Methylenecyclopropane, reaction with nitrones

A positive feature of the reaction is that nitrile oxides are more regioselective, in cycloadditions to methylenecyclopropanes, compared to nitrones. Only traces (up to 5%) of the 4-spirocyclopropane regioisomers are generally observed with methylenecyclopropanes unsubstituted on the exocyclic double bond. The yields are only moderate, but higher with more stable nitrile oxides (Table 27, entries 5, 6, 10-12). 

Dipolar cycloadditions. The reaction of methylenecyclopropane with nitrones generates spirocyclic isoxazolidines that are prone to thermal rearrangement. Thus 4-piperidones can be prepared in a two-step process. 

Dipolar cycloaddition of nitrone 917 to methylenecyclopropane 918 was marginally stereoselective, and gave a 1 1.2 mixture of isoxazolidine adducts 919 and 920 in 80% yield (599). Thermal rearrangement of the mixture afforded the separable ketones 921 (26%) and 922 (38%). Conversion of these into lasubines I and II, respectively, had been reported some years previously (600). The second route, also not very stereoselective, used a Wittig reaction between acetyl-methylenetriphenylphosphorane and A -protected piperidin-2-ol 923 to m e pelletierine (924), Mannich condensation of which with veratraldehyde gave a 1 2.8 ratio of the ketones 921 and 922 (60%) (601). 

---