Synthesis of compounds containing the isoprene unit: condensation of 3-methylbut-3-enoic and 3-methylbut-2-enoic acids with carbonyl compounds

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Abstract
The reaction between benzaldehyde and the lithium tributyltin, lithium di-isobutylaluminium. Dilithium, sodium lithium, potassium lithium, and dipotassium salts obtained bymetalation of 3-methylbut-2- and -3-enoates gives two pairs of isomeric hydroxy-acids, identified as their methyl esters, and a lactone. Two of the acids (IVa and b) and the lactone (VI) are derived by attack at C-4 of the salt; the other two (Va and b) originate from attack at C-2. The ratio of C-4 to C-2 attack depends strongly on the nature of the two metals; the proportion of C-4 attack increases as the ionic character of the organometallic bonds involved in the salts is increased. Only one acid (IVa) was isolated from the dipotassium salt of the 2-enoate; in contrast only the acids (Va and b) were obtained from the lithium tributyltin salt. In all cases studied the reaction shows a remarkable cis stereoselectivity in the configuration of the double bond of the hydroxy-acids (IV). Dehydration of the acid (IVa) with base constitutes a new and convenient method of preparing cis-2,trans-4-dienoates containing an isoprene unit in high yield.

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