RESUMO
A chromatography-free asymmetric synthesis of GDC-6036 (1) was achieved via a highly atroposelective Negishi coupling of aminopyridine 5 and quinazoline 6b catalyzed by 0.5 mol % [Pd(cin)Cl]2 and 1 mol % (R,R)-Chiraphite to afford the key intermediate (Ra)-3. An alkoxylation of (Ra)-3 with (S)-N-methylprolinol (4) and a global deprotection generates the penultimate heterobiaryl intermediate 2. A controlled acrylamide installation by stepwise acylation/sulfone elimination and final adipate salt formation and crystallization delivered high-purity GDC-6036 (1).
RESUMO
The N-terminal nonapeptide domain of the fungal nonribosomal peptide antibiotics cephaibol A and cephaibol C (AcPheAib4LeuIvaGly- Aib) is reported to adopt a right-handed helical conformation in the crystalline state. However, this conformation is at odds with the left-handed helicity observed in solution in related synthetic oligomers capped with Ac-L-PheAib4 fragments. We report the synthesis of four diastereoisomers of the cephaibol N-terminal nonapeptide, and show by NMR and CD spectroscopy that the peptide containing the chiral amino acids Phe and Leu in the naturally occurring relative configuration exists in solution as an interconverting mixture of helical screw-sense conformers. In contrast, the nonapeptide containing the unnatural relative configuration at Phe and Leu adopts a single, stable helical screw-sense, which is left handed when the N-terminal Phe residue is L and right-handed when the N-terminal Phe residue is D.