Syntheses of naturally occurring cytotoxic [7.7]paracyclophanes, (-)-cylindrocyclophane A and its enantiomer, and implications for biological activity

The total syntheses of (-)-cylindrocyclophane A (1), a naturally occurring, cytotoxic [7.7]paracyclophane, and its enantiomer have been achieved in an enantiodivergent manner starting from a chiral propargyl alcohol building block using Smith's cross metathesis/ring-closing metathesis protocol as the key step. The biological evaluation of both enantiomers of cylindrocyclophane A (1 and ent-1) and its analogues indicated that the chirality of 1 is irrelevant to its cytotoxicity, which is attributed to the resorcinol motifs embedded in the robust [7.7] paracyclophane framework.