Scheme 1.

Synthesis of the upper segment 10^a

^aReagents, conditions and yields: (a) Tf₂NPh, 2,4,6-collidine, DMAP, DMF-CH₂Cl₂, reflux, 12 h, 47%; (b) Pd(OAc)₂, DPPP, Et₃N, reflux, 14 h, 81%; (c) LiAlH₄, THF, rt., overnight, 54%; (d) (i) Boc-Asn-OH, DIPCDI, HOBt, DMF, rt, 12 h 87%; (ii) TFA, CH₂Cl₂, rt, 1 h, 79%; (iii) Fmoc-1-amino-cyclohexenecarboxylic acid, EDC, HOBt, DMF, rt, 12 h, 79%; (iv) piperidine, CH₃CN, rt, 2 h, 86%.