A method comprising: (i) separating an S-enantiomer from an R-enantiomer wherein said S-enantiomer and said R-enantiomer are provided in a racemic mixture and wherein each of said S-enantiomer and said R-enantiomer is a compound of Formula I: wherein: R 1 is C 1-4 alkylene-LG wherein LG is a leaving group; R 2 is hydrogen, hydroxyl, halo, cyano, C 1-3 alkyl, C 1-3 alkoxy, C 1-3 fluoroalkyl, or C 1-3 fluoroalkoxy; R 3 and R 4 are independently C 1-3 alkyl, C 7-10 aralkyl, or R 3 and R 4, together with the nitrogen to which they are attached, form a nitrogen-containing C 4-6 aliphatic ring optionally comprising 1 further heteroatom selected from nitrogen, oxygen and sulfur; Y 1 is O, S, SO, SO 2 or CH 2; Y 2 is CH 2, CH 2 -CH 2, CH(CH 3)-CH 2, CH 2 -CH 2 -CH 2 or CH-(CH-CH 2 -CH-CH 2)-CH to provide the following compounds of Formula I: CH 2 CH 2 -CH 2 CH(CH 3)-CH 2 CH 2 -CH 2 -CH 2 CH-(CH-CH 2 -CH-CH 2)-CH said variable bond is above the plane for the S-enantiomer and below the plane for the R-enantiomer as illustrated respectively in Formulas I-S and I-R: (ii) dissolving one of said separated enantiomers in a suitable organic solvent; wherein said method is characterised in that purification of said dissolved enantiomer comprises: (iii) precipitating said dissolved enantiomer out of the solution obtained in step (ii) wherein said precipitating step comprises addition of water; and, (iv) isolating the precipitate obtained in step (iii). (i) separating an S-enantiomer from an R-enantiomer wherein said S-enantiomer and said R-enantiomer are provided in a racemic mixture and wherein each of said S-enantiomer and said R-enantiomer is a compound of Formula I: wherein: R 1 is C 1-4 alkylene-LG wherein LG is a leaving group; R 2 is hydrogen, hydroxyl, halo, cyano, C 1-3 alkyl, C 1-3 alkoxy, C 1-3 fluoroalkyl, or C 1-3 fluoroalkoxy; R 3 and R 4 are independently C 1-3 alkyl, C 7-10 aralkyl, or R 3 and R 4, together with the nitrogen to which they are attached, form a nitrogen-containing C 4-6 aliphatic ring optionally comprising 1 further heteroatom selected from nitrogen, oxygen and sulfur; Y 1 is O, S, SO, SO 2 or CH 2; Y 2 is CH 2, CH 2 -CH 2, CH(CH 3)-CH 2, CH 2 -CH 2 -CH 2 or CH-(CH-CH 2 -CH-CH 2)-CH to provide the following compounds of Formula I: CH 2 CH 2 -CH 2 CH(CH 3)-CH 2 CH 2 -CH 2 -CH 2 CH-(CH-CH 2 -CH-CH 2)-CH said variable bond is above the plane for the S-enantiomer and below the plane for the R-enantiomer as illustrated respectively in Formulas I-S and I-R: R 1 is C 1-4 alkylene-LG wherein LG is a leaving group; R 2 is hydrogen, hydroxyl, halo, cyano, C 1-3 alkyl, C 1-3 alkoxy, C 1-3 fluoroalkyl, or C 1-3 fluoroalkoxy; R 3 and R 4 are independently C 1-3 alkyl, C 7-10 aralkyl, or R 3 and R 4, together with the nitrogen to which they are attached, form a nitrogen-containing C 4-6 aliphatic ring optionally comprising 1 further heteroatom selected from nitrogen, oxygen and sulfur; Y 1 is O, S, SO, SO 2 or CH 2; Y 2 is CH 2, CH 2 -CH 2, CH(CH 3)-CH 2, CH 2 -CH 2 -CH 2 or CH-(CH-CH 2 -CH-CH 2)-CH to provide the following compounds of Formula I: CH 2 CH 2 -CH 2 CH(CH 3)-CH 2 CH 2 -CH 2 -CH 2 CH-(CH-CH 2 -CH-CH 2)-CH said variable bond is above the plane for the S-enantiomer and below the plane for the R-enantiomer as illustrated respectively in Formulas I-S and I-R: (ii) dissolving one of said separated enantiomers in a suitable organic solvent; wherein said method is characterised in that purification of said dissolved enantiomer comprises: (iii) precipitating said dissolved enantiomer out of the solution obtained in step (ii) wherein said precipitating step comprises addition of water; and, (iv) isolating the precipitate obtained in step (iii).