ChemComm
Communication
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Scheme 2 Synthesis of chiral 1,2,4-triazoline derivatives.
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Scheme 3 The synthetic utility of this catalyst system.
containing products. To highlight the synthetic potential of the
1,2,4-triazolines 3, structural elaboration of the heterocycle was
carried out and presented in Scheme 2. When treated with
120 mol% of NaOEt in CHCl3–EtOH at 25 1C, both 3ka and 3kb
were mono-deprotected to provide heterocycles 4a and 4b in
good yields and excellent ee values. The resulting product 4b,
which contains an active hydrogen, then could be transformed
into N-benzyl protected 5a (65% yield, 91% ee) and N-Ts
protected 5b (95% yield, 91% ee), respectively.5
Gram-scale reaction was also evaluated, and cyclization
between isocyano ester 1k and DIAD 2b was performed on a
3 mmol scale with 5 mol% of Fe(acac)2/L6 at 25 1C. As shown in
Scheme 3, the desired product 3kb was obtained with 71% yield
and 93% ee.
In summary, we have developed a highly enantioselective
cyclization of a-isocyano esters and azodicarboxylates cata-
lyzed by a N,N0-dioxide/Fe(acac)2 complex. The excellent yields
(up to 98%), good enantioselectivities (up to 94% ee), broad
substrate scope, mild reaction conditions, and operational
simplicity provided a potential method for the asymmetric
synthesis of 1,2,4-triazoline derivatives. Further investigations
of the N,N0-dioxide–metal complexes system in other reactions
are ongoing.
We appreciate the financial support from the National
Natural Science Foundation of China (No. 21021001 and
20902060), and National Basic Research Program of China
(973 Program: No. 2010CB833300).
´
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ESI†.
c
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