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.