Total Synthesis of Thermoactinoamide A and Its Analogue

When Synthesis Gets It Wrong: Unexpected Epimerization Using PyBOP in the Synthesis of the Cyclic Peptide Thermoactinoamide A

“…To our surprise, when we received and checked the synthetic peptide, its NMR spectra did not match those of the natural thermoactinoamide A ( 1a ) (Figure S1). In contrast, the NMR spectra of the synthetic product matched well those reported for the product of the total synthesis of thermoactinoamide A performed by Sun et al, who however did not question the structure of the natural compound in their paper (possibly because they recorded the NMR spectra of the synthetic compound in a different solvent). Although the most obvious explanation for these data was that the structure determined for natural thermoactinoamide A was incorrect, a thorough study of the synthetic products demonstrated that this was not the case.…”

“…Interestingly, the putative thermoactinoamide A reported in ref and prepared using an independent synthetic procedure was also Ile-α -epi -thermoactinoamide A ( 1b ), as the 1 H and 13 C NMR spectra reported in the paper are identical to those of the commercial synthetic compound we purchased.…”

“…It is also likely that Sun and co-workers (as well as the company from which we purchased the synthetic compound) obtained similar results in their synthesis of thermoactinoamide A. Despite having obtained two reaction products, they unthinkingly (and incorrectly) assigned the structure of their target compound 1a to the most abundant compound from the cyclization reaction, overlooking the less abundant one.…”

“…A full set of 1D and 2D NMR experiments was then recorded to further confirm structure 1b and assign all its 1 H and 13 C NMR signals (Table ). All experiments were recorded using CD 3 OD as solvent, because the reference spectra in ref were recorded in this solvent; for comparison, Table also contains the full assignment of NMR spectra of natural thermoactinoamide A ( 1a ) in CD 3 OD, not reported in the previous papers. Analysis of the TOCSY and COSY spectra allowed the assignment of all protons of the six side chains from the corresponding α-proton signals, confirming the amino acid composition of the peptide.…”

“…Several head-to-tail (or side chain-to-tail) ligation strategies have been developed and employed to circumvent those undesired reactions, but none of them is without drawbacks . This idea was strengthened by the observation that in the synthetic procedure described in ref , the linear peptide YI-6 ( 2 ) that was subjected to cyclization had d - allo -Ile as the C-terminal amino acid, that is, the amino acid that is activated and can epimerize during the reaction.…”