A process for preparing a compound of formula, or a salt thereof, wherein X is H or an alcohol protecting group selected from formyl, acetyl, chloroacetyl, trifluoroacetyl, benzoyl, allyl, propargyl, benzyl, p-methoxybenzyl, p-bromobenzyl or 2,6-dichlorobenzyl; and a silylether; and R is H, a C 1 -C 6 alkyl group optionally substituted by one or more substituents, which are equal or different, selected from hydroxyl or halogen, or an amino protecting group selected from benzyl, benzyloxycarbonyl, tert-butyloxycarbonyl, 9-fluorenylmethyl, 2-chloro-3-indenylmethyl, 2,2,2-trichloroethyl, 2-chloroethyl, 2-adamantyl, 8-quinolyl, 4-methylsulfinylbenzyl, 2-methylthioethyl, 4-azidobenzyl, and N'-phenylaminothiocarbonyl; comprising double reductive amination reaction of a dicarbonyl compound of formula, wherein X is as defined above, with an amine of formula ( VIII), or a salt thereof, NH 2 -R ( VIII) wherein R is as defined above, in the presence of a reducing agent selected from a borane complex with an amine of formula or a borane complex with a pyridine of formula wherein each of substituents Y, being the same or different, is H, or a C 1 -C 6 alkyl optionally substituted by one or more substituents, which are equal or different, selected from hydroxyl or halogen, or an aryl group optionally substituted by one to three substituents, which may be the same or different, selected independently from a straight or branched C 1 -C 4 alkyl group, which in turn is optionally substituted by one to three halogen atoms; a hydroxy group; a C 1 -C 4 alkoxy group; a halogen atom; a cyano group; and a nitro group, or two of Y, taken together with the nitrogen atom to which they are linked, form a C 5 -C 6 cycloalkyl ring optionally containing an oxygen atom or an =NRa group, wherein Ra is hydrogen, an amino protecting group as defined above, or a C 1 -C 4 alkyl group; and each of substituents W, being the same or different, is H, a C 1 -C 6 alkyl group optionally substituted by one or more substituents, which are equal or different, preferably by one to three substituents, selected from hydroxyl or halogen, or a halogen atom; and of a solvent, and, if the case, converting a compound of formula into another compound of formula or a salt thereof, or converting a salt of a compound of formula into its free base, or converting a compound of formula into salt thereof. is H, or a C 1 -C 6 alkyl optionally substituted by one or more substituents, which are equal or different, selected from hydroxyl or halogen, or an aryl group optionally substituted by one to three substituents, which may be the same or different, selected independently from a straight or branched C 1 -C 4 alkyl group, which in turn is optionally substituted by one to three halogen atoms; a hydroxy group; a C 1 -C 4 alkoxy group; a halogen atom; a cyano group; and a nitro group, or two of Y, taken together with the nitrogen atom to which they are linked, form a C 5 -C 6 cycloalkyl ring optionally containing an oxygen atom or an =NRa group, wherein Ra is hydrogen, an amino protecting group as defined above, or a C 1 -C 4 alkyl group; and each of substituents W, being the same or different, is H, a C 1 -C 6 alkyl group optionally substituted by one or more substituents, which are equal or different, preferably by one to three substituents, selected from hydroxyl or halogen, or a halogen atom;