| http://www.w3.org/ns/prov#value | - A particularly suitable method for preparing a compound of formula (I) wherein R4 and/or R5 is other than hydrogen, is reductive alkylation of the corresponding compound wherein R4 and/or R5 represents hydrogen, with an appropriate aldehyde or a ketone (e.g. benzaldehyde or acetone) in the presence of a suitable reducing agent.
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