10.1002/adsc.201800914
Advanced Synthesis & Catalysis
Acknowledgements
served as a suitable nucleophile to give 9d, the sterically
more demanding aliphatic acyclic secondary amines and
aniline gave no conversion towards 9e-g. The same
reaction conditions allowed facile transesterification using
primary (10a, b), phenolic (10c) and secondary (10d, e)
alcohols, although the latter needed elevated
temperatures.[2d] Also here sterics played a role as t-BuOH
gave no conversion. Aliphatic thiolates served as suitable
nucleophiles and gave thioesters 11a, b in high yields.
Unexpectedly, phenyl thiolate did not give any reaction. It
was found that in the absence of a nucleophile and using a
slight excess of the equimolar azole/DBU combination,
hemiaminal esters 12a-e were formed with high
efficiency.[11] By using cyanide as the nucleophile, the
cyanohydrine ester 13 was obtained albeit in moderate but
yet unoptimized yield. Finally, mildly activated vinyl
benzoate (1a) could be reduced in high yield to benzyl
alcohol (14) using NaBH4.
The authors would like to thank The Netherlands Organization
for Scientific Research (NWO-CW, ECHO grant number
711.014.010 to J.v.M). E. Zuidinga and J.M. Ernsting (University
of Amsterdam) are acknowledged for mass spectrometry and
NMR assistance.
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9b: R = NHBn, 98%
9c: R = NHi-Pr, 81%
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9e: R = NEt2, 0%
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10d: R = i-Pr, 79% (80 oC)
10e: R = (+)-menthol, 63% (80 oC)
10f: R = OtBu, 0% (80 oC)
9f: R = NHPh, 0%
9g: R = NH-p-OMePh, 0%
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R-SH
O
O
NaBH4
OH
(1.2 equiv)
SR
K2CO3
O
THF/H2O
(1.2 equiv)
CH3CN
11a: R = Et, 94%
1a
14 93%
11b: R = Bn, 104% (+disulfide)
11c: R = Ph, 0%
azole (1.2 equiv)
DBU (1.2 equiv)
O
O
O
R
O
CN
CH3CN
KCN
DMF
12a: R = pyrazole, 95%
12b: R = imidazole, 92%
12c: R = 1,2,4-triazole, 87%
13
41%
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12d: R = 1,2,3-triazole, 88% (+ regioisomer)
12e: R = benzotriazole, 90% (+ regioisomer)
Scheme 7. Some enol ester follow-up transformations.
In conclusion, we have developed efficient and
scalable CLE-mediated enol esterification methodology of
carboxylic acids from alkenyl boroxine pyridine
complexes and boronic acids. A wide variety of both
aliphatic and aromatic carboxylic acids in combination of
all substitution patterns of alkenyl boroxines showed their
compatibility. In the case of 2-substituted alkenyl
boroxines the double bond configuration was fully retained
in the enol ester product. Also N-hydroxyimides, imides
and saccharine could be transformed in the respective
amidooxy vinyl enol ethers and vinyl enamides. Finally,
with the exception of methionine, all other relevant amino
acids showed their compatibility to give their enol esters in
a stereoselective fashion
4
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