Remote Induction of the Condensation of p-Nitrobenzaldehyde on the Methylene Group of 5-Hydroxy-2-coumaranone

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Abstract
(Z)-5-Hydroxy-3-(p-nitrobenzylidene)-2-coumaranone ((Z)-p-nitro-marginalin) is obtained in 92% yield by reaction of p-nitrobenzaldehyde with 5-hydroxy-2-coumaranone. The same condensation carried out with 2-coumaranone gives a mixture of (Z)- and (E)-p-nitrobenzylidene-2-coumaranones (72% yield) in a 2:1 ratio. In a discussion taking also into account previous results in the series, it is concluded that the 5-hydroxyl group exerts a remote induction in the orientation of the intermediate secondary alcohol, stabilized through chelation with the lactone carbonyl group. Such a long distance effect is in agreement with the requirements of the Cram''s rule, leading in this particular case to a high stereoselectivity in the final dehydrated ene-lactone.
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