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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