Tulearins A, B, and C; structures and absolute configurations

Article abstract of DOI:10.1016/j.tetlet.2009.04.028

The relative configuration of tulearin A (1) is determined by X-ray diffraction analysis of a cyclic carbonate derivative 2 and the absolute configuration (2R,3R,5S,8S,9S,15R,17S) from the 9-MTPA-esters 1R and 1S is determined using the modified Mosher's

Full text of DOI:10.1016/j.tetlet.2009.04.028

Tetrahedron Letters 50 (2009) 3820–3822
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Tetrahedron Letters
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Tulearins A, B, and C; structures and absolute configurations
Ashgan Bishara a, Amira Rudi a, Israel Goldberg a, Maurice Aknin b, Yoel Kashman a,
*
a School of Chemistry, Tel Aviv University, Ramat Aviv 69978, Israel
b Laboratorie de Chimîe des Substances Naturelles et des Aliments, Faculté des Sciences et Techniques, Université de Réunion, 15 Avenue Rene Cassin, B.P. 7151,
97715 Saint Denis, Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The relative configuration of tulearin A (1) is determined by X-ray diffraction analysis of a cyclic carbon-
ate derivative 2 and the absolute configuration (2R,3R,5S,8S,9S,15R,17S) from the 9-MTPA-esters 1R and
1S is determined using the modified Mosher’s method. A mechanism for the unexpected formation of car-
bonate 2 is suggested. Two N-phenyltriazolinedione derivatives 3 and 4 are also prepared. Two additional
tulearins, B and C (5 and 6) are isolated in very small amounts and their structures are elucidated by spec-
troscopic means.
Received 1 March 2009
Revised 30 March 2009
Accepted 7 April 2009
Available online 14 April 2009
Ó 2009 Elsevier Ltd. All rights reserved.
As part of our research on bioactive marine natural products we
recently reported the structures of three new groups of com-
pounds, the salarins, tulearins, and taumycins, isolated from the
Madagascan sponge Fascaplysinopsis sp.1–3
The salarins and tulearins were found to inhibit cell prolifera-
tion and modulate the cycle of cultured cell lines from both mouse
and human origins.
The change of an a-hydroxycarbamate functionality into a cyc-
lic carbonate is reported in the literature as an unexpected side
product under acidic conditions.5
Fortunately, compound 2 gave crystals suitable for X-ray dif-
fraction analysis, confirming its structure and establishing the rel-
ative configuration of all seven chiral centers of 1. Interestingly, it
is reported that a digitoxoside-carbonate ring-opens under aque-
The planar structure, including the configuration of the double
bonds of tulearin A (1), was established on the basis of extensive
2D NMR data interpretation.1
ous ammoniacal conditions to a mixture of the corresponding a-
hydroxycarbamates.6 The molecular structure of compound 24 is
depicted in Figure 2.
Within the framework of a structure–activity relationship (SAR)
study of the tulearins, we investigated the effects of a variety of re-
agents on the molecule in order to change the six functional moi-
eties. Among others, we treated 1 with different bases in order to
transform the carbamate and/or the macrolide-lactone group,
and also attempted to obtain tulearin C (Fig. 1). Treatment of 1
with a mixture of aq ammonia/MeOH (1:1), afforded less polar
compound 2 as colorless crystals in 83% yield.
The molecular geometry of 2 reveals common characteristics of
bond lengths and bond angles and the non-centrosymmetric space
group of the crystal is consistent with the chiral nature of this com-
pound. There are seven asymmetric carbons in 2, and their relative
absolute configurations are R at C-2, C-3, and C-15, and S at C-5, C-
8, C-9, and C-17. Despite this, the absolute configuration of the en-
tire structure could not be determined from diffraction data. There
The molecular ion of 2, m/z 519 [M+H]+ analyzed by FABMS, had
the formula C31H50O6 with seven degrees of unsaturation, in com-
parison to m/z 558 [M+Na]+ of 1 (C31H53NO6). The difference of 17
mass units indicated the loss of NH3, suggesting replacement of the
20
H
18
H3C (CH2)4
13
26
17
H C
O
3
CH3
O
a-hydroxycarbamate of 1, by a cyclic carbonate. The major changes
29
30
1
R3O
10
in the NMR spectra were in the C-7 to C-10 segment [dC 31.5 (t, C-
7), dH 1.65 (m); dC 80.7 (d, C-8), dH 4.20 (td, J = 6.3, 5.3 Hz); dC 79.7
(d, C-9), dH 4.30 (tdd, J = 6.8, 5.3, 1.2 Hz); dC 32.3 (t, C-10), dH 1.95
(m), 1.67 (m); dC 154.5 (s, C-31)] which implied a cyclic 8,9-car-
OR1
H3C
27
4
8
OR2
CH3
bonate functionality4 instead of the
a-hydroxycarbamate of 1.
28
The substitution of the 8-carbamate by an 8,9-carbonate changed,
inter alia, the carbon resonance of C-31, from 157.7 ppm in 1 to
154.5 ppm in 2 (Fig. 1).
1 R1,R3= H R2= CONH2 (C-31)
2 R1= H R2,R3= carbonate (C-31)
5 R1,R2= CONH2 R3= H
6 R1,R2,R3= H
* Corresponding author. Tel.: +972 36408419; fax: +972 3 6409293.
E-mail address: kashman@post.tau.ac.il (Y. Kashman).
Figure 1. Tulearins A–C (1, 5, and 6) and carbonate 2.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.028

A. Bishara et al. / Tetrahedron Letters 50 (2009) 3820–3822
3821
Figure 2. (a) 3D Molecular structure and (b) wire frame model of compound 2 obtained by X-ray analysis.
is an intramolecular hydrogen bond in the structure between O3-
20
H
+0.05
+0.04
18
H3C
H3ꢀꢀꢀO2 [OꢀꢀꢀO 2.791(3) Å, O–HꢀꢀꢀO 139°]. The C–C conformations
in the O1–C17 macrocycle are in the sequence: ac, -sc, ap, ap, sc,
ap, ap, -ac, sc, sc, -ac, ap, -ac, ap, ap, and –sc [where the symbols
sc (synclinal), ap (antiperiplanar), and ac (anticlinal) refer to tor-
sion angles within (30–90°), 180 30°, and (90–150°)], respec-
tively. In this observed conformation, the inward-facing
methylenes (C-4 and C-7) and methines (C-9, C-13, and C-15) fill
the void effectively within the macrocycle. The exocyclic chain res-
idue C-17 to C-26 adopts an extended conformation. The confor-
mation about most of the C–C bonds in this chain is anti-
periplanar, with a single exception of an anti-clinal conformation
about the C-21–C-22 bond.
13
10
+0.04
17
26
H3C
O
CH3
29
O
+0.08
+0.12
30
1
+0.01
MTPAO
OH
H3C
-0.09
+0.01
27
4
O
8
+0.04
O
-0.08
-0.13
31
-0.02 CH3
NH2
28
Figure 3.
D
d values [
D
d (in ppm) = dS ꢁ dR] obtained for the C-9 (S)- and (R)-MTPA
esters of tulearin A.
to have a common biosynthesis) is 2R,3R,5S,8S,9S,15R, and 17S. It
is important to stress that, as required in the modified Mosher’s
method, all the assigned protons with positive and negative
values are actually found on the right and left sides of the MTPA
plane (MTPA-C-9 to C-4), respectively. Also, the absolute values
The absolute stereochemistry of tulearin A (1) was determined
by a modified Mosher’s method.7 The technique utilizes aniso-
Dd
tropic shifts induced in the 1H NMR spectra of
a-methoxy-a-(tri-
fluoromethyl)phenylacetic (MTPA) esters of secondary alcohols to
define the absolute configuration. Both (+)-(R)-(1R) and (ꢁ)-(S)-
of
moiety.10
With the expectation that reducing the conformational mobility
Dd are inversely proportional to the distance from the MTPA
(1S) MTPA esters of compound 1 were prepared8 and the
Dd values
(Fig. 3) from their 500 MHz 1H NMR spectra were calculated
D
d[d(S-MTPA ester) ꢁ d(R-MTPA ester)].9 Using this method, the
of the side chain of tulearin would increase the chances of crystal-
lization, we performed a Diels–Alder reaction of 1 with N-phen-
absolute configuration of C-9 was determined to be S, hence, on
the basis of the X-ray structure, the absolute configuration of the
other chiral centers of tulearins A, B, and C (assuming the three
yltriazolinedione,
a reaction that afforded two cycloaddition
products 3 and 4.11

3822
A. Bishara et al. / Tetrahedron Letters 50 (2009) 3820–3822
38
H3C
H3C
36
26
26
O
33
34
N
H
O
CH3
21
N
Ph
N
32
H
N
21
N
H
H
O
H
O
18
18
33
13
13
N
17
17
O
H
O
N 32
N N
H C
O
3
CH3
O
CH3
Tulearin A (1)
O
+
34
30
29
29
1
1
38
CH2Cl2, r.t.
O
10
10
OH HO
OH HO
H3C
H3C
27
O
O
4
8
4
8
36
O
O
31
31
NH2
NH2
CH3
CH3
28
28
3
4
Scheme 1. Diels–Alder reaction of tulearin A (1) with N-phenyltriazolinedione.
COSY, and HSQC spectra) see Supplementary data.Crystal data:
M = 518.71, orthorhombic, space group P212121, a = 5.4308(3), b = 10.5353(6),
c = 53.487(3) Å, V = 3060.3(3) Å3, Z = 4, T = 110(2) K, Dc = 1.126 g cmꢁ3
(MoK
) = 0.076 mmꢁ1, 3152 unique reflections to 2hmax = 50.7°, 340 refined
C31H50O6,
Characteristics in the NMR spectra of both adducts were the
replacement of the C-18,20-diene system by a single trisubstituted
double bond and the appearance of the expected N-phenyl group.
The differences between 3 and 4 are in the configurations of C-
18 and C-21 as a result of cycloaddition from above or below the
plane of the molecule.12 The stereochemistry of 3 and 4, as de-
picted in Scheme 1, was deduced from NOEs around the C-17/18
bond. Namely, compound 3 exhibits NOEs between H-17 and H-
18, CH3-29, CH3-30, as well as between CH3-27 and the phenyl
H-36 (Scheme 1). While, only two NOEs between H-17 and H-18,
CH3-29 were observed in compound 4. The change in the C-17-
18 coupling constant, that is, 4 Hz and less than 1 Hz for 3 and 4,
respectively, pointed to changes in the rotamer population around
the C-17-18 bond.
Two additional tulearins obtained from the Fascaplysinopsis sp.
in very minor amounts were designated as tulearin B (5) and C
(6).13,14 Tulearin B was determined to be the 3,8-dicarbamate ana-
log of tulearin A and tulearin C (6) the 3,8,9-trihydroxy precursor of
compounds 1 and 5. Both 5 and 6 exhibited the appropriate molec-
ular peaks in the CIMS. Characteristic for 5 was the shift of the C-3
methine to dC 70.7 (d, C-3) and dH 5.10 (dd, J = 8.7, 3.2 Hz) and for 6
was the shift of methine C-8 to dC 72.3 (d, C-8) and dH 3.30 (m).14
In summary, establishment of the relative and absolute config-
urations of tulearins A–C opens the way for synthetic work toward
these biologically interesting compounds.
,
l
a
parameters, R1 = 0.049 for 2142 observations with I > 2
(wR2 = 0.132) for all unique data. CCDC 721796.
r(I), R1 = 0.088
5. Nicolaou, K. C.; Sun, Y. P.; Guduru, R.; Banerji, B.; Chen, D. Y.-K. J. Am. Chem. Soc.
2008, 130, 3633–3644. similar acidic conditions left 1 intact. A suggested
mechanism for the latter transformation under aqueous basic conditions is
depicted in Supplementary data.
6. Koepper, S.; Thiem, J. J. Carbohydr. Chem. 1987, 6, 57–85.
7. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092–4096.
8. Interestingly, as with esterification with MTPA, the reactions of tulearin A (1)
with p-TsCl in pyridine, at rt for 48 h, and with (CF3CO)2O in Et3N, at rt for 24 h,
gave the 9-tosylate and the 9-trifluoroacetate in 70% and 80% yields,
respectively.
9. The similar J8,9 values for 1,1R,1S (3.9, 5.1 and 4.7 Hz, respectively) suggest that
no significant changes in the conformation around the esterification site took
place.
10. For the experimental, preparation, and NMR data, including 2D spectra, see the
Supplementary data.
11. (a) Cycloaddition product (3): colorless oil; ½a D25
ꢂ
ꢁ23 (c 0.2, CHCl3). 1H and 13C
NMR data for the substituted region (C-17)–(C-22): [dC 69.8 (d, C-17), dH 5.62
(dd, J = 11.3, 3.9 Hz); dC 58.1 (d, C-18), dH 4.51 (d, J = 3.9 Hz); dC 131.4 (s, C-19);
dC 123.9 (d, C-20), dH 5.70 (s); dC 56.7 (d, C-21), dH 4.30 (br s); dC 39.4 (t, C-22),
dH 1.74 (m)], dC 21.6 (q, C-30), dH 1.91 (s). N-phenyltriazolinedione substituent:
[dC 153.4 (s, C-32); dC 149.2 (s, C-33); dC 129.3 (s, C-34); dC 125.5 (d, C-35, 39),
dH 7.48 (d, J = 8.4 Hz); dC 129.0 (d, C-36, 38), dH 7.44 (d, J = 8.4 Hz); dC 128.0 (d,
C-37), dH 7.34 (t, J = 7.1 Hz)]. FABMS m/z 711.0 [M+H]+ (80), 733.0 [M+Na]+
(100). (b) Cycloaddition product (4): colorless oil; ½a D25
ꢂ
ꢁ19 (c 0.2, CHCl3). 1H
and 13C NMR data for the substituted region (C-17)–(C-22): [dC 69.4 (d, C-17),
dH 5.32 (d, J = 11.0 Hz); dC 56.5 (d, C-18), dH 4.78 (br s); dC 131.5 (s, C-19); dC
123.5 (d, C-20), dH 5.72 (s); dC 57.3 (d, C-21), dH 4.30 (br s); dC 36.6 (t, C-22), dH
1.90 (m), 1.23 (m)], dC 20.7 (q, C-30), dH 1.95 (s). N-phenyltriazolinedione
substituent: [dC 154.3 (s, C-32); dC 149.0 (s, C-33); dC 128.2 (s, C-34); dC 125.4
(d, C-35, 39), dH 7.54 (d, J = 8.4 Hz); dC 129.0 (d, C-36, 38), dH 7.60 (d, J = 8.4 Hz);
dC 127.9 (d, C-37), dH 7.32 (t, J = 7.1 Hz)]. FABMS m/z 711.0 [M+H]+ (15), 733.0
[M+Na]+ (100). For experimental, syntheses, and NMR data, including 2D
spectra, see the Supplementary data.
Supplementary data
Supplementary data associated with this paper can be found, in
the online version, at doi:10.1016/j.tetlet.2009.04.028.
12. From the observed NOEs between CH3-27 and H-36; and between CH3-30 and
H-17, of 3, requiring a planar rotamer and as no changes were observed with
temperature, atropoisomers were excluded.
References and notes
13. Tulearin B (5): yellow amorphous powder; ½a D26
ꢂ
ꢁ37 (c 0.13, CHCl3); IR (CHCl3)
1. Bishara, A.; Rudi, A.; Aknin, M.; Neumann, D.; Ben-Califa, N.; Kashman, Y. Org.
Lett. 2008, 10, 153–456.
2. Bishara, A.; Rudi, A.; Aknin, M.; Neumann, D.; Ben-Califa, N.; Kashman, Y.
Tetrahedron Lett. 2008, 49, 4355–4358.
mmax 3679, 3430, 3020, 2960, 1726, 1602, 1582 cmꢁ1
.
1H and 13C NMR data
(acetone-d6) of the segment (C-1)–(C-4): [dC 172.1 (C-1); dC 43.9 (d, C-2), dH
2.73 (qd, J = 7.2, 3.2 Hz); dC 70.7 (d, C-3), dH 5.10 (dd, J = 8.7, 3.2 Hz); dC 39.8 (t,
C-4), dH 1.70 (m), 1.29 (m); dC 11.9 (C-27), dH 1.06 (d, J = 7.2 Hz)]. HRCIMS m/z
579.4007 [M+H]+ (calcd for C32H55N2O7 579.4009).
3. Bishara, A.; Rudi, A.; Aknin, M.; Neumann, D.; Ben-Califa, N.; Kashman, Y. Org.
Lett. 2008, 10, 4307–4309.
14. Tulearin C (6): Colorless oil; ½a D18
ꢂ
ꢁ18 (c 0.24, CHCl3); IR (CHCl3) mmax 3480,
4. Cyclic carbonate of tulearin A (2): Colorless crystals (methanol); ½a D25
ꢂ
+12 (c 0.3,
3020, 1750, 1616, 1216, 1032, 926 cmꢁ1
.
1H and 13C NMR data (acetone-d6) of
CHCl3). 1H and 13C NMR data for the changed segment (C-7)–(C-10); dC 31.5 (t,
C-7), dH 1.65 (m); dC 80.7 (d, C-8), dH 4.20 (td J = 6.3, 5.3 Hz); dC 79.7 (d, C-9), dH
4.30 (tdd, J = 6.8, 5.3, 1.2 Hz); dC 32.3 (t, C-10), dH 1.95 (m), 1.67 (m); dC 154.5 (s,
C-31). FABMS m/z 519 [M+H]+ (100), 541 [M+Na]+ (45). For NMR data (1H, 13C,
the segment (C-7)–(C-10): [dC 29.3 (t, C-7), dH 1.23 (m); dC 72.3 (d, C-8), dH 3.30
(dd, J = 8.7, 3.2 Hz); dC 70.4 (d, C-9), dH 3.46 (m); dC 32.1 (m, C-10), dH 1.59 (m)].
HRCIMS m/z 493.3879 [M+H]+ (calcd for C30H52O5 493.3893).