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Weinreb amide to aldehyde in the presence of an ester
Babcock_Hall:
I was looking over a synthesis recently in which a Weinreb amide was made and then reduced with DIBAL. It was done in the presence of a tertiary-butylester. Would there be some chance that the ester would also reduce? Are there any special tricks when making aldehydes with DIBAL? There was a thread here a few years ago, which mentioned this reduction, but there was not much discussion about it.
zarhym:
tert-Butyl esters are less reactive in reduction compare to weinreb amide.
In my experience, reduction of weinreb amide to aldehyde requires low temperature (-78 to 0°C) to provent over reduction.
You can read this paper for more infomation.
https://onlinelibrary.wiley.com/doi/pdf/10.1002/bkcs.10571
You can check the table 3 in this paper to compare the reduction on ester and weinreb amide.
Babcock_Hall:
Thank you; this paper looks quite helpful.
pgk:
Theoretically, DIBAL reduction occurs via formation of a >=O-Al Lewis complex that forces the hydride to attack from the less bulky side, which is impossible in case of a bulky tertiary ester, especially in low temperatures.
(But I have never worked with DIBAL and thus, I cannot know what happens in real life.)
Babcock_Hall:
C. L. Bailey et al. / Tetrahedron Letters 56 (2015) 706–709. http://dx.doi.org/10.1016/j.tetlet.2014.12.066
This paper uses MgAB an alternative to DIBAL in reducing a Weinreb amide. The reaction takes place at room temperature. An ordinary ester is not reduced. So many reducing agents, so little time...
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