Oxidative and reductive transformations of epothilone A.

The C7 hydroxy group of cytotoxic epothilone A was selectively oxidized using PDC. A selective oxidation of the C3 hydroxy group was accomplished with Me2S/(PhCO2)2 after in situ protection of C7-OH. Reduction of epothilone A or of a C5, C7 dioxo derivative with NaBH4 proceeded at the C5 carbonyl group. Oxidation and hydrogenation of the C16-C17 double bond proved to be difficult but it was easily cleaved with ozone and the resulting keto derivative was transformed to epothilone analogs with different side chains.

Derivatization of the C12-C13 functional groups of epothilones A, B and C.

Epothilone A reacted with hydrohalic acids to C12-C13 halohydrin regioisomers (ratios: 2:1-4:1), whereas epothilone B gave under the same conditions the isomerically pure C12 halo C13 hydroxy derivative. With non-nucleophilic Brønstedt acids and with Lewis acids a highly solvent dependent product distribution and some unexpected rearrangement products were observed. Epothilone C bearing a double bond between C12 and C13 was regioselectively dihydroxylated or hydrogenated at that position.

New mild acid-labile protecting groups for the guanidino function of N alpha-fluorenylmethoxycarbonyl-L-arginine in solid-phase peptide synthesis: 10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-yl, 2-methoxy-10,11-dihydoro-5H-dibenzo[a,d]cyclohepten-5-yl and 5H-dibenzo[a,d]cyclohepten-5-yl groups.

10,11-Dihydro-5H-dibenzo[a,d]cyclohepten-5-yl [5-dibenzosuberyl] and 5H-dibenzo[a,d]-cyclohepten-5-yl [5-dibenzosuberenyl] groups have been found to be useful protecting groups for the guanidino function of arginine in solid-phase peptide synthesis on Fmoc chemistry. The arginine derivatives (4a,b,c) derivatized with these groups were easily deprotected with mild acid (less than 30 min with 25% trifluoroacetic acid). Tryptophan-containing peptide sequences, two hexapeptides (6) and (8), were synthesized in good yield by mild acid treatment (50% trifluoroacetic acid in 1 h) of the peptide resins (5a,c-f and 7a,c,d) assembled via 4a,b,c using benzotriazol-1-yl-oxy-tris-(pyrrolidino)-phosphonium hexafluorophosphate-1-hydroxybenzotriazole mediated coupling.