Stereoselective total synthesis of sporiolide B and attempted synthesis of sporiolide A

Article abstract of DOI:10.1080/00397911.2015.1135958

A simple and efficient stereoselective total synthesis of sporiolide B and attempted synthesis of sporiolide A, from epichlorohydrin, using asymmetric synthetic approach is reported. The key reactions involved are Sharpless epoxidation, Jacobsen reaction, syn-allylation, Yamaguchi esterification, and Grubbs ring-closing metatheses reaction to result in the macrocyclic ring system.

Full text of DOI:10.1080/00397911.2015.1135958

Synthetic Communications
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Chemistry
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Stereoselective total synthesis of sporiolide B and
attempted synthesis of sporiolide A
Gattu Sridhar & Gangavaram V. M. Sharma
To cite this article: Gattu Sridhar & Gangavaram V. M. Sharma (2016) Stereoselective total
synthesis of sporiolide B and attempted synthesis of sporiolide A, Synthetic Communications,
46:4, 314-321, DOI: 10.1080/00397911.2015.1135958
To link to this article: http://dx.doi.org/10.1080/00397911.2015.1135958
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Date: 15 March 2016, At: 02:46

SYNTHETIC COMMUNICATIONS®
2016, VOL. 46, NO. 4, 314–321
http://dx.doi.org/10.1080/00397911.2015.1135958
Stereoselective total synthesis of sporiolide B and attempted
synthesis of sporiolide A
Gattu Sridhar and Gangavaram V. M. Sharma
Organic and Biomolecular Chemistry Division, CSIR Indian Institute of Chemical Technology, Hyderabad, India
ABSTRACT
ARTICLE HISTORY
Received 3 November 2015
A simple and efficient stereoselective total synthesis of sporiolide B
and attempted synthesis of sporiolide A, from epichlorohydrin, using
asymmetric synthetic approach is reported. The key reactions
involved are Sharpless epoxidation, Jacobsen reaction, syn-allylation,
Yamaguchi esterification, and Grubbs ring-closing metatheses
reaction to result in the macrocyclic ring system.
KEYWORDS
Ring-closing metathesis;
syn-allylation; sporiolide A;
sporiolide B; Yamaguchi
esterification
GRAPHICAL ABSTRACT
Introduction
Over the past few years, natural products have continued to be of interest on account of
their diverse biological properties. Synthesis of biologically important natural products
has always fascinated organic chemists. Because of their continuous efforts, tremendous
development in the synthesis of complex natural products such as macrolides, steroids,
terpenes, and alkaloids has been possible. Natural products containing a medium ring
lactone framework are found in plants, insects (pheromones), and bacteria (antibiotics),
and they can have a terrestrial or a marine origin.
Sporiolide B (1) and sporiolide A (2) are 12-membered macrolides isolated by Shigemori
et al.[1] from a marine-derived fungus Cladosporium sp., which was separated from an
Okinawan marine brown alga Actinotrichia fragilis. Macrolides 1 and 2 were found to
exhibit cytotoxicity against murine lymphoma L1210 cell line with IC50 values of 0.13
and 0.81 µg/mL, respectively. Further assays revealed that sporiolide A (2) shows moderate
antifungal activity against Candida albicans (MIC 16.7 µg/mL), Cryptococcus neoformans
CONTACT Gattu Sridhar
sridhar1org@yahoo.com
Organic and Biomolecular Chemistry Division, CSIR Indian Institute
of Chemical Technology, Hyderabad, 500107, India.
Supplemental data for this article can be accessed on the publisher’s website.
© 2016 Taylor & Francis

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Figure 1. Structures of sporiolide B (1) and sporiolide A (2).
(MIC 8.4 µg/mL), Aspergillus niger (MIC 16.7 µg/mL), and N. crassa (MIC 8.4 µg/mL)
respectively.
In 2006 the first synthesis of sporiolide B was reported[2] from D-xylose in 15 linear
steps and 3.5% of overall yield. In 2007, Du et al.[3] again reported synthesis of sporiolide
B from D-glucal in 17 steps with 4.8% overall yield, and in 2006[4] reported the first total
synthesis of sporiolide A from D-glucal in 16 steps with 6.1% of overall yield. In 2010
Venkateswarlu et al.[5] reported stereoselective total synthesis of cytotoxic sporiolide A
from D-mannital in 13% steps with 7% of overall yield.
In this study our main aim is to synthesize both the macrolides sporiolide B (1) and
sporiolide A (2) (Fig. 1). Sporiolide B (1) was successfully achieved and met with failure
to give sporiolide A (2).
Results and discussion
As shown in Scheme 1 the retrosynthetic analysis of macrolactone 1 revealed that it could
be obtained from bis-olefin 3, while macrolactone 2 could be obtained from bis-olefin 4, by
ring-closing metathesis (RCM) protocol. Bis-olefin 3 in turn could be realized by esterfica-
tion of acid 5 with alcohol 6 and bis-olefin 4 from acid 7 and alcohol 6. The alcohol 6 could
be realized from propylene epoxide 8.
Acids 5 and 7 could be derived from 9 and 10 respectively, while both the 9 and 10
could be made from 11. Alcohol 11 could be realized from the known alcohol 12, which
can be derived from epichlorohydrin 13.
Accordingly, reaction of the known alcohol 12[6] (prepared from 13), with BnBr and
NaH in tetrahydrofuran (THF) at 0 °C to 35 °C for 6 h afforded the benzyl ether 14 in
85% yield (Scheme 2). Desilylation of 14 with tetrabutylammonium fluoride (TBAF) in
THF for 1 h gave 15 in 85% yield. Swern oxidation of 15 and subsequent Wittig olefination
of 15a afforded the ester 16 in 87% yield.
Diisobutylaluminium hydride (DIBAL-H) reduction of 16 in CH2Cl2 at 0 °C for 2 h furn-
ished the allylic alcohol 17 (86%), which on Sharpless epoxidation[7] with (þ)-diisopropyl
tartrate (DIPT), Ti(OiPr)4, and cumene hydroperoxide in CH2Cl2 afforded 18 (85%). Epox-
ide 18 on opening with Red-Al in dry THF for 3 h afforded 19 (85%), which on treatment
with TBSCl and imidazole in CH2Cl2 for 2 h gave TBS ether 11 in 90% yield, a common
intermediate for 1 and 2.
For the synthesis of sporiolide B (1), 11 was subjected to reaction with methyl iodide and
NaH in THF at 0 °C to 35 °C for 4 h to afford 9 in 85% yield. Oxidative deprotection of
paramethoxybenzyl (PMB) ether in 9 with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)
in aqueous CH2Cl2 at 0 °C to 35 °C for 1 h gave 20 (87%), which on Swern oxidation fol-
lowed by 1,2-syn allylation[8] of aldehyde 20a with allyltributyltin and MgBr2 · Et2O in CH2Cl2
at À 78 °C for 4 h furnished the allylic alcohol 21 in 79% yield. Reaction of 21 with MOMCl

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G. SRIDHAR AND G. V. M. SHARMA
Scheme 1. Retroanalysis of sporiolide B (1) and sporiolide A (2).
and DIPEA in CH2Cl2 at 0 °C to 35 °C for 6 h furnished the methoxy methyl (MOM) ether 22
in 86% yield. Desilylation of 22 with TBAF in THF at 0 °C to 35 °C for 1 h gave 23 (89%),
which on oxidation with 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) and bis(acetoxy)
iodobenzene (BAIB) in CH2Cl2/H2O (1:1) furnished the acid 5 in 81% yield.
Likewise, alcohol 6 was prepared from the known epoxide 8.[9] Accordingly, treatment
of 8 with allylmagnesium chloride in ether at À 78 °C gave alcohol 6.
Esterification of acid 5 and alcohol 6 under Yamaguchi conditions[10] in the presence of
2,4,6-trichlorobenzoyl chloride and Et3N followed by DMAP in toluene for 1 h afforded
bis- olefin 3 in 87% yield. RCM reaction of ester 3 with Grubb’s[11] second-generation
Scheme 2. Synthesis of fragment 11.

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Scheme 3. Synthesis of sporiolide B (1).
catalyst in CH2Cl2 at reflux for 6 h furnished lactone 24 in 76% yield. Hydrogenation of 24
with 10% Pd-C in MeOH for 12 h afforded 25 in 77% yield, by concomitant debenzylation
followed by double-bond reduction.
Oxidation of 25 with Dess–Martin periodinane[12] in anhydrous CH2Cl2 at 0 °C to 35 °C
for 4 h furnished ketone 26 in 80% yield. Finally, deprotection of MOM ether with 10%
HCl in THF for 5 h afforded sporiolide B 1 in 84% yield, whose spectral and optical
rotation data were comparable with the data reported in literature.[1]
Scheme 4. Synthesis of fragment 29.

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G. SRIDHAR AND G. V. M. SHARMA
Scheme 5. Attempted synthesis of sporiolide A (2).
To ascertain the absolute stereochemistry in 21, 11 was subjected oxidative deprotection
of PMB ether with DDQ in aqueous CH2Cl2 at 0 °C to 35 °C for 1 h to give 11a. Selective
oxidation of primary alcohol with TEMPO, TBACl, and NCS and buffer in CH2Cl2 for 1 h
furnished aldehyde 27, which on syn-allylation[8] with allyltributyltin and MgBr2 · Et2O in
CH2Cl2 at À 78 °C for 4 h gave the corresponding allylic alcohol 28 in 79% yield.
Reaction of 28 with 2,2-dimethoxy propane and p-TsOH (cat.) in CH2Cl2 at 0 °C to 35 °C
for 1 h furnished acetonide 29 (73%), whose 13C NMR confirmed the syn-1,3-diol[13] in 29.
For the synthesis of 2, alcohol 11 was subjected to reaction with TBSCl in the presence of imi-
dazole in CH2Cl2 to give TBS ether 10 in 84% yield. Oxidative deprotection of PMB ether 10 with
DDQ in aqueous CH2Cl2 at 0 °C to 35 °C for 1 h gave 30 in 79% yield. Swern oxidation of 30 in
CH2Cl2 for 2 h furnished the aldehyde 30a, which on syn-allylation[8] with allyltributyltin and
MgBr2 · Et2O in CH2Cl2 at À 78 °C for 4 h gave the corresponding allylic alcohol 31 in 76% yield.
Reaction of 31 with MOMCl and DIPEA in CH2Cl2 at 0 °C to 35 °C for 6 h furnished the
MOM ether 32 in 76% yield. Desilylation of 32 with PPTS in MeOH at 0 °C to 35 °C for 1 h
gave 33 in 78% yield, which on oxidation with TEMPO and BAIB in aqueous CH2Cl2 (1:1)
furnished the acid 7 in 72% yield.
Acid 7 on esterification with alcohol 6 under Yamaguchi conditions[10] afforded
bis-olefin 4 in 70% yield. Ester 4 was subjected to RCM reaction with Grubb’s[11] second-
generation catalyst under different reaction conditions, which, however, met with failure to
give macrolactone 34.
Experimental
(4S,5R,6R)-5-(Benzyloxy)-8-(tert-butyldimethylsilyloxy)-6-methoxyoct-1-en-
4-ol (21)
To a solution of oxalyl chloride (0.9 mL, 10.54 mmol) in dry CH2Cl2 (15 mL) at À 78 °C, dry
DMSO (2.8 mL, 21.18 mmol) was added dropwise and stirred for 20 min. A solution of 20
(2.5 g, 7.06 mmol) in dry CH2Cl2 (20 mL) was added and stirred for 2 h at À 78 °C. Workup
as described for 15a furnished the corresponding aldehyde 20a.

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319
To a solution of the aldehyde 20a (2.5 g, 7.1 mmol) in dry CH2Cl2 (25 mL), MgBr2 · OEt2
(3.66 g, 14.20 mmol) was added at À 78 °C. The reaction mixture was stirred at À 78 °C for
45 min. Allyltributyltin (3.26 mL, 10.63 mmol) was added to the reaction mixture dropwise
and stirred for 4 h at À 78 °C. It was quenched by the addition of saturated NH4Cl solution
(15 mL), allowed to warm to room temperature, and extracted with CH2Cl2 (3 � 15 mL).
The combined organic layers were washed with brine (20 mL) and dried (Na2SO4). Solvent
was evaporated and the residue was purified by column chromatography (60- to 120-mesh
silica gel, 9% EtOAc in petroleum ether) to furnish 21 (2.2 g, 79%) as a yellow syrup. The
dr of syn and anti alcohol is 97:3. ½a�25 ¼ þ 73.7 (c 0.5, CHCl3); IR (neat): 3425, 3070, 2929,
D
1
1719, 1642, 1452, 1376, 1316, 1273, 1176, 1099, 1026, 919, 758 cmÀ 1; H NMR (CDCl3,
300 MHz): δ 7.39–7.28 (m, 5H, ArH-Bn), 5.98–5.79 (m, 1H, olefinic), 5.20–5.08 (m, 2H,
olefinic), 4.90 (d, 2H, J ¼ 6.9 Hz, benzylic), 4.27–4.18 (m, 1H, -OCH), 3.90–3.68 (m, 3H,
-OCH2,-OCH), 3.55 (s, 3H, -OCH3), 3.47–3.38 (m, 1H, -OCH), 2.59–2.48 (m, 2H,
-CH2), 1.99–1.78 (m, 2H, -CH2), 0.93 (s, 9H, 3 � -CH3), 0.14 (s, 6H, 2 � -CH3); 13C
NMR (CDCl3, 75 MHz): δ 137.8, 134.6, 128.4, 128.3, 128.1, 128.0, 127.9, 117.4, 104.9,
80.3, 73.9, 70.5, 60.1, 58.7, 39.1, 38.3, 25.6, 17.9, À 3.6; HRMS (ESI): m/z calculated for
C22H38O4NaSi [M þNa]þ 417.1659, found 417.1665.
(3R,4S,5S)-((S)-Hex-5-en-2-yl)4-(benzyloxy)-3-methoxy-5-(methoxymethoxy)oct-7-
enoate (3)
To a solution of acid 5 (0.7 g, 2.07 mmol) and Et3N (0.5 mL, 4.13 mmol) in dry THF (15 mL)
at 0 °C, 2,4,6-trichlorobenzoyl chloride (0.32 mL, 2.07 mmol) in dry THF (5 mL) was added
and stirred the reaction mixture at 35 °C temperature for 2 h under a nitrogen atmosphere. It
was filtered and the filtrate evaporated. The resulting anhydride was dissolved in toluene
(10 mL) and treated with alcohol 6 (0.2 g, 2.07 mmol) in toluene (2 mL), and catalytic amount
of DMAP in dry toluene (10 mL) was added to the reaction mixture and stirred for 1 h at 35 °
C temperature. It was filtered through celite and washed with toluene (2 � 25 mL). Solvent
was evaporated and purified the residue by column chromatography (60- to 120-mesh silica
gel, 6% EtOAc in petroleum ether) to afford 3 (0.75 g, 87%) as a yellow syrup with 100%
diastereoselectivity. ½a�25 ¼ þ 97.6 (c 0.5, CHCl3); IR (neat): 3448, 3073, 2930, 1731, 1639,
D
1453, 1376, 1274, 1161, 1103, 1035, 916, 748, 699 cmÀ 1
;
1H NMR (300 MHz, CDCl3):
δ 7.36–7.27 (m, 5H, ArH-Bn), 5.93–5.73 (m, 2H, olefinic), 5.15–5.04 (m, 4H, olefinic),
4.99–4.93 (m, 2H, benzylic), 4.67 (s, 2H, -OCH2), 3.95–3.88 (m, 2H, 2 � -OCH),
3.80–3.76 (m, 1H, -OCH), 3.68–3.63 (m, 1H, -OCH), 3.39 (s, 6H, 2 � -OCH3), 2.70–2.61
(m, 2H, -CH2), 2.49–2.43 (m, 2H, -CH2), 2.15–2.04 (m, 2H, -CH2), 1.77–1.62 (m, 2H,
-CH2), 1.20 (d, 3H, J ¼ 6.2 Hz, -CH3); 13C NMR (CDCl3, 75 MHz): δ 171.7, 138.4, 137.8,
134.8, 128.2, 127.8, 127.5, 117.4, 114.8, 96.0, 79.8, 78.4, 73.4, 70.4, 57.8, 55.9, 36.4, 35.3, 35.1,
29.6, 19.8; HRMS (ESI): m/z calculated for C24H36O6Na [MþNa]þ 443.1469, found 443.1477.
(4R,5R,6S,12S)-5-(Benzyloxy)-4-methoxy-6-(methoxymethoxy)-12-
methyloxacyclododec-8-en-2-one (24)
To a stirred solution of 3 (0.5 g, 1.19 mmol) in dry CH2Cl2 (1 mg/1 mL, 500 mL), Grubbs’
second-generation catalyst (10 mol%) was added and stirred at reflux for 6 h and then at
35 °C in the open air for an additional 1 h. The reaction mixture was filtered through celite

320
G. SRIDHAR AND G. V. M. SHARMA
and concentrated, and the residue was purified by column chromatography (60- to
120-mesh silica gel, 8% EtOAc in petroleum ether) to afford 24 (0.35 g, 76%) as a yellow
syrup with 100% diastereoselectivity. ½a�25 ¼ þ 61.8 (c 0.5, CHCl3); IR (neat): 3449, 2925,
D
1
2852, 1731, 1633, 1456, 1372, 1247, 1149, 1099, 1032, 958, 917, 749, 699 cmÀ 1; H NMR
(300 MHz, CDCl3): δ 7.49–7.29 (m, 5H, ArH-Bn), 5.40–5.25 (m, 2H, olefinic), 4.69 (m,
4H, benzylic, -OCH2), 4.07–3.83 (m, 2H, 2 � -OCH), 3.74–3.48 (m, 1H, -OCH), 3.38 (s,
6H, 2 � -OCH3), 3.37–3.31 (m, 1H, -OCH), 2.71–2.56 (m, 1H, -CH), 2.35–2.11 (m, 1H,
-CH), 2.35–2.11 (m, 1H, allylic -CH), 2.04–1.84 (m, 1H, allylic -CH), 1.34–1.24 (m, 2H,
allylic -CH2), 1.22 (d, 3H, J ¼ 6.3 Hz, -CH3), 0.98–0.81 (m, 2H, -CH2); 13C NMR (CDCl3,
75 MHz): δ 171.0, 138.8, 133.8, 128.6, 128.1, 127.6, 97.4, 79.6, 74.2, 74.6, 67.3, 67.6, 56.5,
56.2, 38.7, 38.4, 26.8, 28.6, 24.4; HRMS (ESI): m/z calculated for C22H32O6Na [MþNa]þ
415.1132, found 415.1126.
(4R,6S,12S)-6-Hydroxy-4-methoxy-12-methyloxacyclododecane-2,5-dione (1)
A stirred solution of 26 (0.03 g, 0.09 mmol) in THF (20 mL) was cooled to 0 °C, treated with
10% aqueous HCl (1 mL), and stirred at 35 °C for 5 h. The reaction mixture was quenched
with saturated NaHCO3 solution (5 mL) and extracted with EtOAc (2 � 10 mL). The com-
bined organic layers were washed with water (2 � 15 mL) and brine (15 mL), and dried
(Na2SO4). The solvent was evaporated and the residue was purified by column chromato-
graphy (60- to 120-mesh silica gel, 20% EtOAc in petroleum ether) to give 1 (21 mg, 84%)
as a colorless amorphous solid with 100% diastereoselectivity. ½a�2D5 ¼ À 29.1 (c 0.5,
1
CHCl3); IR (neat): 3429, 2934, 1710, 1646, 1220, 1170, 1106, 763, 713 cmÀ 1; H NMR
(300 MHz, CDCl3): δ 4.83–4.81 (m, 1H, -OCH), 4.44 (d, 1H, J ¼ 8.7 Hz, -OCH), 4.30 (d,
1H, J ¼ 5.7 Hz, -OCH), 3.53 (s, 3H, -OCH3), 3.43 (t, 1H, J ¼ 12.6 Hz, -CH), 2.60 (dd,
1H, J ¼ 8.8 Hz, 13.9 Hz, -CH), 2.06–2.04 (m, 1H, -CH), 1.69–1.58 (m, 2H, 2 � -CH),
1.57–1.48 (m, 2H, 2 � -CH), 1.47–1.38 (m, 2H, 2 � -CH), 1.36–1.25 (m, 2H, 2 � -CH),
1.23 (d, 3H, J ¼ 6.4 Hz, -CH3), 1.09–1.07 (m, 1H, -CH); 13C NMR (75 MHz, CHCl3):
δ 208.8, 170.6, 76.1, 74.4, 73.3, 58.4, 41.3, 33.4, 29.9, 26.5, 23.8, 22.1, 20.8; HRMS (ESI):
m/z calculated for C13H22O5Na [MþNa]þ 281.1367, found 281.1369.
(3R,4R,5S)-((S)-Hex-5-en-2-yl)4-(benzyloxy)-3-(tert-butyldimethylsilyloxy)-5-
(methoxymethoxy)oct-7-enoate (4)
To a solution of acid 5 (0.3 g, 0.68 mmol) and Et3N (0.2 mL, 1.36 mmol) in dry THF
(10 mL) at 0 °C, 2,4,6-trichlorobenzoyl chloride (0.1 mL, 0.68 mmol) in dry THF (3 mL)
was added and stirred at 35 °C for 2 h under a nitrogen atmosphere. It was filtered, and
the filtrate evaporated. The resulting anhydride was dissolved in toluene (3 mL) and treated
with alcohol 6 (0.11 g, 0.68 mmol) in toluene (7 mL), and a catalytic amount of DMAP in
dry toluene (5 mL) was added to the reaction mixture and stirred for 1 h at 35 °C. It was
filtered through celite and washed with toluene (2 � 15 mL). The solvent was evaporated
and the residue was purified by column chromatography (60- to 120-mesh silica gel,
7% EtOAc in petroleum ether) to afford 3 (0.25 g, 70%) as a yellow syrup with 100% dia-
stereoselectivity. ½a�25 ¼ þ 31.7 (c 0.5, CHCl3); IR (neat): 2932, 2857, 1740, 1643, 1462,
D
1
1239, 1096, 919, 776, 698 cmÀ 1; H NMR (300 MHz, CDCl3): δ 7.37–7.27 (m, 5H, ArH-
Bn), 5.93–5.74 (m, 2H, olefinic), 5.13–5.06 (m, 2H, olefinic), 5.04–4.94 (m, 2H, olefinic),

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4.92–4.87 (m, 1H, benzylic), 4.82 (m, 1H, J ¼ 11.4 Hz, benzylic), 4.72–4.63 (m, 3H, -OCH,
-OCH2), 4.45–4.40 (m, 1H, -OCH), 3.80–3.77 (m, 1H, -OCH), 3.61–3.57 (m, 1H, -OCH),
3.39 (s, 3H, -OCH3), 2.67-2.61 (m, 2H, -CH2), 2.47–2.43 (m, 2H, -CH2), 2.31–1.72 (m, 4H,
2 � -CH2), 1.20 (d, 3H, J ¼ 6.25 Hz, -CH3), 0.88 (s, 9H, 3 � -CH3), 0.09 (d, 6H, J ¼ 6.6 Hz,
2 � -CH3); 13C NMR (CDCl3, 75 MHz): δ 171.4, 138.6, 137.7, 134.9, 128.2, 127.6, 127.4,
117.2, 114.8, 95.9, 83.2, 73.6, 70.5, 69.8, 68.7, 55.9, 39.1, 35.0, 29.5, 25.8, 19.7, 17.9, 16.5,
À 4.4, À 4.8; HRMS (ESI): m/z calculated for C29H48O6NaSi [MþNa]þ 543.3112, found
543.3101.
Conclusion
Thus, in summary an efficient stereoselective total synthesis of sporiolide B (1) has been
achieved in 22 steps with a 4.35% overall yield, while the attempted synthesis of sporiolide
A (2) from epichlorohydrin failed to give macrolactone at the macrocyclization stage.
Funding
The authors are thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, for
financial support (CSC-0108; BSC-0116). G. S. R. is thankful to CSIR, New Delhi, India, for the
award of a research fellowship.
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