Toward the synthesis of macrolide aspergillide D

Article abstract of DOI:10.1080/00397911.2019.1660900

Stereoselective synthesis of 16-membered macrocyclic core of marine natural product, Aspergillide D is described using linear strategy. The salient features of this synthetic study include the Sharpless asymmetric kinetic resolution followed by regioselective ring-opening reaction of corresponding epoxide to establish the stereo centers and ring-closing metathesis (RCM) by Grubbs'-II catalyst for the construction of macrocyclic ring.

Full text of DOI:10.1080/00397911.2019.1660900

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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Toward the synthesis of macrolide aspergillide D
Satheeshkumar Reddy Kandimalla, B. V. Subba Reddy & Gowravaram
Sabitha
To cite this article: Satheeshkumar Reddy Kandimalla, B. V. Subba Reddy & Gowravaram
Sabitha (2019): Toward the synthesis of macrolide aspergillide D, Synthetic Communications, DOI:
10.1080/00397911.2019.1660900
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https://doi.org/10.1080/00397911.2019.1660900
Toward the synthesis of macrolide aspergillide D
Satheeshkumar Reddy Kandimallaa,b,c, B. V. Subba Reddya,c, and
Gowravaram Sabithab,c
aFluoro-Agrochemicals Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India;
bNatural Products Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India;
cAcademy of Scientific and Innovative Research, New Delhi, India
ABSTRACT
ARTICLE HISTORY
Received 15 July 2019
Stereoselective synthesis of 16-membered macrocyclic core of marine
natural product, Aspergillide D is described using linear strategy. The
salient features of this synthetic study include the Sharpless asym-
metric kinetic resolution followed by regioselective ring-opening
reaction of corresponding epoxide to establish the stereo centers
and ring-closing metathesis (RCM) by Grubbs’-II catalyst for the con-
struction of macrocyclic ring.
KEYWORDS
Macrolide; natural product;
regioselective ring-opening
reaction; ring-closing
metathesis (RCM); sharpless
kinetic resolution
GRAPHICAL ABSTRACT
Introduction
Fourteen- and sixteen-membered macrolides have received a lot of attention from
researchers due to their prominent antibiotic activity,[1,2] and most of them were iso-
lated from fungi and diverse species of Streptomyces. Aspergillide D (1) is a 16-mem-
bered marine macrolide (Figure 1), isolated by Qi et al.[3] from the extract of the
gorgonian allied fungal strain Aspergillus sp. SCSGAF 0076 and the structure was estab-
lished by 1H, 13C NMR, NOESY, DEPT and HRMS analysis. In 2017, Mohapatra
group[4] described the first total synthesis of aspergillide D. Very recently Narsaiah
et al.[5] reported a formal synthesis of aspergillide D.
In continuation of our research interest in the total synthesis of macrolides,[6] herein
we disclose our efforts toward the total synthesis of aspergillide D in which, ring-closing
metathesis (RCM) as the key step to accumulate the macrolactone core and Sharpless
CONTACT B. V. Subba Reddy
basireddy@iict.res.in; Gowravaram Sabitha
gowravaramsr@yahoo.com
Senior
Principal Scientist, D-132, Discovery Laboratory, Natural Products Chemistry Division, CSIR-Indian Institute of Chemical
Technology, Tarnaka, Hyderabad, 500007, India
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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S. R. KANDIMALLA ET AL.
Figure 1. Structure of Aspergillide D (1).
Scheme 1. Retrosynthetic analysis of Aspergillide D.
kinetic resolution followed by regioselective ring-opening reaction of the resultant epox-
ide to generate the required stereo centers.
The retrosynthetic analysis of Aspergillide D (1) is illustrated in Scheme 1, the
macrocyclic core 2 could be constructed from the diene compound 3 via Ring closing
metathesis using Grubbs’-II catalyst. The diene compound 3 could be synthesized from
epoxy alcohol 4 through one carbon homologation of compound 4 by regioselective
epoxide ring opening using Me3SI (trimethylsufonium iodide), TBS-deprotection fol-
lowed by acrylation. The epoxy alcohol 4 could be obtained from alcohol 5 via oxida-
tion on 1ꢀ alcohol and the resultant aldehyde could be coupled with vinylmagnesium
bromide, which would be accessed racemic allylic alcohol, followed by Sharpless kinetic
resolution. The compound 5 would be prepared from epoxy alcohol 6 by benzylation of
2ꢀ alcohol and epoxide ring opening with LAH, followed by deprotection of PMB group
with DDQ. Further, the epoxy alcohol 6 could be derived from commercially available
1,10-decanediol (7) through Sharpless asymmetric kinetic resolution.
Results and discussion
We initiated the synthesis of required alcohol intermediate 5 (Scheme 2) with commer-
cially available 1,10-decanediol (7). Mono-PMB protection of diol 7 with 4-methoxyben-
zyl alcohol using Amberlyst-15 in CH2Cl2 reflux conditions[7] provided PMB ether 8 in
90% yield. The primary alcohol 8 was oxidized by PCC (Pyridinium Chlorochromate)[8]
in CH2Cl2 to get the resultant aldehyde, which was subjected to Grignard reaction with
vinylmagnesium bromide (1 M solution in THF) at ꢁ78 ꢀC to give the racemic allylic
alcohol 9 in 75% yield over two steps. Then, the allylic alcohol 9 was converted to the
chiral epoxy alcohol 6 in 44% yield under Sharpless kinetic resolution conditions[9] by

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Scheme 2. Reagents and conditions: (a) PMB-OH, Amberlyst-15, CH2Cl2, reflux, 12 h, 90%; (b) 1) PCC,
CH2Cl2, rt, 1–2 h, 2) VinylMgBr, THF, ꢁ20 ꢀC, 2 h, 75% yield, over two steps; (c) (-)-DIPT, Ti(OiPr)4,
TBHP, MS (4 Ao), CH2Cl2, ꢁ25 ꢀC, 12 h, 44%; (d) NaH, BnBr, DMF, 0 ꢀC-rt, 4–5 h, 94%; (e) LiAlH4, THF,
0 ꢀC-rt, 2–3 h, 90%; (f) TBSCl, Im, CH2Cl2, 0 ꢀC-rt, 2 h, 95%; (g) DDQ, CH2Cl2:H2O (19:1), pH ¼ 7 buffer,
rt, 1 h, 86%.
the reaction of (-)-DIPT and Ti(OiPr)4 in the presence of 0.5 equivalents of tert-butyl-
hydroperoxide in CH2Cl2 at ꢁ25 ꢀC. The enantiomeric purity of compound 6 was
determined by LCMS analysis (dr ¼ 94:6). The free 2ꢀ alcohol group in 6 was protected
as its benzyl ether to produce 10 in 94% yield by treating it with Benzyl bromide and
Sodium hydride (NaH) in DMF. The terminal epoxide 10 was reductively opened with
LiAlH4 in THF at 0 ꢀC to achieve 11 in 90% yield. Then the corresponding 2ꢀ alcohol
11 was masked with TBS-Cl in the presence of Imidazole in CH2Cl2 to obtain com-
pound 12 in 95% yield. The PMB group in compound 12 was oxidatively removed
under neutral conditions using DDQ in CH2Cl2:H2O (19:1) in phosphate buffer solution
(pH ¼ 7) to get the desired alcohol 5 in 86% yield.[10]
Subsequently, the primary alcohol 5 was oxidized under IBX conditions[11] to obtain
the corresponding aldehyde, which was coupled with vinylmagnesium bromide gave the
resultant racemic allylic alcohol 13 in 70% yield over two steps. Following the similar
chemistry of compound 6, the epoxy alcohol 4 was also prepared as enantiomerically
rich in 42% yield under Sharpless kinetic resolution conditions along with enantio-
merically pure (S)-allylic alcohol (13a) in 46% yield.[9] At this stage to improve the yield
of alcohol 4, successively we converted the (S)-allylic alcohol (13a) into (R)-alcohol
(13c) by inversion of the hydroxyl configuration using the Mitsunobu procedure in 72%
yield through the corresponding ester 13 b.[12] Then, the (R)-alcohol (13c) was trans-
formed to the epoxy alcohol 4 by Sharpless epoxidation using (-)-DIPT/Ti(OiPr)4 in
77% yield (Scheme 3).
Consequently, one carbon homologation on the resultant epoxy alcohol 4 was car-
ried out via regioselective epoxide ring opening with Me3SI/n-BuLi[13] to obtain diol
compound 14 in 75% yield (Scheme 4). The stereo chemistry of 1,2-diol 14 was
determined using a modified Mosher ester analysis.[14] So that to conform the con-
figurations at C-3 and C-4 of diol 14 was treated with R- or S-methoxy-(trifluoro-
methyl)phenylacetyl acid [MTPA, as chiral dervatizing agent (CDA)] and DCC
(coupling reagent) in the presence of DMAP in CH2Cl2 to give the corresponding
1
(S,S)- and (R,R)-bis-MTPA esters respectively.[15] Then, recorded the HNMR spectra
of both bis-MTPA esters and calculated the DdSR values as shown in Figure 2. Thus
the absolute configuration of 1,2-diol (14) was established as 3S, 4 R (See in detail
results in the supporting information).

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S. R. KANDIMALLA ET AL.
Scheme 3. Reagents and conditions: (a) 1) IBX, ACN, reflux, 1 h, 2) VinylMgBr, THF, ꢁ20 ꢀC-rt, 2 h,
70% yield, over two steps; (b) (-)-DIPT, Ti(OiPr)4, TBHP, MS (4 Ao), CH2Cl2, ꢁ25 ꢀC, 12 h, 42%; (c) DIAD,
TPP, P-Nitrobenzoicacid, THF, 0 ꢀC, 1 h; (d) K2CO3, MeOH, rt, 2 h, 72%; (e) (-)-DIPT, Ti(OiPr)4, TBHP, MS
(4 Ao), CH2Cl2, ꢁ25 ꢀC, 12 h, 77%.
Scheme 4. Reagents and conditions: (a) n-BuLi, Me3SI, THF, ꢁ20 ꢀC-rt, 3 h, 75%; (b) 2,2-DMP, PPTS,
CH2Cl2, 0 ꢀC-rt, 3 h 90%; (c) TBAF (1.0 M), THF, 0 ꢀC-rt, 24 h, 84%; (d) Acryloyl Chloride, TEA, CH2Cl2,
0 ꢀC-rt, 30 min., 76%; (e) Grubbs’-II Catalyst, CH2Cl2, reflux, 16 h, 61%; (f) TiCl4, CH2Cl2, 0 ꢀC.
Figure 2. Selected Dd Values of the (S,S)- and (R,R)-MTPA bis-esters of 1,2-diol (14) {DdSR ¼ d [(S,S)-
MTPA bis-ester] - d [(R,R)- MTPA bis-ester] in ppm}{DdSR values (in Hz): DdSR (¼dS – dR) 400MHz}.

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Then, the acetonide compound 15, achieved from 14 by treatment with 2,2-dime-
thoxypropane in the presence of PPTS in CH2Cl2 at room temperature in 90% yield.
The compound 15 further reacted with TBAF (1.0 M) in THF to access the TBS-depro-
tected intermediate 16 in 84% yield, followed by acrylation in the presence of acryloyl
chloride, TEA in DCM produced diene compound 3 in 76% yield. The diene ester 3,
which set the stage for the ring-closing metathesis (RCM) reaction[16] to accumulate the
16-membered macrocyclic lactone 2. The diene ester was dissolved in degassed CH2Cl2
and refluxed with Grubbs’ 2nd generation catalyst under high dilution conditions to
1
deliver the macrolactone 2 in 61% yield. The RCM product 2 was established by H
1
and 13C NMR spectral analysis. H NMR revealed the resonating peaks at d 6.82 (dd, J1
¼ 15.7 Hz, J2 ¼ 7.5 Hz, 1 H) and 6.04 (dd, J1 ¼ 15.7 Hz, J2 ¼ 1.2 Hz, 1 H) for newly
generated internal double bond and coupling constant J ¼ 15.7 Hz shows its trans-geom-
etry. 13C NMR also disclosed the peaks at d 142.6 and 124.2 ppm for the double bond
carbons and a peak at m/z 453.2615 [M þ Na]þ in ESI-HRMS further confirmed the
formation of 16-membered macrocycle 2.
Finally, the total deprotection of cyclic compound 2 having benzyl (Bn) and acetonide
groups tried with TiCl4 to achieve the synthesis of aspergillide D (1). Unfortunately, we
could not get the final product and observed the decomposition of cyclised product 2.
At this stage, though we had a small quantity of 2, started further investigation to com-
plete the total synthesis of 1 as well as we thought that Bn group may create the prob-
lem in final step. Then, a selective protection of the secondary hydroxyl group also
required instead of Bn group to achieve the target molecule. These efforts are under
progress in our group and will be reported in due course.
Conclusion
In this report, we have accomplished a linear synthesis of macrocyclic ring of aspergil-
lide D starting from 1,10-decanediol in 14 steps. Key steps in the synthetic sequence
include Sharpless kinetic resolution followed by regioselective ring-opening reaction of
corresponding epoxide to create the stereo centers and ring-closing metathesis (RCM)
to assemble the macrolactone core.
Experimental
General: All reactions were carried out using standard syringe, septa and cannula tech-
niques under inert atmosphere. All glassware apparatus used for reactions are perfectly
oven/flame dried. Anhydrous solvents were distilled prior to use: THF from Na and
benzophenone; CH2Cl2 from CaH2; MeOH from Mg cake. Commercial reagents were
used without purification. All organic extracts were dried with anhydrous Na2SO4.
Organic solutions were concentrated in a rotary evaporator under reduced pressure at a
bath temperature below 35 ꢀC. The diastereomeric ratio of the products were measured
with a chiral-phase HPLC using Chiralpak AS column. Column chromatography was
carried out by using silica gel (60–120 mesh) unless otherwise mentioned. Analytical
thin layer chromatography (TLC) was run on silica gel 60 F254 pre-coated plates
(250 lm thickness). Optical rotations [a]D were measured on a polarimeter and given in

6
S. R. KANDIMALLA ET AL.
10ꢁ1 degcm2gꢁ1. Infrared (IR) spectra were recorded in CHCl3/KBr or neat (as men-
tioned) and reported in wave number (cmꢁ1). Mass spectral data were obtained using
MS (EI) ESI, HRMS mass spectrometers. High resolution mass spectra (HRMS) [ESIþ]
1
were obtained using either a TOF or a double focusing spectrometer. H NMR spectra
were recorded at 400, 500 MHz and 13C NMR spectra at 100 MHz in CDCl3 solution
unless otherwise mentioned, chemical shifts are in ppm downfield from tetramethylsi-
lane and coupling constants (J) are reported in hertz (Hz). The following abbreviations
are used to designate signal multiplicity: s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet,
m ¼ multiplet, br ¼ broad.
(3aS,8S,9R,17aR,E)-9-(benzyloxy)-2,2,8-trimethyl-3a,8,9,10,11,12,13,14,15,16,17,17a-
dodeca hydro-6H[4,5-e][1]oxacyclohexadecin-6-one (macrolactone core 2): Grubbs’-II
catalyst (0.031 g, 0.037 mmol) was added to a solution (degassed with nitrogen) of com-
pound 3 (0.170 g, 0.371 mmol) in CH2Cl2 (150 mL) at room temperature. The resultant
reaction mixture was stirred overnight at 40 ꢀC. The solvent was evaporated under rota
and the crude product was purified by column chromatography using silica gel
(EtOAc:hexane ¼ 1:9) to provide lactone 2 (0.087 g, 61% yield) as a pale-yellow oil.
20
[a]D ¼ ꢁ3.81 (c 0.41, CHCl3); IR (neat): 2927, 2855, 1939, 1717, 1637, 1452, 1373,
1
1263, 1170, 1066, 979, 739, 696 cmꢁ1; H NMR (500 MHz, CDCl3) d 1.20–1.56 (m,
25 H), 3.38–3.44 (m, 1 H), 3.50–3.59 (m, 1 H), 4.18–4.25 (m, 1 H), 4.55 (d, J ¼ 11.6 Hz,
1 H), 4.60 (d, J ¼ 11.6 Hz, 1 H), 5.03–5.10 (m, 1 H), 6.04 (dd, J1 ¼ 15.7 Hz, J2 ¼ 1.2 Hz,
1 H), 6.82 (dd, J1 ¼ 15.7 Hz, J2 ¼ 7.5 Hz, 1 H), 7.30–7.38 (m, 5 H); 13C NMR (100 MHz,
CDCl3) d 17.9, 22.7, 23.2, 25.7, 26.2, 26.9, 27.7, 28.4, 29.3, 29.4, 29.6, 78.5, 78.7, 79.7,
80.7, 80.9, 108.1, 124.2, 127.9(2 C), 128.3, 128.4(2 C), 139.3, 142.6, 164.7; HRMS calcd
for C26H38O5Na 453.2617 [M þ Na]þ, found 453.2615.
Full experimental detail, 1H and 13C NMR spectra can be found via the
“Supplementary Content” section of this article.
Acknowledgments
KSR thank UGC, India for the award of fellowship. All the authors thank Council of Scientific
and Industrial Research (CSIR), New Delhi, India and CSIR-Indian Institute of Chemical
Technology (IICT) for financial support.
IICT Communication No: IICT/Pubs./2019/273.
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