Scheme 1.

Synthesis of photocaged MG132‐Cage (1). The synthetic precursor MG132‐Ester‐Cage (6) is used as control compound in the biological experiments. The caging group DMNB is introduced via an ester link early in the synthesis. After successive peptide couplings a final reductive step yields MG132‐Cage with the aldehyde protected as acetal. Conditions: i) Leucine methyl ester x HCl, PyBOP, DIPEA; ii) NaOH 57 % yield over two steps; iii) DMNB‐alcohol, DCC, DMAP, 69 % yield; iv) 5, TFA; v) 3, DIPEA, DCC, HOBt, 77 % yield over two steps; vi) DIBAL‐H, acetic anhydride, 17 % yield.