Synthesis of the JK/LM-ring model of prymnesins, potent hemolytic and ichthyotoxic polycyclic ethers isolated from the red tide alga Prymnesium parvum: confirmation of the relative configuration of the K/L-ring juncture

Article abstract of DOI:10.1016/j.tetlet.2006.06.032

Synthesis of the JK/LM-ring model of prymnesins, polycyclic ether toxins isolated from the red tide phytoflagellate, Prymnesium parvum, is described. Comparison of the ¹H and ¹³C NMR data of the synthetic model with those reported fo

Full text of DOI:10.1016/j.tetlet.2006.06.032

Tetrahedron Letters 47 (2006) 5687–5691
Synthesis of the JK/LM-ring model of prymnesins, potent
hemolytic and ichthyotoxic polycyclic ethers isolated from the
red tide alga Prymnesium parvum: confirmation of the
relative configuration of the K/L-ring juncture
Makoto Sasaki,* Naoki Takeda, Haruhiko Fuwa, Ryuichi Watanabe,
Masayuki Satake and Yasukatsu Oshima
Graduate School of Life Sciences, Tohoku University, Tsutsumidori-amamiya, Aoba-ku, Sendai 981-8555, Japan
Received 21 April 2006; revised 30 May 2006; accepted 2 June 2006
Available online 27 June 2006
Abstract—Synthesis of the JK/LM-ring model of prymnesins, polycyclic ether toxins isolated from the red tide phytoflagellate,
1
Prymnesium parvum, is described. Comparison of the H and 13C NMR data of the synthetic model with those reported for pry-
mnesins confirmed the reported configurational assignment of the K/L-ring juncture.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Prymnesium parvum is a unicellular alga that blooms in
brackish water and causes massive fish kills worldwide.
Prymnesin-1 (PRM1, 1) and prymnesin-2 (PRM2, 2)
were isolated as the causative toxins from the cultured
cells of the phytoflagellate. These toxins possess extre-
mely potent hemolytic activity, which is about 5000-fold
greater than that of Merck saponin on a molar basis,
and also exhibit potent ichthyotoxicity.3 Their gross
structures including partial stereochemistry were deter-
mined by Igarashi and co-workers.1,2 Prymnesins pos-
sess unique structural features: an unbranched single
chain of 90 carbons except for a single methyl group,
a fused polycyclic ether ring system (ABCDE-ring), four
distinct 1,6-dioxadecalin units (FG-, HI-, JK- and LM-
rings), conjugated double and triple bonds, chlorine
atoms and an amino group, and glycosidic residues
including an uncommon L-xylose. The relative stereo-
chemistry of the fused polycyclic ether domain and four
1,6-dioxadecalin units was determined by extensive
NMR analysis. Recently, the absolute configuration of
the amino group at C14 in PRM2 was determined to
be S by using a chiral anisotropic reagent and that at
chlorinated C85 to be S by fluorimetric chiral HPLC
comparison between a degradation product and syn-
thetic references (Fig. 1).4 However, the configuration
of C76–C85 acyclic portion still remained undetermined.
Based on the coupling constant, J61,62 = 8.5 Hz, and
NOE data, the K/L-ring juncture was inferred to take
C60–C63 anti conformation. However, a weak NOE be-
tween 61-H and 62-H observed in the NOESY spectra of
PRM2, along with the intense NOE between 61-H and
equatorial oriented 63-Ha in the NOESY spectra of N-
acetylprymnesin-2 (NAPRM2), suggested the presence
of a minor gauche conformer. Hence, stereochemistry
of this part should be confirmed by synthetic methods.
We have previously demonstrated that the synthesis of
model fragments coupled with NMR J-based configura-
tion analysis (JBCA)5 have successfully elucidated the
relative configuration of the acyclic portions of a com-
plex natural product, maitotoxin, the most toxic and
largest non-biopolymer.6 A similar approach was suc-
cessfully applied to prymnesins, wherein the relative
configuration of the I/J- and E/F-ring junctures was
confirmed and revised, respectively.7,8 As part of our
studies toward complete stereochemical assignment of
prymnesins, we describe herein the synthesis of the
JK/LM-ring model 3 for comparison of the NMR data
with those of prymnesins, which led to confirmation of
the reported stereochemical assignment of the K/L-ring
juncture of the natural products.
*
Corresponding author. Tel.: +81 22 717 8828; fax: +81 22 717
8897; e-mail: masasaki@bios.tohoku.ac.jp
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.032

5688
M. Sasaki et al. / Tetrahedron Letters 47 (2006) 5687–5691
Me
H
H
H
H
H
H
H
H
H
O
O
B
E 37
O
H
F
O
H
H
A
O
C
O
D
Cl
Cl
H
20
O
G
H
H
1
H
O
H
O
H
J
OH
H
H
H
HO
H
NH2
OH
H
OH
O
K
L
OH
O
H
14S
H
OH
O
M
HO
OH
N
OH
H
HO
74
H
O
OH
OH
H
1: prymnesin-1 (PRM1)
HO
OH
OH
OH
H
O
O
OH OH OH OH OH
R
OH
2: prymnesin-2 (PRM2): R =
O
O
O
90
O
76
HO
OH
O
O
OH OH Cl
HO
HO
HO
OH
HO
Cl
HO
OH
85S
Figure 1. Structures of prymnesin-1 (1) and prymnesin-2 (2).
Retrosynthetic analysis of the JK/LM-ring model 3 is
outlined in Scheme 1. It was envisioned that target 3
would be derived from alkynyl ketone 4 through reduc-
tive cyclization of the K- and L-rings, respectively. The
key intermediate 4 would be synthesized via convergent
union of the lithium acetylide generated from alkyne 5,
corresponding to the J-ring, and aldehyde 6, represent-
ing the M-ring.
OAc
OH
Cl
H
OAc
OAc
O
Ph
OH
OH
a-c
d,e
O
O
H
7
O
O
H
H
8
9
Cl
Cl
OTBS
OH
OTBS
f,g
h,i
R
J
O
O
H
H
11: R = TMS
5 : R = H
j
10
The synthesis of alkyne 5 started with the known alcohol
8,9 which was readily obtained from commercially avail-
able tri-O-acetyl-D-glucal (7) in three steps (Scheme 2).
Stereoselective installation of the C5610 chlorine atom
was performed by treatment with CCl4 and PPh3 in
the presence of K2CO3, and subsequent removal of the
benzylidene acetal provided diol 9 in 89% overall yield.
Protection as the TBS ethers, followed by selective
removal of the primary TBS group, gave alcohol 10 in
90% yield over two steps. Triflation of the primary
hydroxy group and subsequent treatment with lithium
trimethylsilylacetylide afforded 11 in 84% yield over
two steps. Finally, desilylation with K2CO3 provided
alkyne 5 in a quantitative yield.
Scheme 2. Reagents and conditions: (a) H2, Pd/C, EtOAc; (b)
NaOMe, MeOH; (c) PhCH(OMe)2, CSA, DMF, 94% (three steps);
(d) PPh3, K2CO3, CCl4, 80 ꢁC, 89%; (e) CSA, MeOH, quant.; (f)
TBSOTf, 2,6-lutidine, CH2Cl2, 0 ꢁC; (g) CSA, MeOH, 0 ꢁC, 90% (two
steps); (h) Tf2O, 2,6-lutidine, CH2Cl2, ꢀ78 ꢁC; (i) n-BuLi, TMS-
acetylene, THF/HMPA, ꢀ78 ꢁC, 84% (two steps); (j) K2CO3, MeOH,
quant.
H
H
H
H
O
O
M
OBn
OBn
c,d
OBn
OBn
R
66
RO
TBSO
e
OBn
OBn
12: R = Bn
13: R = H
14: R = H,OH
6 : R = O
a,b
The synthesis of aldehyde 6 commenced with the known
compound 1211 (Scheme 3). Regioselective removal of
the benzyl ether at C6610 was successfully achieved by
the method of Nicotra.12 Thus, treatment of 12 with
iodine to effect debenzylative iodoetherification, followed
by reduction with zinc and acetic acid, produced alcohol
13 in 91% yield over two steps. After TBS protection,
the terminal olefin was subjected to hydroboration with
9-BBN to give primary alcohol 14 in 92% overall yield.
Scheme 3. Reagents and conditions: (a) I2, CH2Cl2; (b) Zn, HOAc,
THF/MeOH, 91% (two steps); (c) TBSOTf, 2,6-lutidine, CH2Cl2, 0 ꢁC,
98%; (d) 9-BBN, THF; then H2O2, NaHCO3, 0 ꢁC, 94%; (e) SO3Æpyr-
idine, DMSO, Et3N, CH2Cl2, 0 ꢁC, 95%.
Oxidation with SO3Æpyridine/DMSO completed the syn-
thesis of the desired aldehyde 6 in 95% yield.
With the requisite fragments in hand, we next carried out
their convergent union. The lithium acetylide generated
from alkyne 5 (n-BuLi, THF/HMPA, ꢀ78 ꢁC) was re-
acted with aldehyde 6 to give propargyl alcohol 15 in
81% yield as a mixture of diastereomers (Scheme 4). Oxi-
dation of 15 under Swern conditions yielded the corre-
sponding ketone 4 in a quantitative yield. Subsequent
removal of the TBS ethers was best carried out by the ac-
tion of HF in acetonitrile to provide dihydroxy ketone 16
in 79% yield. Exposure of 16 to Et3SiH and BF3ÆOEt2 in
acetonitrile effected reductive cyclization to form the L-
ring (94%), and the remaining secondary alcohol was
protected as the TBS ether to yield 17 in a quantitative
yield. The stereochemistry at the newly generated stereo-
H
H
O
M
Cl
OH
OH
62L
O
H
H
H
O
K
61H
H
H
H
J
O
OHC
O
OH
OBn
OBn
OH
3
M
TBSO
OBn
Cl
6
Cl
+
O
OTBS
H
H
O
J
O
OBn
OBn
OTBS
M
J
O
H
TBSO
4
OBn
H
5
Scheme 1. Retrosynthesis of model compound 3.

M. Sasaki et al. / Tetrahedron Letters 47 (2006) 5687–5691
5689
Cl
OTBS
Cl
Cl
J
O
OH
O
OTBS
H
H
OR
H
H
O
O
H
5
J
O
J
O
OBn
OBn
OBn
OBn
a
b
M
M
H
H
H
TBSO
H
RO
OHC
Cl
O
M
OBn
OBn
15
OBn
OBn
H
4 : R = TBS
16: R = H
c
TBSO
OBn
6
d,e
H
H
H
H
H
O
O
OBn
OBn
Cl
O
Cl
OBn
OBn
H
O
K
OH
OTBS
HO
L
O
M
OBn
L
O
M
OTBS
L
M
f,g
h,i
J
O
H
H
J
O
O
OBn
J
O
H
H
OBn
H
H
OBn
O
OBn
NOE
OH
H
H
H
17
19
18
Scheme 4. Reagents and conditions: (a) 5, n-BuLi, THF/HMPA, ꢀ78 ꢁC; then 6, 81%; (b) (COCl)2, DMSO, Et3N, CH2Cl2, ꢀ78 ꢁC ! rt, quant.; (c)
HF, MeCN, 0 ꢁC, 79%; (d) Et3SiH, BF3ÆOEt2, MeCN, 0 ꢁC, 94%; (e) TBSOTf, 2,6-lutidine, CH2Cl2, 0 ꢁC, quant.; (f) H2, Lindlar catalyst, quinoline,
EtOAc, quant.; (g) OsO4, NMO, THF/H2O, 95%; (h) (CF3CO)2O, DMSO, Et3N, CH2Cl2, ꢀ60 ꢁC; (i) HF, MeCN, 0 ꢁC, 91% (two steps).
center was confirmed by NOE, as shown in Scheme 4.
Half-reduction of alkyne 17 with Lindlar catalyst,
followed by dihydroxylation of the resulting cis-olefin,
provided diol 18 as an approximately 2:1 mixture of
diastereomers (95% yield, two steps). Subsequent oxida-
tion of 18 under the influence of trifluoroacetic anhy-
dride, DMSO, and triethylamine (CH2Cl2, ꢀ60 ꢁC) led
to the corresponding diketone,13,14 which without purifi-
cation was subjected to HF in acetonitrile to generate
hemiacetal 19 in 91% yield over two steps.
respectively (Scheme 5). At this stage, stereochemistry of
each compound could not be assigned. Treatment of the
major alcohol 20 with p-toluenesulfonic acid in metha-
nol at 70 ꢁC produced the corresponding methyl acetal
(65% yield), which was then reduced with Et3SiH/
BF3ÆOEt2 to give tetracyclic ether 22 in 86% yield. The
stereostructure of 22 was established by the coupling
constant, J60,61 = 12 Hz, and NOE, as shown. On the
other hand, reduction of methyl acetal, derived from
21 (p-toluenesulfonic acid, methanol, 70 ꢁC, 75%), with
Et3SiH/BF3ÆOEt2 afforded the desired 23 in only low
yield (22%). Hence, we decided to invert the C60 stereo-
chemistry of 22 by an oxidation/reduction sequence.
Dess–Martin oxidation15 of 22, followed by reduction
with L-Selectride, generated the desired 23 in 83% yield
over two steps. Finally, removal of the benzyl ethers
under hydrogenolysis furnished the targeted JK/LM-ring
model 3 in 92% yield.16 In addition, the corresponding
tetraacetate 24 was prepared under the usual conditions
(acetic anhydride, pyridine, room temperature).17
Subsequent treatment of 19 with DIBALH provided
b-alcohol 20 and a-alcohol 21 in 65% and 22% yield,
H
H
H
O
Cl
OBn
OBn
H
H
OH
L
O
M
O
K
J
O
H
OBn
Cl
O
19
a
H
H
H
H
O
O
The JK/LM-ring model 3 and its acetate 24 were sub-
Cl
OBn
+
OBn
OBn
1
jected to the NMR studies, and their H and 13C NMR
H
H
OH
H
H
OH
L
M
L
O
M
O
K
O
K
O
OBn
J
O
J
O
H
H
H
H
chemical shifts for the C59–C64 portion were compared
with those of N-acetylprymnesin-2 (NAPRM2) and per-
acetylprymnesin-2 (PAPRM2). The NMR chemical shift
values observed for 3 matched the reported values for
NAPRM2 (Table 1). In particular, differences in the
13C NMR chemical shifts were within Dd = 0.3 ppm. In
addition, the chemical shift values for tetraacetate 24 also
corresponded well with those for PAPRM2 (Table 2).
Thus, the formerly assigned configuration of the K/L-
ring juncture was confirmed to be represented by struc-
ture 3. Also, the coupling constant, J61,62 = 8.4 Hz, and
NOE data of compound 3 (Fig. 2) reproduced those ob-
served for the natural products.18 These results show that
the K/L-ring juncture adopts mostly a C–C anti confor-
mation (Fig. 2),19 although only a minor gauche con-
former, which shows NOE between 61-H and 62-H, is
present.18
OBn
OBn
OH
OH
20
21
b,c
b,c
NOE
H
H
O
H
H
O
Cl
H
OBn
OBn
Cl
OBn
OBn
H
H
H
H
H
L
O
M
L
O
M
O
O
K
61H
H
H
H
J
K 60
J
O
OBn
OBn
O
OH
OH
H
H
22
23
J60,61 = 12 Hz
f
d,e
H
H
H
O
Cl
OR
OR
H
H
L
O
M
O
K
3 : R = H
24: R = Ac
J
O
H
g
OR
OR
Scheme 5. Reagents and conditions: (a) DIBALH, CH2Cl2, ꢀ78 ꢁC,
65% for 20; 22% for 21; (b) p-TsOH, MeOH, 70 ꢁC, 65% for 20; 75%
for 21; (c) Et3SiH, TMSOTf, CH2Cl2, 0 ꢁC, 86% for 22; 22% for 23; (d)
DMP, CH2Cl2, quant.; (e) L-Selectride, THF, ꢀ78 ꢁC, 83%; (f) H2,
Pd(OH)2/C, MeOH, 92%; (g) Ac2O, pyridine, quant.
In conclusion, we have synthesized the JK/LM-ring
model 3 of prymnesins through convergent union of

5690
M. Sasaki et al. / Tetrahedron Letters 47 (2006) 5687–5691
Table 1. 1H and 13C NMR chemical shifts of C59–C64 regions in N-acetylprymnesin-2 (NAPRM2) and model compound 3 (600 MHz, 1:1 C5D5N/
CD3OD)a
Position
59
NAPRM2
3
Ddb
1H (pattern)
13C
1H (pattern)
13C
1H
0.09
0.06
0.04
ꢀ0.05
ꢀ0.02
ꢀ0.13
ꢀ0.08
ꢀ0.09
ꢀ0.19
13C
0.1
1.59
2.23
4.23
38.7
1.50 (ddd, 12.0, 12.0, 3.0)
2.17 (m)
38.6
60
61
62
63
66.2
84.9
77.4
30.3
4.19 (brs)
3.25 (brd, 8.4)
3.64 (m)
66.3
84.7
77.1
30.3
ꢀ0.1
0.2
0.3
3.20 (dd, 8.5, 2)
3.62 (ddd, 10, 8.5, 2)
1.22
1.96
1.32
1.76
1.35 (m)
0.0
2.04 (brdd, 12.6, 3.0)
1.41 (m)
64
30.8
30.8
0.0
1.95 (m)
a Referred to internal CHD2OD (d 3.31) for 1H NMR and 13CD3OD (d 50.0) for 13C NMR.
b Dd = d(NAPRM2) ꢀ d(model 3).
Table 2. 1H and 13C NMR chemical shifts of C59–C64 regions in peracetylprymnesin-2 (PAPRM2) and compound 24 (600 MHz, CDCl3)a
Position
PAPRM2
24
Ddb
1H
13C
1H (pattern)
1.49 (m)
2.19 (ddd, 13.8, 4.8, 3.6)
5.03 (brs)
3.26 (dd, 9.0, 1.2)
3.38 (brdd, 9.0, 9.0)
1.28 (m)
13C
1H
0.12
0.02
0.05
0.04
0.05
0.04
0.04
13C
59
1.61
2.21
5.08
3.30
3.43
1.32
2.12
1.54
2.02
33.3
33.4
ꢀ0.1
60
61
62
63
67.3
80.4
74.8
28.2
67.4
80.4
74.7
28.2
ꢀ0.1
0.0
0.1
0.0
2.08 (m)
1.53 (m)
2.04 (m)
64
28.2
28.3
0.01
ꢀ0.02
ꢀ0.1
a Referred to internal CHCl3 (d 7.26) for 1H NMR and 13CDCl3 (d 77.0) for 13C NMR.
b Dd = d(PAPRM2) ꢀ d(compound 24).
Acknowledgements
We are grateful to Drs. T. Yasumoto and T. Igarashi
(Japan Food Research Laboratories) for helpful discus-
sion. This work was supported by a Grant-in-Aid from
the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
Figure 2. Major conformation for C61–C62 of model compound 3.
Double-headed arrows indicate NOEs.
References and notes
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1785; (e) Nonomura, T.; Sasaki, M.; Matsumori, N.;
Murata, M.; Tachibana, K.; Yasumoto, T. Angew. Chem.,
Int. Ed. 1996, 108, 1786–1789; For the work from Kishi’s
the J- and M-rings, followed by reductive ring-closure of
the K- and L-rings. Comparison of the NMR chemical
shifts for model 3 and its tetraacetae 24 with those re-
ported for the natural products and its peracetate deriv-
ative allowed the unambiguous confirmation of the
assigned relative configuration of the K/L-ring juncture
of prymnesins. To establish the complete stereochemis-
try of the polycyclic portion (C20–C74) of prymnesins,
the reported NMR-based configurational assignment
of the M/N-ring juncture has to be confirmed by a
synthetic approach. Stereochemical assignment of the
acyclic polyol portion (C76–C85), together with the
absolute configuration and total synthesis of prymne-
sins, will pose new challenges to synthetic chemists. Fur-
ther studies toward the complete stereostructure of
prymnesins are underway and will be reported in due
course.

M. Sasaki et al. / Tetrahedron Letters 47 (2006) 5687–5691
5691
group, see: (f) Zheng, W.; DeMattei, J. A.; Wu, J.-P.;
Duan, J. J.-W.; Cook, L. R.; Oinuma, H.; Kishi, Y. J. Am.
Chem. Soc. 1996, 118, 7946–7968.
3.05 (ddd, J = 10.5, 9.6, 4.2 Hz, 1H), 2.17 (m, 1H), 2.13
(m, 1H), 2.04 (brdd, J = 12.6, 3.0 Hz, 1H), 1.95 (m, 1H)
1.78 (brd, J = 15.0 Hz, 1H), 1.50 (ddd, J = 12.0, 12.0,
3.0 Hz, 1H), 1.41 (m, 1H), 1.35 (m, 1H); 13C NMR
(125 MHz, C5D5N/CD3OD) d 84.7, 82.0, 81.2, 81.0, 78.2,
77.1, 74.9, 72.3, 69.3, 66.3, 63.9, 63.5, 60.1, 38.6, 36.1, 30.8,
30.3; HRMS (ESI) calcd for C17H28O8Cl [(M+H)+]
395.1473, found 395.1485.
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2001, 42, 5725–5728.
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Org. Lett. 2004, 6, 1501–1504.
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letter corresponds to that of prymnesins.
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14. Oxidation of 18 under usual Swern conditions [(COCl)2,
DMSO, Et3N, CH2Cl2, ꢀ78 ꢁC] resulted in low yield of
the corresponding diketone.
20
17. Data for compound 24: ½aꢁD +29.4 (c 0.40, CHCl3); IR
(film) 2916, 2874, 1745, 1431, 1371, 1240, 1129, 1102,
1
1051, 1029, 980, 755 cmꢀ1; H NMR (600 MHz, CDCl3)
d 5.28 (brd, J = 3.6 Hz, 1H), 5.03 (brs, 1H), 4.84 (dd,
J = 10.2, 3.6 Hz, 1H), 4.37 (brd, J = 3.0 Hz, 1H), 3.96 (d,
J = 6.6 Hz, 2H), 3.81 (dd, J = 12.0, 10.8 Hz, 1H), 3.77
(dd, J = 6.6, 6.6 Hz, 1H), 3.72 (ddd, J = 12.0, 9.0, 4.8 Hz,
1H), 3.65 (dd, J = 12.0, 4.8 Hz, 1H), 3.38 (brdd, J = 9.0,
9.0 Hz, 1H), 3.26 (dd, J = 9.0, 1.2 Hz, 1H), 3.22 (dd,
J = 10.8, 10.2 Hz, 1H), 3.18 (dd, J = 9.0, 3.0 Hz, 1H), 3.08
(ddd, J = 10.8, 9.0, 4.2 Hz, 1H), 2.19 (ddd, J = 13.8, 4.8,
3.6 Hz, 1H), 2.13 (m, 1H), 2.08 (m, 1H), 2.04 (m, 1H), 2.02
(s, 3H), 1.99 (s, 3H), 1.94 (s, 3H), 1.93 (s, 3H), 1.87 (brd,
J = 15 Hz, 1H), 1.53 (m, 1H), 1.49 (m, 1H), 1.28 (m, 1H);
13C NMR (125 MHz, CDCl3) d 170.7, 170.5, 170.1, 169.8,
80.4, 78.5, 76.3 (·2), 74.70, 74.66, 71.1, 68.3, 67.4, 66.9,
62.0, 61.8, 57.2, 34.0, 33.4, 28.3, 28.2, 20.9, 20.8, 20.6, 20.5;
HRMS (ESI) calcd for C25H35O12ClNa [(M+Na)+]
585.1715, found 585.1707.
15. Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–
4156.
19
16. Data for compound 3: ½aꢁD +54.6 (c 0.43, MeOH); IR
(film) 3379, 2926, 2871, 1725, 1584, 1304, 1098, 976,
753 cmꢀ1 1H NMR (600 MHz, C5D5N/CD3OD) d 4.47
;
(d, J = 2.4 Hz, 1H), 4.19 (brs, 1H), 4.11 (d, J = 3.0 Hz,
1H), 4.06 (ddd, J = 12.0, 9.0, 4.2 Hz, 1H), 3.96 (dd,
J = 11.4, 6.6 Hz, 1H), 3.89 (dd, J = 11.4, 5.1 Hz, 1H), 3.77
(brdd, J = 12.0, 12.0 Hz, 1H), 3.73 (dd, J = 9.6, 3.6 Hz,
H), 3.58–3.65 (m, 3H), 3.53 (dd, J = 9.6, 9.6 Hz, 1H), 3.25
(dd, J = 8.4, 1.2 Hz, 1H), 3.23 (dd, J = 9.0, 3.0 Hz, 1H),
18. A weak NOE observed between 61-H and 62-H in the
NOESY spectra of NAPRM22 was also reproduced in the
model compound 3.
2
19. JC61,H-62 value, ꢀ5.1 Hz, also indicated the predominant
gauche orientation for C61-O/H-62.