3-Oxanitrones 10 and 3-azanitrones 11 were prepared starting from (S)-ethyl lactate 5 and (S)-N-benzyl alaninol 8, respectively. Both nitrones underwent spontaneously an intramolecularcycloaddition affording the tricyclic compounds 12 and 13. The diastereomeric forms of 12 and of 13 were separated by column chromatography and identified by NMR spectroscopy.
The N-benzylated bicyclic compounds 5 were converted to C-phenyl nitrones 6 by highly regioselective oxidative opening of the isoxazolidine ring with MCPBA. One or both of the isoxazolidine rings of compound 8 were opened in the same way affording a mixture of compounds 9 and 10. Compounds 13a and b were formed by oxidation of N-methylated bicyclic compounds 11a and 4 respectively, with MCPBA, while Ile was decomposed under these conditions. A mixture of 15 and 16 was isolated after oxidation of the tricyclic compound 14, whereas oxidation of the diasteromeric 17 provided the oxime 19 as the single product. The reason for the different course of the oxidation is discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.
Formation of enantiopure tricyclic compounds by intramolecular 1,3-dipolar cycloaddition of nitrones
Nitrones 6 prepared from ester 5 underwent an intramolecularcycloaddition affording diastereomeric mixtures of tricyclic compounds 7A/B. These were separated to give the enantiopure compounds 7A and 7B. Starting from aminoalcohol 8 compounds 10A and 10B were formed via nitrones 9. Nitrone 9a yielded the trans-product 10aC in addition. X-ray analyses confirm the structure of 7aB and 10aA. Catalytic