Synthesis and antiproliferative activity of (-)-cleistenolide, (6S)-cleistenolide and 4-substituted cleistenolide analogues

A new total synthesis of the natural δ-lactone cleistenolide (1) and its (6S)-stereoisomer 2 was achieved starting from d-glucose. Key steps in the synthesis of 1 involved: oxidative cleavage of the C₁-C₂ bond in partially protected d-glucose derivative (20), and chain extension of resulting aldehyde 20a with a single C₂ fragment using (Z)-selective Wittig olefination. Synthesis of 2 involves the following key steps: periodate cleavage of the C₅-C₆ bond in the commercially available monoacetone d-glucose (24), followed by C₂ chain elongation by using the (Z)-selective Wittig olefination. This new approach is also applied to prepare a few new 4-substituted cleistenolide analogues (3 - 18). Compounds 3 - 7 were designed using molecular hybridization, while the remaining eleven analogues were designed using the bioisosterism method. MTT assay showed that most analogues were more active than lead 1 against several malignant cells, but were completely inactive in the culture of normal foetal lung fibroblasts (MRC-5). The K562 cells appeared to be the most sensitive to the synthesized analogues. The strongest antiproliferative activity against this cell line was shown by 4-O-cinnamoyl derivative 3 and 4,6-di-O-benzyl derivative 17, with submicromolar IC₅₀ values (0.76 and 0.67 μM, respectively). Structural features important for the activity of this class of compounds were identified by SAR analysis.