An efficient synthesis of the C14-C21 acid fragment of cytochalasin Z 8 was accomplished in 10 steps with 14% overall yield. Boron-mediated anti-selective aldol condensation and Pd(OAc) 2 -Aphos-Y-catalysed Balkyl Suzuki-Miyaura cross-coupling were employed to construct the requisite C17 and C18 stereogenic centres and alkene subunit.Cytochalasins are secondary fungal metabolites with a wide range of biological activities that target cytoskeletal processes.
1Cytochalasins Z 7 -Z 9 (1-3, Chart 1) were isolated from the marine-derived fungus Spicaria elegans, and their structures and absolute congurations were established by Zhu et al. Cytochalasin Z 8 (2, Chart 1) is structurally related to cytochalasins Z 7 and Z 9 and features highly substituted hydroisoindol-1-one fused with a 12-membered macrolactone ring at the C-8 and C-9 positions. Cytochalasin Z 8 has been reported to exert cytotoxicity against P388 and A-549 cell lines with IC50 values of 56 and 21 mM, respectively, and therefore has signicant potential in cell biology and medicine. A number of laboratories have worked towards total synthesis of the cytochalasin family and developed linear 3 or convergent 4 strategies for their total synthesis. Total synthesis of cytochalasin congeners was accomplished by the laboratories of Stork, 3a,4a Thomas, 3b,3c,3e,3f Trost, 4d Vedejs (zygosporin E), 4b,4c,4e Myers, 5 Liu and Tang (periconiasins A-E) 6 and Nay (periconiasin G).7 To the best of our knowledge, total synthesis of cytochalasin with a 12-membered macrocyclic ring has not been reported. The intriguing molecular architecture and potent biological activity of cytochalasin Z 8 prompted us to pursue its total synthesis and render it to be readily available for biological investigations.The retrosynthetic strategy is depicted in Scheme 1. Intramolecular ring-closing metathesis (RCM) strategy 8 which is a promising tool for constructing macrolactone is oen used for synthesising macrolides. 9 We envisioned an RCM reaction at C13 and C14 positions and an esterication for assembling a 12-membered macrolactone. Thus, acid fragment 4 was required for the total synthesis of 2. Our strategy was exible and it allowed rapid access to structural analogues. In this study, we report the synthesis of C14-C21 acid fragment 4 via a highly anti-selective aldol condensation 10 of aldehyde 6 with Abiko's chiral norephedrine-derived propionate (1R,2S)-7 (ref. 11) and B-alkyl Suzuki-Miyaura cross-coupling 12 of chiral alkyl iodide 5 with (Z)-1-bromoprop-1-ene.Our rst task was to construct C16-C18 syn-anti stereotriad.13 The aldehyde functionality in 6 was expected to undergo an anti-selective aldol reaction with the (E)-boron enolate generated from Abiko's chiral propionate 7 for installing C17-C18 anti stereochemistry according to our synthetic strategy in Scheme 1. We initially prepared crude aldehyde 6 from commercially available (S)-methyl 3-hydroxy-2-methyl propionate (Roche ester) 14 by tosylation and partial ester reduction
15(Scheme 2). The unstable crude aldeh...