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MedChemComm
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conducted on the onychocin analogue conformers determined
the stability of the conformers in solution to decrease in the
order tctc>>tttc≥tttt.14 It was assumed that a similar trend
would hold for the endolide conformers, given their similar
structures and experimental observations.
DOI: 10.1039/C9MD00050J
When linear peptide 6* containing an unnatural C-terminal D-
valine residue was subjected to the same reaction conditions,
epimerisation was observed to occur far less readily than with
the C-terminal L-valine peptide 6, again affording 8 as the
major product (26%), with minor epimerisation to yield
endolide B (2) (3%) (Figure 2, green traces). This suggests that
formation of endolide B (2) was disfavoured using PyAOP
regardless of the stereochemistry of the C-terminal amino acid
of the linear peptide precursor. A similar observation was
recently made by Gunjal and Reddy during their attempted
synthesis of the N-methylated cyclic tetrapeptide
pseudoxyallemycin B, where 3-epi-pseudoxyallemycin was
formed as the sole product during the T3P mediated
cyclisation of linear precursors with both C-terminal L- and D-
tyrosine residues.16 The authors proposed that the selective
formation of the undesired epimeric product could have
resulted from introduction of a turn-inducing D-amino acid by
C-terminal epimerisation and/or external conformational
assistance of the activated T3P ester intermediate; both
plausible explanations for this observed phenomenon. As such,
it was thought that altering the coupling reagent, and hence
the conformation of active ester intermediate, could facilitate
cyclisation of the all-L linear peptides to provide endolide B (2).
To this end, we turned our attention to the use of T3P as a
cyclisation reagent, since this has been reported to reduce the
rate of epimerisation in difficult couplings.14,25,26 Linear
peptides 5 and 6 were subjected to the same conditions as
used previously [1 mM in DMF with DIPEA (6 equivalents) at 40
°C] but with T3P as the cyclisation reagent (3 equivalents)
(Scheme 3).
observed to occur, affording 2 as the major product (60%),
with minor formation of the D-valine epimer 8 (5%) (Figure 3,
green traces). These results were in direct contrast with those
obtained using PyAOP, where formation of the epimerised
Figure 3 RP-HPLC (214 nm) profiles for the T3P mediated cyclisation of 5 (red, top), 6
(blue, middle) and 6* (green, bottom) at 10 min, 1 h (for 6*), 5 h and 23 h. Linear
gradient of 20–80% B over 40 min (~1% B min-1) using a Luna C18 column (4.6 × 250
mm, 5 µm). A = 0.1% TFA in H2O and B = 0.1% TFA in MeCN. KPs = kinetic products.
Pleasingly, monitoring of the reactions by analytical HPLC
(after reaction times of 10 min, 5 hours and 23 hours) showed
both linear peptides 5 and 6 formed a common major product,
which possessed the same retention time (25.2 min) as that of
the minor product formed using PyAOP (Figure 3, red and blue
traces respectively). This was identified as the desired product,
endolide B (2), following purification by preparative HPLC and
MS and NMR analysis. Gratifyingly, the 1H and 13C NMR data of
synthetic 2 was found to be in excellent agreement with the
isolation report (see Tables S1 and S2 in the ESI).1 The HPLC
chromatograms of the cyclisation reaction profiles of 5 and 6
showed clean conversion of kinetic products 9a and 9b to the
desired product 2, with little formation of epimerised side
products 7 and 8 respectively, albeit with reduced reaction
kinetics (complete conversion after 23 hours). Excellent
conversion was observed with peptide 6, forming endolide B
(2) in 91% purity, while conversion from peptide 5 was only
20% due to formation of additional unidentified side products.
products 7 or 8 was favoured regardless of the C-terminal
stereochemistry of the linear peptide precursor. While the
stereoselective cyclisation of peptides has been reported to
proceed independently of starting material configuration,16 to
the best of our knowledge, this is the first report of this
phenomenon being reagent-controlled. The conformations of
the different T3P and 1-hydroxy-7-azabenzotriazole (HOAt)
active ester intermediates are speculated to mediate this
process.
With endolide B (2) successfully in hand, we turned our
attention to the preparation of endolide A (1). Since peptide 6
with a C-terminal valine residue gave the best results in the
cyclisation to form endolide B (2), we chose to prepare linear
peptide 10 with the sequence MeHN-3-(3-furyl)Ala-Leu-NMe-
3-(3-furyl)Ala-Val-OH using the same procedures as outlined
previously in Scheme 2. Peptide 10 was then cyclised using
both T3P and PyAOP under the same conditions used in the
As expected, when linear D-valine peptide 6* was subjected to
the same reaction conditions, slow epimerisation was
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Davison, Emma K.
