1
95
particular interest as the ring opening of allylic epoxy alcohols gives the 3-iodo 1,2-diols. Enol ethers are
also excellent substrates for iodohydroxylation using NIS as exemplified by entry 4. Indeed, dihydropyran
reacted readily at −20°C to give the desired product as a single regio- and trans-stereoisomer. The non-
conjugated unsaturated ketone hex-5-en-2-one reacted in a Markownikov fashion in high yield (entry
5
). The less reactive conjugated ketone, hex-4-en-3-one remained unchanged under standard conditions.
However, at room temperature, the reaction proceeded slowly to give quantitatively after 12 h the erythro
-hydroxy-2-iodo-hexanone (entry 6). The regioselectivity for this reaction can be rationalised by attack
3
of H O on the iodonium intermediate at the β-carbon and not at the α-carbon which is deactivated by the
2
presence of the electron-withdrawing carbonyl group. When pseudo-ionone or carvone were subjected
to iodohydroxylation, in both cases, the reactions were chemoselective with only the more reactive non-
conjugated double bond being converted into the corresponding iodohydrin (entries 7 and 8). The less
reactive α,β-unsaturated esters and acids could not be transformed into the desired iodohydrins. Indeed,
at −20°C, no reaction occurred and 100% of the starting material was recovered. However, in contrast to
an α,β-unsaturated conjugated ketone (entry 6), when the reaction was carried out at room temperature,
only unidentified products were formed (entries 9 and 11). The non-conjugated unsaturated esters and
acids did react smoothly under standard conditions to give the expected iodohydrins in a Markownikov
fashion and in high yields (entries 10 and 12).
In summary, this paper describes a very simple route to iodohydrins. Our methodology presents
several advantages including mild reaction conditions, short reaction times, high chemo-, and regio-
stereoselectivities as well as very high yields.
Acknowledgements
We thank l’Ecole Normale Superieure (MENRT) for generous financial support to M.S and A. Valleix
for running mass spectra. We also thank Dr. Matteo Zanda for helpful suggestions.
References
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1
1
1
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3. Only one example of iodohydroxylation using NIS/H
converted into the corresponding iodohydrin; Guindon, Y.; Guerin, B.; Chabot, C.; Olgivie, W. J. Am. Chem. Soc. 1996,
2528.
2 4
SO /acetone is reported in the literature. Methyl cinnamate was
1