Total Synthesis and Structure Revision of Diplobifuranylone B

Article abstract of DOI:10.1021/acs.joc.9b01613

An asymmetric total synthesis of diplobifuranylone B was achieved in 10 steps for the longest linear sequence and in 15.8% overall yield from commercially available methyl (R)-(+)-lactate and l-glutamic acid. This synthesis features a stereoselective construction of the key 2,5-dihydrofuran ring in the natural product via a recently developed asymmetric gold catalysis. The stereochemical flexibility offered by the catalysis enables an expedient revision of the reported structure of diplobifuranylone B, where the relative stereochemistry of the 2,5-dihydrofuran moiety was previously misassigned as cis instead of trans.

Full text of DOI:10.1021/acs.joc.9b01613

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Article
Total Synthesis and Structure Revision of Diplobifuranylone B
Xinpeng Cheng, Carlos D Quintanilla, and Liming Zhang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01613 • Publication Date (Web): 31 Jul 2019
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Total Synthesis and Structure Revision of Diplobifuranylone B
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Xinpeng Cheng, Carlos D. Quintanilla, and Liming Zhang*
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Department of Chemistry and Biochemistry, University of California, Santa Barbara, California
93106, USA
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TOC graphic
ABSTRACT: An asymmetric total synthesis of diplobifuranylone B was achieved in 10 steps for
the longest linear sequence and in 15.8% overall yield from commercially available methyl (R)-
(+)-lactate and L-glutamic acid. This synthesis features a stereoselective construction of the key
2,5-dihydrofuran ring in the natural product via a recently developed asymmetric gold catalysis.
The stereochemical flexibility offered by the catalysis enables an expedient revision of the reported
structure of diplobifuranylone B, where the relative stereochemistry of the 2,5-dihydrofuran
moiety was previously misassigned as cis instead of trans.
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INTRODUCTION
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Diplobifuranylone B (2) was isolated from Diplodia corticola, a fungi species well known as a
phytotoxin producer, in 2006.1 Compounds of its family also include diplobifuranylone A (1),1 and
diplobifuranylone C (3).2 The structures of these natural products (Figure 1) were proposed based
on UV, 1H NMR, 13C NMR, COSY, HMQC, HMBC, and NOESY spectroscopic experiments. The
absolute configuration of the stereogenic carbinol C-6 was deduced via the Mosher’s method.3 The
other stereogenic centers were determined using chiroptical methods in 2017.4 To date, the total
synthesis of these secondary metabolites has not been reported.
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Figure 1. Diplobifuranylones produced by Diplodia corticola
Recently, our group reported a stereoselective strategy for the construction of chiral 2,5-
dihydrofurans from chiral propargylic alcohols via asymmetric gold catalysis. In this chemistry, a
designed chiral bifunctional biphenyl-2-yl phosphine ligand (L) enables a gold-catalyzed
asymmetric isomerization of an alkyne into an allene.5 With a chiral propargylic alcohol as the
substrate, the gold-catalyzed isomerization generates an alcohol intermediate featuring a chiral
allene motif, which undergoes tandem gold-catalyzed stereospecific cyclization to form a chiral
2,5-dihydrofuran with excellent diastereoselectivity (Scheme 1). Herein, we report the application
of this chemistry as a key step toward a concise total synthesis of nominal diplobifuranylone B (2),
which possesses a cis 2,5-dihydrofuran ring. The stereochemical flexibility offered by our gold
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catalysis permits a rapid structural revision of this reported natural product, which instead features
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a trans-dihydrofuran moiety.
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Scheme 1. Gold-Catalyzed Asymmetric Cycloisomerization of Chiral Propargylic Alcohol
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RESULTS AND DISCUSSION
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Our retrosynthetic analysis of nominal diplobifuranylone B (2) is shown in Scheme 2. A
key step is the installation of the cis-2,5-dihydrofuran ring by the gold-catalyzed asymmetric
isomerization/cyclization of the lactone 4 at the final stage. The lactone motif of 4 could be easily
constructed through lactonization of the dihydroxyester 5, which could, in turn, be prepared
through an epoxide ring opening reaction between the epoxide 6 and the alkyne 7.
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Scheme 2. Retrosynthetic Analysis
We commenced the synthesis with the preparation of the two fragments 6 and 7 (Scheme
3). 6 was prepared from L-glutamic acid (8) by following a reported four-steps sequence.6 7 was
synthesized from methyl (R)-(+)-lactate (9) in five steps. The synthetic sequence began with a TBS
group protection of 9, followed by a DIBAL-H reduction. The α-siloxypropanal 10 was isolated
in 78% overall yield. An asymmetric nucleophilic addition to 10 by ethynyltrimethylsilane was
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achieved in the presence of a stoichiometric Ti(OiPr)4-BINOL complex7 to afford the propargylic
alcohol 11 in 70% isolated yield and d.r. >50:1 upon careful column separation. THP protection
of 11 followed by selective removal of the TMS group gave the desired terminal alkyne 7 in a
combined 94% yield.
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Scheme 3. Preparation of the chiral epoxide 6 and the alkyne 7.
With these two chiral fragments in hand, we carried out the planned end game. As shown in
Scheme 4, the BF3-mediated ring opening8 of the chiral epoxide 6 by deprotonated 7 delivered the
γ-hydroxy ester 12 with the requisite carbon skeleton of the natural product. Removal of the THP
group and lactonization were achieved in one step to give the lactone 13 in 90% overall yield.
However, when 13 was subjected to the asymmetric gold catalysis for the construction of the 2,5-
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dihydrofuran ring, the reaction was sluggish with most of the starting material 13 remained after
12 h; moreover, no desired product 14 was found in the reaction mixture.
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Scheme 4. Initial Attempt on the Total Synthesis of Diplobifuranylone B
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We speculated that the strong electron-withdrawing nature of the lactone motif might affect the
gold-catalyzed isomerization of alkyne to allene and hence decided to modify the synthesis
sequence to have the 2,5-dihydrofuran ring installed before the lactonization step. To this end, we
converted the γ-hydroxy esters 12 to the desired propargylic alcohol 16 in 76% combined yield
through a two-step sequence, i.e., TBS protection of the free hydroxyl group to avoid potential
gold-catalyzed 5-endo-trig cyclization and subsequent selective deprotection of the THP group in
the presence of magnesium bromide.9 To our delight, subjecting 16 to our asymmetric gold
catalysis with (S)-L as the ligand afforded smoothly the 2,5-dihydrofuran product 17 in 60% yield
and with a diastereomeric ratio of 95/5. One-pot removal of both TBS groups of 17 and
lactonization in the presence of PTSA/MeOH completed the synthesis of the nominal
diplobifuranylone B (2) in 80% yield (Scheme 5). The absolute stereochemistry of C2’ is assigned
based on our previous report on chiral 2,5-dihydrofuran synthesis.5 To our surprise, there are
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obvious discrepancies between the 1H and 13C NMR spectra (Table 1) as well as optical rotation
{[α]20D = +24.7 (c = 1.06, CHCl3); lit. [α]20D = -90.7 (c = 0.55, CHCl3)}of our synthetic compound
(2) and what were reported.1
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Scheme 5. Revised Route of Total Synthesis of Norminal Diplobifuranylone B
Our asymmetric gold catalysis permits easy access to the C2’-epimer of 2, i.e., the trans-2,5-
dihydrofuran 19 (Scheme 5) by simply employing the ligand enantiomer, i.e., (R)-L, in the
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conversion of 16. Indeed, by following the same two-step endgame, compound 19 was synthesized
with comparable diastereoselectivity and efficiency to 2. Much to our delight, the spectroscopic
data including 1H NMR, 13C NMR, 1H spin decouple, COSY, HMQC, and HMBC and the HR-MS
measurement of 19 match those reported for diplobifuranylone B well except one surprising outlier
in the 13C data. The 13C chemical shifts of 2, 19, and the parent γ-lactone10 and those reported in
the isolation paper are listed in Table 1 for comparison. All the 13Cchemical shifts of 19 are within
0.11 ppm difference from the literature data, which are reported with accuracy down to the 0.1
ppm level, and hence can be considered as identical except that of the lactone C4. For this outlier,
the reported value is 22.9 ppm, but our measured value is 27.9 ppm, which is identical to that of
the parent γ-lactone.10 In addition, the chemical shift of the lactone C4 of diplobifuranylone A1 is
28.2 ppm. We also performed a rather comprehensive literature search of γ-lactones possessing
only one substituent at C-2, as in the case of diplobifuranylone B and found that the chemical shifts
of the lactone C4 range from 26.9 ppm to 29.5 ppm.11 In no example, the 13C signal could shift to
as high a field as 22.9 ppm. Unfortunately, we could not obtain the original 1H and 13C spectra of
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diplobifuranylone B. Considering that all the other C chemical shifts are essentially identical
between the reported data and those of 19, we feel confident to conclude that the reported C4 13C
chemical shift is a typo and should be 27.9 ppm.
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Table 1. 13C NMR Comparison
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Position
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Lit. Δ(2-Lit.) a Δ(19-Lit.) a
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81.04 80.12 68.49 80.1
23.54 23.75 22.06 23.7
27.74 27.97 27.70 22.9
176.39 177.24 177.81 177.2
+0.94
-0.16
+4.84
-0.85
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+0.80
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+0.10
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+1.02
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+5.07
+0.04
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+0.09
+0.03
+0.11
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+0.04
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2’
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87.11 87.97
129.50 128.79
127.20 127.33
91.00 91.01
69.08 69.09
18.92 17.94
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128.7
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90.9
69.1
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a Calculated with the accuracy of the literature data extended to 0.01 ppm.
To understand the misassignment of the natural product’s 2,5-dihydrofuran stereochemistry, we
carefully examined the original reports.1,4 It was based on the NOESY, ROESY and double
decoupling experiments. Our DFT calculations reveal that the distances between H-2’ and H-5’ in
the optimized structures of the cis (2) and the trans (19) isomers at the B3LYP//cc-pVDZ level are
3.50 Å and 3.99 Å, respectively. With both of these measurements less than 4 Å, the observed nOe
might not be a reliable indicator of a cis configuration. The double coupling experiment revealed
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that the long-distance coupling constant between H-2’ and H-5’ is 5.5 Hz, which was suggested to
corroborate the cis stereochemistry. However, this large long-range coupling constant is
characteristic of trans-2,5-dihydrofurans.12 Typically, the J values oftrans-2,5-dihydrofurans are >
5 Hz, while those of cis-2,5-dihydrofurans are <4 Hz. Some of the examples we found in the
literature are listed in Figure 2.13-17 Our decoupling experiments reveal that the coupling constants
between H-2’ and H-5’ are 3.8 Hz and 5.8 Hz for 2 and 19, respectively, which is consistent with
our stereochemistry assignments. A related case is furanomycin (29) (Figure 3). The relative
stereochemistry of its featured 2,5-dihydrofuran ring was initially assigned incorrectly as cis in 30
based on the coupling constant of 5.7 Hz between H-2 and H-5.18 However, it was later determined
to be trans upon its total synthesis12 and X-ray diffraction studies.19
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Figure 2. The long-range coupling constant between H-2 and H-5 (J2,5) in the 2,5-dihydrofuran
systems
Figure 3. Structure of (+)-furanomycin and its initially misassigned cis-structure 30.
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CONCLUSION
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In conclusion, an asymmetric total synthesis of diplobifuranylone B was accomplished in
10 steps for the longest linear sequence and in 14 total steps from the commercially available
methyl (R)-(+)-lactate and L-glutamic acid. The overall yield was 15.8%. The key 2,5-
dihydrofuran moiety of the natural product is constructed via a recently published asymmetric gold
catalysis. This work allows the revision of the structure of diplobifuranylone B, in which the
relative stereochemistry of its 2,5-dihydrofuran moiety is established as trans instead of the
originally assigned cis. The total synthesis is convergent and should be applicable to the synthesis
of other diplobifuranylones.
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EXPERIMENTAL SECTION
General Information: Ethyl acetate (ACS grade), hexanes (ACS grade) and diethyl ether (ACS
grade) were purchased from Fisher Scientific and used without further purification. Anhydrous
dichloromethane (HPLC grade), 1,2-dichloroethane (HPLC grade) was purified by distillation
over calcium hydride. Tetrahydrofuran, toluene, and o-xylene were distilled over
sodium/benzophenone. Commercially available reagents were used without further purification.
Reactions were monitored by thin layer chromatography (TLC) using Silicycle precoated silica
gel plates. Flash column chromatography was performed over Silicycle silica gel (230-400 mesh).
1H NMR and 13C NMR spectra were recorded on Varian 400 MHz, 500 MHz and 600 MHz
spectrometers using residue solvent peaks as internal standards (CDCl3, 1H: 7.26 ppm; 13C: 77.00
ppm).
(R)-2-((tert-butyldimethylsilyl)oxy)propanal (10). Aldehyde (10) was synthesized from Methyl
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(R)-(+)-lactate (9) according to literature procedure . 78% yield, colorless oil; H NMR (600
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MHz, CDCl3) δ 9.61 (d, J = 1.3 Hz, 1H), 4.09 (qd, J = 6.8, 1.3 Hz, 1H), 1.28 (d, J = 6.8 Hz, 3H),
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0.92 (s, 9H), 0.10 (s, 3H), 0.09 (s, 3H); 1H NMR is in accordance with literature20.
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(3S,4R)-4-((tert-butyldimethylsilyl)oxy)-1-(trimethylsilyl)pent-1-yn-3-ol (11). Propargylic alcohol
(11) was synthesized from aldehyde (10) according to a modified literature procedure21. Under N2
protection, TMS acetylene (5.54 mL, 40 mmol) and 14 mL toluene were added into a Schlenk
flask. 1.5 M Et2Zn in toluene (26.7 mL, 40 mmol) was added to the solution carefully. The mixture
was heated to reflux for 1 h, during which time a large amount of white precipitate formed in the
reaction flask. The mixture was cooled to room temperature, and (S)-BINOL (1.14 g, 4 mmol),
Et2O (80 mL), and Ti(OiPr)4 (2.96 mL, 10 mmol) were added. After stirring for 1 h, aldehyde
9(1.88 g, 10 mmol) was added, and the mixture was stirred overnight. 1.0 M tartaric acid was
slowly added into the reaction mixture to quench the reaction and further stirring for 30 min. The
mixture was partitioned in a separatory funnel, and the aqueous portion was extracted three times
with Et2O. The combined organic extracts were washed with brine and dried over MgSO4.
Filtration and concentration, followed by flash column chromatography on silica gel (hexane/Et2O
= 100/1 to hexane/Et2O = 10:1 gradient), afforded propargylic alcohol 11 (2.03 g, 70% yield, d.r >
50 :1) as a white solid.1H NMR (500 MHz, CDCl3) δ 4.23 (dd, J = 5.1, 3.8 Hz, 1H), 3.91 (qd, J =
6.2, 3.9 Hz, 1H), 2.34 (d, J = 5.4 Hz, 1H), 1.22 (d, J = 6.2 Hz, 3H), 0.90 (s, 9H), 0.17 (s, 9H), 0.10
(s, 3H), 0.09 (s, 3H).13C{1H} NMR (126 MHz, CDCl3) δ 103.7, 90.8, 71.0, 67.5, 25.8, 18.2, 18.0,
-0.2, -4.4, -4.8. These data are in accordance with literature21.
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tert-butyldimethyl(((2R,3S)-3-((tetrahydro-2H-pyran-2-yl)oxy)-5-(trimethylsilyl)pent-4-yn-2-
yl)oxy)silane (11-1). Propargylic alcohol (11) (631.5 mg, 2.2 mmol) was dissolved in 10 mL DCM.
3,4-dihydro-2H-pyran (0.3 mL, 3.3 mmol) and pyridinium p-toluenesulfonate (28 mg, 0.11 mmol)
were added into this solution and stirring overnight. After reaction going to completion, sodium
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bicarbonate solid was added into the reaction mixture and stirring for 30 min. The solvent was
removed under reduced pressure and the residue was purified by flash chromatography on silica
gel (hexane/Et2O = 50:1), afforded compound 11-1 (805.6 mg, 99% yield, d.r. = 1/1) as a colorless
liquid. 1H NMR (500 MHz, CDCl3) δ 5.03 (t, J = 3.1 Hz, 1H), 4.85 (t, J = 3.2 Hz, 1H), 4.15 (d, J
= 5.9 Hz, 1H), 4.13 (d, J = 5.1 Hz, 1H), 4.07 – 4.01 (m, 1H), 3.94 – 3.90 (m, 2H), 3.92 – 3.87 (m,
1H), 3.83 – 3.76 (m, 2H), 1.97 – 1.46 (m, 12H), 1.24 (d, J = 6.1 Hz, 3H), 1.21 (d, J = 6.2 Hz, 3H),
0.89 (s, 9H), 0.88 (s, 9H), 0.15 (s, 9H), 0.15 (s, 9H), 0.10 – 0.06 (m, 12H); 13C{1H} NMR (126
MHz, CDCl3) δ 104.4, 103.2, 99.4, 94.6, 90.7, 89.8, 72.9, 71.3, 70.4, 61.9, 61.6, 30.2, 30.2, 25.8,
25.8, 25.52, 25.51, 20.4, 19.1, 18.9, 18.8, 18.1, 18.0, -0.08, -0.11, -4.51, -4.53, -4.7.
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tert-butyldimethyl(((2R,3S)-3-((tetrahydro-2H-pyran-2-yl)oxy)pent-4-yn-2-yl)oxy)silane
(7).
Compound 11-1 (0.80g, 2.2 mmol) was dissolved in 10 mL MeOH, followed by adding K2CO3
(450 mg, 3.3 mmol) into this reaction mixture. The solution was stirred at room temperature for 1
h. 30 mL Et2O was added into this reaction mixture, and then solid was removed via filtration
through silica gel pad (Et2O as eluent). The solvent was removed under reduced pressure to afford
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terminal alkyne (7) (612.1 mg, 94% yield) as a colorless liquid. H NMR (600 MHz, CDCl3) δ
5.02 (t, J = 3.3 Hz, 1H), 4.84 (t, J = 3.4 Hz, 1H), 4.21 (t, J = 4.5 Hz, 1H), 4.21 (t, J = 4.6 Hz, 1H),
4.02 (ddd, J = 11.4, 9.5, 3.1 Hz, 1H), 3.98 – 3.94 (m, 1H), 3.94 – 3.90 (m, 1H), 3.82 (ddd, J = 11.0,
9.7, 2.9 Hz, 1H), 3.57 – 3.49 (m, 2H), 2.41 (d, J = 2.2 Hz, 1H), 2.33 (d, J = 2.1 Hz, 1H), 1.91 –
1.48 (m, 12H), 1.26 (d, J = 6.2 Hz, 3H), 1.23 (d, J = 6.2 Hz, 3H), 0.89 (s, 9H), 0.88 (s, 9H), 0.09
(s, 3H), 0.09 (s, 3H), 0.08 (s, 3H), 0.07 (s, 3H); 13C{1H} NMR (126 MHz, CDCl3) δ 99.7, 94.8,
82.2, 81.1, 74.1, 73.4, 72.0, 71.3, 70.3, 69.9, 62.2, 61.7, 30.2, 30.2, 25.8, 25.8, 25.5, 25.4, 20.2,
20.0, 18.9, 18.7, 18.2, 18.0, -4.55, -4.59, -4.59, -4.8.
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Methyl (S)-3-(oxiran-2-yl)propanoate (6). Epoxide (6) was synthesized from L-glutamic acid (8)
according to literature procedure 8. 20% overall yield, colorless oil; 1H NMR (600 MHz, CDCl3)
δ 3.63 (s, 3H), 2.95 – 2.90 (m, 1H), 2.72 – 2.69 (m, 1H), 2.46 – 2.43 (m, 1H), 2.43 – 2.38 (m, 2H),
1.96 – 1.87 (m, 1H), 1.76 – 1.67 (m, 1H). 1H NMR is in accordance with literature8.
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Methyl
(4S,8S,9R)-9-((tert-butyldimethylsilyl)oxy)-4-hydroxy-8-((tetrahydro-2H-pyran-2-
yl)oxy)dec-6-ynoate (12). Compound 12 was synthesized according to a modified literature
procedure 8. Under nitrogen at -78 ℃, terminal alkyne (7) (2.09 g, 7.0 mmol) was dissolved in dry
THF(5 mL), then n-BuLi (2.5 M in hexane, 2.8 mL, 7.0 mmol) was added. After 5 min, BF3·Et2O
(0.86 mL, 7.0 mmol) was added, and 30 min later, epoxide (6) (650.7 mg, 5.0 mmol) was added.
The reaction media was stirred at -78 ℃ for 3 h, then quenched with sat. NaHCO3(aq). The
aqueous layer was extracted with Et2O for three times. The combined organic layers were washed
with brine, dried over Na2SO4. The solvent was remove under reduced pressure and the residue
was purified by flash column chromatography on silica gel (hexane/EtOAc = 20:1 to
hexane/EtOAc = 10 : 1 to hexane/EtOAc = 1:1), afforded compound 12 (1.68 g, 78% yield) as a
colorless liquid. 1H NMR (400 MHz, CDCl3) δ 4.97 (t, J = 3.2 Hz, 1H), 4.80 (t, J = 3.5 Hz, 1H),
4.19 (dt, J = 3.8, 1.8 Hz, 1H), 4.13 (dt, J = 4.1, 2.0 Hz, 1H), 4.02 – 3.92 (m, 2H), 3.88 (td, J = 6.2,
4.3 Hz, 2H), 3.79 – 3.70 (m, 2H), 3.66 (s, 6H), 3.53 – 3.46 (m, 2H), 2.53 – 2.26 (m, 8H), 1.96 –
1.44 (m, 16H), 1.20 (d, J = 6.5 Hz, 3H), 1.19 (d, J = 6.5 Hz, 3H), 0.87 (s, 9H), 0.87 (s, 9H), 0.07
– 0.04 (m, 12H); 13C{1H} NMR (101 MHz, CDCl3) δ 174.1, 174.1, 99.7, 94.8, 82.5, 82.0, 81.6,
80.1, 73.1, 71.2, 70.7, 70.3, 69.2, 69.2, 62.3, 61.8, 51.6, 51.6, 31.1, 31.0, 30.5, 30.38, 30.36, 30.2,
27.97, 27.93, 25.76, 25.72, 25.42, 25.33, 20.32, 19.18, 19.12, 19.01, 18.1, 18.0, -4.56, -4.67, -4.75,
-4.75; HRMS (ESI-TOF) m/z : [M+Na]+ calculated for C22H40O6SiNa 451.2492; Found 451.2494.
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Methyl (4S,8S,9R)-4,9-bis((tert-butyldimethylsilyl)oxy)-8-((tetrahydro-2H-pyran-2-yl)oxy)dec-6-
ynoate (15). TBSCl (370.8 mg, 2.46 mmol) was added into a solution of 12 (878.9 mg, 2.05 mmol)
and imidazole (279.1 mg, 4.10 mmol) in DMF (10 mL) at room temperature and stirred for 24 h.
The reaction was quench by adding sat. NaHCO3 (aq) into the mixture. The aqueous phase was
extracted with Et2O for four times. The combined organic layer was washed with brine, dried over
Na2SO4. The solvent was removed under reduced pressure and the residue was purified by flash
column chromatography on silica gel (hexane/EtOAc = 20:1), afforded compound 15 (1.01 g, 91%
yield) as a colorless liquid. 1H NMR (600 MHz, CDCl3) δ 4.99 (t, J = 3.3 Hz, 1H), 4.85 (t, J = 3.4
Hz, 1H), 4.23 (dt, J = 4.1, 2.1 Hz, 1H), 4.19 (dt, J = 4.9, 1.9 Hz, 1H), 4.00 (ddd, J = 11.4, 9.5, 3.0
Hz, 1H), 3.89 (dqd, J = 12.4, 6.2, 4.4 Hz, 2H), 3.85 – 3.79 (m, 3H), 3.65 (s, 6H), 3.49 (dqd, J =
12.0, 4.1, 1.5 Hz, 2H), 2.44 – 2.29 (m, 8H), 2.03 – 1.95 (m, 2H), 1.89 – 1.77 (m, 4H), 1.75 – 1.45
(m, 10H), 1.22 (d, J = 6.2 Hz, 3H), 1.20 (d, J = 6.2 Hz, 3H), 0.89 – 0.85 (m, 36H), 0.09 – 0.03 (m,
24H); 13C{1H} NMR (126 MHz, CDCl3) δ 174.0, 173.9, 99.4, 94.7, 82.6, 82.1, 79.2, 72.1, 71.7,
70.6, 70.2, 69.9, 69.8, 62.1, 61.6, 51.4, 31.4, 31.3, 30.26, 30.23, 29.62, 29.62, 27.63, 27.57, 25.80,
25.75, 25.72, 25.52, 25.46, 20.1, 19.02, 18.99, 18.5, 18.1, 18.00, 17.96, -4.56, -4.57, -4.65, -4.80,
-4.88, -4.89; HRMS (ESI-TOF) m/z : [M+Na]+ calculated for C28H54O6Si2Na 565.3357; Found
565.3362
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Methyl (4S,8S,9R)-4,9-bis((tert-butyldimethylsilyl)oxy)-8-hydroxydec-6-ynoate (16). MgBr2·Et2O
(1.44 g, 5.57 mmol) was added into a solution of 15 (859.8 mg, 1.58 mmol) in Et2O and the reaction
mixture was stirred at room temperature until reaction completion. The solvent was removed and
the residue was purified by flash column chromatography (hexane/EtOAc = 20 : 1 to
hexane/EtOAc = 10 : 1) to give 16 (609.3 mg, 84% yield) as a colorless oil. [α]20D = -22.7 (c =
1.19, CHCl3); 1H NMR (500 MHz, CDCl3) δ 4.27 – 4.22 (m, 1H), 3.89 (qd, J = 6.2, 3.6 Hz, 1H),
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3.84 (tt, J = 7.6, 4.3 Hz, 1H), 3.66 (s, 3H), 2.44 – 2.29 (m, 5H), 2.00 (dddd, J = 13.8, 9.1, 6.7, 3.9
Hz, 1H), 1.82 (dddd, J = 13.7, 8.9, 7.4, 6.1 Hz, 1H), 1.21 (d, J = 6.2 Hz, 3H), 0.89 (s, 9H), 0.87 (s,
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9H), 0.08 (s, 3H), 0.08 (s, 3H), 0.06 (s, 3H), 0.05 (s, 3H); C{1H} NMR (101 MHz, CDCl3) δ
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174.0, 83.0, 80.0, 71.2, 69.8, 67.1, 51.5, 31.4, 29.6, 27.5, 25.8, 18.0, 18.0, -4.47, -4.55, -4.84, -
4.86; HRMS (ESI-TOF) m/z : [M+Na]+ calculated for C23H46O5Si2Na 481.2781; Found 481.2796.
Methyl
(S)-4-((tert-butyldimethylsilyl)oxy)-4-((2R,5S)-5-((R)-1-((tert-butyldimethylsilyl)
oxy)ethyl)-2,5-dihydrofuran-2-yl)butanoate (17). To a 2-dram vial were added sequentially 16
(68.0 mg, 0.15 mmol), 10 mol % (S)-LAuCl (13.2 mg), 20 mol% NaBArF (26.4 mg) and 0.75 mL
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dry dichloroethane (DCE). The reaction was stirred at 60 ℃ for 50 h. Upon completion, the
reaction mixture was concentrated under reduced pressure. The residue was purified by silica gel
flash column chromatography (hexane/Et2O = 20 : 1) to yield 17 (40.8 mg, 60% yield, d.r. = 95/5)
as a colorless oil. [α]20D = -13.7 (c = 1.11, CHCl3); 1H NMR (600 MHz, CDCl3) δ 5.99 – 5.96 (m,
1H), 5.95 – 5.92 (m, 1H), 4.58 – 4.52 (m, 1H), 4.47 – 4.37 (m, 1H), 3.66 (s, 3H), 3.48 (qd, J = 4.3,
2.0 Hz, 2H), 2.47 (t, J = 8.0 Hz, 2H), 2.02 – 1.84 (m, 2H), 1.20 (d, J = 6.0Hz, 3H), 0.89 (s, 9H),
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0.89 (s, 9H), 0.06 (s, 3H), 0.05 (s, 3H), 0.04 (s, 3H), 0.03 (s, 3H); C{1H} NMR (126 MHz,
CDCl3) δ 174.3, 129.6, 129.5, 91.2, 88.2, 74.6, 72.4, 51.5, 29.3, 28.5, 25.8, 21.1, 18.04, 18.03, -
4.23, -4.24, -4.5, -4.7; HRMS (ESI-TOF) m/z: [M+Na]+ calculated for C23H46O5Si2Na 481.2781;
Found 481.2770.
Methyl (S)-4-((tert-butyldimethylsilyl)oxy)-4-((2S,5S)-5-((R)-1-((tert-butyldimethylsilyl)
oxy)ethyl)-2,5-dihydrofuran-2-yl)butanoate (18). To a 2-dram vial were added sequentially 16
(68.0 mg, 0.15 mmol), 10 mol % (R)-LAuCl (13.2 mg), 20 mol% NaBArF (26.4 mg) and 0.75 mL
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dry dichloroethane (DCE). The reaction was stirred at 60 ℃ for 50 h. Upon completion, the
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reaction mixture was concentrated under reduced pressure. The residue was purified by silica gel
flash column chromatography (hexane/Et2O = 20 : 1) to yield 18 (40.8 mg, 62% yield, d.r. = 95/5)
as a colorless oil. [α]20D = -124.0 (c = 1.08, CHCl3); 1H NMR (600 MHz, CDCl3) δ 5.96 (dt, J =
6.3, 1.8 Hz, 1H), 5.85 (dt, J = 6.3, 1.8 Hz, 1H), 4.76 (tt, J = 5.2, 1.8 Hz, 1H), 4.55 (tt, J = 6.0, 1.8
Hz, 1H), 3.73 – 3.65 (m, 2H), 3.65 (s, 3H), 2.46 (ddd, J = 15.8, 10.2, 5.4 Hz, 1H), 2.30 (ddd, J =
16.1, 10.0, 6.1 Hz, 1H), 1.79 (tdt, J = 9.9, 6.1, 3.4 Hz, 1H), 1.64 – 1.54 (m, 1H), 1.15 (d, J = 6.2
Hz, 3H), 0.88 (s, 9H), 0.87 (s, 9H), 0.07 (s, 3H), 0.05 (s, 3H), 0.04 (s, 3H), 0.03 (s, 3H); 13C{1H}
NMR (126 MHz, CDCl3) δ 174.2, 129.5, 127.6, 90.9, 89.4, 73.5, 71.3, 51.5, 30.5, 27.3, 25.9, 25.8,
20.3, 18.11, 18.07, -4.32, -4.43, -4.84, -4.88; HRMS (ESI-TOF) m/z : [M+Na]+ calculated for
C23H46O5Si2Na 481.2781; Found 481.2786.
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(S)-5-((2R,5S)-5-((R)-1-hydroxyethyl)-2,5-dihydrofuran-2-yl)dihydrofuran-2(3H)-one (2). p-
toluenesulfonic acid monohydrate (1.0 mg, 0.0055 mmol) was added into a solution of 17 (12.5
mg, 0.03 mol) in MeOH (0.3 mL). The mixture was stirred at room temperature until starting
material 17 has been consumed. The solvent was removed under reduced pressure and the residue
was dissolved in DCM and stirred for 1 h. Then the solvent was removed again, and the residue
was purified by flash column chromatography (hexane/EtOAc = 1/2) to yield 2 (4.5 mg, 83%
yield) as a colorless oil. [α]20D = +24.7 (c = 1.06, CHCl3); 1H NMR (600 MHz, CDCl3) δ 6.07 (dt,
J = 6.6, 1.7 Hz, 1H), 5.96 (dt, J = 6.3, 1.8 Hz, 1H), 4.85 – 4.81 (m, 1H), 4.74 – 4.71 (m, 1H), 4.53
(dt, J = 7.3, 5.8 Hz, 1H), 3.89 (dq, J = 6.5, 3.8 Hz, 1H), 2.61 – 2.49 (m, 2H), 2.39 – 2.31 (m, 1H),
2.19 – 2.11 (m, 1H), 1.22 (d, J = 6.5 Hz, 3H); 13C{1H} NMR (101 MHz, CDCl3) δ 176.4, 129.5,
127.2, 91.0, 87.1, 81.0, 69.1, 27.7, 23.5, 18.9; HRMS (ESI-TOF) m/z: [M+Na]+ calculated for
C10H14O4Na 221.0790; Found 221.0789; J(H2’-H5’) = 3.8 Hz.
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(S)-5-((2S,5S)-5-((R)-1-hydroxyethyl)-2,5-dihydrofuran-2-yl)dihydrofuran-2(3H)-one (19). p-
toluenesulfonic acid monohydrate (2.0 mg, 0.01 mmol) was added into a solution of 18 (22.4 mg,
0.05 mol) in MeOH (0.5 mL). The mixture was stirred at room temperature until starting material
18 has been consumed. The solvent was removed under reduced pressure and the residue was
dissolved in DCM and stirred for 1 h. Then the solvent was removed again, and the residue was
purified by flash column chromatography (hexane/EtOAc = 1/2) to yield diplobifuranylone B (19)
(7.0 mg, 73% yield) as a colorless oil. [α]20D = -132.4 (c = 0.55, CHCl3); Literature1 : [α]20D = -
90.7 (c = 0.55, CHCl3); 1H NMR (600 MHz, CDCl3) δ 6.01 (dt, J = 6.3, 2.0 Hz, 1H), 5.90 (dt, J =
6.3, 2.0 Hz, 1H), 4.97 (dtd, J = 6.1, 2.5, 1.7 Hz, 1H), 4.79 (dddd, J = 5.9, 3.7, 2.3, 1.5 Hz, 1H),
4.54 (ddd, J = 8.0, 5.3, 2.8 Hz, 1H), 3.90 (dq, J = 6.5, 3.4 Hz, 1H), 2.66 (ddd, J = 17.7, 10.1, 7.0,
1H), 2.47 (ddd, J = 17.7, 10.3, 6.4, 1H), 2.34 – 2.27 (m, 1H), 1.64 (br, s, 1H), 1.17 (d, J = 6.5 Hz);
13C NMR{1H} (126 MHz, CDCl3) δ 177.2, 128.8, 127.3, 91.0, 88.0, 80.1, 69.1, 28.0, 23.8, 17.9;
HRMS (ESI-TOF) m/z: [M+Na]+ calculated for C10H14O4Na 221.0790; Found 221.0785; J(H2’-
H5’) = 5.8 Hz, J(H2-H2’) = 2.8 Hz; Literature1: J(H2’-H5’) = 5.5 Hz; J(H2-H2’) = 2.8 Hz.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
DFT Calculation and Spectra Data (PDF)
AUTHOR INFORMATION
Corresponding Author
* zhang@chem.ucsb.edu
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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The authors thank NSF (CHE 1800525) and NIGMS (R01GM123342) for generous financial
support and NIH shared instrument grant S10OD012077 for the acquisition of a 400 MHz NMR
spectrometer. The authors thank Dr. Hongjun Zhou for NMR assistance. We also acknowledge
support from the Center for Scientific Computing at the California NanoSystems Institute and
Materials Research Laboratory, an NSF MRSEC (DMR1720256).
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