Method for the purification of 23-E isomers of 23-imino derivatives of LL-F28249 compounds

A method for increasing the purity of the 23-E isomer of a 23-imino derivative of a LL-F28249 a compound present in a first mixture which comprises reacting the first mixture comprising the 23-E isomer of the 23-imino derivative of the LL-F28249 a compound having the structural formula I ##STR8## wherein R is methyl, ethyl or isopropyl;

• R.sub.1 is hydrogen or methyl;
• and
• R.sub.2 is C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.4 alkoxymethyl, benzyl, allyl, propargyl, phenyl, CH.sub.2 CO2-(C.sub.1 -C.sub.4 alkyl), N-(C.sub.1 -C.sub.6 alkyl)carbamoyl, N-(allyl)carbamoyl, N-(propargyl)carbamoyl, N-(phenyl)carbamoyl, N-(chlorophenyl)carbamoyl, N-(dichlorophenyl)carbamoyl, N-(benzyl)carbamoyl, C.sub.1 -C.sub.6 alkanoyl, chloroacetyl, methoxyacetyl, phenylacetyl optionally substituted on the phenyl ring with one or two halogen atoms, C.sub.1 -C.sub.4 alkyl groups, C.sub.1 -C.sub.4 alkoxy groups or nitro groups, phenoxyacetyl optionally substituted on the phenyl ring with one or two halogen atoms, C.sub.1 -C.sub.4 alkyl groups, C.sub.1 -C.sub.4 alkoxy groups or nitro groups, or benzoyl optionally substituted with one or two halogen atoms, C.sub.1 -C.sub.4 alkyl groups, C.sub.1 -C.sub.4 alkoxy groups or nitro groups, with a protecting agent and a base to form a second mixture comprising a solid compound having the structural formula II ##STR9## wherein R, R.sub.1 and R.sub.2 are as described above for formula I, and R.sub.3 is a protecting group, mixing the second mixture with methanol, separating the formula II compound and deprotecting the formula II compound.