A method for the preparation of a compound of formula (VI): in which R.sub.2 is an optionally substituted C1 C6 alkyl; R.sub.4 is H, OH, an optionally substituted C1 C6 alkyl or an optionally substituted C1 C6 alkoxy; R.sub.5 is an optionally substituted aryl, an optionally substituted aralkyl, or an optionally substituted alkyl; and R.sub.6 is H or an optionally substituted C1 C6 alkyl; the method including the step of subjecting a ketone of formula (V) in which R.sub.2, R.sub.4 and R.sub.5 are as defined above, to reductive amination with an amine of formula R.sub.3NH.sub.2 in which R.sub.3 is an optionally substituted alkyl, and a reductant to form the compound of formula (VI), wherein the reaction is conducted in the presence of a supercritical fluid or a liquefied gas.