Method of preparing compound of formula comprising the steps a) providing compound of formula b) adding at least one silver or gold salt or complex and at least one acid which is selected from the group consisting of BX 3, the adducts of BX 3 with alcohols or ethers, HBX 4, H[PX 6], H[SbF 6], HClO 4, H 2 SO 4, trifluoroacetic acid, sulfonic acids and sulfonimides of formula (HAN-1); and/or at least one silver or gold metal salt or complex having an anion which is selected from the group consisting of [BX 4] -, [PX 6] -, [SbF 6] -, [ClO 4] -, CF 3 COO -, sulfonates, tetra(3,5-bis(trifluoromethyl)phenyl)borate (BAr F -), tetraphenylborate, and anions of formula (AN-1); wherein X represents a halogen atom; and Y 1 represents a phenyl or a C 1-8 -alkyl group which preferably is substituted by at least one halogen atom to compound of formula forming a mixture; c) submitting said mixture to a temperature in the range between -15°C and 40°C, preferably between 15°C and 40°C; wherein R 1 and R 3 are independently from each other hydrogen or methyl groups; R 2 represents either hydrogen or a methyl group or a group OR 2' where R 2' represents hydrogen or a phenol protection group; R 5 represents either completely saturated C 1-25 -alkyl group or a C 1-25 -alkyl group comprising at least one carbon-carbon double bond; R 6 represents either H or a C 1-25 -alkyl group, particularly a methyl group; with the proviso that R 5 is different from R 6; and wherein each * represents a chiral centre and this chiral centre has the same configuration in formula and formula and wherein the compound of formula is obtained by the reaction a) providing compound of formula b) adding at least one silver or gold salt or complex and at least one acid which is selected from the group consisting of BX 3, the adducts of BX 3 with alcohols or ethers, HBX 4, H[PX 6], H[SbF 6], HClO 4, H 2 SO 4, trifluoroacetic acid, sulfonic acids and sulfonimides of formula (HAN-1); and/or at least one silver or gold metal salt or complex having an anion which is selected from the group consisting of [BX 4] -, [PX 6] -, [SbF 6] -, [ClO 4] -, CF 3 COO -, sulfonates, tetra(3,5-bis(trifluoromethyl)phenyl)borate (BAr F -), tetraphenylborate, and anions of formula (AN-1); wherein X represents a halogen atom; and Y 1 represents a phenyl or a C 1-8 -alkyl group which preferably is substituted by at least one halogen atom to compound of formula forming a mixture; c) submitting said mixture to a temperature in the range between -15°C and 40°C, preferably between 15°C and 40°C; wherein R 1 and R 3 are independently from each other hydrogen or methyl groups; R 2 represents either hydrogen or a methyl group or a group OR 2' where R 2' represents hydrogen or a phenol protection group; R 5 represents either completely saturated C 1-25 -alkyl group or a C 1-25 -alkyl group comprising at least one carbon-carbon double bond; R 6 represents either H or a C 1-25 -alkyl group, particularly a methyl group; with the proviso that R 5 is different from R 6; and wherein each * represents a chiral centre and this chiral centre has the same configuration in formula and formula and wherein the compound of formula is obtained by the reaction