Abstract
Chemically, N1 nitrogen of piperazic acid is more nucleophilic than N2
nitrogen, but amide bonds predominantly formed at N2 nitrogen are
prevalent in piperazic acid-containing natural products, with only one
exception of sanglifehrin. Thus two orthogonal protecting groups of
nitrogen are often employed to realize selective coupling of N2
nitrogen, resulting in increased synthetic steps and low synthetic
efficiency. However, we developed selective deprotection of N2-Cbz from
the N1,N2-diCbz piperazic acid-containing peptide to form the N2 amide
exclusively, avoiding the tedious orthogonal protection strategy
commonly applied to the easily-accessible N1,N2-diCbz piperazic acid as
the building block. We employ this method to achieve an efficient
synthesis of piperazic acid-containing cyclodepsipeptide core of
verucopeptin with an overall yield of 21%. The key steps include late
stage coupling of piperazic acid with 3-hydroxyleucine derivatives, and
HATU-mediated macrolactamization of 19-membered macrocycle at N9 and
C10. The selective deprotection of N2-Cbz from the N1,N2-diCbz-piperazic
acid at late-stage would greatly facilitate the total syntheses of
piperazic acid-containing cyclodepsipeptides of biological interest.