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The reaction between 2-alkylpyrazino[2,1-b]quinazoline-3,6-diones and aromatic aldehydes in the presence of KO t Bu afforded the corresponding 1-arylmethylene derivatives, preferently as the Z isomers. This diastereoselectivity was the result of thermodynamic control, as shown by ab initio calculations. These aldol reactions proceeded with partial racemization of the C-4 stereocenter, in contrast with the condensations of the related 2-acetylpyrazino[2,1b]quinazoline-3,6-diones. Catalytic hydrogenation of the 1-arylmethylene derivatives obtained through these condensations afforded cis-1,4-disubstituted pyrazino[2,1-b]quinazoline-3,6diones, normally with complete diastereoselectivity, but these reactions had compounds from partial reduction of the benzene ring as side products.

Aldol condensations in pyrazino[2,1-b]quinazolines. Fast access to seco-ardeemin derivatives