Proline isomerisation

Figure 1. Schematic representation of the free-energy landscape of a polypeptide displaying different kinds of substates and energy barriers, such as cis-trans proline isomerisation, side chain dihedral transition, and to P backbone transition.

Calabrese, M., Stancher, B., and Del Signore, A. Proline isomerisation in traditional balsamic vinegar from Modena as a function of ageing period, Ital. Food Beverage Techno ., 27,11, 2002. 

Isomerisation studies indicate that the iminium ion formed initially prior to isomerisation as the amount of ( ) product increased over time. Only the (Z) isomer was observed after 10 min. Notably, nonenolisable aldehydes i.e. acrylaldehyde and cinnamaldehyde) were unreactive, indicative of an enamine-type mechanism as opposed to a typical aza-MBH route. Several procedures for the asymmetric, proline-catalysed MBH reaction between methyl vinyl ketone (MVK) and aromatic aldehydes have been developed to date. ... 

On the other hand, a chiral ruthenium catalyst, prepared from a chiral P/N ligand derived from L-proline, was applied in 2005 to the asymmetric isomerisation of racemic allylic alcohols via DKR. This new type of reaction was applicable to the asymmetric synthesis of muscone, as shown in Scheme 2.45. 

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