Stereochemistry and total synthesis of janolusimide, a tripeptide marine toxin

Article abstract of DOI:10.1016/S0040-4039(00)00533-5

The stereochemistry of janolusimide, a lipophilic tripeptide marine toxin, has been fully elucidated by stereoselective synthesis of the lactam component (5S)-3,3'-dimethyl-5-isopropylpyrrolidin-2,4-dione. The peptide was then synthesized in 13 steps and in 0.8% total yield. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00533-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3979–3982
Stereochemistry and total synthesis of janolusimide, a tripeptide
marine toxin
Assunta Giordano, Carmela Della Monica, Francesco Landi, Aldo Spinella ∗ and
Guido Sodano ∗
Dipartimento di Chimica, Università di Salerno, Via S. Allende, 84081 Baronissi (SA), Italy
Received 14 February 2000; accepted 28 March 2000
Abstract
The stereochemistry of janolusimide, a lipophilic tripeptide marine toxin, has been fully elucidated by stereo-
selective synthesis of the lactam component (5S)-3,30-dimethyl-5-isopropylpyrrolidin-2,4-dione. The peptide was
then synthesized in 13 steps and in 0.8% total yield. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: marine metabolites; molluscs; peptides; imides.
Several unusual peptides endowed with interesting biological activities have been isolated from marine
organisms.1 Some of them have been synthesized both for their interesting biological activities and in
order to prove (or disprove) structure and stereochemistry.2
Janolusimide (1) is a lipophilic tripeptide neurotoxin isolated from the nudibranch mollusc Janolus
cristatus.3 From a structural point of view, 1 is a unique peptide since, besides N-methyl-L-alanine, it
contains two rather unusual constituents, a 4-amino-3-hydroxy-2-methylpentanoic acid and a previously
undescribed lactam, the latter being linked to the remaining part of the molecule by means of a
labile imide linkage. Moreover, the other naturally occurring peptide having a 4-amino-3-hydroxy-2-
4
methylpentanoic acid constituent is the antitumor antibiotic bleomicin A2 in which the amino acid
occurs as the 2S,3S,4R isomer.
In the paper describing the gross structure of janolusimide,3 only the stereochemistry of N-methyl-
L-alanine was established, while the stereochemistry of the other two constituents remained to be
elucidated. Recently,5 in the course of a synthesis of four stereoisomers of 4-amino-3-hydroxy-2-
methylpentanoic acid, the stereochemistry of the amino acid occurring in janolusimide was established
∗
Corresponding author. Tel: +39-089-965230; fax: +39-089-965296; e-mail: sodano@unisa.it (G. Sodano)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00533-5
tetl 16812

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to be 2R,3S,4S. We report here the elucidation of the absolute configuration of the lactam portion (2)
which establishes the complete stereochemistry of janolusimide to be 1, as well as the total synthesis of
the tripeptide.
In order to elucidate the stereochemistry of the single stereogenic center in the lactam ring, we decided
to synthesize the S-isomer 2 starting from L-valine (Scheme 1). The carboxyl group of N-carboxybenzyl-
L-valine (3) was activated by means of 1,10-carbonyldiimidazole (CDI) and the resulting adduct was
reacted in situ with the carbanion generated from butyllithium and ethyl acetate to afford the β-ketoester
4,6 [α]D−29.8; c 0.3, CH2Cl2 (lit.6b [α]D −28.7; c 1.22, CH2Cl2), in 60% yield. Alkylation of 4 was
carried out with methyl iodide and potassium carbonate, using less than the required stoichiometric
amount of base in order to avoid racemization of the resulting product. The optically active dimethyl
derivative 5, [α]D −4.4 (c 3.9, CHCl3), was obtained in 87% yield. In order to avoid racemization of 5
during the deprotection–cyclization step,7 5 was reduced with NaBH4 to a chromatographically separable
mixture of diastereomers 6a and 6b (ca. 1:1) which were deprotected and cyclized to the diastereomeric
hydroxylactams 7 and 8.8 Oxidation of the 7 and 8 mixture with PDC in the presence of molecular sieves9
afforded 2, whose optical rotation ([α]D −30.5; c=0.3, CHCl3; natural 23 [α]D –38.3; c=0.7, CHCl3)
establishes the absolute configuration of the lactam occurring in janolusimide and of the tripeptide itself
as depicted in 1.
Scheme 1.
The introduction of the central amino acid with the desired stereochemistry into the growing
janolusimide molecule was carried out via the alkene 10 which was prepared following the re-
cently described procedure5 (Scheme 2). Thus, N-BOC-L-alaninal (9), prepared by reduction of N-
BOC-L-alanine methyl ester,10 was reacted with the organoborane reagent prepared from (−)-B-
methoxydiisopinocampheylborane and (E)-2-butene11 to afford the homoallylic alcohol 10. Deprotection
of 10 afforded the aminoalcohol 11 which was coupled with N-BOC-N-methyl-L-alanine (12) using
EDC12 as the coupling reagent. The resulting product 13 was protected as the N,O-acetonide derivative
14
(14)13 which, in turn, was oxidized with RuCl3/NaIO4 to the protected dipeptide 15.13 Compound 15
was methylated with CH2N2 and deprotected (p-TsOH, CH3OH) to afford 16 which was identical to the
dipetide methyl ester obtained from degradation of the natural product.3
Coupling of 15 with the lactam 2 proved to be difficult. After several attempts, 15 was transformed into
the pentafluorophenyl ester 1713,15 which was reacted with the N-anion obtained from 2 and butyllithium
affording two diastereomeric protected tripeptides 1813 and 1913 in a 2.8:1 ratio and in 63% yield.
Apparently, the conditions used for the preparation of the imidate (n-BuLi) caused partial racemization of
2. Compounds 18 and 19 were separated by repeated silica gel chromatography and deprotected to afford
janolusimide 1 (from 18), ([α]D −8.5; c=0.8, CHCl3; lit.3 [α]D –10.3; c=2.5, CHCl3) and its epimer 20
(from 19). The identification of the deprotection product coming from 18 as 1 was made on the basis of

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Scheme 2.
a mixed proton NMR spectrum of the synthetic compound with the natural one, since small differences
in the chemical shift values were observed for the individual samples, as previously esperienced for the
4-amino-3-hydroxy-2-methylpentanoic acids.5
Acknowledgements
This work has been supported by MURST (PRIN Chimica dei Composti Organici di Interesse
Biologico) and CNR, Rome. The mass spectra were obtained from the Servizio di Spettrometria di Massa
del CNR e dell’Università di Napoli; whose staff are gratefully acknowledged.
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