A new pyridine-2,6-bis(oxazoline) for efficient and flexible lanthanide-based catalysts of enantioselective reactions with 3-alkenoyl-2-oxazolidinones

A new pyridine-2,6-bis(oxazoline) (4) has been easily synthesised from the reaction of (1S,2S)-2-amino-1- phenylpropane-1,3-diol (1) and dimethyl pyridine-2,6-dicarboximidate (2), followed by TIPS (TIPS=triisopropylsilyl) protection of the 4’-CH2OH group. The catalysts derived from 4 and eight lanthanide(iii) triflates have been tested over three reactions involving 3- acryloyl- and 3-crotonoyloxazolidinones (5a,b): the Diels–Alder (DA) reaction with cyclopentadiene, the 1,3-dipolar cycloaddition with diphenyl nitrone and the Mukaiyama–Michael reaction with 2-trimethylsilyloxyfuran. Several reactions exhibit very good enantioselectivity (ee>90%), and the opposite enantiomers can be easily obtained simply by changing the cation. This specific feature of the ligand can be appreciated in the DA reaction of 5a, since the catalyst [ScIII(4)] gives the adduct (2’S)-9a with 99% ee, whereas the catalyst [YIII(4)] gives the opposite enantiomer with 95% ee. A rationale of the enantioselectivity is proposed on the basis of the NMR spectra of La based complexes involving 4 and 5 as ligands.