Polyether octacyclic

RCM was also used in Yamamoto s total synthesis of the marine neurotoxin gambierol (81) [62], to close the central seven-membered E ring, thereby completing the octacyclic polyether core 80 (Scheme 15). Following previously developed methodology [63], metathesis precursor 79 was produced as the major epimer, by boron trifluoride etherate-mediated intramolecular allylation of a-chloroacetoxy ether 78. Subsequent treatment of 79 with catalyst C produced the octacyclic ether 80 in 88% yield. 

Fuwa, H., Sasaki, M. and Tachibana, K., Synthetic studies toward gambierol. Convergent synthesis of the octacyclic polyether core, Org. Lett., 3, 3549, 2001. 

N-methylmorpholine N-oxide (NMO) [93] to afford ketone 72. Oxidative removal of the PMB group followed by treatment with ethanethiol and zinc triflate effected cycUzation of the D-ring as the mixed thioketal to yield, after acetylation, 73. Finally, desulfurization under radical reduction conditions (triphenyltin hydride, AIBN, toluene, 110 °C) [94] proceeded cleanly to furnish the octacychc polyether core 74 in high yield. Thus, the synthesis of the octacycle 74 was achieved in just seven steps from the couphng of two advanced intermediates 67 and 68. 

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