Synthesis of chiral spiro 3-oxazolin-5-one 3-oxides (chiral nitrones) via a nitrosoketene intermediate and their asymmetric 1,3-dipolar cycloaddition reactions leading to the EPC synthesis of modified amino acids
Cycloaddition of chiral cyclic ketones such as (−)-menthone, (+)-nopinone, and (+)-camphenilone to nitrosoketene generated by thermolysis of 5-hydroxyimino-2,2-dimethyl-1,3-dioxane-4,6-dione gave the corresponding chiralspiro 3-oxazolin-5-one 3-oxides (chiral cyclic nitrones). These nitrones underwent asymmetric 1,3-dipolar cycloaddition reactions with electron rich olefins to give the corresponding
Chiral spiro nitrone 3, treated with (3-trimethylsilyl)cyclopent-1-ene in the presence of BF3 . Et(2)O, gives the 1,3-dipolar cycloadduct 4 as a single isomer, which is converted to (2S,1'S)-(cyclopent-2-enyl)glycine (2S,1'S)-7 by alkaline hydrolysis, catalytic reduction and BF3 mediated alkene formation.