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33.4.6.1.1.3 Variation 3: By the Action of Protic Acid Catalysis

DOI: 10.1055/sos-SD-033-00516

Sammakia, T.; Abramite, J. A.; Sammons, M. F.Science of Synthesis, (200733409.

The formation of enamines is accelerated by protic acids as first described for the formation of the enamine 17 from progesterone (16) (Scheme 8).[‌22‌] It is observed that no water is collected after heating a solution of pyrrolidine with progesterone (16) in benzene to reflux for 1 hour. However, upon addition of a catalytic amount of 4-toluenesulfonic acid, water begins to form immediately and the enamine 17 is provided in 97% yield. A variety of other acids can be employed including Dowex-50,[‌24‌] montmorillonite K 10,[‌25‌] and acetic acid.[‌18‌] This procedure is especially useful for the formation of hindered enamines.

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