (5:95) mixture to afford, tert-butyl (4-
iodobutoxy)dimethylsilane (1.84 g, 98%) as a colorless oil. IR (neat): υ 2928, 2855, 1738, 1365, 1226, 1033,
836, 774 cm-1; 1H NMR (400 MHz, CDCl3): δ 3.63 (t, 2H, J 6.2 Hz), 3.22 (t, 2H, J 7.1 Hz), 1.94 - 1.87 (m, 2H), 1.65
- 1.58 (m, 2H), 0.89 (s, 9H), 0.05 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 61.9, 33.5, 30.2, 25.9, 18.3, 7.1, -5.3;
HRMS (ESI): m/z [M + Na]+ calcd. for C10H23INaOSi: 337.01545; found: 337.19723.
(5S,6R)-6-(Methoxymethoxy)-2,2,5,14,14,15,15-heptamethyl-3,3-diphenyl-4,13-dioxa-3,14-disilahexadec-7-
yne (9). To a stirred solution of alkyne 8 (1.5 g, 4.5 mmol) in a THF-HMPA (2:1, 10 mL) mixture was added
o
dropwise n-BuLi (1.8mL, 4.5 mmol) at -78 C under a nitrogen atmosphere. While adding n-BuLi, the reaction
mixture converted to a brown color and was then stirred for 30 min at the same temperature. Then tert-
butyl(4-iodobutoxy)dimethylsilane (1 g, 3.2 mmol), which was dissolved in THF (5 mL), was added. The
reaction mixture was stirred for 1 h, and allowed to warm up to room temperature. After completion of the
reaction (confirmed by TLC), the reaction waq quenched with saturated NH4Cl and the solvent was removed
under reduced pressure. The residue was extracted with EtOAc (2x20 mL) and the combined organic layers
were washed with brine, dried over Na2SO4 and the solvent was evaporated under reduced pressure. The
crude product was purified by column chromatography using silica gel, 60-120 mesh by eluting with a EtOAc-
hexane (5:95) mixture to afford the coupled product 9 (1.76 g, 70%) as a color less liquid. Optical rotation
[α]D25 -90.1 (c = 1.0, CHCl3); IR (neat): 3045, 2953, 2926, 2856, 2240, 1540, 1465, 1107, 1039, 703 cm-1; 1H NMR
(400 MHz, CDCl3): δ 7.75 - 7.69 (m, 4H), 7.44 - 7.33 (m, 6H), 4.92 (d, 1H, J 6.6 Hz), 4.59 (d, 1H, J 6.7 Hz), 4.29 -
4.27 (m, 1H), 3.99 - 3.94 (m, 1H), 3.59 (t, 2H, J 12.2, 6.5 Hz), 3.35 (s, 3H), 2.18 (td, 2H, J 7.0, 1.8 Hz), 1.61 - 1.49
(m, 3H), 1.13 (d, 3H, J 6.3 Hz), 1.07 (s, 9H), 0.88 (s, 9H), 0.03 (s, 6H); 13C NMR (100 MHz, CDCl3): δ = 135.9,
135.9, 134.4, 134.0, 129.4, 127.4, 94.1, 86.9, 71.8, 70.8, 62.6, 55.4, 31.9, 29.6, 26.9, 25.9, 25.0, 19.2, 18.7,
18.6, -5.3; HRMS (ESI): m/z [M + Na]+ calcd. for C33H52O4Si2Na: 591.32975; found: 591.32055.
(7R,8S)-8-[(t-Butyldiphenylsilyl)oxy]-7-(methoxymethoxy)non-5-yn-1-ol (10). To a stirred solution of
compound 9 (1.7 g, 3.0 mmol) in anhydrous MeOH (15 mL) was added PPTS (0.82 g, 3.3 mmol) at r.t. and was
stirred for 1 h. After completion of the reaction, the solvent was removed and the residue was quenched with
NH4Cl solution and extracted with EtOAc (2x10 mL), then the organic layers were washed with brine, dried
over Na2SO4 and evaporated under reduced pressure. The crude product was purified by column
chromatography using silica gel (60-120 mesh) by eluting with EtOAc-hexane (2:8) mixture to afford, alcohol
25
10 (1.22 g, 90%) as a colourless oil. Optical rotation [α]D -81.0 (c = 0.5, CHCl3); IR (neat): 3561, 3054, 2925,
1
2855, 2220, 1697, 1463, 1108, 741 cm-1; H NMR (400 MHz, CDCl3): δ 7.74 - 7.68 (m, 4H), 7.45 - 7.33 (m, 6H),
4.91 (d, 1H, J 6.6 Hz), 4.59 (d, 1H, J 6.7 Hz), 4.29 - 4.25 (m, 1H), 4.00 -3.93 (m, 1H), 3.63 (t, 2H, J 12.6, 6.3 Hz),
3.35 (s, 3H), 2.20 (td, 2H, J 6.8, 1.9 Hz), 1.65 - 1.51 (m, 4H), 1.13 (d, 3H, J 6.3 Hz), 1.06 (s, 9H); 13C NMR (100
MHz, CDCl3): δ = 35.9, 135.9, 134.4, 133.9, 129.5, 127.4, 94.1, 86.7, 77.2, 71.7, 70.8, 62.3, 55.5, 31.8, 26.9,
24.8, 19.3, 18.8, 18.5; HRMS (ESI): m/z [M+Na]+calcd. for C27H38O4SiNa: 477.24316; found: 477.24332.
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(7R,8S)-8-[(tert-Butyldiphenylsilyl)oxy]-7-(methoxymethoxy)nonan-1-ol (11). To a stirred solution of alcohol
10 (1.1 g, 2.7 mmol) in EtOAc (10 mL) was added Pd/C (10%, 10 mg) at r.t. and was stirred for 10 h, under a
hydrogen atmosphere. After completion of the reaction, it was filtered through celite and the filtrate was
evaporated under reduced pressure. The crude product was purified by column chromatography using silica
gel by eluting with a EtOAc-hexane (2:8) mixture to afford alcohol 11 (0.98 g, 93%) as a colourless oil. Optical
25
rotation [α]D +12.25 (c = 0.5, CHCl3); IR (neat): 3560, 3053, 2922, 2854, 1459, 1108, 1039 cm-1; 1H NMR (400
MHZ, CDCl3): δ 7.70 - 7.65 (m, 4H), 7.45 - 7.39 (m, 6H), 4.83 (d, 1H, J 6.6 Hz), 4.65 (d, 1H, J 6.6 Hz), 3.82 (qd, 1H,
J 6.4, 2.7 Hz), 3.62 (t, 2H, J 13.3, 6.7 Hz), 3.55 - 3.49 (m, 1H), 3.37 (s, 3H), 1.64 - 1.44 (m, 4H), 1.36 - 1.15 (m,
6H), 1.06 (s, 9H), 1.00 (d, 3H, J 6.23 Hz); 13C NMR (100 MHz, CDCl3): δ 135.9, 135.9, 134.5, 133.8, 129.6, 129.5,
127.5, 127.4, 96.4 81.3, 71.6, 62.9, 55.7, 32.7, 31.2, 29.4, 26.9, 25.8, 25.6, 19.2, 17.9; HRMS (ESI): m/z [M +
Na]+ calcd. for C27H42O4SiNa: 481.27446; found: 481.27490.
(5R,6S)-5-(6-Iodohexyl)-6,9,9-trimethyl-8,8-diphenyl-2,4,7-trioxa-8-siladecane (12). To a stirred solution of
compound 11 (0.9 g, 2.0 mmol) in a toluene-acetonotrile (3:1, 15 mL) mixture were added TPP (0.46 g, 1.76
mmol), imidazole (0.2 g, 2.7 mmol) and iodine (0.75 g, 1.8 mmol). The mixture was then stirred for 30 min at
r.t. and changed to a thick brown color. After completion of the reaction (confirmed by TLC), the reaction
o
mixture was quenched by adding a saturated Hypo at 0 C and was stirred for 15 min. The mixture was
extracted with EtOAc (2x10 mL) and the combined organic layers were washed with brine, dried over Na2SO4
and the solvent was evaporated under reduced pressure. The crude compound was purified by column
chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (5:95) mixture to afford, iodide
25
compound 12 (1.05 g, 95%) as a color less oil. Optical rotation [α]D +11.80 (c = 0.5, CHCl3); IR (neat): 3054,
1
2926, 2855, 1320, 1107, 1037, 739, 703 cm-1; H NMR (400 MHz, CDCl3): δ 7.79 - 7.66 (m, 4H), 7.45 - 7.35 (m,
6H), 4.82 (d, 1H, J 6.5 Hz), 4.64 (d, 1H, J 6.7 Hz), 3.84 - 3.79 (qd, 1H, J 6.4, 2.7 Hz), 3.53 - 3.49 (m, 1H), 3.37 (s,
3H), 3.17 (t, 2H, J 14.2, 7.1 Hz), 1.82 - 1.75 (m, 2H), 1.44 - 1.50 (m, 1H) 1.38 -1.8 (m, 7H), 1.06 (s, 9H), 1.01 (d,
3H, J 6.25 Hz); 13C NMR (100 MHz, CDCl3): δ 135.9, 135.9, 134.5, 133.8, 129.6, 129.5, 127.5, 127.4,
96.4,
81.2, 71.6, 55.7, 33.4, 31.1, 30.4, 28.5, 26.9, 25.6, 19.2, 17.9, 7.2; HRMS (ESI): m/z [M+Na]+calcd. for
C27H41O3INaSi: 591.17619; found: 591.17642.
(5R,6S)-5-{8-[(4R,5S)-5-((Benzyloxy)methyl)-2,2-dimethyl-1,3-dioxolan-4-yl]oct-7-yn-1-yl}-6,9,9-trimethyl-
8,8-diphenyl-2,4,7-trioxa-8-siladecane (13). To a stirred solution of alkyne 3 (0.56 g, 2.3 mmol) in a THF-
o
HMPA (2:1, 5 mL) mixture was added dropwise n-BuLi (0.78mL, 2.0 mmol) at -78 C under a nitrogen
atmosphere. While adding n-BuLi dropwise, the reaction mass converted to a brown color and was then
stirred for a further 30 min at the same temperature. Then, a solution of iodo compound 12 (0.9 g, 1.5 mmol),
which was dissolved in THF (5 mL), was added. The reaction mixture was stirred for a further 1 h, and gradually
warmed to room temperature. After completion of the reaction, the reaction was terminated with saturated
NH4Cl and extracted with EtOAc (2x20 mL). The combined organic layers were washed with brine, dried over
Na2SO4 and concentrated under reduced pressure. The residue was purified by column chromatography using
silica gel (60-120 mesh) by eluting with a EtOAc-hexane (0.5:9.5) mixture to afford 13 (0.76 g, 73%) as a color
25
less liquid. Optical rotation [α]D +31.37 (c = 1, CHCl3); IR (neat): υ 2926, 2855, 2250, 1428, 1107, 1038 cm-1;
1H NMR (400 MHz, CDCl3): δ 7.71 - 7.65 (m, 4H), 7.45 - 7.30 (m, 10H), 4.85 - 4.81 (m, 2H), 4.67 - 4.60 (m, 2H),
4.57 - 4.51 (m, 1H), 4.33 - 4.27 (m, 1H), 3.82 (qd, 1H, J 6.2, 2.6 Hz), 3.74 - 3.68 (m, 2H), 3.54 - 3.48 (m, 1H),
3.36 (s, 3H), 2.20 - 2.14 (td, 2H, J 7.1, 1.9 Hz), 1.53 (s, 3H), 1.50 - 1.40 (m, 4H), 1.38 - 1.12 (m, 10H), 1.06 (s, 9H),
1.01 (d, 3H, J 6.4 Hz); 13C NMR (100 MHz, CDCl3): δ 137.9, 135.9, 135.9, 134.5, 133.8, 129.6, 129.5, 128.3,
127.8, 127.7, 127.5, 127.4, 109.9, 96.4, 88.4, 81.3, 76.6, 75.4, 73.6, 71.6, 70.2, 67.9, 55.7, 31.2, 29.1, 28.8,
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28.4, 27.7, 26.9, 25.9, 25.7, 19.2, 18.7, 17.9; HRMS (ESI): m/z [M+Na]+calcd. for C42H58O6SiNa70: 709.38949;
found: 709.38949.
{(4S,5R)-5-[(9R,10S)-10-((tert-Butyldiphenylsilyl)oxy)-9-(methoxymethoxy)undecyl]-2,2-dimethyl-1,3-di
oxolan-4-yl}methanol (14). To a stirred solution of compound 13 (0.65 g, 0.95 mmol) in EtOAc (5 mL) was
added Pd/C (10%, 10 mg) at r.t. and was stirred under a hydrogen atmosphere for 10 h. After completion of
the reaction (confirmed by TLC), it was filtered through a celite bed and the filtrate was concentrated under
vacuum pressure. The residue was purified by column chromatography using silica gel (60-120 mesh) by
eluting with a EtOAc-hexane (3:7) mixture to afford, alcohol 14 (0.51 g, 90%) as a color less oil. Optical rotation
25
[α]D +8.0 (c = 0.5, CHCl3); IR (neat): 3542, 3026, 2924, 2855, 1533, 1460, 1107, 1039 cm-1; 1H NMR (400 MHz,
CDCl3): δ 7.71 - 7.65 (m, 4H), 7.44 - 7.34 (m, 6H), 4.83 (d, 1H, J 6.6 Hz), 4.65 (d, 1H, J 6.6 Hz), 4.18 - 4.11 (m,
1H), 3.92 - 3.85 (m, 1H), 3.85 - 3.77 (m, 1H), 3.76 - 3.70 (m, 1H), 3.60 - 3.57 (m, 1H), 3.54 - 3.50 (m, 1H), 3.37 (s,
3H), 1.51 - 1.36 (m, 8H), 1.34 - 1.13 (m, 14H), 1.06 (s, 9H), 1.00 (d, 3H, J 6.4 Hz); 13C NMR (100 MHz, CDCl3): δ
135.9, 135.9, 134.6, 133.8, 129.5, 129.5, 127.5, 127.4, 108.5, 96.4, 81.5, 81.3, 77.9, 76.9, 71.7, 62.0, 61.8, 55.6,
33.1, 31.3, 29.6, 29.4, 28.8, 28.2, 27.3, 26.9, 26.6, 25.9, 25.8, 25.5, 19.1, 17.8; HRMS (ESI): m/z [M+Na]+calcd.
for C35H56O6NaSi: 623.37384; found: 623.37484.
Ethyl-(E)-3-{(4S,5R)-5-[(9R,10S)-10-((tert-butyldiphenylsilyl)oxy)-9-(methoxymethoxy)undecyl]-2,2-di
methyl-1,3-dioxolan-4-yl}acrylate (15). To a stirred solution of alcohol 14 (0.45 g, 0.75 mmol) in anhydrous
CH2Cl2 (10 mL) was added DMP (0.41 g, 0.97 mmol) and NaHCO3 (110 mg, 0.44 mmol) at 0 oC for 15 min, and
was then slowly warmed to room temperature for 1 h. The completion of the reaction was monitored by TLC,
o
then cooled to 0 C and the reaction was quenched with crushed ice and was extracted with CH2Cl2 (2x10 mL).
The combined organic layers were washed with brine, dried over Na2SO4, the solvent was evaporated under
reduced pressure and the obtained aldehyde was used for the following Wittig reaction without purification.
To a stirred solution of the above aldehyde in toluene (10 mL) was added Ph3P=CHCO2Et (0.29 g, 0.83 mmol)
and the resulting mixture was refluxed for 2 h. After completion of the reaction (confirmed by TLC), the
reaction was terminated with water and extracted with EtOAc (2×15 mL). The combined organic layer was
dried over Na2SO4, the solvent was evaporated under reduced pressure and the residue product was purified
by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (1:9) mixture to
25
afford, ester compound 15 (0.19 g, 75%) as a colorless liquid. Optical rotation [α]D +5.25 (c = 0.5, CHCl3); IR
1
(neat): 3045, 2924, 2854, 1525, 1462, 1217, 772 cm-1; H NMR (400 MHz, CDCl3): δ 7.70 - 7.65 (m, 4H), 7.44 -
7.34 (m, 6H), 6.86 (dd, 1H, J 15.7, 5.7 Hz), 6.11 (dd, 1H, J 15.5, 1.3 Hz), 4.83 (d, 1H, J 6.6 Hz), 4.65 (d, 1H, J 6.6
Hz), 4.21 (q, 2H), 4.17 - 4.12 (m, 1H), 3.85 - 3.79 (qd, 1H, J 6.2, 2.7 Hz), 3.76 - 3.70 (m, 1H), 3.54 - 3.49 (m, 1H),
3.36 (s, 3H), 1.42 (d, 6H, J 10.8 Hz), 1.36 (m, 19H), 1.05 (s, 9H), 1.00 (d, 3H, J 6.4 Hz); 13C NMR (100 MHz,
CDCl3): δ = 165.9, 143.8, 135.9, 134.6, 133.5, 128.6, 129.5, 127.5, 127.4, 122.9, 108.8, 96.4, 81.3, 78.4, 71.7,
60.5, 55.7, 31.9, 31.3, 30.5, 25.7, 29.7, 29.4, 28.0, 26.9, 26.3, 25.8, 25.3, 19.2, 17.9, 14.2; HRMS (ESI): m/z
[M+Na]+calcd. for C39H60O7SiNa: 691.40005; found 691.40123.
Ethyl(E)-3-{(4S,5R)-5-[(9R,10S)-10-hydroxy-9-(methoxymethoxy)undecyl]-2,2-dimethyl-1,3-dioxolan-4-yl}
acrylate (16). To a stirred solution of ester compound 15 (0.3 g, 0.44 mmol) in anhydrous THF (5 mL) in a
o
plastic vial was added the HF•Py complex (70%, 0.1 mL) at 0 C. The reaction mixture was slowly warmed to
room temperature and was stirred for 24 h. The completion of the reaction was confirmed by TLC, then
carefully poured in a saturated NaHCO3 solution (5 mL) and was stirred for 30 min and extracted with EtOAc
(2×10 mL). The combined organic layers were further washed with a saturated CuSO4 (5 mL) solution, water (5
mL), brine (5 mL) and then the organic layer was dried over Na2SO4 and concentrated under pressure. The
residue product was purified by column chromatography using silica gel, (60-120 mesh) by eluting with a
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EtOAc-heaxane (1:9) mixture to afford the secondary alcohol 16 (0.14 g, 75%) as a colorless liquid. Optical
rotation [α]D25 +5.30 (c = 1, CHCl3). Lit[α]D20 +7.18 (c = 1, CHCl3)4; IR (neat): 3556, 2924, 2854, 1725, 1432, 1254,
1
1034, 772 cm-1; H NMR (400 MHz, CDCl3): δ 6.84 (dd, 1H, J 15.7, 6.2 Hz), 6.06 (dd, 1H, J 15.7, 1.5 Hz), 4.74 (d,
1H, J 6.9 Hz), 4.67 - 4.61 (m, 2H), 4.24 - 4.18 (m, 4H), 3.79 - 3.7 (m, 1H), 3.52 - 3.47 (m, 1H), 3.43 (s, 3H), 3.06
(brs, 1H), 1.51 (s, 3H), 1.38 (s, 3H), 1.32 - 1.23 (m, 18H) 1.14 (d, 3H, J 6.5 Hz); 13C NMR (100 MHz, CDCl3): δ
166.0, 143.8, 122.9, 108.8, 97.6, 85.2, 78.3, 77.2, 68.9, 60.5, 55.8, 30.9, 30.5, 29.7, 29.6, 29.4, 28.0, 26.3, 26.0,
25.5, 17.1, 14.2. HRMS (ESI): m/z [M+Na]+calcd. for C23H42O7SiNa: 453.28227; found: 453.28375.
Acknowledgements
Authors AT and KB are thankful to UGC-New Delhi and CSIR-New Delhi respectively for providing fellowships.
We also thank the Minister of Science and Technology for providing financial support under DST-SERB-
EEQ/2018/001311. We are grateful to the Director CSIR-IICT for providing facilities.
IICT Communication No: IICT/Pubs./2018/185.
Supplementary Material
1
Copies of H and 13C NMR of compounds 2-16 are available in the Supplementary Material file associated with
this manuscript.
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