Publication: ALPHA-Bromination of linear enals and cyclic enones.
Issued Date
2007
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eng
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Mahidol University
Bibliographic Citation
Tetrahedron Letters. Vol.48, No.9 (2007), 8607-8610
Suggested Citation
Pakorn Bovonsombat, Rungkarn Rujiwarangkul, Thanathip Bowornkiengkai, Juthamard Leykajarakul ALPHA-Bromination of linear enals and cyclic enones.. Tetrahedron Letters. Vol.48, No.9 (2007), 8607-8610. Retrieved from: https://repository.li.mahidol.ac.th/handle/20.500.14594/32873
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Title
ALPHA-Bromination of linear enals and cyclic enones.
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Abstract
In a prior report, -iodination of cyclic and linear enones was found to be
effected by pyridine and stoichiometric iodine. However, with the same
combination of iodine and pyridine, refluxing was necessary for reactions of
linear enones. For example, (Z)-4-iodo-4-hexen-3-one was obtained from trans-
4-hexen-3-one in 62% yield in boiling acetonitrile. Other -iodinations of
cyclic and linear enones include the use iodine and excess quantities of
pyridine, iodine and morpholine, pyridinium dichromate (PDC) with iodine and
IN3, prepared in situ from ICl and NaN3. Extending the aforementioned
reactions to the bromination of enones is not practical due to the handling
difficulties of bromine, and its oxidative nature. Thus milder conditions are
required that will tolerate sensitive groups such as an aldehyde in -
bromination reactions. Previous examples of -bromination involve synthesis
of -bromo analogues of flavones using a combination of iodobenzene
diacetate and tetrabutylammonium bromide. Another bromination method
involves the use of a rhodium (III) complex and acid halides or benzyl halides,
via halogenation of diazodicarbonyl analogues. The same combination was also
effective for -chloroenones.
Herein we report a methodology for the formation of cyclic -bromo enones
and linear enals, which are useful templates for synthesis. Enones or enals are
converted directly to their respective -bromoenones or -bromoenals using a
combination of pyridine-N-oxide and N-bromosuccinimide (NBS) by stirring in
typical organic solvents at room temperature.1 Unlike the previously reported
iodination methodology, which gave excellent yields with pyridine as catalyst,
-bromination is more effective with pyridine-N-oxide. -Halogenations of
linear enals were found to proceed with double bond geometry retention.