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SePh
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TBSO
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PhSe
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H
N
i
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CH2Ph
Ph
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4j; 79%, 70/29/1/>0
3j; 90%ee
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Scheme 4. Reagents and conditions: (i) TBSOTf, 2,6-lutidine,
0°C; (ii) Bu3SnH, AIBN, toluene, 110°C; (iii) TBAF, THF.
monosubstituted secondary 2-pyrrolidones via radical
cyclization strategy; neither a temporary N-protective
group10 nor an a-substituent that affords chiral induc-
tion are required.8 Optically active 4-benzyl-2-
pyrrolidone 6b was obtained in a similar manner. With
this procedure in hand, 4,5-disubstituted-2-pyrrolidone
6c was examined (Scheme 4). Although
a
diastereomeric mixture was formed during the conver-
sion to 4 and 5, desired (4R,5S)-2-pyrrolidone 6c was
isolated as a single isomer with 90% ee;13 no separation
of diastereomers in 4 and 5 was needed. Thus, the
stereoselectivity of the radical cyclization step is quite
high.6c
In conclusion, we have found a simple and stereoselec-
tive method to convert lactic or mandelic amides to
2-oxy-1,3-oxazolidin-4-ones, which serve as a useful
precursor to prepare optically active chiral secondary
2-pyrrolidones. This method provides the first use of
mandelic acid as a chiral orthester-equivalent auxiliary.
Each step of the procedure takes place under mild and
neutral conditions, and products are isolated in good
yields in a stereoselective manner. Recovered mandelic
acid maintained its optical purity so that it opens a way
for recycled use. Further investigation and applications
are now underway in our laboratory.
Acknowledgements
The present work was partially supported by the Grant-
in-Aid (11640536) from the Ministry of Education,
Science, Sports and Culture, Japan. Financial support
from Sasakawa Scientific Research Grant (to Y.O.) is
also appreciated.
References
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