Synthesis of the Unnatural Amino Acid AGDHE, a Constituent of the Cyclic Depsipeptides Callipeltins A and D

Article abstract of DOI:10.1021/ol0269852

(Matrix Presented) The novel amino acid residue (2R,3R,4S)-4-amido-7-guanidino-2,3-dihydroxyheptanoic acid (AGDHE, 3), a constituent of the cyclic depsipeptides callipeltins A and D, and its (2S,3S,4S) diastereomer were synthesized from a protected L-orni

Full text of DOI:10.1021/ol0269852

ORGANIC
LETTERS
2002
Vol. 4, No. 25
4455-4458
Synthesis of the Unnatural Amino Acid
AGDHE, a Constituent of the Cyclic
Depsipeptides Callipeltins A and D
Jason C. Thoen, AÄ ngel I. Morales-Ramos, and Mark A. Lipton*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393
lipton@purdue.edu
Received September 27, 2002
ABSTRACT
The novel amino acid residue (2R,3R,4S)-4-amido-7-guanidino-2,3-dihydroxyheptanoic acid (AGDHE, 3), a constituent of the cyclic depsipeptides
callipeltins A and D, and its (2S,3S,4S) diastereomer were synthesized from a protected L-ornithine derivative in 13 steps (15% overall yield),
and its configurational assignment was reexamined by 1H NMR.
Callipeltin A (1, Figure 1), a cyclic depsidecapeptide isolated
from the marine lithisda sponges Callipelta sp.1 and Latrun-
cula sp.,2 was shown to possess antifungal and anti-HIV
activity and cytotoxicity against selected human carcinoma
cell lines,3 as well as powerful inhibition of the Na/Ca
exchanger in guinea pig left atria.4 The structure of 1 was
determined by Minale and co-workers;1 it contains a number
of novel amino acids and a novel fatty acid, though the
stereochemistry of the â-methoxytyrosine residue could not
be determined. Current efforts in our laboratory are focused
on the synthesis and study of 1 and related compounds.
Comparison of the bioactivity of 1 with the related
compound callipeltin B indicates that the side chain attached
to the macrocycle, recently isolated as callipeltin D (2),2 is
essential for anti-HIV activity.3 A key residue in the side
chain is the novel amino acid (2R,3R,4S)-4-amido-7-guani-
dino-2,3-dihydroxyheptanoic acid (AGDHE, 3).5 The con-
figuration of the vicinal diol was proposed based on 1H NMR
coupling constants and molecular mechanics calculations,
assuming a six-membered-ring hydrogen-bonded conforma-
tion of the â-hydroxyamide. This same assumption was used
to assign the configuration of TMHEA ((2R,3R,4R)-3-
hydroxy-2,4,6-trimethylheptanoic acid) in 1 and later was
revised,6 calling into question the stereochemical assignment
of AGDHE 3. The possible bioactivity of 3, as well as the
need for a protected form of the residue for the synthesis of
1, has prompted us to develop a protocol for its construction
and reexamine its configurational assignment.
The synthesis of 3 started from Nδ-benzyloxycarbonyl-
NR-tert-butyloxycarbonyl-L-ornithine (4, Scheme 1), a com-
mercially available ornithine derivative. The original con-
ception of the synthesis envisaged stereoselective addition
of an R-hydroxyacetate synthon to an R-aminoaldehyde
derived from a protected ornithine derivative. In practice,
the carboxylic acid was converted to a mixed anhydride using
isobutyl chloroformate and immediately reduced with NaBH4
to give alcohol 5 in 95% yield.7 Swern oxidation of 5 yielded
(1) Zampella, A.; D’Auria, V.; Paloma, L.; Casapullo, A.; Minale, L.
Debitus, C.; Henin, Y. J. Am. Chem. Soc. 1996, 118, 6202-6209.
(2) Zampella, A.; Randazzo, A.; Borbone, N.; Luciani, S.; Trevisi, L.;
Debitus, C.; D’Auria, M. V. Tetrahedron Lett. 2002, 43, 6163-6166.
(3) D’Auria, V.; Zampella, A.; Paloma, L.; Minale, L. Tetrahedron 1996,
52, 9589-9596.
(4) Trevisi, L.; Bova, S.; Carnelli, G.; Danieli-Betto, D.; Floreani, M.;
Germinario, E.; D’Auria, M. V.; Luciani, S. Biochem. Biophys. Res.
Commun. 2000, 279, 219-222.
(5) Synthesis of (2R,3R,4S)-4,7-diamino-2,3-dihydroxyheptanoic acid has
been previously published. Chandrasekar, S.; Ramachandar, T.; Ven-
kateswara Rao, B. Tetrahedron: Asymmetry 2001, 12, 2315-2321.
(6) Zampella, A.; D’Auria, M. V. Tetrahedron: Asymmetry 2002, 13,
1237-1239.
10.1021/ol0269852 CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/16/2002

tert-butyl dicarbonate and DMAP to effect regioselective
addition of a Boc protecting group to the side chain nitrogen,
giving intermediate 9 in 97% yield. TBAF deprotection of
the TBS group provided alcohol 10 in essentially quantitative
yield.
Scheme 2a
a Reagents and conditions: (a) TBSCl, imidazole, DMF (97%);
(b) Boc2O, DMAP, CH3CN (97%); (c) TBAF, THF (99%); (d) (i)
(COCl)2, DMSO, CH2Cl2 -65 °C: NEt3, (ii) (CF3CH2O)2POCH2-
CO2CH3, K2CO3, 18-C-6, PhCH3-CH3CN, -15 °C (92%).
Oxidation of 10 provided the desired aldehyde, which was
not prone to the cyclization observed with 5. Although a
number of literature procedures for stereospecific addition
of R-alkoxyenolates and R-alkoxyenolate equivalents to
aldehydes were examined,9-11 our eventual solution was to
osmylate a Z-alkene derived from a Horner-Emmons
olefination.12 Thus, the aldehyde was subjected without
purification to Z-selective Horner-Emmons olefination
conditions (Scheme 2).13 Trace amounts of E-alkene in the
crude product mixture were removed by flash chromatog-
raphy, yielding the desired Z-alkene 11 in 92% yield.14,15
Alkene 11 was subjected to a number of Sharpless asym-
metric dihydroxylation16 conditions in an effort to obtain the
desired diol 12a (Table 1). Reetz has previously examined
the stereoselectivity of dihydroxylation of R-amino-E-alkenes
and shown that S-configured amines (as in this case)
underwent â-dihydroxylation,17 opposite the selectivity de-
sired in our synthesis. It was expected that employment of
Sharpless asymmetric dihydroxylation conditions might
reverse the intrinsic selectivity of addition. In all cases, this
Figure 1.
the aldehyde 6, but this compound rapidly underwent
cyclization to the cyclic aminal 7, which was unreactive and
not a viable synthetic intermediate.8
Scheme 1a
(7) Rodriguez, M.; Llinares, M.; Doulut, S.; Heitz, A.; Martinez, J.
Tetrahedron Lett. 1991, 32, 7, 923-926.
(8) Salituro, G. F.; Agarwal, N.; Hofman, T.; Rich, D. H. J. Med. Chem.
1987, 30, 286-295.
(9) Barret, A. G. M.; Malecha, J. W. J. Org. Chem. 1991, 56, 5243-
5245.
i
a Reagents and conditions: (a) (i) PrOCOCl, NEt3, THF, -15
(10) Brown, H. C.; Narla, G. J. Org. Chem. 1995, 60, 4686-4687.
(11) Kobayashi, S.; Hayashi, Y. J. Org. Chem. 1995, 56, 1098-1099.
(12) All new compounds exhibited spectroscopic (IR, 1H, and 13C NMR)
and analytical data in accord with the assigned structures.
(13) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405-4408.
(14) Pihko, P. M.; Koskinen, A. M. P. J. Org. Chem. 1998, 63, 92-98.
(15) 1H NMR indicated >60:1 ratio of Z/E alkenes.
(16) Wang, L.; Sharpless, K. B. J. Am. Chem. Soc. 1992, 114, 7568-
7570.
°C, (ii) NaBH4, H2O (95%, two steps); (b) (COCl)2, DMSO, NEt3,
THF, -45 °C.
The undesired cyclization event was prevented by place-
ment of an additional protecting group on the side-chain
nitrogen of the ornithine residue (Scheme 2). Alcohol 5 was
protected as the TBS ether 8, which was treated with di-
(17) Reetz, M. T.; Strack, T. J.; Mutulis, F.; Goddard, R. Tetrahedron
Lett. 1996, 37, 9293-9296.
4456
Org. Lett., Vol. 4, No. 25, 2002

Scheme 3a
Table 1. Survey of Ligands for the Asymmetric
Dihydroxylation of 11
ligand
oxidant
solvent
12a /12b
(DHQD)2PHAL K3Fe(CN)6 tBuOH/H2O
1:5
<1:2
1:5
2:3
1:1
1:2
1:2
1:5
<1:2
1:1
(DHQD)2PYR
(DHQD)2AQN
DHQD-IND
(DHQ)2PHAL
(DHQ)2PYR
(DHQD)2AQN
DHQD-IND
DHQD-IND
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 tBuOH/H2O
K3Fe(CN)6 acetone/tBuOH/H2O
NMO
NMO
iPrOH
acetone/tBuOH/H2O
1:1
chemistry failed to afford 12a as the major product. The best
results (a 1:1 ratio) were obtained using catalytic osmium
tetraoxide with NMO as the stoichiometric oxidant.
Alkene 11 was dihydroxylated (Scheme 3) using OsO4-
NMO to afford a 1:1 mixture of the two diastereomeric diols
12a and 12b. After chromatographic separation of diaster-
eomers, desired diol 12a was obtained in 49% isolated yield.
Diol 12a was treated with TFA to remove both Boc
protecting groups, followed by reprotection of the N-terminal
amine with Fmoc in 82% yield over two steps. Protection
of the diol was accomplished with 2,2-dimethoxypropane
and catalytic CSA in DMF. The reaction was extremely
sluggish at room temperature, but when run at 35 °C
overnight yielded the desired acetonide 13a in 78% yield.
Hydrogenolysis of the Z group provided the amine for the
guanylation reaction. This transformation was accomplished
by treatment of the acetate salt of the amine with Et3N and
N,N′-di-Z-N′′-trifluoromethanesulfonyl-guanidine,18 yielding
the guanylated product 14a in 66% yield over two steps.
Saponification of the methyl ester with 0.2 M aq LiOH in
THF at 0 °C yielded 15a, a protected version of AGDHE
suitable for incorporation into the solid-phase synthesis of
callipeltin A. Compound 15a is somewhat unstable due to
the presence of a carboxylic acid and the acid-labile acetonide
protecting group in the same molecule, prompting the
immediate use of 15a without purification in the synthesis
of callipeltin A. Isomer 12b was subjected to analogous
conditions providing 15b in almost the same yield as shown
for its diastereomer 15a.
a Reagents and conditions: (a) OsO4, NMO, MeSO2NH2, acetone-
tBuOH-H2O (12a, 49%; 12b, 49%); (b) (i) 33% TFA-CH2Cl2,
(ii) Fmoc-OSu, NEt3, CH2Cl2 (82%; 73%, two steps), (iii) CSA,
(CH3)2C(OCH3)2, DMF, 35 °C (13a, 78%; 13b, 89%); (c) (i) H2,
Pd/C, 5% AcOH-MeOH, (ii) N,N′-di-Z-N′′-trifylguanidine, NEt3,
CHCl3 (14a, 66%; 14b, 72%, two steps); (d) LiOH, THF, 0 °C
(15a, 90%; 15b, 90%); (e) Dowex resin, 90% MeOH-H2O (16a
and 16b in quantitative yield); (f) H2, Pd/C, 2.5% AcOH-MeOH
(17a, 78%; 17b, 70%).
after determination of the vicinal coupling constants between
the protons on these carbons (J2,3 ) 9.1 Hz, J3,4 ) 2.1 Hz)
and comparison to the calculated value expected for J3,4 (2.0
Hz) in this configuration as determined computionally.1 The
product of the dihydroxylation procedure, 12a, showed nearly
identical coupling between H-3 and H-4 (2.0 Hz), suggesting
that this intermediate has the same configuration at these
stereogenic centers. The actual C-2, C-3 configuration of 12a
was confirmed as (R,R) through spectroscopic analysis of
oxazolidinone and lactam structures derived from 12a.19-21
(19) The configuration of the dihydroxylation products (12a and 12b)
was determined by conversion of the aminodiol to an oxazolidinone and
examination of the coupling constant between H-3 and H-4:
With both isomers 12a and 12b in hand, it was decided
to examine the stereochemical assignment of Minale et al.
1
by correlation of H NMR data. The methines of C-2, C-3,
and C-4 were assigned the 2R,3R,4S configuration by Minale
(18) Feichtinger, K.; Zapf, Z.; Sings, H. L.; Goodman, M. J. Org. Chem.
1998, 63, 3804-3805.
Org. Lett., Vol. 4, No. 25, 2002
4457

More evidence for this configuration was found upon
deprotection of the acetonides in both diastereomers 14a and
14b, giving diols 16a and 16b (Scheme 3). Comparison of
the coupling constants again showed closer values for the
2R,3R,4S configuration rather than the 2S,3S,4S isomer
(Table 2). Closer relation to the natural product was achieved
when the benzyloxycarbonyl groups in 16a and 16b were
successfully deprotected, yielding guanidines 17a and 17b.
Examination of vicinal coupling constants in these isomers
lent further support for the assignment of AGDHE as the
(2S,3R,4R) diastereomer.
1
Table 2. Comparison of Selected H NMR Data for Diols
16a,b, 17a,b, and Callipeltins A (1) and D (2)
compd
H-2 d, ppm (J , Hz)
H-3d, ppm (J , Hz)
1
2
16a
16b
17a
17b
3.99 d (9.1)
4.02 d (7.1)
3.87 d (8.4)
4.11 d (3.0)
3.87 d (8.5)
4.11 d (3.8)
3.65 dd (9.1, 2.2)
3.76 dd (7.1, 2.7)
3.58 d d (8.5, 2.0)
3.71 s
3.58 d d (8.4, 1.9)
3.71 d (3.7)
(20) Futagawa, S.; Inui, T.; Shiba, T. Bull Chem. Soc. Jpn. 1993, 46,
3308-3310.
(21) Further confirmation of the (2S,3R,4R) stereochemical assignment
was obtained from the observation of NOE enhancements in the lactam
derived from 11a:
In summary, a protected derivative of (4S,3R,2R)-4-amido-
7-guanidino-2,3-dihydroxyheptanoic acid (AGDHE, 15a)
suitable for use in solid-phase synthesis was synthesized in
13 steps and 15% overall yield. In addition, the configura-
tional assignment of this fragment was supported on the basis
1
of a comparison of H NMR coupling constants with those
of the natural products callipeltins A and D.
OL0269852
4458
Org. Lett., Vol. 4, No. 25, 2002