Cycloadditions. Part III. Steric effects in the addition of 2-diazopropane to conjugated allenic esters and nitriles

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Abstract
Esters of buta-2-,3-dienoic acid and its methyl-substituted analogues react with 2-diazopropane at more electrophilic (αβ) double-bond, but the orientation of addition is dependent on the degree of substitution at the γ-carbon atom. Allenes monosubstituted or unsubstituted at the γ-position give 4-alkylidene-1-pyrazolines (i.e. the electronically preferred adducts). The initial adduct from methyl buta-2,3-dienoate isomerises under the reaction conditions to 3-methoxycarbonyl-5,5-dimethyl-4-methylene-2-pyrazoline, which reacts readily with a second molecule of 2-diazopropane to give a spiran. The orientation of addition is reversed with γγ-disubstituted allenic esters and nitriles, which give solely 3-isopropylidene-1-pyrazolines. The reversals are attributed to a highly unfavourable ‘wedging’ interaction in the transition states for normal addition to the γγ-dimethylallenes. The orientation in several other allene-diazoalkane cycloadditions is discussed. The 3-isopropylidene-1-pyrazolines give good yields of isopropylidenecyclopropanes on photolysis.

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