Guanidines polymer-supported

Ishikawa and co-workers also reported a class of structurally modified guanidines for promotion of the asymmetric Michael reaction of ierf-butyl-diphenylimino-acetate to ethyl acrylate [124,125]. In addition to a polymer support design (Scheme 69), an optical resolution was developed to achieve chiral 1,2-substituted ethylene-l,2-di-amines, a new chiral framework for guanidine catalysis. The authors discovered that incorporating steric bulk and aryl substituents in the catalyst did improve stereoselec-tivitity, although the reactivity did suffer (Scheme 70, Table 4). 

Shey, J.-Y. Sun, C.-M. Soluble Polymer-Supported Synthesis of N,N-di(Boc)-Protected Guanidines, Synlett 1998, 1423-1425. 

A quaternary ammonium hydroxide ion exchange resin 6 was shown to sequester phenols, hydroxypyrazoles, and other weakly acidic heterocycles.25 The sequestered nucleophiles could also be used as polymer-supported reactants. Similarly, the guanidine-functionalized resin 7 was also shown to be a useful capture agent for weakly acidic nucleophiles, including phenols and cyclic iV-acyl sulfonamides.26... 

Polymer supported carbodiimides are also used in the reaction of N,N -bis(t-butoxy-carbonyl)thiourea with polymer supported triamines to give N,N -bis(t-butoxycarbonyl)-protected guanidines." " ... 

In more recent work a different polystyrene supported dehydrating agent (Fig. 16.4a) was used for synthesis of 1,3,4-oxadiazoles under thermal and microwave conditions [107]. Addition of a homogeneous base, for example guanidine, improved the cyclodehydration reaction and led to completion under thermal and, even more significantly, microwave conditions. This required subsequent purification steps to remove the base for clean and quantitative isolation of the desired products, however. On the other hand, use of a polymer supported phosphazene base (Fig. 16.4b) as a base additive circumvents the need for additional purification of the reaction mixture for product isolation. 

Surprisingly, there have been only few synthetic studies on polymer-supported asymmetric superbase reagents. Recently, Wannaporn and Ishikawa prepared a new chiral guanidine based polymer catalyst and applied it to the asymmetric Michael addition reaction of iminoacetate with methyl vinyl ketone [39] (Scheme 6.7). Although the catalyst shows only moderate levels of reactivity and enantioselectivity, the result demonstrates the possibility of expanding an exciting field of asymmetric synthesis using polymer-supported chiral superbase catalysts. 

Tamura, Y, Fukuda, W., Tomoi, M. and Tokuyama, S. (1994) Polymer-supported bases. XII. Regioselective synthesis of lysophospholipids using polymer-supported bicyclic amidines or guanidines. Synthetic Communications, 24, 2907-2914. 

Wannapom, D. and Ishikawa, T. (2005) Polymer-supported and polymeric chiral guanidines preparation and application to the asymmetric Michael reaction of iminoacetate with methyl vinyl ketone. Molecular Diversity, 9, 321-331. 

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