钯促进的碳氢键活化在齐墩果酸D和E环脱氢烯化中的应用研究

Regio-selective Dehydrogenation on the D or E Rings of Oleanolic Acid by Pd-Promoted C—H Activation

齐墩果烷型的三萜及其糖苷化合物广泛分布于陆地植物和一些海洋生物中,是许多传统药物中的活性成分.这类五环三萜化合物的D和E环常发生羟基化的修饰,这些羟基及进一步的酰化或糖基化对这些化合物的活性非常重要.报道了利用最近发展的钯促进的碳氢键活化的方法,以齐墩果酸C28位羧基上缩合的8-胺基喹啉或2-氨甲基吡啶酰胺作为导向基团实现了对齐墩果酸D和E环的选择性脱氢烯化.这为进一步引入其他官能团奠定了基础.在此,N,N-双齿配体对于反应的顺利进行十分重要.

Oleanane-type triterpenes and their glycosides are a structurally and biologically diverse class of metabolites that are widely distributed in terrestrial plants and some marine organisms.Many of these compounds bear functional groups and modifications on the D and/or E rings of the triterpenes.These triterpenes compounds are of growing interest for drug research as they are active constituents of folk medicines and provide valuable pharmacological profiles.For their limited availability and accessibility,chemical synthesis provides a realistic way to determine the availability of homogenous natural products and their derivatives.Here,we report the selective dehydrogenation on the D or E rings of oleanoic acid by palladium promoted C—H activation with 8-aminoquinoline amide(substrate 12) and 2-aminomethylpyridine amide(substrate 16) as the directing groups.Thus,upon treatment with thionyl chloride,the 28-COOH of oleanoic acid was converted into 28-COCl,which coupled with the corresponding amines to provide the substrates readily for dehydrogenation.Notably,treatment of 12 with optimized conditions(1.0 equiv.Pd(OAc)2,1.5 equiv.Oxone,1,2-dichloroethane,80 ℃,24 h) led to the dehydrogenated products 13 [double bond at C(15)—C(16)] and 14 [double bond at C(20)—C(21)] in 55% and 2% yields respectively.Treatment of 16 with optimized conditions(1.0 equiv.Pd(OAc)2,2.0 equiv.Oxone,1,2-dichloroethane,microwave 85 ℃,50 min) provided 17 [double bond at C(20)—C(21)] and 18 [double bond at C(15)—C(16)] in 42% and 20% yields respectively.Moreover,the distribution of the products varied in different solvents.It is worth mentioning that the N,N-bidentate ligands(two nitrogen atoms as the coordination sites in 12 and 16) are crucial for the palladium promoted olefination.