Construction of the isocopalane skeleton: application of a desulfinylative 1,7-hydrogen atom transfer strategy
Xiong Xiao1, ZhongYu Xu1, Qian-Ding Zeng2
1Shanghai Key Laboratory of New Drug Design, School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237 (P. R. China), Fax: (+86) 21-64253689.
Abstract:
Two attractive chirons, aldehyde 6 and chloride 7, exhibiting functionalized ent-spongiane-type tricyclic skeletons (ABC ring system), have been constructed and their absolute configurations have been studied by NMR spectroscopy and confirmed by single-crystal X-ray diffraction. Both of these chirons are derived from commercially available andrographolide in good yield. Aldehyde 6 is obtained through a novel K2 S2 O8 -catalyzed aquatic ring-closing reaction of allylic sodium sulfonate and intramolecular 1,7-hydrogen atom transfer process. Further mechanistic investigations demonstrate that the 1,7-hydrogen atom transfer is a free-radical process, whereby hydrogen migrates from C18 to C17, as evidenced by double-18- deuterium-labeled isotope experiments. Prospective applications of these two chiral sources are also discussed.
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