Total synthesis of siomycin A: completion of the total synthesis.

The total synthesis of siomycin A (1), a representative compound of the thiostrepton family of peptide antibiotics, was achieved by incorporating the five synthetic segments A (2), B (3), C (4), D (5), and E (6). The dehydropiperidine segment A (2) was esterified with the dihydroquinoline segment C (4), and the subsequent coupling with the beta-phenylselenoalanine dipeptide segment D (5) at the segment C portion followed by lactamization between the segments A and D gave segment A-C-D (27). This was amidated with the pentapeptide segment B (3) at the segment A portion followed by one-pot cyclization (between segments A and B) and elongation (with the beta-phenylselenoalanine dipeptide segment E (6) at the segment A portion), thus furnishing siomycin A (1).

Total synthesis of siomycin A: construction of synthetic segments.

The five practical segments for the total synthesis of siomycin A, that is, the dehydropiperidine segment A (5), the pentapeptide segment B (3), the dihydroquinoline segment C (6), and the beta-phenylselenoalanine dipeptide segments D (7) and E (4), were synthesized. Segment A (5) was constructed by the coupling of the azomethine ylide and the chiral sulfinimine, followed by the stereoselective reduction of the six-membered imine function. Segment B (3) was synthesized by the phenylselenylation of the beta-lactone, stereoselective vinylzinc addition to the chiral sulfinimine, and oxazoline-thioamide conversion. Segment C (6) was prepared by the one-pot olefination of the tetrahydroquinoline N-oxide using triflic anhydride and triethylamine, stereoselective reduction of the methyl ketone function, and regioselective Yb(OTf)(3)-catalyzed epoxide opening by the amino group. Segments D (7) and E (4) were synthesized by coupling of the properly protected beta-phenylselenoalanines.