Polymer supported guanidine base

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. 

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