開催日: 2017/09/20 - 2017/09/22
The structurally complicated and interesting terpenens pallambins A-D were isolated from the liverwort Pallavicinia ambigua. These natural products have 4-6 rings, 7-10 contiguous stereogenic centers which can be considered as challenging architectures in organic synthetic chemistry. Two elegant total syntheses of pallambins have been reported, one from the Wong group and the other from Carreira group. The former route furnished pallambins C and D in 38 steps from the Wieland-Miescher ketone, and the latter gave pallambins A and B in 23 steps from a fluvene. We have achieved an efficient and concise total synthesis of pallambins C and D in only 11 pots, wherein protecting group manipulations is absent. Our total synthesis of pallambins C and D commences with inexpensive furfuryl alcohol 10. Eschenmoser-Claisen rearrangement of 10 followed by reduction gave aldehyde 9 in a single vessel with decent yield. Robinson annulation of 9 and ethyl vinyl ketone afforded the corresponding cyclohexenone derivative 11 in moderate yield. TMS-enol ether 12 was obtained via Michael addition of a vinyl cuprate species followed by trapping the resulted enolate by TMSCl. Chemoselective oxidative cleavage of furan moiety was carried out in the presence of methylene blue with oxygen to give keto-aldehyde 13. The intramoleculer Mukaiyama aldol reaction was successfully accomplished in good yield and with excellent diastereoselectivity. Surprisingly, conversion of 6 to acetal 14 was achieved by treatment with CH(OMe)3 and SnCl4, and the intermediate 14 could be transformed into alkyl bromide 15 by successive addition of AcBr in the same reaction vessel. A reduction of the alkyl bromide followed by desaturation and elimination of methanol proceeded smoothly delivering enol ether 15 in good yield. Fortunately, we established the new transformation of enol ethers into the corresponding difunctionalized adducts by using SnCl4 and I2. The desired iodo-diester 18 was obtained in excellent yield and with perfect diastereoselectivity under this condition. Finally, the total synthesis of pallambins C and D was accomplished by engineering four transformations in one-pot.