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Catalytic enantioselective annulations via 1,4-addition-aldol cyclization of functionalized organozinc reagents Naasz, R.; Arnold, L.A.; Pineschi, M.; Keller, E; Feringa, B.L.

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Abstract:Various diazo compounds, nitrile oxides, nitrones and azomethine ylides were examined in 1,3-dipolar cycloadditions to enantiomerically pure 5-(R)-menthyloxy-2(5H)-furanone 1a. Pyrazoline 9 was obtained in 100% c.y. as a mixture of 2 diastereoisomers in ratios up to 72 : 28, whereas pyrazoline 16 was obtained in 100 % c.y. as a single enantiomer. Photochemically pyrazolines 9 and 10 have been converted to cyclopropanes 11 and 13. Under thermal conditions pyrazoline 9 is converted to 4-methyl-5-menthyloxy-2(5H)-furanone. Isoxazoles 21a-24a were obtained enantiomerically pure via nitrile oxide addition to 1a in 64-67% yield. Nitrone addition afforded isoxazolidines 27, 28 and 34 with complete anti-facial-and regiochemistry, but with endo-exo selectivities up to 76%. Enantiomerically pure isoxazolidines were obtained in 25-75% yield. Pyrrolidine 36 was obtained diastereomerically pure in 81% c.y. Pyrrolidines 42 and 45, however, were obtained as diastereomeric mixtures in 37% resp. 6% yield. IntroductionCycloadditions are undoubtedly a cornerstone in synthetic methodology. In order to control the absolute stereochemistry of the ring systems which are formed, regio-, facial-, and endo/exo-selectivity are decisive factors. Therefore it is not surprising that a variety of asymmetric Diels-Alder reactions and 1,3-dipolar cycloadditions have been developed in the last decade.1,2 The 1,3-dipolar cycloaddition reactions of nitrones and nitrile oxides to alkenes have been extensively used for the preparation of isoxazolidines and isoxazoles.3 Further transformations offer access to a variety of functional intermediates for synthesis, in many cases with multiple stereogenic centers introduced during the cycloaddition

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