Results and Discussion
We started the synthesis of the
cyclodepsipeptide 2 from the building blocks described in
Figure 3. Among them, N-protected sarcosine 11a and11b , dipeptide 12 are commercially available.
Protected 3-hydroxyleucine 7 , [20,21]tert -butyl (benzyloxy)glycinate9 ,[22] N1-Cbz,N2-Cbz Piz10a ,[11] and N1-Cbz Piz10b [11] were synthesized via previously
reported methods.
Figure 3 Building blocks to construct the cyclodepsipeptide
core.
With these building blocks in
hand, we firstly explored the direct coupling of 9with N1-Cbz Piz 10b in
which N2 nitrogen was not protected, in the presence of HATU and DIEA.
However, the yield of coupling was only 19% because of nucleophilic
competition between N2 nitrogen
of 10b and the
hydroxylamine of 9 . To
avoid this side reaction, Fmoc group was introduced to protect N2
nitrogen before the coupling of 9 and 13 . But Fmoc
deprotection of the dipeptide 14 under the treatment of
piperidine also gave rise to a low yield, probably because of the
undesired cleavage of N-O bond in hydroxylamine motif. These
experimental results indicated that tedious handlings of protecting
groups or direct coupling in the presence of free N2 nitrogen could
impair the synthetic efficiency, especially for the condensation of Piz
and hydroxylamine derivatives. Then we tried to realize the selective
hydrogenation of N2-Cbz over the N1-Cbz and the benzyl group in
hydroxylamide, after smooth coupling of 10a and 9 .
Fortunately, we can control the reaction time to maximize the amount of5 under the condition of 10% Pd/C and H2. Five
grams of 5 could be obtained in one batch in 58% yield when
hydrogenation time was 2 h and the
solvent was THF. Further prolonging the reaction time or replacing THF
with MeOH as the solvent could result in excessive hydrogenation, while
insufficient reaction time would lead to low conversion rate. (Scheme 2)
Scheme 2 Synthetic routes to Piz-containing dipeptide5 a
a Reagents and
conditions: (a) TMSCl, FmocCl, DIEA, THF, rt, 5 h, 76%; (b) oxalyl
chloride, toluene, 60oC, 1 h; (c) 9 , Et3N,
toluene, 60 oC, 1 h; (d) piperidine, THF, rt, 4 h,
52%; (e) 9 , HATU, DIEA, THF, rt, 4 h; (f) H2,
10% Pd/C, solvents, rt.
Then amidation of 12 and 11a under EDCI and HOBt
conditions proceeded smoothly to deliver tripeptide 13 in 86%
yield. After hydrolysis of methyl ester 13 , the resulting acid6a was subjected to esterification reaction with 7a(the simplified derivative of 7 ), which was synthesized from
commercially available reagents, to yield isopeptide 14 . The
treatment of 14 with Pd(PPh3)4and N-methylaniline removed the allyl group to give acid 15 . We
have attempted to carry out the chlorination of 15 with excess
oxalyl chloride and amide condensation under AgCN-assisted
conditions[19] between acid chloride and dipeptide5 . However, no desired product 16 was observed
probably due to the lability of Boc group in excess oxalyl chloride.
(Scheme 3)
Scheme 3 First attempt
to generate the cyclodepsipeptide corea
a Reagents and conditions: (a) 11a , EDCI,
HOBt, CH2Cl2, rt, 4 h, 86%; (b) LiOH,
THF/H2O=3/1, rt, 2 h, 89%; (c) 7a , DIC, DMAP,
CH2Cl2, rt, 4 h, 76%; (d)
Pd(Ph3)4, N-methylaniline, rt, 3 h; (e)
oxalyl chloride, CH2Cl2, rt, 1 h; (f)5 , AgCN, toluene, 60 oC, 30 min.
Correspondingly, acid-sensitive Boc group in 6a was replaced
with Fmoc group. The acid 6b was obtained in 2 steps from12 , and then transformed to isopeptide 17 in a yield
of 85%. Then, the allyl group of 17 was removed and the
resulting acid 18 was subjected to the chlorination under
excess oxalyl chloride condition.
However, only trace of desired product 3 was obtained after
amidation with dipeptide 5 under AgCN-assisted conditions.
Oxalylation at N6 site was deemed
to be the major side reaction, which was detected by liquid
chromatography-mass spectrometry after quenching of chlorination
reaction with H2O. The undesired oxalylation reaction
could be circumvented by using thionyl chloride instead of oxalyl
chloride condition. The chlorination of 18 under thionyl
chloride and amidation with dipeptide 5 afforded 3 .
Terminal Fmoc group and tertiary butyl group in cyclization precursor3 were removed successively under diethylamine and TFA
conditions to produce trifluoroacetate 19 . Notably, the Fmoc
deprotection should be monitored carefully to avoid
much prolonging the reaction, and the
use of more reactive piperidine instead of diethylamine was not
recommended, as N-OBn moiety was not stable under these treatments.
Macrolactamization was conducted using Carpino’s HATU reagent in high
dilution to afford 20 in 76% yield over 3 steps. After
detaching the Troc group with Zn powder in aqueous acid, we directly
removed benzyl and Cbz protecting groups by hydrogenolysis under Hale’s
condition.[17]Cyclodepsipeptide core 2was obtained as a hydrochloride, and the spectral data were consistent
with that previously reported. [18] (Scheme 4)
Scheme 4 Construction of the cyclodepsipeptide
corea
a Reagents and
conditions: (a) 11b , EDCI, HOBt,
CH2Cl2, rt, 4 h; (b) LiOH,
THF/H2O=3/1, rt, 2 h, 82% over 2 steps; (c) 7 ,
EDCI, DMAP, CH2Cl2, rt, 4 h, 85%; (d)
Pd(Ph3)4, N-methylaniline, THF, rt, 2 h,
91%; (e) oxalyl chloride, CH2Cl2, rt, 1
h; (f) 5 , AgCN, toluene, 60 oC, 30 min, 85%
over 2 steps; (g) 2%(v/v) diethylamine in MeCN, rt, 4 h; (h) TFA/
CH2Cl2=3/1, rt; (i) HATU, DIEA, THF, rt,
24 h 65% over 3 steps; (j) Zn powder, AcOH/H2O=9/1, rt,
2 h; (k) H2, 10% Pd/C, 0.01 M HCl in MeOH, 24 h, 60%
over 2 steps.
Conclusions
Piz-containing natural products display potent biological activities and
Piz residue potentially exerts significant structural and biological
effects. However, the assembly of Piz-containing peptides suffered from
tedious handlings of protecting groups on the Piz residue. We found that
selective deprotection of N2-protecting group at late-stage posterior to
the direct coupling of carboxylic acid within N1,N2-diCbz Piz would be
efficient method, facilitating the syntheses of Piz-containing natural
products. As such, we developed an efficient synthetic method to
construct cyclodepsipeptide core 2 of VE in total 13 steps with
an overall yield of 21%. Our strategy also features late stage coupling
of Piz and 3-hydroxyleucine derivatives, which could reduce the
synthetic steps to handle substrates bearing the infrequent unnatural
amino acids. Macrolactamization at N9 and C10 realized the cyclization
of 19-membered macrocycle in high yield.
Experimental
Experimental procedures are available in the Supporting information.
Supporting Information
The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement
This work was financially supported by National Natural Science
Foundation of China (No. 22207081), National Science & Technology Major
Project “Key New Drug Creation and Manufacturing Program”, China (No.
2018ZX09711002-006-004), China National Postdoctoral Program for
Innovative Talents (No. BX20200232), China Postdoctoral Science
Foundation (No. 2020M681725).
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