ASYMMETRIC 1,3-DIPOLAR CYCLOADDITIONS TO 5-(R)-MENTHYLOXY-2(5H)-FURANONE

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 la 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.