Total synthesis and assignment of configuration of the thiazoline-based cyclopeptide cyclodidemnamide isolated from the sea squirt Didemnum molle

Article abstract of DOI:10.1039/a909363j

A total synthesis of the proposed structure 1 for the oxazoline/thiazoline-based cyclopeptide cyclodidemnamide, isolated from the sea squirt Didemnum molle, is described. The synthesis features a novel double cyclodehydration, sequential formation of chiral thiazoline and oxazoline rings in a preformed cyclopeptide intermediate, as a key stratagem. The NMR spectroscopic data for synthetic 1 and natural cyclodidemnamide did not correlate. The configuration of the natural product was re-assigned as 15, the C-15-(R)-Val epimer, which was confirmed by total synthesis.

Full text of DOI:10.1039/a909363j

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Total synthesis and assignment of configuration of the thiazoline-
based cyclopeptide cyclodidemnamide isolated from the sea squirt
Didemnum molle
1
Christopher D. J. Boden, Mark Norley and Gerald Pattenden*
School of Chemistry, Nottingham University, Nottingham, UK NG7 2RD
Received (in Cambridge, UK) 26th November 1999, Accepted 21st December 1999
A total synthesis of the proposed structure 1 for the oxazoline/thiazoline-based cyclopeptide cyclodidemnamide,
isolated from the sea squirt Didemnum molle, is described. The synthesis features a novel double cyclodehydration,
sequential formation of chiral thiazoline and oxazoline rings in a preformed cyclopeptide intermediate, as a key
stratagem. The NMR spectroscopic data for synthetic 1 and natural cyclodidemnamide did not correlate. The
configuration of the natural product was re-assigned as 15, the C-15-(R)-Val epimer, which was confirmed by
total synthesis.
Cyclodidemnamide 1 is a thiazoline/oxazoline containing
cyclopeptide which has been isolated from the sea squirt
Didemnum molle.1 The compound exhibits moderate toxicity
toward human colon cells, and it is related structurally to other
thiazoline-based marine cyclopeptides e.g. lissoclinamide 4 22
and mollamide 3.3 Chiral amino acid substituted thiazolines of
the type contained within the structures 1, 2, and 3, are well
known to be configurationally labile under a variety of acid and
base conditions.4 In earlier work we described a total synthesis
of the thiazoline-containing cyclopeptide lissoclinamide 4 2,
whereby the thiazoline ring was produced simultaneously with
the oxazoline ring in 2 via a novel “one-pot” double dehydra-
tion sequence from a preformed cyclothioamide intermediate
as a key step.5 We have now developed this overall strategy
towards the synthesis of both cyclodidemnamide 16 and moll-
amide 3.7 In this paper we report the full details of our synthesis
of the structure 1 reported for cyclodidemnamide leading to the
reassignment of its configuration to 15, and the total synthesis
of the latter.
In their structure elucidation of cyclodidemnamide Toske
and Fenical assigned the S-stereochemistries at both C15 and
C27 based on hydrolysis of the cyclopeptide followed by GC-
MS analysis of the configuration of the resulting (S)-valine
derivative.1 As mentioned earlier and elsewhere,4 however,
there can be problems with this analysis owing to the ease with
which thiazole and thiazoline based amino acid units of the
type displayed in structures 1–3 undergo epimerisation during
hydrolysis. Nevertheless, we accepted the assignment of the
structure proposed for cyclodidemnamide and based our syn-
thetic strategy to 1 on double dehydration from the cyclo-
thioamide precursor 4, as a key step, which we planned to
obtain from the known (S)-Val substituted thiazole 78 via the
acyclic (endo-thio) tetra- and hepta-peptides 6 and 5, respec-
tively (Scheme 1). Thus, removal of the Boc group in 7 followed
by acylation of the resulting amine with Boc-(S)-Ser-OH in the
presence of DCC–HOBt first gave the tripeptide 8 in 91% yield.
The Boc group in 8 was next deblocked and the free amine was
then thioacylated with the proline-derived reagent 99 in 56%
yield from 8. The three amino acid residues (S)-Val, (S)-aThr,†
and (R)-Phe were next introduced to the chain in sequence, by
a cycle of Boc removal followed by acylations using the DCC–
HOBt–iPr2NEt coupling method, leading to the compounds
10, 11 and 5 in 68%, 73% and 71% yield respectively (Scheme 2).
Saponification of 5 and removal of the Boc group now gave
the amino acid 12, but all attempts to effect macrocyclisation of
12 using diphenylphosphorazide (DPPA) or pentafluorophenyl
diphenylphosphinate (FDPP) failed, and instead led to prod-
ucts of decomposition. We suspected that the problem was due
to the poor solubility of the amino acid 12 in DMF, occasioned
by H-bonding involving the serine and threonine hydroxy
groups. We therefore decided to convert the heptapeptide 5 into
the diacetate 13 corresponding to 12 prior to macrocyclisation,
and this was smoothly accomplished in three straightforward
steps (Scheme 3). Treatment of the diacetate 13 with DPPA–
iPr2NEt in DMF (0–25 ЊC), and work-up after 3 days, then
produced the diacetylated cyclopeptide 14a in a highly satisfy-
ing 60–70% overall yield. Finally, saponification of the acetate
groups in 14a followed by treatment of the resulting diol 14b
† aThr = -allo-threonine.
DOI: 10.1039/a909363j
J. Chem. Soc., Perkin Trans. 1, 2000, 883–888
This journal is © The Royal Society of Chemistry 2000
883

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Scheme 1
Scheme 2 Reagents: i, 50% TFA–CH2Cl2, 0 ЊC, 1 h; ii, DCC, HOBt, iPr2NEt, CH2Cl2, 0 ЊC → rt, 18 h; iii, aq. NaHCO3–CH2Cl2; iv, 9, DMF,
0 ЊC → rt, 18 h.
with Burgess’ reagent10 in anhydrous THF gave the cyclodi-
demnamide structure 1, as a foam, in approximately 30% yield.
Much to our chagrin, although the synthetic cyclodidemn-
amide showed closely similar NMR spectroscopic data to those
reported for the natural product, the two compounds were
clearly different at one or more stereocentres. We surmised that
the amino acid sequence in the natural product was assigned
correctly, but that the difference between it and our synthetic
compound was most likely associated with a wrongly assigned
configuration at C15, the valine-derived thiazole centre, in the
natural product. We therefore re-assigned this C15 centre as
(R)-Val, and hence natural cyclodidemnamide as structure 15,
and then repeated the entire synthesis using exactly the same
strategy and order of steps to those earlier described, but start-
ing from the (R)-Val enantiomer of the thiazole 7. In this
manner we synthesised the stereostructure 15 which gratifyingly
Experimental
For general experimental details see ref. 5a.
showed NMR spectroscopic and other data which were identi-
cal to those reported for natural cyclodidemnamide isolated
Boc-(S)-Ser-Val-Thz-OMe; (R)-Val epimer, 8
from D. molle.
Trifluoroacetic acid (50 ml) was added dropwise over 0.5 h to a
884 J. Chem. Soc., Perkin Trans. 1, 2000, 883–888

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Scheme 3 Reagents: i, aq. NaOH, THF–MeOH (3:1), 0 ЊC, 1 h; ii, 50% TFA–CH2Cl2, 0 ЊC, 1 h; iii, Ac2O, Et3N, cat. DMAP, DMF, rt, 2 h; iv,
DPPA, iPr2NEt, DMF, 0 ЊC → rt, 3 days; v, aq. K2CO3, MeOH, 0 ЊC, 1 h; vi, Burgess’ reagent, THF, reflux, 2 h.
stirred solution of the R-enantiomer of 7 (7.77 g, 24.71 mmol)5
in dichloromethane (50 ml) at 0 ЊC and the mixture was stirred
at 0 ЊC for 1 h. The solution was concentrated in vacuo and the
solid residue was dissolved in dichloromethane (150 ml), cooled
to 0 ЊC and treated dropwise over 5 min with iPr2NEt (10.8 ml,
62.0 mmol). The mixture was stirred at 0 ЊC for 15 min and then
Boc-(S)-Ser-OH (5.58 g, 27.19 mmol) was added followed by
HOBt (4.01 g, 29.7 mmol). The resulting cloudy mixture was
stirred at 0–4 ЊC for a further 15 min, then treated with DCC
(6.63 g, 32.1 mmol) and stirred at 0 ЊC for 90 min followed by a
further 18 h at room temperature. The suspension was concen-
trated in vacuo and then EtOAc (150 ml) was added to the
residue. The mixture was filtered and the filtrate was washed
with 10% w/v citric acid solution and saturated NaHCO3, then
dried (MgSO4), and evaporated to dryness in vacuo. The residue
was purified by flash chromatography on silica gel (40%
EtOAc–Et2O eluant) to give the tripeptide (9.04 g, 22.5 mmol,
91%) as a white foam: δH (500 MHz, DMSO, 90 ЊC): 8.36 (1H,
s), 8.00 (1H, d, J 8.5 Hz), 6.31 (1H, br s), 5.04 (1H, dd, J 6.6, 8.5
Hz), 4.52 (1H, br s), 4.13 (1H, dt, J 8.0, 5.7 Hz), 3.87 (3H, s),
3.65 (2H, m), 2.36 (1H, app octet, J 6.7 Hz), 1.42 (9H, s), 0.97
(3H, d, J 6.8 Hz), 0.94 (3H, d, J 6.8 Hz); δC (125 MHz, DMSO,
90 ЊC): 172.4 (s), 170.3 (s), 160.9 (s), 154.8 (s), 145.4 (s), 127.9
(d), 78.1 (s), 61.5 (t), 56.8 (d), 56.2 (d), 51.3 (q), 31.9 (d), 27.8
(q), 18.8 (q), 17.6 (q); HRMS, m/z for C17H27N3O6SNa
(Mϩ ϩ Na), calcd: 424.1520; found 424.1518.
NaHCO3 (50 ml) for 15 min. The separated aqueous layer was
extracted a further 15 times with dichloromethane (100 ml each
time) and the combined extracts were then dried (MgSO4) and
concentrated in vacuo to leave an off-white foam which was
immediately dissolved in DMF (100 ml) and cooled to 0 ЊC.
Boc-Pro-Bin9 (Bin = benzimidazolone) was added portionwise
over 10 min and the resulting bright yellow solution was stirred
at 0 ЊC for 90 min and then for a further 18 h at room temper-
ature. The solution was diluted with EtOAc (500 ml), then
washed with water (3 × 200 ml), dried (MgSO4) and concen-
trated in vacuo. The residue was purified by flash chrom-
atography on silica gel (Et2O eluant) to give the tetrapeptide
(4.50 g, 8.74 mmol, 56%) as a white foam: δH (500 MHz, DMSO,
90 ЊC): 9.27 (1H, d, J 7.0 Hz), 8.37 (1H, s), 8.18 (1H, d, J 8.4
Hz), 5.11 (1H, dt, J 7.2, 5.2 Hz), 5.06 (1H, dd, J 6.6, 8.4 Hz),
4.68 (1H, br t, J 5.2 Hz), 4.63 (1H, dd, J 3.8, 8.7 Hz), 3.87 (3H,
s), 3.89–3.81 (2H, m), 3.52–3.42 (2H, m), 2.35 (1H, app octet,
J 6.7 Hz), 2.31–2.23 (1H, m), 1.99–1.89 (2H, m), 1.84–1.76 (1H,
m), 1.40 (9H, s), 0.97 (3H, d, J 6.8 Hz), 0.94 (3H, d, J 6.8 Hz);
δC (125 MHz, DMSO, 90 ЊC): 204.9 (s), 172.3 (s), 168.5 (s),
160.9 (s), 153.4 (s), 145.3 (s), 128.1 (d), 78.7 (s), 66.9 (d), 60.5 (t),
59.8 (d), 56.5 (d), 51.3 (q), 46.8 (t), 33.1 (t), 31.9 (d), 27.8 (q),
22.7 (t), 18.8 (q), 17.6 (q); HRMS, m/z for C22H34N4O6S2 ϩ Na
(Mϩ ϩ Na), calcd: 537.1842; found 537.1817.
NMR data for 6, (S)-Val epimer: δH (400 MHz, DMSO,
90 ЊC): 9.22 (1H, d, J 7.0 Hz), 8.37 (1H, s), 8.10 (1H, d, J 8.4
Hz), 5.11 (1H, dt, J 7.3, 5.2 Hz), 5.06 (1H, dd, J 6.5, 8.4 Hz),
4.65 (1H, dd, J 8.7, 3.8 Hz), 4.58 (1H, br s), 3.87 (3H, s), 3.89–
3.81 (2H, m), 3.52–3.42 (2H, m), 2.35 (1H, app octet, J 6.7 Hz),
2.31–2.23 (1H, m), 1.99–1.89 (2H, m), 1.84–1.76 (1H, m), 1.41
(9H, s), 0.99 (3H, d, J 6.8 Hz), 0.95 (3H, d, J 6.8 Hz); δC (100
MHz, DMSO, 90 ЊC): 204.9 (s), 172.2 (s), 168.5 (s), 160.8 (s),
153.4 (s), 145.4 (s), 127.8 (d), 78.7 (s), 66.9 (d), 60.5 (t), 59.7 (d),
56.5 (d), 51.2 (q), 46.8 (t), 33.0 (t), 31.85 (d), 27.7 (q), 22.7 (t),
18.7 (q), 17.6 (q).
NMR data for 8, (S)-Val epimer: δH (400 MHz, DMSO,
80 ЊC): 8.37 (1H, s), 8.00 (1H, br s), 6.31 (1H, br s), 5.04 (1H,
dd, J 6.6, 8.5 Hz), 4.53 (1H, br s), 4.13 (1H, dt, J 8.0, 5.7 Hz),
3.87 (3H, s), 3.66 (2H, m), 2.36 (1H, app octet, J 6.7 Hz), 1.42
(9H, s), 0.97 (3H, d, J 6.8 Hz), 0.94 (3H, d, J 6.8 Hz); δC (100
MHz, DMSO, 80 ЊC): 172.5 (s), 170.3 (s), 160.9 (s), 154.85 (s),
145.4 (s), 127.95 (d), 78.2 (s), 61.5 (t), 56.8 (d), 56.2 (d), 51.4 (q),
31.9 (d), 27.9 (q), 18.8 (q), 17.6 (q).
Boc-Pro-ꢀ(CS-NH)-(S)-Ser-Val-Thz-OMe; (R)-Val epimer, 6
Boc-(S)-Val-Pro-ꢀ(CS-NH)-(S)-Ser-Val-Thz-OMe; C-15(R)-
Val epimer, 10
Trifluoroacetic acid (50 ml) was added dropwise over 30 min
to a stirred solution of the (R)-Val epimer of 8 (6.21 g, 15.47
mmol) in dichloromethane (50 ml) at 0 ЊC and the resulting
solution was stirred at 0 ЊC for 1 h. The solution was concen-
trated in vacuo and the solid residue was dissolved in dichloro-
methane (150 ml) and the solution shaken vigorously with sat.
Trifluoroacetic acid (45 ml) was added dropwise over 30 min to
a stirred solution of the (R)-Val epimer of 6 (4.50 g, 8.74 mmol)
in dichloromethane (45 ml) at 0 ЊC and the solution was then
stirred at 0 ЊC for 1 h. The solution was concentrated in vacuo,
J. Chem. Soc., Perkin Trans. 1, 2000, 883–888
885

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and the solid residue was dissolved in dichloromethane (50 ml),
cooled to 0 ЊC and treated dropwise with iPr2NEt (3.8 ml, 21.8
mmol). The mixture was stirred at 0 ЊC for 15 min and then
Boc-(S)-Val-OH (2.09 g, 9.62 mmol) was added followed by
HOBt (1.42 g, 10.51 mmol). The resulting cloudy mixture was
stirred for a further 15 min at 0 ЊC, then treated with DCC
(2.34 g, 11.34 mmol) and stirred at 0 ЊC for 90 min followed by a
further 18 h at room temperature. The suspension was concen-
trated in vacuo, EtOAc (50 ml) was added to the residue and the
mixture was then filtered. The filtrate was washed with 10% w/v
citric acid solution and saturated NaHCO3, then dried (MgSO4)
and concentrated in vacuo. The residue was purified by flash
chromatography on silica gel (10% acetone–Et2O eluant) to give
the pentapeptide (3.65 g, 5.95 mmol, 68%) as a white foam:
δH (500 MHz, DMSO, 90 ЊC): 9.45 (1H, br s), 8.38 (1H, s), 8.10
(1H, br d, J 7.7 Hz), 6.08 (1H, br s), 5.07 (1H, m), 5.04 (1H, dd,
J 6.5, 8.5 Hz), 4.92 (1H, br s), 4.64 (1H, br s), 4.13 (1H, br s),
3.85 (3H, s), 3.84 (2H, m), 3.79 (1H, br s), 3.68 (1H, m), 2.35
(1H, app octet, J 6.7 Hz), 2.20 (1H, m), 2.14–2.00 (3H, m),
1.89 (1H, m), 1.42 (9H, s), 0.98 (3H, d, J 6.8 Hz), 0.97 (3H, d,
J 6.7 Hz), 0.93 (3H, d, J 6.7 Hz), 0.88 (3H, d, J 6.7 Hz); δC (125
MHz, DMSO, 90 ЊC): 204.6 (s), 172.4 (s), 170.4 (s), 168.4 (s),
160.9 (s), 145.3 (s), 128.0 (d), 108.1 (s), 78.0 (s), 66.0 (d),
60.4 (t), 60.2 (d), 57.0 (d), 56.5 (d), 51.4 (q), 47.4 (d), 47.1 (t),
33.0 (t), 31.9 (d), 27.9 (q), 24.0 (t), 19.2 (q), 18.8 (q), 17.7
(q), 17.3 (q); HRMS, m/z 636.2502 (M ϩ Na) C27H43N5O7S2
requires: 636.2560.
NMR data for 10, C-15(S)-Val epimer: δH (500 MHz,
DMSO, 90 ЊC): 9.4 (1H, br s), 8.37 (1H, s), 8.09 (1H, br d,
J 6.8 Hz), 6.05 (1H, br s), 5.07 (1H, m), 5.05 (1H, dd, J 6.6,
8.4 Hz), 4.92 (1H, br s), 4.63 (1H, br s), 4.13 (1H, br s), 3.87
(3H, s), 3.84 (2H, m), 3.79 (1H, br s), 3.68 (1H, app q, J 6.6
Hz), 2.35 (1H, app octet, J 6.7 Hz), 2.20 (1H, m), 2.14–2.00
(3H, m), 1.89 (1H, m), 1.42 (9H, s), 0.98 (3H, d, J 6.7 Hz),
0.97 (3H, d, J 6.6 Hz), 0.94 (3H, d, J 6.8 Hz), 0.89 (3H, d,
J 6.7 Hz); δC (125 MHz, DMSO, 90 ЊC): 204.6 (s), 172.3 (s),
170.4 (s), 168.4 (s), 160.9 (s), 145.3 (s), 128.0 (d), 108.1 (s),
78.0 (s), 66.0 (d), 60.4 (t), 60.2 (d), 57.0 (d), 56.4 (d), 51.3 (q),
47.4 (d), 47.0 (t), 33.0 (t), 31.2 (d), 27.9 (q), 24.0 (t), 19.2 (q),
18.8 (q), 17.6 (q), 17.2 (q).
170.1 (s), 170.0 (s), 168.4 (s), 160.9 (s), 154.9 (s), 145.4 (s),
128.0 (d), 78.1 (s), 66.6 (d), 64.4 (t), 60.4 (d), 60.3 (d), 56.5
(d), 55.3 (d), 52.4 (q), 47.2 (t), 32.3 (d), 31.9 (t), 29.8 (d),
27.9 (q), 23.8 (t), 19.3 (q), 18.8 (q), 17.7 (q), 17.3 (q), 14.7
(q); HRMS, m/z for C31H50N6O9S2 ϩ Na (Mϩ ϩ Na), calcd:
737.2964; found 737.2978.
NMR data for 11, C-15(S)-Val epimer: δH (400 MHz,
DMSO, 35 ЊC): 9.75 (1H, d, J 4.0 Hz), 8.42 (1H, s), 8.38 (1H, d,
5.3 Hz), 7.60 (1H, d, J 8.4 Hz), 6.81 (1H, d, J 8.0 Hz), 5.00 (2H,
m), 4.85 (1H, dd, J 4.4, 8.9 Hz), 4.67 (1H, d, 4.9 Hz), 4.49 (1H,
app t, J 7.9 Hz), 3.95–3.79 (5H, m), 3.82 (3H, s), 3.65 (1H, m),
2.31 (1H, app octet, J 6.7 Hz), 2.26–1.99 (4H, m), 1.87 (1H,
m), 1.4 (9H, s), 1.06 (3H, d, J 6.2 Hz), 0.95–0.82 (12H, m);
δC (100 MHz, DMSO, 35 ЊC): 205.1 (s), 173.4 (s), 170.5 (s),
170.0 (s), 168.9 (s), 161.3 (s), 155.4 (s), 145.5 (s), 129.0 (d), 78.3
(s), 66.7 (d), 60.8 (d), 60.4 (d), 56.7 (d), 55.5 (d), 52.0 (q), 47.6
(t), 32.3 (d), 32.0 (t), 30.0 (d), 28.3 (q), 24.2 (t), 19.9 (q), 19.8 (q),
18.0 (q), 17.8 (q).
Boc-(R)-Phe-(S)-aThr-(S)-Val-Pro-ꢀ(CS-NH)-(S)-Ser-Val-
Thz-OMe; C-15(R)-Val epimer, 5
Trifluoroacetic acid (10 ml) was added dropwise over 15 min to
a stirred solution of the C-15-(R)-Val epimer of 11 (920 mg,
1.29 mmol) in dichloromethane (10 ml) at 0 ЊC and the solution
was then stirred at 0 ЊC for 1 h. The solution was concentrated
in vacuo and the solid residue was dissolved in dichloromethane
(10 ml), cooled to 0 ЊC and treated dropwise with iPr2NEt
(0.56 ml, 3.21 mmol). The mixture was stirred at 0 ЊC for 15
min and then Boc-(R)-Phe-OH (376 mg, 1.42 mmol) was added
followed by HOBt (209 mg, 1.55 mmol). The resulting cloudy
mixture was stirred at 0 ЊC for a further 15 min then treated
with DCC (346 mg, 1.68 mmol) and stirred at 0 ЊC for 90 min
followed by a further 18 h at room temperature. The suspen-
sion was concentrated in vacuo and EtOAc (10 ml) was added
to the residue. The suspension was filtered and the filtrate was
then washed with 10% w/v citric acid solution and saturated
NaHCO3, then dried (MgSO4) and concentrated in vacuo. The
residue was purified by flash chromatography on silica gel (30%
acetone–Et2O eluant) to give the heptapeptide (790 mg, 0.916
mmol, 71%) as a white foam: δH (500 MHz, DMSO, 90 ЊC):
9.41 (1H, br s), 8.37 (1H, s), 8.09 (1H, d, J 7.3 Hz), 7.68 (1H, d,
J 8.2 Hz), 7.52 (1H, br s), 7.27–7.18 (5H, m), 6.46 (1H, br s),
5.08–5.03 (2H, m), 4.89 (1H, br s), 4.67 (1H, br s), 4.46 (2H,
br s), 4.32–4.28 (2H, m), 3.88–3.76 (4H, m), 3.86 (3H, s), 3.69
(1H, m), 3.07 (1H, dd, J 5.1, 14.0 Hz), 2.83 (1H, dd, J 9.3, 14.0
Hz), 2.35 (1H, app octet, J 6.7 Hz), 2.21–1.99 (4H, m), 1.87
(1H, m), 1.35 (9H, s), 1.07 (3H, d, J 6.3 Hz), 0.98 (6H, d, J 6.7
Hz), 0.93 (3H, d, J 6.7 Hz), 0.92 (3H, d, J 6.3 Hz); δC (125
MHz, DMSO, 90 ЊC): 204.5 (s), 172.4 (s), 171.2 (s), 169.9 (s),
169.6 (s), 168.4 (s), 160.9 (s), 154.6 (s), 145.3 (s), 137.7 (s), 128.8
(d), 128.0 (d), 127.6 (d), 125.8 (d), 78.1 (s), 66.9 (d), 66.2
(d), 60.4 (t), 60.3 (d), 58.1 (d), 56.5 (d), 55.7 (d), 51.4 (q),
47.2 (t), 37.5 (t), 33.0 (d), 31.9 (q), 31.6 (d), 29.5 (d), 27.8
(q), 24.0 (t), 23.8 (t), 19.3 (q), 18.8 (q), 17.7 (q), 17.5 (q);
HRMS, m/z for C40H59N7O10S2 ϩ Na (Mϩ ϩ Na), calcd:
884.3663; found 884.3661.
Boc-(S)-aThr-(S)-Val-Pro-ꢀ(CS-NH)-(S)-Ser-Val-Thz-OMe;
C-15(R)-Val epimer, 11
Trifluoroacetic acid (25 ml) was added dropwise over 20 min to
a stirred solution of the C-15-(R)-Val epimer of 10 (2.91 g, 4.74
mmol) in dichloromethane (25 ml) at 0 ЊC and the resulting
solution was then stirred at 0 ЊC for 1 h. The solution was
concentrated in vacuo and the solid residue was then dissolved
in dichloromethane (25 ml), cooled to 0 ЊC and treated drop-
wise with iPr2NEt (2.1 ml, 12.06 mmol). The mixture was
stirred at 0 ЊC for 15 min and then Boc-(S)-aThr-OH (1.14 g,
5.20 mmol) was added followed by HOBt (769 mg, 5.69 mmol).
The resulting cloudy mixture was stirred at 0 ЊC for a further
15 min then treated with DCC (1.27 g, 6.16 mmol) and stirred
at 0 ЊC for 90 min followed by a further 18 h at room temper-
ature. The suspension was concentrated in vacuo and EtOAc (25
ml) was added to the residue. The mixture was filtered and the
filtrate was washed with 10% w/v citric acid solution and satur-
ated NaHCO3, then dried (MgSO4), concentrated in vacuo and
the residue purified by flash chromatography on silica gel (30%
acetone–Et2O eluant) to give the hexapeptide (2.47 g, 3.46
mmol, 73%) as a white foam: δH (500 MHz, DMSO, 90 ЊC):
9.42 (1H, br s), 8.38 (1H, s), 8.10 (1H, br s), 7.39 (1H, br s), 6.38
(1H, br s), 5.06 (2H, m), 4.88 (1H, br s), 4.67 (1H, br s), 4.48
(2H, br s), 3.95–3.79 (5H, m), 3.86 (3H, s), 3.69 (1H, m), 2.35
(1H, app octet, J 6.7 Hz), 2.26–1.99 (4H, m), 1.88 (1H, br s),
1.42 (9H, s), 1.12 (3H, d, J 6.2 Hz), 0.98 (3H, d, J 6.8 Hz),
0.97 (3H, d, J 6.6 Hz), 0.94 (3H, d, J 6.8 Hz), 0.91 (3H, d,
J 6.7 Hz); δC (125 MHz, DMSO, 90 ЊC): 204.5 (s), 172.4 (s),
NMR data for 5, C-15(S)-Val epimer: δH (400 MHz, DMSO,
80 ЊC): 9.41 (1H, br s), 8.36 (1H, s), 8.09 (1H, d, J 7.3 Hz), 7.68
(1H, d, J 8.2 Hz), 7.52 (1H, br s), 7.27–7.18 (5H, m), 6.46 (1H,
br s), 5.08–5.03 (2H, m), 4.89 (1H, br s), 4.67 (1H, br s), 4.46
(2H, br s), 4.32–4.28 (2H, m), 3.88–3.76 (4H, m), 3.87 (3H, s),
3.69 (1H, m), 3.08 (1H, dd, J 4.0, 14.1 Hz), 2.84 (1H, dd, J 9.3,
14.1 Hz), 2.35 (1H, app octet, J 6.7 Hz), 2.21–1.99 (4H, m),
1.9 (1H, m), 1.35 (9H, s), 1.13 (3H, d, J 6.3 Hz), 0.98 (6H, d,
J 6.8 Hz), 0.93 (3H, d, J 6.7 Hz), 0.92 (3H, d, J 6.3 Hz); δC (100
MHz, DMSO, 90 ЊC): 204.6 (s), 172.4 (s), 171.2 (s), 169.9 (s),
169.6 (s), 168.5 (s), 160.9 (s), 154.7 (s), 145.4 (s), 137.7 (s), 128.8
(d), 128.0 (d), 127.8 (d), 125.8 (d), 78.1 (s), 67.1 (d), 66.2 (d),
60.4 (t), 60.3 (d), 58.1 (d), 56.6 (d), 55.6 (d), 51.4 (q), 47.2 (t),
886
J. Chem. Soc., Perkin Trans. 1, 2000, 883–888

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37.3 (t), 31.9 (d), 31.6 (d), 29.5 (d), 27.8 (q), 23.8 (t), 19.3 (q),
18.8 (q), 17.7 (q), 17.5 (q).
Hz), 4.03 (1H, app t, J 6.1 Hz), 3.88 (1H, app q, J 8.1 Hz), 3.73–
3.63 (1H, m), 3.43 (1H, dd, J 4.9, 13.9 Hz), 3.13 (1H, dd, J 8.2,
13.9 Hz), 2.41 (1H, m), 2.32 (1H, m), 2.25 (1H, m), 2.09 (3H, s),
1.98 (3H, s), 2.06–1.92 (2H, m), 1.79 (1H, m), 1.17 (3H, d, J 6.4
Hz), 1.06 (3H, d, J 6.5 Hz), 0.95 (3H, d, J 6.75 Hz), 0.84 (3H, d,
J 6.7 Hz); δC (125 MHz, CDCl3): 204.0 (s), 173.3 (s), 173.2 (s),
171.5 (s), 170.0 (s), 168.3 (s), 167.7 (s), 166.0 (s), 161.1 (s), 148.7
(s), 136.3 (s), 129.6 (d), 129.0 (d), 127.2 (d), 124.4 (d), 69.8 (d),
69.5 (d), 63.9 (d), 61.1 (d), 61.0 (d), 56.2 (d), 55.4 (d), 55.2 (d),
48.6 (t), 37.45 (t), 32.7 (t), 32.5 (d), 30.1 (d), 24.8 (t), 21.2 (q),
21.0 (q), 20.2 (q), 19.8 (q), 19.0 (q), 17.3 (q), 14.25 (q).
Cyclo-(R)-Phe-(S)-aThr(Ac)-(S)-Val-Pro-ꢀ(CS-NH)-(S)-
Ser(Ac)-Val-Thz; C15(R)-Val epimer, 14a
Aqueous NaOH (1 M, 1.8 ml, 1.8 mmol) was added to a solu-
tion of the C-15-(R)-Val epimer of 5 (314 mg, 0.36 mmol) in
3:1 THF–MeOH (8 ml) at 0 ЊC and the solution was stirred
at 0 ЊC for 1 h. The solution was concentrated in vacuo, and the
residue was then treated with EtOAc (10 ml) and cooled to 0 ЊC.
HCl (2 M, 5 ml) was added dropwise over 10 min and the layers
were separated. The aqueous layer was extracted with EtOAc
(3 × 25 ml) and the combined organic layers were washed with
brine, dried (MgSO4) and concentrated to leave a white solid.
The solid was dissolved in DMF (4 ml) and the solution was
treated successively with Et3N (0.30 ml, 2.15 mmol), Ac2O (0.12
ml, 1.27 mmol) and DMAP (2.6 mg, 0.021 mmol). The result-
ing solution was stirred at room temperature for 2 h, cooled
to 0 ЊC, then treated with water (5 ml) and allowed to warm to
room temperature and stirred for a further 1 h. The solution
was diluted with EtOAc (25 ml), then recooled to 0 ЊC and
treated dropwise with HCl (2 M, 2 ml). The two layers were
separated and the aqueous layer was extracted with EtOAc
(3 × 25 ml). The combined organic layers were washed with
water (3 × 10 ml) and brine, then dried (MgSO4) and concen-
trated in vacuo to leave a solid. The residue was dissolved in
dichloromethane (3 ml) and the solution was cooled to 0 ЊC.
TFA (3 ml) was added dropwise over 5 min and the resulting
solution was stirred at 0 ЊC for 1 h and then concentrated in
vacuo to leave the diacetate 13 [C-15-(R)-Val epimer] as a solid.
The solid was dissolved in DMF (80 ml), and the solution was
cooled to 0 ЊC and treated with iPr2NEt (0.22 ml, 1.26 mmol).
The mixture was stirred at 0 ЊC for 15 min and then DPPA (0.12
ml, 0.557 mmol) was added and stirring was continued at 0 ЊC
for a further 90 min. The solution was then allowed to warm to
room temperature, stirring was stopped and the mixture was
allowed to stand at room temperature for 3 days. It was then
diluted with EtOAc (250 ml), poured into ice-cold water (50 ml)
and the two layers were separated. The aqueous layer was
extracted with EtOAc (3 × 100 ml) and then the combined
organic extracts were washed with water (5 × 50 ml), dried
(MgSO4) and concentrated in vacuo. The residue was purified
by flash chromatography on silica gel (30% acetone–Et2O
eluant) to give the diacetylated cyclopeptide 14a (175 mg, 0.215
mmol, 59%) as a foam: δH (500 MHz, CDCl3): 8.32 (1H, d, J 8.4
Hz), 8.21 (1H, d, J 7.4 Hz), 7.95 (1H, s), 7.87 (1H, d, J 7.4 Hz),
7.30–7.20 (5H, m), 6.78 (1H, d, J 9.1 Hz), 6.49 (1H, d, J 8.9 Hz),
5.44 (1H, app dt, J 2.8, 7.6 Hz), 5.28 (1H, dq, J 3.3, 6.6 Hz),
5.03 (2H, m), 4.88 (1H, app dt, J 5.9, 8.7 Hz), 4.77 (1H, dd,
J 7.8, 12.3 Hz), 4.72 (1H, dd, J 3.2, 8.9 Hz), 4.50 (1H, dd, J 3.3,
7.4 Hz), 4.40 (1H, dd, J 2.8, 12.3 Hz), 3.78–3.68 (2H, m), 3.39
(1H, dd, J 5.9, 14.4 Hz), 3.08 (1H, dd, J 9.0, 14.4 Hz), 2.46 (1H,
m), 2.39 (1H, m), 2.18 (1H, m), 2.07 (3H, s), 2.04 (3H, s), 2.02–
1.92 (2H, m), 1.80 (1H, m), 1.36 (3H, d, J 6.6 Hz), 1.04 (3H, d,
J 6.8 Hz), 0.88 (3H, d, J 6.7 Hz), 0.84 (3H, d, J 6.7 Hz), 0.12
(3H, d, J 6.8 Hz); δC (125 MHz, CDCl3): 205.0 (s), 173.3 (s),
172.4 (s), 171.8 (s), 170.2 (s), 168.1 (s), 167.6 (s), 165.0 (s), 162.4
(s), 149.1 (s), 136.9 (s), 129.7 (d), 128.8 (d), 127.0 (d), 123.5 (d),
69.8 (d), 69.3 (d), 63.7 (t), 59.3 (d), 57.8 (d), 56.1 (d), 54.1 (d),
53.7 (d), 48.3 (t), 35.0 (t), 33.7 (d), 33.0 (t), 30.7 (d), 24.7 (t), 21.2
(q), 21.0 (q), 20.2 (q), 19.5 (q), 18.9 (q), 16.3 (q), 14.7 (q); a
satisfactory MS could not be obtained for this compound.
NMR data for 14a, C-15(S)-Val epimer: δH (500 MHz,
CDCl3): 8.45 (1H, d, J 6.2 Hz), 8.04 (1H, s), 7.87 (1H, d, J 8.7
Hz), 7.30–7.20 (5H, m), 7.03 (1H, d, J 8.5 Hz), 6.96 (1H, br s),
6.93 (1H, d, J 8.7 Hz), 5.42 (1H, app quintet, J 6.1 Hz), 5.07
(1H, dt, J 2.5, 5.9 Hz), 5.00 (1H, t, J 9.1 Hz), 4.92 (1H, app dt,
J 5.9, 8.7 Hz), 4.71 (1H, dd, J 8.2, 12.1 Hz), 4.60 (1H, dd, J 5.2,
8.8 Hz), 4.55 (1H, dd, J 4.7, 8.8 Hz), 4.32 (1H, dd, J 2.5, 12.2
Cyclodidemnamide, 15
K2CO3 (1 M, 1.2 ml) was added dropwise over 5 min to a solu-
tion of the C-15-(R)-Val epimer of 14a (96 mg, 0.118 mmol) in
MeOH (5 ml) at 0 ЊC and the solution was stirred at 0 ЊC for
1 h. Ethyl acetate (25 ml) was added and the solution was then
poured into water (10 ml). The layers were separated and the
aqueous layer was extracted with EtOAc (3 × 25 ml). The com-
bined organic layer and extracts were dried (MgSO4) and then
concentrated in vacuo to leave the corresponding diol as a white
powder. The powder was dissolved in THF (20 ml) and Burgess’
reagent (70 mg, 0.294 mmol) was added (Note: it was necessary
to use rigorously dried THF in this reaction). The solution was
heated under reflux for 2 h and then concentrated in vacuo to
leave a residue which was purified by flash chromatography
on silica gel (2% MeOH–dichloromethane as eluant) to give
cyclodidemnamide (23 mg, 0.033 mmol, 28%) as a white foam
whose NMR spectroscopic data agreed completely with liter-
ature values.1
Data for 1: δH (500 MHz, CDCl3): 8.41 (1H, d, J 7.9 Hz), 7.97
(1H, s), 7.48 (1H, d, J 7.5 Hz), 7.25 (5H, m), 7.08 (1H, d, J 7.5
Hz), 5.15 (1H, dd, J 9.8, 1.9 Hz), 5.12 (1H, m), 4.91 (1H, t, J 7.2
Hz), 4.83 (1H, quintet, J 6.3 Hz), 4.66 (1H, dd, J 9.9, 2.6 Hz),
4.35 (1H, t, J 7.5 Hz), 4.03 (1H, d, J 6.3 Hz), 3.70 (1H, m), 3.64
(1H, dd, J 11.2, 1.9 Hz), 3.52 (1H, m), 3.51 (1H, app t, J 10.6
Hz), 3.25 (1H, dd, J 13.7, 5.7 Hz), 3.18 (1H, dd, J 13.7, 6.5 Hz),
3.03 (1H, m), 2.32 (1H, m), 2.17 (1H, m), 2.00 (2H, m), 1.80
(1H, m), 1.39 (3H, d, J 6.3 Hz), 1.07 (3H, d, J 6.5 Hz), 0.76 (3H,
d, J 6.7 Hz), 0.72 (3H, d, J 6.7 Hz), 0.14 (3H, d, J 6.4 Hz);
δC (125 MHz, CDCl3): 180.5 (s), 171.2 (s), 171.0 (s), 170.6 (s),
169.9 (s), 169.3 (s), 160.8 (s), 147.3 (s), 135.8 (s), 129.7 (d), 128.5
(d), 127.2 (d), 124.9 (d), 82.0 (d), 77.5 (d), 73.9 (d), 63.1 (d), 60.2
(d), 54.2 (d), 48.3 (d), 47.8 (t), 39.6 (t), 36.9 (t), 31.2 (d), 30.7 (d),
30.2 (t), 25.5 (t), 21.9 (q), 20.4 (q), 20.3 (q), 20.2 (q), 14.6 (q);
MS (FAB), m/z (%): 716 (Mϩ ϩ Na, 55), 694 (Mϩ ϩ H, 100),
597 (14), 415 (13), 356 (6), 285 (18); HRMS, m/z for C34H44-
N7O5S2 (Mϩ ϩ H), calcd: 694.2845; found: 694.2813.
Acknowledgements
We thank the EPSRC for support of this work (Fellowships to
C. D. J. B. and M. N.), and Professor W. Fenical for providing
NMR spectra for naturally derived cyclodidemnamide.
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