Post-condensation transformations

Fig. 2 General sketch of Ugi reaction followed by a post-condensation transformation...

We propose here a compact classification of these diverse options. To each particular post-condensation transformation a code is given ... 

In this chapter we focus only on post-condensation transformations that follow classical Ugi or Passerini reactions (including the intramolecular ones) and that lead to heterocycles. Therefore, we will not report the many examples of postcondensation reactions applied to non-conventional Ugi or Passerini scaffolds generated by variants of these venerable reactions. Also post-IMCR transformations that involve the inclusion, in the final cyclic system, of sub-structures not initially present in the starting component will be overlooked. 

Post-Condensation Transformations Involving the Acid-Derived Carbonyl... 

The amide derived from the carboxylic acid in Ugi adducts is in most cases tertiary, and therefore it cannot serve as nucleophilic partner in post-condensation transformations, unless a post-Ugi rearrangement converts it into a free amine [52, 54]. An exception is represented by Ugi adducts derived from ammonia, which give rise to two secondary amides, each of them potentially involved, as nucleophile, in nucleophilic substitution processes. Four competitive pathways are in principle possible (N- or 0-alkylations of the two amides), and the reaction is mainly driven by the stability of the formed rings. In the example shown in Fig. 12, 0-alkylation of the carboxylic-derived amide is favoured as it generates a 5-membered ring (oxazoline 62), while the alternative cyclization modes would have formed 3- or 4-membered rings [49]. When R C02H is phthalic acid, however, acylaziridines are formed instead via Walkylation [49]. In both cases, the intramolecular 8 2 reactions takes place directly under the Ugi conditions. 

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

The combination of pept(o)id-introducing MCRs with subsequent and efficient post-condensation transformations, especially ring-closing protocols, is an efficient concept to produce (cyclic) pseudopeptides. The most important versions make use of protected or convertible functional building blocks to allow later condensation, specially cycfization. Most relevant are Ugi-deprotection-cyclization (UDC), Ugi-activation-cyclization (UAC), and the Ugi-deprotection-activation-cycfization (UDAC), which take advantage of the diverse functionalities incorporated into the previously synthesized MCR-adduct as bi- or polyfunctional building block (Fig. 3) [15, 16]. 

The use of resin-bound convertible isocyanides such as the universal Rink isocyanide-resin [18], the safety-catch linker isocyanide-resin [8b, 19] the cyclo-hexenyl isocyanide-resin [8b], and the carbonate convertible isocyanide-resin [20] has found interesting applications in solid-phase Ugi-4CR and post-condensation transformations [21] (Scheme 2.9). 

Scheme 3 Reissert reactions and post-condensation transformations...

Efficient assembly of iminodicarboxamides by a truly four-component reaction. For the Ugi-4CR post-condensation reaction of this transformation, see (b) M. K. Sinha, K. Khoury, E. Herdtweckb, A. Ddmling, Org. Biomol. Ghent. 2013,11,4792-4796. Various cyclization scaffolds by a truly Ugi4-CR. 

High-temperature thermal treatment of hazardous waste offers a reduction in volume as well as a conversion of toxic organic constituents to harmless or less harmful forms [1]. However, hazardous metals can neither be generated nor destroyed in the waste thermal process, but they can be transformed both chemically and physically [2]. There is therefore a potential for hazardous metals to emit if they vaporize at high temperatures [3]. Many matals and their salts will form vapors at temperatures reached by flame and post-flame zones of a combustion chamber. When the vapors cool, they condense to form submicron particles, which tend to be relatively difficult to capture in air polution control equipments. These emissions of submicron metallic particles have been identified as one of the greatest health risks associated with waste incineration [4]. 

---