Dibenzofulvene (Fm) adduct on N-α
Fmoc deblock releases dibenzofulvene (DBF), normally trapped by piperidine. If trapping is incomplete (poor solvent, weak base), DBF Michael-adds to the freshly liberated α-amine. +178 Da N-terminal Fm cap stops elongation.
Why it happens (mechanism)
Piperidine attacks Fmoc Cβ-H (E1cB), generating DBF + decarboxylating carbamate → free amine. DBF is highly Michael-electrophilic; piperidine normally traps it as DBF-piperidine adduct that washes out. But if piperidine is sluggish (e.g., DBU is nonnucleophilic — doesn't trap), DBF persists and adds to the α-amine instead of getting captured. Result: Nα-Fm peptide (+178 Da, irreversible).
When it strikes (triggers)
DBU-only Fmoc deblock in batch mode (no scavenger for DBF). Liquid-phase synthesis without DBF scavenger. Continuous-flow with too-fast resin rinse. Long peptides where the Fmoc deblock is repeated dozens of times.
How to spot it (MS signature)
+178 Da on N-terminal residue. UV-active (Fm has 290-300 nm absorbance, useful for HPLC detection).
How to prevent it
- Use piperidine (a good DBF scavenger) for Fmoc deblock; if DBU is required (continuous-flow), add it together with piperidine.
- Sufficient resin washing (≥6× DMF) after deblock — removes residual DBF.
- For liquid-phase synthesis: use 4-AMP or TAEA — these form aqueous-extractable DBF adducts.
- Octanethiol-functionalized resin can scavenge DBF in liquid phase.
If it already happened (salvage)
- Fm cap is stable; can sometimes be removed by very strong base (large excess piperidine, hot, hours) but yields are poor. Re-synthesize.
Source
Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 7, §7.4.2.