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2005
Multi-membered O,S-heterocycles
R 0691
Ring Fission of Chiral Cyclic Acetals Plus Intramolecular [4 + 2] Cycloaddition:
Sequential Access to Medium-Size Lactones. Application to the Synthesis of
35- 191 A
Carbasugars. — A set of α,β-unsaturated cyclic acetals (I) are ring opened with
trifluoroethyl acrylate in the presence of tBuLi and TMEDA. The resulting acryloyl diethylethers are obtained with high stereoselectivity. Subsequent cycloaddition under
high pressure affords the corresponding products when carried out with dioxepanes
under well-defined conditions. Depending on the structure of the linkage between the
diene and the acrylate (syn/anti relationship between the substituents), the expected lactones are observed with total endo- or exo-selectivities. However, good diastereoselectivity is only be obtained in the case of the endo-adducts. Cycloaddition products derived from dioxolanes are never be obtained. Finally, a five-step diastereoselective
transformation of lactone (VII) into modified carbasugar (VIII) is achieved in 23%
overall yield. — (LEMIEGRE, L.; STEVENS, R. L.; COMBRET, J.-C.;
MADDALUNO*, J.; Org. Biomol. Chem. 3 (2005) 7, 1308-1318; Lab. Fonct.
Azotees Oxygenees Complexes, Inst. Rech. Chim. Org. Fine, Univ. Rouen, F-76821
Mont-Saint-Aignan, Fr.; Eng.) — Bartels
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