Figure 1.
single, six-membered, cyclic hydrogen-bonded conformation.2a
This same method was originally used to assign the config-
uration of TMHEA as (2R,3R,4S) and was later revised,5c
raising concerns about the reliability of the methodology
used. During our synthesis of AGDHE, both the (2R,3R,4S)
and (2S,3S,4S) diastereomers were made and their 1H NMR
spectra were compared to those of AGDHE in callipeltins
A and D. Although those studies supported the original
configurational assignment, it was felt that a complete and
unambiguous verification of the configurational assignment
of AGDHE would require the synthesis of callipeltin D for
spectral comparison to the natural product. Herein we report
the efficient solid-phase synthesis of 1 and its spectral
correlation with the natural product.
Early model studies of the coupling conditions revealed that
it was necessary to have the diol of AGDHE protected during
peptide coupling to reduce the formation of byproducts, and
thus the diol of a previously reported intermediate3b (3) was
protected as a benzylidene acetal (Scheme 1), and the methyl
ester was saponified to afford 4 in 26% yield over two steps.
The saponification reaction led to an incomplete consumption
of starting material and partial Fmoc and benzyl carbamate
deprotection. The single diastereomer of 4 shown in Scheme
1 was the only diastereomer that could be isolated in pure
form and was used in the synthesis of 1. It was also shown
in our model studies that the hydroxy group on D-allothreo-
nine did not need to be protected, because it did not interfere
in subsequent peptide coupling reactions.
Our approach to the synthesis of callipeltin D uses an
Fmoc-based solid-phase strategy. It is envisaged that the
coupling conditions used in the synthesis of callipeltin D
will also be employed in the synthesis of callipeltin A. Due
to the acid labile (2R,3R)-â-methoxytyrosine present in
callipeltin A, the synthesis, cleavage from resin, and removal
of protecting groups must be performed under either mild
or nonacidic conditions. For these reasons, the 2-chlorotrityl
chloride resin6 was chosen for use in the synthesis. It is also
known that this resin discourages the formation of dike-
topiperazines during the attachment of the third residue owing
to the bulky nature of the linker.7 The protecting groups used
in the synthesis were chosen for their facile removal by
hydrogenolysis.
The synthesis of 1 (Scheme 2) started with the activation
of the 2-chlorotrityl chloride resin (5) by treatment with
thionyl chloride before use.8 Following activation, com-
mercially available Fmoc-D-allothreonine was attached to the
resin followed by the remaining residues. It was found in
model studies that piperidine can lead to partial loss of the
benzyl carbamates of the guanidine residue of 4, and
therefore DBU was used in Fmoc deprotection reactions of
the tri- and tetrapeptide intermediates. Peptide couplings were
accomplished using 1.7-4.0 equiv of HBTU/HOBt and 1.7-
4.0 equiv of the Fmoc-protected amino acid. All reactions
were monitored by deprotection of small (∼1 mg) quantities
of resin and analysis of the crude cleavage product by
reverse-phase HPLC and MALDI-TOF mass spectrometry.
Coupling 5 with D-allothreonine, DiMeGlu, AGDHE, and
D-alanine, respectively, afforded the resin-bound tetrapeptide
6 in 81% purity as judged by HPLC analysis.
The component residues of 1 were either purchased as their
Fmoc-protected derivatives or synthesized in our laboratories.
The Fmoc group was removed from tetrapeptide 6,
followed by treatment with the acyl chloride of benzyl-
protected TMHEA (7, Figure 2) and DMAP, yielding the
fully protected, resin-bound version of 1. It was found
Scheme 1
(6) Barlos, K.; Chatzi, O.; Dimitrios, G.; Stavropoulos, G. Int. J. Pept.
Protein Res. 1991, 38, 513.
(7) Barlos, K.; Gatos, D.; Kapolos, S.; Poulos, C.; Schafer, W.; Wenqing,
Y. Int. J. Pept. Protein Res. 1991, 38, 555.
(8) Harre, M.; Nickisch, K.; Tilstam, U. React. Funct. Polym. 1999, 41,
111.
5882
Org. Lett., Vol. 7, No. 26, 2005
Cranfill, David C.
