Redundant coupling Δm +1 residue insertion severity: moderate

NCA-mediated dipeptide impurity (from Boc/Z amino acid activation)

Boc-AA-OH activated by DCC can cyclize to an N-carboxyanhydride (NCA), which then ring-opens and self-couples → a Boc-AA-AA-OH dipeptide forms in the activator. When this dipeptide is the actual coupling partner, you get a +1 residue insertion at the resin position.

Affected residue(s): any

Why it happens (mechanism)

DCC + Boc-AA → O-acylisourea → cyclizes to 2-tert-butoxy-5(4H)-oxazolone. In acidic environment (HOBt, 4-nitrophenol), the oxazolone undergoes detert-butylation → an N-carboxy-anhydride (NCA). NCA + amino acid → carbamate → CO₂ → free amino-AA-ester → couples to another Boc-AA-O-acylisourea → Boc-AA-AA dipeptide. This dipeptide is now the dominant active species in the cocktail.

When it strikes (triggers)

DCC + Boc-AA in DCM specifically (NCA is favored in DCM, suppressed by added pyridine). Long pre-activation. Boc-AA-Cl preparation (Boc-AA-Cl is the worst — fast NCA formation). Z-AA also susceptible.

How to spot it (MS signature)

+1 residue at the targeted coupling position (Δm = residue mass). E.g., for Gly: +57 Da insertion.

How to prevent it

Use Fmoc chemistry, not Boc, when feasible — Fmoc-AA active esters don't NCA-cyclize (no Boc to cleave).
If Boc activation is required: add 1 eq pyridine to the activation mixture in DCM — strongly suppresses oxazolone/NCA formation.
Use Boc-AA fluorides (acid fluorides) instead of Boc-AA-Cl when difficult coupling is needed — Boc-AA-F has much lower NCA tendency.
Switch activator from DCC to EDC (in DCM) or HATU (in DMF, no Boc cleavage path).

If it already happened (salvage)

Insertion is permanent; truncate-and-restart usually.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 5, §5.4.