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Transition structures for the aldolization reaction of the histidine-enamine of isobutyraldehyde and isovaleraldehyde, in which a hydrogen bond is donated by the imidazolium ring (6a, 6b) or the carboxylic acid (6c, 6d). 6a and 6d lead to the major (S)-enantiomer, while 6b and 6c give rise to the (R)-isomer.
