Total synthesis of cyclosporin O both in solution and in the solid phase using novel thiazolium-, immonium-, and pyridinium-type coupling reagents: BEMT, BDMP, and BEP

Cyclosporin O (1), an extensively N-methylated immunosuppressive cyclic undecapeptide isolated from Tolypocladium inflatum Gams, was synthesized in 20-23% overall yield via 4 + 7 segment condensation and cyclization by the combined utilization of novel thiazolium- and immonium-type peptide coupling reagents 2-bromo-3-ethyl-4-methyl thiazolium tetrafluoroborate (BEMT) and 5-(1H-benzotriazol-1-yloxy)-3,4-dihydro-1-methyl 2H-pyrrolium hexachloroantimonate (BDMP) as well as compound 2-bromo-1-ethyl pyridinium tetrafluoroborate (BEP). BEMT and BEP, which have been proven to be very efficient for the coupling of peptide segments containing N-alkylated amino acid residues with respect to the fast reaction speed, low racemization, and high yields, were used to construct hindered amide bonds in CsO with the addition of HOAt, whereas the most efficient HOBt-derived immonium type reagent, BDMP, was used to perform the coupling of coded amino acids in CsO. Thus, the highly hindered protected 8-11 tetrapeptide 25 was successfully synthesized using BEMT in 65% yield, and the 1-7 heptapeptide 21 was obtained in 52-55% yield by the rationally combined utilization of BDMP, BEMT, and BEP. The synthesis of the linear undecapeptide 27 of CsO in the solid phase using BEMT and BEP was accomplished for the further evaluation of the effectiveness of these reagents.