the attachment to the solid support that still allows subsequent
chemical modification of the N- and C-termini of the glutamic
aldehyde equivalent. We envisioned that a novel amino acid
consisting of a double bond as a masked aldehyde function
and a free carboxylic acid for linking to the resin via its side
chain could serve these requirements, as the Meldal group has
previously shown that oxidative cleavage of double bonds on
solid support efficiently generates aldehydes.6
aldehyde 7 under Dess–Martin conditions proceeded smoothly
in 84% yield. 7 was then immediately converted by a Wittig
reaction to obtain building block 8 for the subsequent solid
phase synthesis.
With 8 in hand, the assembly of the cyclodepsipeptide on
solid support could be initiated (Scheme 3). To prevent
unwanted side reactions during the on resin macrolactamiza-
tion, the reaction conditions were optimized towards a low
loading (0.24 mmol gꢀ1 resin) of 8 to a NovaPEG amino resin,
resulting in 9. Deprotection of the Boc-protecting groups with
50% TFA in DCM sets the stage for the subsequent amino
acid couplings. Couplings were performed with HBTU
(3.5 eq.) and HOBt (4 eq.) as the coupling reagents, DIPEA
(3 eq.) as the base and the corresponding Fmoc- or
Boc-protected amino acids (4 eq.). First, Fmoc-citrulline was
coupled, followed by Fmoc-Thr-OH, Boc-Glu-OH and finally
butyric acid, leading to intermediate 10. For cleavage of
the Fmoc- or Boc-protecting groups, either 20% piperidine
in DMF or 50% TFA in DCM were used. The adjacent
esterification of the threonine hydroxyl group side chain with
Fmoc-Val-OH (10 eq.), thereby yielding 11, was achieved with
DMAP (1 eq.) and di-isopropyl carbodiimide (10 eq.) as the
activating reagents. Fmoc-deprotection followed by coupling
with N-Boc-N,O-dimethyl-3-bromotyrosine using standard
coupling conditions and subsequent Boc-deprotection yielded
an N-methyl intermediate, to which Fmoc-isoleucine (5 eq.)
was coupled, using PyBrOP (4.9 eq.) as the coupling reagent
and DIPEA (10 eq.) as the base and a reaction time of 24 h.
This intermediate 12 was deprotected at the N-terminus by
Fmoc-cleavage and the C-terminus by removal of the allyl
protection group by the protocol of Vaz and Brunsveld11 led
to the cyclization precursor. Macrolactamization was obtained
by PyBOP (4 eq.)/HOBt (8 eq.)/DIPEA (12 eq.) activation
and a reaction time of 24 h, thereby yielding 13. The final
conversion of the double bond of 13 into the aldehyde moiety
for establishing the Ahp residue in SymA and for simultaneous
cleavage from the resin was achieved with OsO4 and NaIO4 as
oxidation reagents and DABCO to prevent the formation of
hydroxymethyl ketones.6 To the best of our knowledge, this
is the first time that these reagents are used for cleaving
aldehydes from a resin. Subsequent chromatographic purifica-
tion of the crude product yielded the desired natural product
Symplocamide A (SymA). The resulting 1H and 13C NMR
spectra of the synthetic compound corresponded well to
the data published for the natural product.5 Moreover, the
synthetic SymA demonstrated potent chymotrypsin inhibition,
displaying a Ki value of 0.32 ꢁ 0.09 mM, in accordance with
the literature report (lit.,5 IC50 = 0.38 ꢁ 0.08 mM).
Consequently, our synthetic investigations started with the
solution phase synthesis of the Ahp precursor molecule 8
(Scheme 2). To this end, N-Boc protected glutamic acid benzyl
ester was activated as a mixed anhydride and then reduced
with sodium borohydride using the protocol by Shioiri et al.,4
thereby yielding alcohol 1. 1 was then protected as a silyl ether
and the resulting intermediate 2 was converted to the di-Boc
protected derivative 3.7 Removal of the benzyl ester protecting
group by hydrogenation led to the acid 4,8 which was repro-
tected as an allyl ester using the well-established protocol by
Kunz et al.,9 thereby obtaining the fully protected intermedi-
ate 5. The subsequent removal of the silyl ether to obtain
alcohol 6 emerged to be demanding, as the standard cleavage
conditions using fluoride or mild acidic conditions led to the
loss of one of the two Boc protecting groups. This issue was
solved with the very mild copper-mediated cleavage conditions
described by Wang et al.10 The following oxidation of 6 to
We have established the first solid phase synthesis of an
Ahp cyclodepsipeptide by synthesizing the natural product
Symplocamide A. Essential for the overall synthesis strategy
was the development of the masked aldehyde precursor 8
that proved to be a useful building block to install the Ahp
moiety at the last stage of the synthesis. The established
synthetic route holds promise for the synthesis of custom-
made Ahp depsipeptides, thereby opening the possibility
to generate non-covalent S1 serine protease inhibitors as
chemical biology tools in a fast and efficient manner for
protease research.
Scheme
2
Solution synthesis of building block 8, the general
precursor for the solid phase synthesis of Ahp containing
cyclodepsipeptides.
c
8858 Chem. Commun., 2010, 46, 8857–8859
This journal is The Royal Society of Chemistry 2010
Stolze, Sara C.
