“…On the other hand, organic solution phase synthesis (SPS), which uses similar chemical methodology to SPPS, is the main method for small peptide manufacture. It also suffers poor green credentials: protecting groups, atom inefficiency, a high E-factor, and low process mass intensity, Scheme b, Table . − Biosynthesis methods are leading the way in synthesis of longer peptides, for example in insulin manufacture, but are unproductive for short peptides. For example proteases are used to couple protected amino acids, but rely on high enzyme loadings and displacing unfavorable equilibria with either excess reagents, solubility effects, or organic solvents, Scheme c. ,− Surprisingly, the pioneering work done in the 1950–60s by Bartlett and Hirschmann describing the coupling of N -carboxyanhydride (NCA or Leuch’s anhydride) to amino acids has not been widely adopted, despite it being productive, cost-effective, and green, Scheme . − …”