Lobocyclamide B from Lyngbya confervoides. Configuration and Asymmetric Synthesis of β-Hydroxy-α-amino Acids by (-)-Sparteine-Mediated Aldol Addition

Article abstract of DOI:10.1021/ol025876k

(Matrix Presented) Lobocyclamide B, a cyclododecapeptide containing five β-hydroxy-α-amino acid residues, was isolated from Lyngbya confervoides. This is the first reported occurrence of γ-hydroxythreonine in a natural peptide. Optically active β-hydroxy-α-amino acids required for configurational analysis of the title compound were prepared using a novel (-)-sparteine-mediated asymmetric aldol addition of N-(diphenylmethylene)glycine tert-butyl ester to aldehydes. The method is general for aliphatic and aryl aldehydes and notable for operational simplicity.

Full text of DOI:10.1021/ol025876k

ORGANIC
LETTERS
2002
Vol. 4, No. 11
1883-1886
Lobocyclamide B from Lyngbya
confervoides. Configuration and
Asymmetric Synthesis of
â-Hydroxy-r-amino Acids by
(−)-Sparteine-Mediated Aldol Addition
John B. MacMillan and Tadeusz F. Molinski*
Department of Chemistry, UniVersity of California, One Shields AVenue,
DaVis, California 95616
tfmolinski@ucdaVis.edu
Received March 14, 2002
ABSTRACT
Lobocyclamide B, a cyclododecapeptide containing five â-hydroxy-r-amino acid residues, was isolated from Lyngbya confervoides. This is
the first reported occurrence of γ-hydroxythreonine in a natural peptide. Optically active â-hydroxy-r-amino acids required for configurational
analysis of the title compound were prepared using a novel (−)-sparteine-mediated asymmetric aldol addition of N-(diphenylmethylene)glycine
tert-butyl ester to aldehydes. The method is general for aliphatic and aryl aldehydes and notable for operational simplicity.
Marine cyanobacterium of the genus Lyngbya produce a wide
variety of novel cyclopeptides with potent biological activity.1
Related genera of cyanobacteria are often associated as
symbiotic assemblages with marine sponges and other
invertebrates. In this report we disclose the structure and
configurational assignment of a novel antifungal cyclodo-
decapeptide, lobocyclamide B (1), isolated from a benthic
mat of Lyngbya conferVoides collected in the Bahamas.2 The
structure of 1 incorporates five â-hydroxy-R-amino acids,
including 2 equiv of â-hydroxyleucine (â-Hle) and γ-hy-
droxythreonine (Hth). Although Hth has been reported as
an intermediate in the biosynthesis of viamin B6,3 to the best
of our knowledge this is the first report of its occurrence in
(1) For recent examples, see: (a) Williams, P. G.; Yoshida, W. Y.;
Moore, R. E.; Paul, V. J. J. Nat. Prod. 2002, 65(1), 29-31. (b) Luesch, H.;
Yoshida, W. Y.; Moore, R. E.; Paul, V. J. J. Nat. Prod. 2002, 65(1), 16-
20. For a recent review, see: (c) Gerwick, W. H.; Tan, L. T.; Sitachitta, N.
In The Alkaloids, Vol. 57; Cordell, G. A., Ed.; Academic Press: San Diego,
2001; pp 75-184.
(2) Partly presented: Ernst-Russell, M. A.; Molinski, T. F. 36th Western
Regional Meeting of the American Chemical Society, San Francisco, CA,
October 2000.
a natural peptide. Lobocyclamide B exhibits antifungal
activity against fluconazole-resistant Candida albicans.
Configurational assignment of 1 must meet the challenge
of determining 10 asymmetric centers embodied in the
10.1021/ol025876k CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/30/2002

â-hydroxy amino acid residues. For the purpose of deter-
mination of the absolute stereochemistry of 1, we required
a general method for preparation of several enantioenriched
diastereomers of â-hydroxy-R-amino acids, in particular,
â-hydroxyleucine (2) and γ-hydroxythreonine (3), as stan-
dards for chiral amino acid analysis. Recent strategies for
â-hydroxy amino acid synthesis include chiral catalytic
asymmetric aldol additions of benzophenone glycinates,4
Sharpless epoxidation of allylic alcohols followed by a benzyl
isocyanate addition-ring opening,5 and Strecker synthesis
of protected glyceraldehydes.6
Scheme 1a
We found aldol that addition of the lithium enolate of
N-(diphenylmethylene)glycine tert-butyl ester (4) to alde-
hydes, in the presence of (-)-sparteine, produced erythro
and threo diastereomers of â-hydroxy-R-amino esters in good
yield and each with good levels of asymmetric induction.
The method is general for aliphatic and aryl aldehydes and
is notable for its operational simplicity and easy transforma-
tion into optically active â-hydroxy-R-amino acid standards.
Aldol addition of the lithium enolate of 4 (-78 °C LDA,
THF) to isobutyraldehyde gave a mixture of racemic threo
oxazolidine (()-5a and erythro imine (()-5b (78%, 1:1.8
ratio, respectively).7 When the reaction was carried out in
toluene using n-BuLi/(-)-sparteine as base (-78 °C, 3 h,
Scheme 1), compounds 5a and 5b were obtained in com-
parable yield (67%) but reversed diastereoselectivity (2:1).
It is notable that deprotonation of 4 was efficient under these
conditions with no detectable byproducts (NMR, <2%)
arising from addition of n-BuLi.
a Reagents and conditions: (a) n-BuLi, (-)-sparteine, -78 °C,
toluene; (b) isobutyraldehyde, -78 °C, 3 h; (c) TFA, H2O, CH2Cl2,
25 °C; (d) 6 M HCl aq, 1 h.
by comparison of the measured optical rotations with
literature values ((+)-2a, [RD] +2.3°; lit.4a +3.5°; (-)-2b,
[RD] -18.5°, lit.4a -37°) and showed a preference for the
D-amino acid configuration in each diastereomer.
The method was applied to the synthesis of γ-hydroxy-
threonine isomers 3a and 3b (Scheme 2). Aldol addition of
4 to O-benzylglyoxal (6), in the presence of (-)-sparteine/
n-BuLi, gave oxazolidine 7a and imine 7b (62%, dr 2.3:1).
The enantioselectivity of the latter reaction was similar to
the above-described aldol addition and, again, gave amino
esters of predominantly D-configuration (58% ee and 56%
ee for 7a and 7b, respectively). Esters 7a and 7b were
converted to the γ-hydroxythreonine diastereomers 3a and
3b, employing the procedure described above, and their
configurations were secured by comparison of optical rota-
tions with samples prepared using an independent route.11
The scope of the asymmetric aldol addition was briefly
surveyed with selected alkyl and aryl aldehydes (Table 1).
Chiral HPLC analysis of purified isomers 5a and 5b, which
were easily separated by silica chromatography, showed
enantioselectivities of 60% and 56% ee, respectively.8 The
individual compounds were readily converted by hydrolysis-
hydrogenolysis to (+)-(2R,3S)-â-hydroxyleucine (2a) and
(-)-(2R,3R)-â-hydroxyleucine (2b), respectively (Scheme
1).9,10
The absolute configuration of each amino acid, and by
correlation, the aldol products 5a and 5b, was determined
(3) Hill, R. E.; Himmeldirk, K.; Kennedy, I. A.; Pauloski, R. M.; Sayer,
B. G.; Wolf, E.; Spenser, I. D. J. Biol. Chem. 1996, 271(48), 30426-30435.
(4) (a) O’Donnell, M. J. Aldrichimica Acta 2001, 34, 3-15 and references
cited within. (b) Horikawa, M.; Busch-Petersen, J.; Corey, E. J. Tetrahedron
Lett. 1999, 40, 3843-3846. (c) Belokon, Y. N.; Kochetkov, K. A.;
Ikonnikov, N. S.; Strelkova, T. V.; Harutyunyan, S. R.; Saghiyan, A. S.
Tetrahedron: Asymmetry 2001, 12, 481-485
(5) (a) Nagamitsu, T.; Sunazuka, T.; Tanaka, H.; Omura, S.; Sprengeler,
P. A.; Smith, A. B., III. J. Am. Chem. Soc. 1996, 118, 3584-3590. (b)
Nagamitsu, T.; Sunazuka, T.; Tanaka, H.; Omura, S.; Sprengeler, P. A.;
Smith, A. B., III. Tetrahedron Lett. 1993, 34, 4447-4448.
Manaviazar, S.; Delisser, V. M. Tetrahedron 1994, 50, 9181-9188. (h)
Corey, E. J.; Lee, D. H.; Choi, S. Y. Tetrahedron Lett. 1992, 33, 6735-
6738. (i) Caldwell, C. G.; Bondy, S. S. Synthesis 1990, 34-36. (j) Jung,
M. E.; Jung, Y. H. Tetrahedron Lett. 1989, 30, 6637-6640.
(6) Cataviela, C.; Diaz-de-Villegas, M. D.; Galvez, J. A.; Garcia, J. I.
Tetrahedron 1996, 52, 9563-9574.
(11) The (2S,3S)-L-threo and (2R,3S)-D-erythro â-hydroxythreonines were
synthesised as follows. Strecker synthesis with 2,3-isopropylidene-D-
glyceraldehyde and 9-aminofluorene (KCN, NaHSO3, MeOH aq) under
thermodynamic conditions (Inaba, T.; Fujita, M.; Ogura, K. J. Org. Chem.
1991, 56, 1274-1279) gave a mixture of (2S,3R)-(-)-2-N-(9′-aminofluo-
renyl)-3,4-O-isopropylidene-3,4-dihydroxybutyronitrile and the correspond-
ing (2R,3R)-isomer (81%, 3:1 ratio, respectively) which was separated by
normal phase HPLC (2.5% i-PrOH:hexane). Separate acid hydrolysis of
the aminonitriles (1:1 HCl:AcOH, rt) followed by hydrogenolysis (H2,1 atm,
Pd(OH)2, MeOH) gave the γ-hydroxythreonine diastereomers (2S,3S)-
(-)-2-amino-3,4-dihydroxybutyric acid and (2R,3S)-(+)-2-amino-3,4-di-
hydroxybutyric acid, respectively. For a synthesis of the enantiomer of the
latter compound, see: Pirrung, M. C.; Nunn, D. S.; McPhail, A. T.; Mitchell,
R. E. Bioorg. Med. Chem. Lett. 1993, 3, 2095-2098. For the definitive
configurational assignments of â-hydroxythreonine stereoisomers, see:
Hamel, E. E.; Painter, E. P. J. Am. Chem. Soc. 1953, 75, 1362-1368.
Niemann, C.; Nichols, P. L. J Biol. Chem. 1942, 143, 191-199.
(7) The threo aldol products were isolated consistently in the form of
the cyclized oxazolidines. Cf. Corey, E. J.; et al.4
(8) Optical purities of all purified threo and erythro diastereomers were
determined by chiral HPLC (Chiral Pak OD column, 1-2.5% i-PrOH:
hexanes, UV and evaporative light scattering detectors).
(9) Jung, M. E.; Jung, Y. H. Tetrahedron Lett 1989, 48, 6637-6640.
(10) For recent syntheses of â-hydroxyleucine stereoisomers, see: (a)
Cardillo, G.; Gentilucci, L.; Gianotti, M.; Tolomelli, A. Tetrahedron:
Asymmetry 2001, 12, 563-569. (b) Davis, F. A.; Srirajan, V.; Fanelli, D.
L.; Portonovo, P. J. Org. Chem. 2000, 65, 7663-7666. (c) Laib, T.;
Chastanet, J.; Zhu, J. P. J. Org. Chem. 1998, 63, 1709-1713. (d) Laib, T.;
Chastanet, J.; Zhu, J. P. Tetrahedron Lett. 1997, 38, 1771-1772. (e)
Williams, L.; Zhang, Z. D.; Ding, X. B.; Joullie, M. M. Tetrahedron Lett.
1995, 36, 7031-7034. (f) Yadav, J. S.; Chandrasekhar, S.; Reddy, Y. R.;
Rao, A. V. R. Tetrahedron 1995, 51, 2749-2754. (g) Hale, K. J.;
1884
Org. Lett., Vol. 4, No. 11, 2002

(dr 1:1). Enantiomeric excesses of aldol products 10-12
derived from aryl aldehydes were consistently in the range
of 50-56% ee and somewhat lower for other alkanals
(entries 3 and 4; 39-49% ee). No significant differences in
% ee’s were observed between paired threo-erythro isomers.
Although several asymmetric (-)-sparteine-mediated reac-
tions have been rationalized by mechanisms involving
enantioselective removal of enantiotopic protons and alky-
lation of the resultant “chiral” sp3 hybridized anions,12-14this
mechanism seems unlikely in the present case as a stabilized
enolate is generated from 4 and expected to adopt planar
sp2 enolate geometry. The low polarity of the solvent and
ability of C2 symmetric (-)-sparteine to coordinate the
lithium counterion suggests another mechanism based on
π-face selection mediated by a complex chiral amine-
coordinated lithium enolate rather than a chiral base mech-
anism.15 Such an alternate reaction pathway is consistent with
the observed reversal of diastereoselectivity in the aldol
addition of lithium enolate of 4 in the presence of (-)-
sparteine, together with induction of a consistent level of
asymmetry in the product that is only weakly dependent upon
the structure of the aldehyde.16 A structure for such a putative
complex chiral enolate is speculative at this point, and a
detailed analysis of the reaction mechanism is beyond the
scope of this work.
Scheme 2a
a Reagents and conditions: (a) NaH, BnBr, DMF, 85%; (b) O3,
MeOH, -78 °C, 82% yield; (c) n-BuLi, (-)-sparteine, 4, toluene,
-78 °C; (d) TFA, H2O, CH2Cl2:THF (1:1), 1 h, 94%; (e) 6M HCl,
106 °C, 1 h, 87%; (f) H2, Pd(C) 10%, MeOH, 85%.
Procurement of optically pure enantiomers of â-amino-
decanoic acid (â-Ada, 13) allowed completion of configu-
rational analysis of 1 (Scheme 3). Michael addition of
phthalimide to the chiral 2-decenoate menthyl ester 14 (DBU,
DMF, 110 °C) gave â-phthalimido esters (+)-15a and (-)-
15b (65% combined yield, dr 1.2), which were separated by
HPLC.17 Separate hydrazinolysis of (+)-15a and (-)-15b
followed by saponification (2 M NaOH aq) gave S-(+)-13a
and R-(-)-13b, respectively (∼80% yield, two steps).18
The absolute stereochemistry of 1 was determined by a
combination of chiral HPLC and Marfey’s methods.19 The
The yields of products 8-12 derived from aryl aldehydes
(52-75%) were comparable to those of 5 and 7. Stereo-
chemical analysis revealed a consistent preference for the
threo diastereomer with a selectivity of ∼2:1 with the
exceptions of cyclohexanecarboxaldehyde (entry 3) and
n-heptaldehyde (entry 4) which lacked diastereoselectivity
Table 1. (-)-Sparteine-n-BuLi-Mediated Aldol Additions of
4a
(12) Derdau, V.; Snieckus, V. J. Org. Chem. 2001, 66, 1992-1998.
(13) Kim, B. J.; Park, Y. S.; Beak, P. J. Org. Chem. 1999, 64, 1705-
1708 and references cited within. For a review of enantioselective
deprotonations using chiral bases, see: O’Brien, P. J. Chem. Soc., Perkin
Trans. 1998, 1439.
(14) Deiters, A.; Hoppe, D. J. Org. Chem. 2001, 66, 2842-2849. The
use of (-)-sparteine in TiCl4-promoted aldol reactions of chiral N-
acyloxazolidinethiones has been reported (Crimmins, M. T.; King, B. W.;
Tabet, E. A.; Chaudhary, K. J. Org. Chem. 2001, 66, 894-902). In view
of the fact that similar, very high diastereoselectivities were obtained under
these highly coordinating conditions, even when achiral tert-amines were
substituted for (-)-sparteine, the role of the chiral diamine in enantiodif-
ferentiation is somewhat masked.
(15) For enantioselective additions of aggregates of organolithiums/chiral
aminoalkoxides to aldehydes and ketones, see: Briggs, T. F.; Winemiller,
M. D.; Xiang, B.; Collum, D. B. J. Org. Chem. 2001, 66(19), 6291-6298
and references cited within.
%
time
% ee % ee
entry
R
product yieldb (h)
a :b
2:1
2.3:1
1:1
1:1
1.9:1
3:1
2.5:1
of a of b
1
2
3
4
5
6
7
i-Pr
5
7
8
68
62
63
52
71
69
75
3
2.5
5
5
2.5
6
60
58
42
45
57
52
53
56
56
39
49
56
55
50
BnOCH2
c-C6H11
n-C6H13
Ph
2-naphthyl
2-furyl
9
10
11
12
(16) Since the aldol reaction mixtures were observed to be heterogeneous
and contained sparingly soluble white precipitates, we cannot discount the
possibility that reactions occur at the surface of heterogeneous chiral
aggregates which influence the stereochemical outcome.
4
a General Procedure: Freshly distilled (-)-sparteine was stirred with
solution of n-BuLi (1.2 equiv) in toluene at -78 °C (45 min) followed by
addition of a solution of 4 (1.0 equiv) in toluene. After stirring for 30 min
at -78 °C, the heterogeneous mixture was treated with a solution of
aldehyde (2.0 equiv) in toluene and further stirred for the requisite time
before work up in the usual manner. See Supporting Information for full
details and stereochemical analysis. b Yields are for combined diastereomers,
a + b. For typical reactions, unreacted 4 (∼10-20%, NMR) was observed.
(17) Normal phase HPLC(10 × 250 mm, 0.5% i-PrOH:hexane, 2.0 mL/
min).
(18) For an alternate five-step synthesis of (S)-N-BOC-â-aminodecanoic
acid from nonanoyl chloride, see: Expo´sito, A.; Ferna´ndez-Sua´rez, M.;
Iglesias, T.; Mun˜oz, L.; Riguera, R. J. Org. Chem. 2001, 66, 4206-4213.
(19) Marfey, P. Carlsberg Res. Commun. 1984, 49, 591-596. Briefly,
amino acid standards and the crude hydrolyzate of 1 (6 M HCl, 110 °C, 16
h) were derivatized using (2,4-dinitro-5-fluorophenylamino)-L-alaninamide.
HPLC retention times were measured under standard conditions (4.6 × 250
Org. Lett., Vol. 4, No. 11, 2002
1885

decanoic acid residue was found to have the 3R configura-
tion, consistent with the configurations of â-aminooctanoic20
and â-aminodecanoic acids found in related lipopeptides.21,22
In conclusion, a simple, practical method has been
developed for synthesis of â-hydroxy-R-amino acids and
applied to complete determination of configuration of the
novel peptide, lobacyclamide B (1). The method provides
facile access to both respective enantioenriched diastereomers
each amino acid required as standards in chiral amino acid
analysis.
Scheme 3a
Acknowledgment. We thank Michael Ernst-Russell for
isolation of 1 and preparation of 2-decenoic acid, Karen
Smith for the Strecker synthesis of Hth precursors, Valerie
Paul, (University of Guam) for identification of L. confer-
Voides, Joe Pawlik (UNC Wilmington), the captain and crew
of the R/V Seward Johnson for ship support, and the
Government of the Bahamas for permission to collect. The
400 MHz NMR instrument was partially funded by the NSF
(CHE-9808183). Ship-time on the R/V Seward Johnson was
funded by NSF (OCE-9711255 to J. Pawlik). This research
was supported by the NIH (AI 39987 to T.F.M.).
a Reagents and conditions: (a) (+)-menthol, DCC, DMAP,
CH2Cl2. 72%; (b) phthalimide, DBU, DMF, 120 °C, 65%; (c)
NH2NH2‚H2O, EtOH; (d) 2 M NaOH aq, 1 h, then H+, ∼80% for
2 steps.
Supporting Information Available: Experimental pro-
cedures for synthesis and full characterization of (()-2a, (()-
2b, and optically active isomers, a and b, of 5, 7-13, and
15 and a chiral HPLC chromatogram of 5a. This material is
available free of charge via the Internet at http://pubs.acs.org.
results showed that both â-hydroxyleucine residues in 1 have
the threo-(2R,3S) configuration while the γ-hydroxythreonine
has the threo-(2R,3R) configuration. The rare â-amino-
OL025876K
(20) Gerwick, W. H.; Jiang, Z. D.; Agarwal, S. K.; Farmer, B. T.
Tetrahedron 1992, 48, 2313-2324.
mm C18 column; gradient of 20:40 f 60:40 CH3CN:50 mM triethyl-
ammonium phosphate, aq, 1.0 mL/min). To obtain the retention times of
ent-2a,2b and ent-3a,3b the amino acids 2a,2b and 3a,3b were reacted with
the “ent-” Marfey’s reagent [(2,4-dinitro-5-fluorophenylamino)-D-alanin-
amide], prepared from D-alaninamide according to Marfey. L-Valine and
D-glutamine (as D-glutamate) were determined separately using chiral HPLC
(D-penicillamine-based column).
(21) (a) Bonnard, I.; Rolland, M.; Francisco, C.; Benaigs, B. Lett. Pept.
Sci. 1997, 4, 289-292. (b) Frankmo¨lle, W. P.; Larsen, L. K.; Caplan, F.
R.; Patterson, G. M. L.; Knu¨bel, G.; Levine, I. A.; Moore, R. E. J. Antibiot.
1992, 45, 1451-1457.
(22) The remainder of the amino acid residues in 1 were determined by
Marfey’s procedure as L-Ala, N-Me-L-Ile, allo-L-Thr (×2), and trans-L-
Hpro using commercially available standards.
1886
Org. Lett., Vol. 4, No. 11, 2002