Total synthesis of pro-resolving and tissue-regenerative Protectin sulfido-conjugates

Article abstract of DOI:10.1016/j.tetlet.2015.09.020

The stereospecific total synthesis of the pro-resolving and tissue-regenerative Protectin sulfido-conjugates: 16R,17S-PCTR1, 16R,17S-PCTR2, and 16R,17S-PCTR3, derived from docosahexaenoic acid, has been achieved. The key intermediate 16S,17S-epoxy-Protectin methyl ester was synthesized using the Sharpless catalytic asymmetric epoxidation to generate the chiral centers at C16 and C17. A Cs₂CO₃ promoted coupling provided the skipped diyne intermediate. Wittig reactions and epoxide opening with glutathione, l-cysteinylglycine, and l-cysteine methyl ester hydrochloride, respectively, were the key steps in the synthesis.

Full text of DOI:10.1016/j.tetlet.2015.09.020

Tetrahedron Letters 56 (2015) 5811–5815
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of pro-resolving and tissue-regenerative Protectin
sulfido-conjugates
⇑
Ana R. Rodriguez, Bernd W. Spur
Department of Cell Biology, Rowan University—SOM, Stratford, NJ 08084, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The stereospecific total synthesis of the pro-resolving and tissue-regenerative Protectin sulfido-
conjugates: 16R,17S-PCTR1, 16R,17S-PCTR2, and 16R,17S-PCTR3, derived from docosahexaenoic acid,
has been achieved. The key intermediate 16S,17S-epoxy-Protectin methyl ester was synthesized using
the Sharpless catalytic asymmetric epoxidation to generate the chiral centers at C16 and C17. A
Cs2CO3 promoted coupling provided the skipped diyne intermediate. Wittig reactions and epoxide
Received 7 August 2015
Revised 3 September 2015
Accepted 5 September 2015
Available online 7 September 2015
opening with glutathione,
were the key steps in the synthesis.
L
-cysteinylglycine, and
L
-cysteine methyl ester hydrochloride, respectively,
Keywords:
Protectin sulfido-conjugates
Total synthesis
Ó 2015 Elsevier Ltd. All rights reserved.
Inflammation resolution
Wittig reaction
Sharpless epoxidation
Dess–Martin oxidation
It has been extensively described in the literature that
x
-3
The proposed biosynthesis of the Protectin sulfido-conjugates is
based on the identified precursor and its products formed as
shown in Figure 1. Docosahexaenoic acid is converted to the 17S-
hydroperoxide intermediate that undergoes epoxide formation to
produce 16S,17S-epoxy-Protectin. Enzymatic hydrolysis gave
Protectin D1 whereas glutathione S-transferase gives the 16,17S-
Protectin conjugate in tissue regeneration (PCTR1). The authors
pointed out that it is most likely that the stereochemistry at C16
is R similar as described for the peptido-leukotrienes via SN2 epox-
ide opening by glutathione.16–19 PCTR1 is further metabolized by a
polyunsaturated fatty acids mainly eicosapentaenoic and docosa-
hexaenoic acids have anti-inflammatory activities and decrease
the seriousness of infection.1,2 These fatty acids have been investi-
gated in a variety of diseases including rheumatoid arthritis,
asthma, and cardiovascular diseases. Their effects on neutrophil
function and leukotriene generation in humans have been
described.3–5 The molecules responsible for these beneficial actions
were discovered by Serhan and collaborators to be lipoxygenase
derivatives of these fatty acids. In their pioneering experiments
they identified in the exudates of self-resolving inflammation
several polyhydroxylated metabolites of eicosapentaenoic and
docosahexaenoic acids with powerful anti-inflammatory and pro-
resolution activities: Resolvins, Protectins, and Maresins, collec-
c
-glutamyl transpeptidase to PCTR2 and finally by a dipeptidase to
PCTR3.15
Due to the limited availability from natural sources these sul-
fido-conjugates have to be prepared by total synthesis to make
them available for further pharmacological evaluation. So far a
total synthesis of the Protectin sulfido-conjugates has not been
reported. We recently reported the total syntheses of the Maresin
sulfido-conjugates.20
In this Letter we wish to report the first total synthesis of the
17-series sulfido-conjugated mediators 16R,17S-PCTR1 [(4Z,7Z,
10Z,12E,14E,16R,17S,19Z)-16-glutathionyl-17-hydroxy-4,7,10,12,
14,19-docosahexaenoic acid (1)], 16R,17S-PCTR2 [(4Z,7Z,10Z,
12E,14E,16R,17S,19Z)-16-cysteinylglycinyl-17-hydroxy-4,7,10,12,
14,19-docosahexaenoic acid (2)], and 16R,17S-PCTR3 [(4Z,7Z,
10Z,12E,14E,16R,17S,19Z)-16-cysteinyl-17-hydroxy-4,7,10,12,14,19-
docosahexaenoic acid (3)]. As shown in the retrosynthetic
approach (Fig. 2) 1, 2, and 3 have been prepared from the chiral
tively named specialized pro-resolving mediators (SPMs).6–12
A
new class of SPMs derived from docosapentaenoic acid has just
been reported.13
Recently, Serhan and collaborators investigated self-resolving
exudates in Escherichia coli infected mice, human spleen, and blood
of sepsis patients, and discovered the novel anti-inflammatory, pro-
resolving, and tissue regenerative sulfido-conjugates of Maresin,
Protectin, and Resolvin.14,15 These compounds promote the patho-
gen clearance and could provide a new way to treat infections.
⇑
Corresponding author. Tel.: +1 856 566 7016; fax: +1 856 566 6195.
E-mail address: spurbw@rowan.edu (B.W. Spur).
http://dx.doi.org/10.1016/j.tetlet.2015.09.020
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

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A. R. Rodriguez, B. W. Spur / Tetrahedron Letters 56 (2015) 5811–5815
Figure 1. Biosynthesis of Protectin D1 and proposed biosynthesis of 16R,17S-PCTR1, 16R,17S-PCTR2 and 16R,17S-PCTR3 from DHA.15
16S,17S-epoxy-Protectin methyl ester (4) via epoxide opening with
glutathione, -cysteinylglycine, and -cysteine methyl ester
hydrochloride, respectively. The key intermediate 4 has been
prepared from intermediates 5, 6, and 7. The chiral centers of the
epoxy-aldehyde 7 were generated using the Sharpless catalytic
1.1 equiv of triphenylphosphine in acetonitrile at 75 °C for 36 h in
93% yield. The excess of triphenylphosphine was removed by
extracting the acetonitrile phase with hexane.
L
L
The synthesis of 16S,17S-epoxy-Protectin methyl ester (4) is
described in Scheme 2. Sharpless cat. AE of (2E)-2-octen-5-yn-1-ol
(15) gave the chiral epoxy alcohol 16 as previously described.25
The crystalline nature of 16 allowed to obtain this key intermediate
with >98% ee after recrystallization. Lindlar reduction of 16 in hex-
ane/ethyl acetate in the presence of triethylamine produced 17
that was oxidized using the Dess–Martin periodinane reagent in
CH2Cl2 affording the epoxy aldehyde 7 in 93% yield.26 The four-
carbon homologation of 7 with recrystallized (2E)-4-triphenylphos-
phoranylidene-2-butenal (6) in CH2Cl2 at rt for 17 h followed by
treatment with catalytic iodine in benzene afforded the E,E-dienal
18.27,28 Next, Z-selective Wittig reaction of 18 with the ylide
prepared in situ from the phosphonium iodide 5 with KHMDS at
À78 °C in THF afforded the key epoxy ester 4 in 65% isolated yield
after alkaline quench and flash chromatography purification in the
presence of triethylamine. Compound 4 was characterized by 1H
NMR, 13C NMR, COSY, HSQC, and UV.29 The geometry of the
asymmetric epoxidation. Phosphonium iodide
5 has been
synthesized from the alkynes 8 and 9.
The synthesis of 5 is outlined in Scheme 1. 5-[(Tetrahydro-2H-
pyran-2-yl)oxy]-2-pentyn-1-ol (9) was converted to the iodide 10
with triphenylphosphine, imidazole, and iodine.21 Cross coupling
of iodide 10 with 4-pentynoic acid methyl ester (8)22 in the presence
of CuI, Cs2CO3, and TBAI in DMF gave the skipped diyne 11 in 77%
yield.23 The semi-hydrogenation of 11 using Lindlar catalyst turned
out to be challenging. It was difficult to avoid partial and over-
reduction. The best results were achieved using Lindlar catalyst in
hexane/ethyl acetate in the presence of pyridine to give Z,Z-skipped
diene 12 in 40% isolated yield. Other base modifiers were less effec-
tive.24 Cleavage of the THP group in 12 was achieved in 92% yield
with PPTS in methanol to give 13. Compound 13 was transformed
to the iodide 14 and then converted to the phosphonium salt 5 with

A. R. Rodriguez, B. W. Spur / Tetrahedron Letters 56 (2015) 5811–5815
5813
Figure 2. Retrosynthetic approach to 16R,17S-PCTR1, 16R,17S-PCTR2 and 16R,17S-PCTR3.
Scheme 1. Reagents and conditions: (a) I2, Ph3P, imidazole, THF, 75%; (b) Cs2CO3, CuI, TBAI, DMF, rt, 77%; (c) H2, Lindlar cat., pyridine, EtOAc, hexane, 40%; (d) PPTS, CH3OH, rt,
92%; (e) I2, Ph3P, imidazole, i-Pr2EtN, CH3CN, ether; (f) Ph3P, CH3CN, 75 °C, 93% (over two steps).
Scheme 2. Reagents and conditions: (a) Ref. 25; (b) H2, Lindlar cat., Et3N, EtOAc, hexane, 74%; (c) Dess–Martin periodinane, NaHCO3, CH2Cl2, rt, 93%; (d) 6, CH2Cl2, rt, 46%;
(e) cat. I2, benzene, rt, 84%; (f) 5, KHMDS, THF, À78 °C to À10 °C, 65%.

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A. R. Rodriguez, B. W. Spur / Tetrahedron Letters 56 (2015) 5811–5815
Scheme 3. Reagents and conditions: (a) Glutathione, Et3N, H2O, CH3OH, rt; (b) 1 N LiOH, H2O, 0 °C to rt, 55% (over two steps).
Scheme 4. Reagents and conditions: (a) L-cysteinylglycine, Et3N, H2O, CH3OH, rt; (b) 1 N LiOH, H2O, 0 °C to rt, 50% (over two steps).
Scheme 5. Reagents and conditions: (a) L-cysteine methyl ester hydrochloride, Et3N, H2O, CH3OH, rt, 67%; (b) 1 N LiOH, H2O, CH3OH, 0 °C to rt, 49%.
12E,14E-diene unit in 18 (J12,13 = 15.3 and J14,15 = 15.3) and the
10Z,12E,14E-triene in 4 (J10,11 = 11.4, J12,13 = 15.0 and J14,15 = 15.3)
was confirmed by the 1H–1H coupling constants.29
pharmacological testing. The synthesis of other specialized pro-
resolving mediators (SPMs) will be reported in due course.
The conversion of epoxy ester 4 to 16R,17S-PCTR1 (1) was
achieved in two steps as described in Scheme 3.20,30,31 Reaction
of 4 with 3 equiv of glutathione in CH3OH/triethylamine/H2O gave
the monomethyl ester 19. Mild hydrolysis of crude 19 with 1 N
LiOH in H2O for 30 min afforded 16R,17S-PCTR1 (1) that was puri-
fied by HPLC [Zorbax SB-C18 250 Â 21.2 mm, 280 nm, CH3OH/H2O
(0.1% NH4OAc, pH 5.6, 0.05% EDTA disodium) 65/35]. The fraction
containing 1 was desalted using a reversed phase C-18 cartridge
to give pure 16R,17S-PCTR1 (1). The 1H NMR, COSY, UV, and
HPLC/UV/MS analyses were consistent with the structure of 1.29
Acknowledgements
Financial support of this research in part by the DOD – United
States (AS073084P1) and NJCBIR is gratefully acknowledged.
References and notes
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2120–2136.
Compound 4 was reacted with L-cysteinylglycine to obtain com-
pound 20 that was converted to 16R,17S-PCTR2 (2) using the same
conditions as described for 1 (Scheme 4). 16R,17S-PCTR2 (2) was
purified by HPLC [Zorbax SB-C18 250 Â 21.2 mm, 280 nm,
CH3OH/H2O (0.1% NH4OAc, pH 5.6, 0.05% EDTA disodium) 65/35].
Compound 4 was reacted with L-cysteine methyl ester hydrochlo-
ride to obtain 16R,17S-PCTR3-dimehtyl ester (21)32 that was
converted to 16R,17S-PCTR3 (3) using similar conditions as
described for 1 and 2 (Scheme 5). 16R,17S-PCTR3 (3) was purified
by HPLC [Zorbax SB-C18 250 Â 21.2 mm, 280 nm, CH3OH/H2O
(0.1% NH4OAc, pH 5.6, 0.05% EDTA disodium) 65/35]. The
1H NMR, COSY, UV, and HPLC/UV/MS analyses were consistent
with the structures of 2 and 3.29
In summary,
a concise total synthesis of 16R,17S-PCTR1,
16R,17S-PCTR2, and 16R,17S-PCTR3 has been achieved,29 making
these pro-resolving and tissue-regenerative lipid mediators
from docosahexaenoic acid available for further biological and

A. R. Rodriguez, B. W. Spur / Tetrahedron Letters 56 (2015) 5811–5815
5815
16. Corey, E. J.; Clark, D. A.; Goto, G.; Marfat, A.; Mioskowski, C.; Samuelsson, B.;
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25. Rodriguez, A. R.; Spur, B. W. Tetrahedron Lett. 2012, 53, 86–89.
26. Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277–7287.
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29. Satisfactory spectroscopic data were obtained for all compounds. Selected
physical data: Compound 10: 1H NMR (CDCl3, 300 MHz): d 4.7–4.6 (br t,
J = 3.3 Hz, 1H), 3.9–3.8 (m, 1H), 3.8–3.7 (dt, J = 9.9, 6.9 Hz, 1H), 3.7–3.6 (t,
J = 2.4 Hz, 2H), 3.6–3.5 (dt, J = 9.9, 6.9 Hz, 1H), 3.6–3.4 (m, 1H), 2.5 (tt, J = 6.9,
2.4 Hz, 2H), 1.9–1.5 (m, 6H); 13C NMR (CDCl3, 75.5 MHz): d 98.68, 83.33, 77.87,
65.23, 62.15, 30.49, 25.37, 20.59, 19.33, À17.33. Compound 11: 1H NMR (CDCl3,
300 MHz): d 4.6 (br t, J = 3.3 Hz, 1H), 3.9–3.8 (m, 1H), 3.8–3.7 (dt, J = 9.6, 7.2 Hz,
1H), 3.6 (s, 3H), 3.6–3.4 (dt, J = 9.6, 7.2 Hz, 1H), 3.5–3.4 (m, 1H), 3.1–3.0 (quint,
J = 2.4 Hz, 2H), 2.6–2.4 (m, 6H), 1.8–1.4 (m, 6H); 13C NMR (CDCl3, 75.5 MHz): d
172.40, 98.68, 78.37, 77.28, 75.19, 75.09, 65.71, 62.17, 51.71, 33.32, 30.51,
25.38, 20.13, 19.38, 14.59, 9.70. Compound 12: 1H NMR (CDCl3, 300 MHz): d
5.5–5.3 (m, 4H), 4.6 (dd, J = 4.5, 2.7 Hz, 1H), 3.9–3.8 (m, 1H), 3.7 (dt, J = 9.6,
7.2 Hz, 1H), 3.6 (s, 3H), 3.5–3.4 (m, 1H), 3.4–3.3 (dt, J = 9.6, 7.2 Hz, 1H), 2.8 (br t,
J = 5.7 Hz, 2H), 2.4–2.3 (m, 6H), 1.9–1.4 (m, 6H); 13C NMR (CDCl3, 75.5 MHz): d
173.54, 129.54, 129.37, 127.75, 126.20, 98.72, 66.89, 62.26, 51.53, 33.97, 30.66,
27.94, 25.64, 25.43, 22.75, 19.54. Compound 13: 1H NMR (CDCl3, 300 MHz): d
5.5–5.3 (m, 4H), 3.7 (s, 3H), 3.7–3.6 (m, 2H), 2.9–2.8 (br t, J = 6.4 Hz, 2H), 2.5–
2.3 (m, 6H); 13C NMR (CDCl3, 75.5 MHz): d 173.65, 130.93, 129.17, 127.95,
125.77, 62.20, 51.59, 33.94, 30.85, 25.71, 22.81. Compound 14: 1H NMR (CDCl3,
300 MHz): d 5.6–5.3 (m, 4H), 3.7 (s, 3H), 3.2–3.1 (t, J = 7.2 Hz, 2H), 2.8 (br t,
J = 6.0 Hz, 2H), 2.7–2.6 (br q, J = 7.2 Hz, 2H), 2.4–2.3 (m, 4H); 13C NMR (CDCl3,
75.5 MHz): d 173.50, 130.25, 128.82, 128.34, 128.14, 51.59, 33.92, 31.39, 25.73,
22.79, 5.26. Compound 5: 1H NMR (CDCl3, 300 MHz): d 7.9–7.7 (m, 15H), 5.7–
5.6 (m, 1H), 5.5–5.3 (m, 1H), 5.3–5.2 (m, 2H), 3.9–3.8 (m, 2H), 3.6 (s, 3H), 2.6
(br t, J = 6.0 Hz, 2H), 2.6–2.4 (m, 2H), 2.4–2.1 (m, 4H); 13C NMR (CDCl3,
75.5 MHz): d 173.51, 135.12 (d, J = 2.8 Hz, 3C), 133.80 (d, J = 10.3 Hz, 6C),
130.52 (d, J = 12.0 Hz, 6C), 130.33, 128.44, 128.29, 126.39 (d, J = 14.3 Hz),
118.10 (d, J = 85.3 Hz, 3C), 51.58, 33.74, 25.57, 23.31 (d, J = 48.1 Hz), 22.75,
20.38 (d, J = 3.5 Hz); Compound 16: 1H NMR (CDCl3, 300 MHz): d 3.9 (dd,
J = 12.6, 2.1 Hz, 1H), 3.6 (dd, J = 12.6, 4.2 Hz, 1H), 3.1 (m, 2H), 2.6–2.5 (ddt,
J = 17.4, 4.2, 2.4 Hz, 1H), 2.5–2.4 (ddt, J = 17.4, 4.8, 2.4 Hz, 1H), 2.2–2.0 (qt,
J = 7.5, 2.4 Hz, 2H), 2.1 (br s, 1H), 1.1 (t, J = 7.5 Hz, 3H); 13C NMR (CDCl3,
198.29, 136.08, 120.79, 58.52, 55.98, 28.62, 20.70, 14.07. Compound 18: 1H
NMR (CDCl3, 300 MHz): d 9.5 (d, J = 7.8 Hz, 1H), 7.1–7.0 (dd, J = 15.3, 11.1 Hz,
1H), 6.6 (dd, J = 15.3, 11.1 Hz, 1H), 6.2–6.1 (dd, J = 15.3, 7.8 Hz, 1H), 6.0–5.9 (dd,
J = 15.3, 7.2 Hz, 1H), 5.6–5.5 (dtt, J = 10.8, 7.2, 1.5 Hz, 1H), 5.4–5.2 (dtt, J = 10.8,
7.5, 1.5 Hz, 1H), 3.2 (dd, J = 7.2, 2.1 Hz, 1H), 2.9 (td, J = 5.4, 2.1 Hz, 1H), 2.5–2.2
(m, 2H), 2.1–1.9 (m, 2H), 1.0 (t, J = 7.5 Hz, 3H); 13C NMR (CDCl3, 75.5 MHz): d
193.53, 149.99, 141.20, 135.25, 132.05, 130.82, 121.67, 60.72, 56.70, 29.41,
20.63, 14.09. Compound 4: 1H NMR (CDCl3, 300 MHz): d 6.6–6.5 (dd, J = 15.0,
11.4 Hz, 1H), 6.5–6.4 (dd, J = 15.3, 10.8 Hz, 1H), 6.2 (dd, J = 15.0, 10.8 Hz, 1H),
6.0 (br t, J = 11.4 Hz, 1H), 5.6–5.2 (m, 8H), 3.6 (s, 3H), 3.2–3.1 (dd, J = 8.2, 2.1 Hz,
1H), 3.0–2.9 (br t, J = 5.8 Hz, 2H), 2.9 (td, J = 5.4, 2.1 Hz, 1H), 2.9–2.8 (br t,
J = 5.4 Hz, 2H), 2.5–2.2 (m, 6H), 2.0–2.1 (br quint, J = 7.5 Hz, 2H), 0.95 (t,
J = 7.5 Hz, 3H); 13C NMR (CDCl3, 75.5 MHz): d 173.54, 134.89, 134.49, 131.58,
131.17, 130.23, 129.15, 128.69, 128.52 (2C), 127.98, 127.58, 122.27, 60.36,
58.06, 51.57, 33.97, 29.68, 26.23, 25.60, 22.80, 20.68, 14.18; UV (hexane) kmax
271, 280, 292 nm. 16R,17S-PCTR1 (1): 1H NMR (CD3OD, 300 MHz): d 6.6 (dd,
J = 13.5, 11.1 Hz, 1H), 6.3–6.2 (m, 2H), 6.1–6.0 (t, J = 11.1 Hz, 1H), 5.8–5.6 (dd,
J = 13.8, 10.2 Hz, 1H), 5.5–5.3 (m, 7H), 4.6–4.5 (dd, J = 9.3, 5.1 Hz, 1H), 3.9–3.7
(2d ABsystem, J = 17.4 Hz, 2H), 3.7 (td, J = 6.3, 3.9 Hz, 1H), 3.7–3.6 (t, J = 6.3 Hz,
1H), 3.4 (dd, J = 10.2, 3.9 Hz, 1H), 3.1–2.8 (m, 5H), 2.7 (dd, J = 13.8, 9.3 Hz, 1H),
2.6–2.5 (m, 2H), 2.4–2.2 (m, 5H), 2.2–2.1 (m, 3H), 2.0 (br quint, J = 7.5 Hz, 2H),
0.95 (t, J = 7.5 Hz, 3H); UV (CH3OH) kmax 272, 281, 292 nm; HPLC/UV: Zorbax
SB-C18, 1.8
lm, 50 Â 2.1 mm, 280 nm, CH3OH/H2O (0.1% formic acid) 35:65–
70:30, 0.2 mL/min, tR = 13.1 min; HPLC/MS (m/z): 648.2 [MÀH]À. 16R,17S-
PCTR2 (2): 1H NMR (CD3OD, 300 MHz): d 6.7–6.6 (dd, J = 14.4, 11.1 Hz, 1H),
6.4–6.3 (m, 2H), 6.1–6.0 (t, J = 11.1 Hz, 1H), 5.7 (dd, J = 14.1, 10.2 Hz, 1H), 5.6–
5.3 (m, 7H), 4.0 (d, J = 17.4 Hz, 1H), 3.9–3.8 (t, J = 7.2 Hz, 1H), 3.8–3.7 (m, 1H),
3.6 (d, J = 17.4 Hz, 1H), 3.5–3.4 (dd, J = 10.2, 3.3 Hz, 1H), 3.1–2.8 (m, 5H), 2.8
(dd, J = 14.4, 7.2 Hz, 1H), 2.4–2.1 (m, 6H), 2.1–2.0 (br quint, J = 7.5 Hz, 2H), 0.95
(t, J = 7.5 Hz, 3H); UV (CH3OH) kmax 272, 281, 292 nm; HPLC/UV: Zorbax SB-
C18, 1.8
lm, 50 Â 2.1 mm, 280 nm, CH3OH/H2O (0.1% formic acid) 35:65–
70:30, 0.2 mL/min, tR = 11.9 min; HPLC/MS (m/z): 519.2 [MÀH]À. 16R,17S-
PCTR3-dimethyl ester (21): 1H NMR (CD3OD, 300 MHz): d 6.6 (dd, J = 13.5,
11.1 Hz, 1H), 6.4–6.2 (m, 2H), 6.1–6.0 (t, J = 11.1 Hz, 1H), 5.7–5.6 (dd, J = 13.8,
10.2 Hz, 1H), 5.5–5.3 (m, 7H), 3.8–3.7 (m, 1H), 3.7 (s, 3H), 3.6 (s, 3H), 3.6 (dd,
J = 7.5, 5.1 Hz, 1H), 3.4–3.3 (dd, J = 10.2, 3.9 Hz, 1H), 3.0 (m, 2H), 2.9–2.8 (m,
3H), 2.7–2.6 (dd, J = 13.8, 7.5 Hz, 1H), 2.4–2.1 (m, 6H), 2.1–2.0 (br quint,
J = 7.5 Hz, 2H), 0.95 (t, J = 7.5 Hz, 3H); 13C NMR (CD3OD, 75.5 MHz): d 175.39,
175.28, 135.03, 134.87, 133.37, 131.35, 131.19, 130.24, 129.82, 129.46, 129.15,
129.06, 128.69, 125.63, 74.62, 55.15, 54.90, 52.72, 52.09, 36.09, 34.82, 34.27,
27.11, 26.50, 23.82, 21.78, 14.60; UV (CH3OH) kmax 272, 281, 292 nm. 16R,17S-
PCTR3 (3): 1H NMR (CD3OD, 300 MHz): d 6.6 (dd, J = 14.0, 11.1 Hz, 1H), 6.4–6.2
(m, 2H), 6.1–6.0 (t, J = 11.1 Hz, 1H), 5.7 (dd, J = 14.4, 10.2 Hz, 1H), 5.5–5.3 (m,
7H), 3.8–3.7 (ddd, J = 7.5, 6.0, 3.6 Hz, 1H), 3.7–3.6 (dd, J = 9.0, 3.6 Hz, 1H), 3.5
(dd, J = 10.2, 3.6 Hz, 1H), 3.1–2.8 (m, 6H), 2.5–2.1 (m, 6H), 2.1–2.0 (br quint,
J = 7.5 Hz, 2H), 0.95 (t, J = 7.5 Hz, 3H); UV (CH3OH) kmax 272, 281, 292 nm;
HPLC/UV: Zorbax SB-C18, 1.8
lm, 50 Â 2.1 mm, 280 nm, CH3OH/H2O (0.1%
formic acid) 35:65–70:30, 0.2 mL/min, tR = 14.6 min; HPLC/MS (m/z): 462.2
[MÀH]À.
30. Cohen, N.; Banner, B. L.; Lopresti, R. J.; Wong, F.; Rosenberger, M.; Liu, Y.-Y.;
Thom, E.; Liebman, A. A. J. Am. Chem. Soc. 1983, 105, 3661–3672.
31. Rodriguez, A.; Nomen, M.; Spur, B. W.; Godfroid, J.-J.; Lee, T. H. Eur. J. Org. Chem.
2000, 2991–3000.
75.5 MHz):
d 84.18, 73.27, 61.26, 57.83, 53.61, 21.67, 13.96, 12.31;
[a
]
D
25 = À17.7 (c 0.73, CHCl3). Compound 17: 1H NMR (CDCl3, 300 MHz): d
5.6–5.4 (dtt, J = 10.8, 7.2, 1.5 Hz, 1H), 5.4–5.2 (dtt, J = 10.8, 7.5, 1.5 Hz, 1H), 3.9
(dd, J = 12.6, 2.4 Hz, 1H), 3.6 (dd, J = 12.6, 4.2 Hz, 1H), 3.0–2.9 (m, 2H), 2.5–2.2
(m, 2H), 2.1–1.9 (m, 2H), 1.9–1.8 (br s, 1H), 0.95 (t, J = 7.5 Hz, 3H); 13C NMR
(CDCl3, 75.5 MHz): d 134.92, 122.36, 61.75, 57.95, 55.29, 29.23, 20.66, 14.06;
32. In a flame dried flask under argon compound 4 (8.6 mg, 0.024 mmol) was
dissolved in Et3N (100
hydrochloride (12.4 mg, 0.072 mmol) previously dissolved in CH3OH/Et3N/
H2O (200 l/100 l/20 l). The reaction mixture was stirred in the dark at rt for
l
l) and treated with
L-cysteine methyl ester
[
a
]
25 = À27.0 (c 0.64, CHCl3). Compound 7: 1H NMR (CDCl3, 300 MHz): d 9.0 (d,
l
l
l
D
J = 6.3 Hz, 1H), 5.7–5.6 (dtt, J = 10.8, 7.2, 1.5 Hz, 1H), 5.4–5.2 (dtt, J = 10.8, 7.5,
1.5 Hz, 1H), 3.3 (td, J = 5.4, 2.1 Hz, 1H), 3.2 (dd, J = 6.3, 2.1 Hz, 1H), 2.6–2.4 (m,
2H), 2.2–2.0 (m, 2H), 1.0 (t, J = 7.5 Hz, 3H); 13C NMR (CDCl3, 75.5 MHz): d
5 h. Concentration followed by purification by flash chromatography ethyl
acetate/hexane 80:20 (1% Et3N) afforded 16R,17S-PCTR3-dimethyl ester (21)
(7.9 mg, 67%).