Study On The Synthesis Of The Two Classes Of Natural Products

This Master Degree thesis is about studying on the synthesis of the two classes of natural products.One is oxazole-containing natural products.They are Melanoxadin,MR-93 A,Melanoxazal and MR-93B.They are all the melanin biosynthesis inhibitors.The other is New 6-Alkylated a-Pyrones(=2H-Pyran-2-ones)which was extracted through the stem bark dichloromethane from Ravensara crassifolia.It showed antifungal activity against the phytopathogenic fungus Cladosporium cucumerinum.This thesis has two chapters:The first part of this thesis was about studying on the total synthesis of oxazole-containing natural products.The literature structure of Melanoxadin,MR-93A,Melanoxazal and MR-93B were firstly synthesized in enantiopure forms through chiral pool based routes.The absolute configurations of four key intermediates were established by single-crystal X-ray crystallography.Melanoxadin,MR-93A were both syn(S,S)configurations.Melanoxazal was an(R)configuration.MR-93 B was a(S)configuration.The large discrepancies in the optical rotations between the natural MR-93A,MR-93B and the pure synthetic samples were discussed.The previous assignments for the C-1'/H-1' and C-2'/H-2' made for MR-93 A should be inverted.The second part of this thesis was about studying on the synthesis of natural product from Ravensara crassifolia.This work was based on the synthesis technology of enatiopure epoxy chiral building block which had a 1,3-diol fragment from D-glucolactone which was inexpensive and easily available.This eight steps'synthetic route had a easy post-processing without any anhydrous and anaerobic operation.Only the last reaction needs be purified by chromatography on silica gel.What's more,the scale of it could be increased to 10 g scale stably.The research result showed that our synthetic route was proved conclusively and some condition of the reactions were optimized.What's more,The absolute configuration of the compound which was the diastereoisomer of natural product was confirmed with an anti(R,R)configuration,although the optical rotation of our synthetic compound could not match with that work of Venkateswarlu's.On the other hand,the optical rotation of our synthetic compound could match with natural product.But there's a discrepancie in NMR between our synthetic compound and natural product.It needs a further research to bring to light whether our synthetic compound is a six-membered a,P-unsaturated lactone structure as reported or an eight-membered ?,?-unsaturated lactone structure with our guess.