Total Synthesis and Stereochemical Revision of Stereocalpin A: Mirror-Image Approach for Stereochemical Assignments of the Peptide–Polyketide Macrocycle

Abstract

Stereocalpin A is a cyclic depsipeptide with cytotoxic activity isolated from the Antarctic lichen Stereocaulon alpinum. Although a number of synthetic investigations of the unprecedented 12-membered macrocycle of stereocalpin A with a dipeptide segment and a polyketide substructure have been conducted, the configurational assignment has not been completed. In this study, we achieved the first total synthesis and stereochemical revision of stereocalpin A. To facilitate the comprehensive assessment of eight possible stereocalpin A isomers, four stereoisomers of polyketide precursors were conjugated with l-Phe-l-MePhe and d-Phe-d-MePhe dipeptides (MePhe: N-methylphenylalanine) to provide four possible isomers and four mirror-image structures of the remaining isomers, respectively. The comparative NMR analysis of a series of stereoisomers revealed that stereocalpin A possesses 2 R,4 S,5 R-configurations, which is unique among the related 12-membered hybrid peptide-polyketide natural products reported recently. The NOE correlations in the polyketide substructure of stereocalpin A were also retrospectively analyzed among the eight possible stereoisomers.

Stereocalpin B, a New Cyclic Depsipeptide from the Antarctic Lichen Ramalina terebrata

“…The nuclear Overhauser effects (NOEs) were observed between NH/H-13β, NH/H-21, H-21/H 3 -25, and H 3 -22/H-24, as reported for stereocalpin A by Seo et al in 2008 [14]. The reported absolute configuration at C-21 for naturally occurring stereocalpin A is R; however, stereocalpin A possesses a potentially labile α-methyl β-keto carboxamide, where C-22 methyl functionality is capable of experiencing configurational change and converts into the 21-epi form via the enol (enolate) forms E1 and/or E2 during purification processes (Figure S12), clarifying the misleading NOE correlations of H-21 and H 3 -22 [18]. This also implies the directions of C-22 and C-25 methyl groups toward the opposite sides, from the macrocycle ring [18].…”

“…The reported absolute configuration at C-21 for naturally occurring stereocalpin A is R; however, stereocalpin A possesses a potentially labile α-methyl β-keto carboxamide, where C-22 methyl functionality is capable of experiencing configurational change and converts into the 21-epi form via the enol (enolate) forms E1 and/or E2 during purification processes (Figure S12), clarifying the misleading NOE correlations of H-21 and H 3 -22 [18]. This also implies the directions of C-22 and C-25 methyl groups toward the opposite sides, from the macrocycle ring [18]. NOEs were also observed between H-24/H-26, H 3 -25/H-26, and H 3 -25/H 2 -27 posing difficulty in corroborating the stereochemistry at C-24 and C-26.…”

“…NOEs were also observed between H-24/H-26, H 3 -25/H-26, and H 3 -25/H 2 -27 posing difficulty in corroborating the stereochemistry at C-24 and C-26. Instead, vicinal J coupling constants between H-24 and H-26 were indicative of cis configuration ( 3 J H-24/H-26 = 3.6 Hz in 1a and 2.7 Hz in 1b), suggesting close proximity of the two protons with a small dihedral angle [18]. Due to the conformational flexibility of the macrocyclic nature of 1, we could not obtain suitable ECD calculations, as flexible natural products are known to have a large number of stable conformers [19].…”