1-Pyrroline 1-oxide reaction with methyl crotonate

Another possible route to thienamycin (487) has utilized the dipolar cycloaddition of 1-pyrroline 1-oxide (482) with methyl crotonate (79TL4359). The reaction is highly stereoselective due to the operation of secondary orbital effects. The isoxazolidine (483), produced in 90% yield, was subjected to hydrogenolysis, and the resulting amino alcohol (484) was selectively blocked with hexamethyldisilazane to give (485). Treatment with ethylmagnesium bromide then gave /3-lactam (486 Scheme 107). 

With cyclic nitrones, where ( /Z) isomerization is not possible, the exo transition state is usually steri-cally preferred (equation 19-21). When steric factors in the endo and exo transition states are nearly the same, however, the existence of bonding secondary orbital interactions in the endo transition state can tip the balance in its favor this appears to be the case for the reaction of methyl crotonate and l-pyrroline 1-oxide (equation 22). ... 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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