Triene side chain

The final steps to complete the total synthesis required stereoselective construction of the triene side chain and global deprotection. After extensive experimentation, it was found that the Pd(PPh3)4/ CuCl/LiCl-promoted modified Stille coupling conditions" were quite suitable for the construction... 

Oxidation of cyclic enol ether 35 with dimethyl dioxirane (DMDO) followed by in situ reduction of the intermediate epoxide with DIBALH gave secondary alcohol as a 10 1 mixture of diastereom-ers. Oxidation of these alcohols with TPAP/NMO afforded a 10 1 mixture of ketone 37 and its C16 epimer. The isomers were separated and the minor isomer was recycled to a 4 1 mixture of isomers by treatment with imidazole. Subsequent construction of the D-ring was performed by radical reduction of mixed thioacetal in the same way as that adopted by the Sasaki group, leading to octacycle 38. Stereoselective installation of the triene side chain was then carried out via (Z)-vinyl iodide 39... 

In the total synthesis of (-)-gambierol, a marine polycyclic ether toxin, by two groups [28,29], the sensitive terminal (Z, Z)-triene side chain in 53 was constructed as a crucial step by stereoselective coupling of (Z)-alkenyl iodide 51 with (Z)-l,4-pentadienylstannane 52 in DMSO to afford 53 in 72% yield without isomerization of the triene system. Pd2(dba)3 and Cul as catalysts and TFP (1-3) as a ligand were used [29]. Similar coupling using the less reactive alkenyl bromide provided 53 in 43 % yield [28]. 

Subsequently, compound 91 was elaborated to primary alcohol 92 with the H-ring functionality in a similar manner to the Sasaki synthesis (Scheme 11). For the introduction of the triene side chain, a simple and practical method for the stereoselective synthesis of (Z)-vinyl iodide, which is expected to be more reactive than the bromide counterpart, was developed [105]. Thus, PCC oxidation of alcohol 92 followed by treatment of the resultant aldehyde with tetraiodomethane and triphenylphosphine gave diiodoalkene 93. Reduction of 93 with the zinc-copper couple in the presence of acetic acid provided (Z)-vinyl iodide 94 in high yield. Since deprotection of the fully protected... 

By a series of studies of the enzymatic and non-enzymatic polymerization mechanism of urushiol. Scheme 2 has been established. Urushiol (1) is oxidized into the corresponding quinone (2)26 d formed quinone (2) undergoes C-C and C-0 coupling reaction with catechol nucleuse or the triene side chain urushiol, a major component of urushiol, giving dimeric urushiol (3), (4) and (5) 27-30 ... 

Furthermore, as substituents for urushiol, we found that catechol derivative, made by the Friedel-Craftes type alkylation of catechol with linseed oil, linseed oil alcohol made by the reduction of linseed oil or oligomeric butadiene can be used.Though there are number of papers relating to synthesis of urushiol components39 synthesis of the triene side chain urushiol is faced with difficulties. On the other hand, recently, a report appeared about the formation of urushiol by tissue cell structures of Rhus vernicifera stock This method seems to be the most interesting to synthesize urushiol, suggesting a need for a further studying. 

Smith, A.B., Lin, Q.Y, Pettit, G.R., Chapuis, J.C., and Schmidt, J.M. (1998) Synthesis and in vitro cancer cell growth inhibitory activity of monocyclic model compounds containing spongistatin triene side-chains. Bioorg. Med. Chem. Lett., 8, 567-568. 

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