Synthesis, Photoreactivity and Cytotoxic Activity of Caged Compounds of L‐Leucyl‐L‐Leucine Methyl Ester, an Apoptosis Inducer

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Abstract
I,‐Leucyl‐L‐leucine methyl ester (Leu‐Leu‐OMe), an apoptosis inducer in natural killer cells and macro‐phages, was caged with trans‐o‐hydroxycinnamoyl (3ad), trans‐o‐mercaptocinnamoyl (4) and o‐nitrobenzyl derivatives (5a, b), and the photochemical reactivity of these derivatives in phosphate‐buffered saline containing 1% dimethyl sulfoxide and their immunological properties were studied. All of the derivatives exhibited absorplion at wavelengths longer than the UVB region. Although 3a–d and 4 were expected to isomerize to a cis isomer, which thgn cyclizes intramolecularly to give Leu‐Leu‐OMe and a coumarin derivative, cyclization efficiency was not satisfactory except for 3a. However, 3a itself caused necrosis (cell swelling) of U937 cells (a myeloid cell line). In contrast, 5a and b released Leu‐Leu‐OMe quickly and efficiently and did not affect U937 cells. Although irradiated 5b induced necrosis, irradiated 3a and 5a induced apoptosis in these cells, as evidenced by a decrease in cell size.