開催日: 2017/09/20 - 2017/09/22
Casuarinin and stachyurin are ellagitannins isolated from chestnut and oak. Castalagin and vescalagin are known as analogues thereof. Structural features common to these compounds are the presence of D-glucose in an open chain form and hexahydroxydiphenoyl (HHDP) or nonahydroxytriphenoyl (NHTP) groups linked to the glucose moiety through a C-glycosidic bond. The presence of the C-glycosidic bond has greatly influenced generation of their analogues and expression of biological activities. Here, we describe the synthetic study of these ellagitannins, including the development of efficient method for construction of the C-glycosidic bond and the total synthesis of casuarinin. The following transformations completed the total synthesis of casuarinin. For the comprehensive synthesis of C-glycoside ellagitannins, we embraced the synthetic strategy based on the presumed biosynthesis of casuarinin, in which pedunculagin and liquidambin are the key intermediates. Thus, we synthesized an equivalent of pedunculagin using the oxidative coupling of 4-O-benzylated gallates as the key reaction. The opening of the pyranose ring in the pedunculagin-equivalent as an oxime followed by introduction of the galloyl group at O-5 provided an equivalent of liquidambin. Treatment of this compound with acid constructed the C-glycosidic bound to furnish the skeleton of casuarinin as a single diastereomer. Finally, we achieved the first total synthesis of casuarinin by removal of the benzyl groups.