Synthetic strategy and concept of multi-step synthesis of biologically important compounds are discussed with special reference under stereochemical control. A-strain in cyclic exo-olefin was expanded to acyclic cases to allow high stereoselectivity in conjugate addition, epoxidation, etc. A special synthetic methodology named heteroconjugate addition has been improved in various aspects (i) preparation of heteroolefin, (ii) diastereo-switching of
syn or
anti product, (iii) enantio-switching of D or L from D-sugar analog and (iv) coupling with other fragments in the total syntheses of maytansinoids, okadaic acid, tautomycin, etc. Recent advances on sugar acetylene chemistry are also introduced to use the corresponding biscobalthexacarbonyl complexes and Nicholas type reaction for medium-size ether ring formation. These syntheses request multiple organic reactions such as carbanions, carbenium cations, organometallic reagents, heteroatom chemistry for bond formation in good harmony with functional group preparation of the target natural products.
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