Total Synthesis of the Annonaceous Acetogenin (+)-Asimicin. Development of a New Bidirectional Strategy

The total synthesis of the Annonaceous acetogenin (+)-asimicin is described. The approach employs the (R)-α-OSEM allylic stannane 7 of >95% ee and the dialdehyde 8 obtained from (S,S)-diethyl tartrate. Addition of 7 to 8 in the presence of InCl3 afforded the bis-adduct 9 in 71% yield. Tosylation and treatment with TBAF led to the core bis-tetrahydrofuran intermediate, diol 11, in 78% yield. Mono tosylation (n-BuLi, TsCl, THF−DMSO) and subsequent hydrogenolysis with LiBEt3H gave alcohol 14. The iodide 15 was coupled with the higher-order vinylcyanocuprate to afford olefin 30. This was converted to diol 31 of high ee by the Sharpless protocol. This diol yielded the epoxide 33 via the mono-trisylate 32. Addition of (R)-lithio-2-(OTBS)-3-butyne in the presence of BF3·OEt2 afforded the alcohol 34. The SEM derivative 35 was treated with TBAF, and the resulting alcohol was converted to the butenolide 38 by a sequence involving treatment with (CF3CO)2O, then Pd(PPh3)4, CO, THF−H2O, and finally AgNO3/silica gel. C...