Synthesis of the Allyl Ethers

The synthesis of the allyl ethers in nitrogen heterocyclic systems presents an element of complication in that the allylation could occur on the oxygen atom or the basic nitrogen atom. This is a feature of alkylation of ambident anions.3 However, this applies only when the allylation is effected by reacting the oxo or hydroxy derivative of the compound with an allyl halide in the presence of a base.3 The alternative method is to react the appropriate halo derivative with sodium allyloxide in allyl alcohol. The latter approach provides not only better yields of the allyl ethers but also certainty of the constitution of the ethers obtained. A diagnostic tool in deciding between the 1-allyl derivative and the O-allyl compound that has commonly been employed is the infrared absorption of the amide carbonyl in the case of the former which is clearly absent in the latter. 

More recently, NMR spectra have also been employed to determine the positions of the allyl functions.6 

An additional element of complication in preparing the allyl ethers utilizing the salts of hydroxypyridines lies in the intervention of SN2 displacements. Thus the sodium salt of 2-pyridone on treatment with crotyl chloride or with 3-chloro-1 -butene afforded the same compound, viz., l-crotyl-2-pyridone5 [Eq. (1)]. 

Allyloxy derivatives from pyridine-1-oxides posed still a further element of uncertainty. This arose from the facile isomerization of 2-alkoxypyridine-l-oxides into the corresponding l-aIkoxy-2-pyri-dones even under the conditions of synthesis. Thus, treatment of 2-chloropyridine-l-oxide with sodium allylalcoholate afforded the 2-allyloxypyridine-l-oxide whereas treatment of the same starting compound with sodium benzyloxide gave l-benzyloxy-2-pyridone. A closer investigation revealed that 2-benzyloxypyridine-l-oxide rearranged readily under the experimental conditions of synthesis.6 

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