Condensation modifications

Neo AG, Marcos CF, Marcaccini S, Pepino R (2005) Studies on isocyanides. A facile synthesis of 4, 5-dihydro-l, 4-benzothiazepin-3(2H)-ones via post-condensation modifications of the Ugi reaction. Tetrahedron Lett 46 7977-7979... 

Further applications for the preparation of jfi-turn mimetics have also been reported by Martens et al. with the synthesis of pipecolic acid derivatives 88 (presumably proceeding via a cyclic Schiff base - not a true post-condensation modification ) [74] and Golebiowski and co-workers who have reported the solid-supported synthesis of bicyclic diketopiperazines with the generic structure 89, via the multi-step synthesis shown in Scheme 11.18 [75]. 

The time consuming chromatographical purification of heterocycles 28 and 29 slowed down the rate of library production. A phase separation using fluorous chemistry was employed by Zhang and Lu to address the workup and purification of fused 3-aminoimidazo[ l,2-a]pyridines (such as 30) [54]. Thus, attachment of a perfluorooctanesulfonyl tag to aldehydes and subsequent Ugi three-component microwave-assisted condensations with 2-aminopyridines and isocyanides furnished the desired heterocycles 30, which were conveniently isolated by fluorous solid-phase extraction. The fluorous tag could be subsequently used as an activating group in the post-condensation modifications, such as Suzuki-Miyaura cross-coupling reactions. 

Fluorous aminoesters have been employed in 3-component 1,3-dipolar cycloaddition reactions and post-condensation modifications in the synthesis of a bridged-tricyclic ring system 30 (Scheme 21) [49]. 

Marcaccini S, Torroba T (2005) Post-condensation modifications of the Passerini and Ugi reactions. In Zhu J, Bienayme H (eds) Multicomponent reactions. Wiley-VCH, Weinheim, pp 33-75... 

B. Piqani, W. Zhang, Beilstein J. Org. Ghem. 2011, 7, 1294— 1298. Synthesis of diverse dihydropyrimidine-related scaffolds by fluorous benzaldehyde-based Biginelli reaction and post-condensation modifications. 

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