A method of preparing a compound of Formula II-a'-1 or Formula II-b'-1, wherein R I represents a linear, branched and/or cyclic alkyl or alkenyl group having 2 to 10 carbon atoms; R" represents methyl; R III represents hydrogen or a linear, branched and/or cyclic alkyl or alkenyl group having 1 to 10 carbon atoms or a linear, branched and/or cyclic alkoxy group having 1 to 10 carbon atoms; R IV represents hydrogen or a linear, branched and/or cyclic alkyl group having 1 to 10 carbon atoms or an optionally substituted aryl or alkylaryl group having 6 to 40 carbon atom or acetyl or silyl or a protective group; and R V represents hydrogen or a methyl group; and the stereochemistry at the position marked with * is S in case the carbon atom in R I next to the carbon atom marked with * is a carbon atom with a higher priority in the Cahn Ingold Prelog system as the thebaine-derived ring system, or the stereochemistry at the position marked with * is R in case the carbon atom next to the carbon atom marked with * in R I is a carbon atom with a lower priority in the Cahn Ingold Prelog system as the thebaine-derived ring system; or wherein R I represents a hydrogen or a methyl group; R II represents a linear, branched and/or cyclic alkyl or alkenyl group having 2 to 10 carbon atoms; R III, R IV and R V have the same meanings as above; and the stereochemistry at the position marked with * is S in case the carbon atom in R" next to the carbon atom marked with * is a carbon atom with a lower priority in the Cahn Ingold Prelog system as the thebaine-derived ring system, or the stereochemistry at the position marked with * is R in case the carbon atom next to the carbon atom marked with * in R" is a carbon atom with a higher priority in the Cahn Ingold Prelog system as the thebaine-derived ring system, involving: reacting a compound of Formula I-a or Formula I-b wherein R I, R III, R IV and R V have the same meanings as above, with a reagent chosen from R II MgX and R II Li, wherein R II has the same meaning as above and X is chosen from a halogen or pseudohalogen ion, in the presence of a compound of formula LnY3 · nLiY, wherein Ln is chosen from lanthanide ions and Y is chosen from halogenide or hydroxide ions, and n is 0, 1, 2 or 3, preferably n = 0, 2, wherein the reaction is carried out in an ether as solvent. R" represents methyl; R III represents hydrogen or a linear, branched and/or cyclic alkyl or alkenyl group having 1 to 10 carbon atoms or a linear, branched and/or cyclic alkoxy group having 1 to 10 carbon atoms; R IV represents hydrogen or a linear, branched and/or cyclic alkyl group having 1 to 10 carbon atoms or an optionally substituted aryl or alkylaryl group having 6 to 40 carbon atom or acetyl or silyl or a protective group; and R V represents hydrogen or a methyl group; and the stereochemistry at the position marked with * is S in case the carbon atom in R I next to the carbon atom marked with * is a carbon atom with a higher priority in the Cahn Ingold Prelog system as the thebaine-derived ring system, or the stereochemistry at the position marked with * is R in case the carbon atom next to the carbon atom marked with * in R I is a carbon atom with a lower priority in the Cahn Ingold Prelog system as the thebaine-derived ring system; or wherein R I represents a hydrogen or a methyl group; R II represents a linear, branched and/or cyclic alkyl or alkenyl group having 2 to 10 carbon atoms; R III, R IV and R V have the same meanings as above; and the stereochemistry at the position marked with * is S in case the carbon atom in R" next to the carbon atom marked with * is a carbon atom with a lower priority in the Cahn Ingold Prelog system as the thebaine-derived ring system, or the stereochemistry at the position marked with * is R in case the carbon atom next to the carbon atom marked with * in R" is a carbon atom with a higher priority in the Cahn Ingold Prelog system as the thebaine-derived ring system, involving: reacting a compound of Formula I-a or Formula I-b wherein R I, R III, R IV and R V have the same meanings as above, with a reagent chosen from R II MgX and R II Li, wherein R II has the same meaning as above and X is chosen from a halogen or pseudohalogen ion, in the presence of a compound of formula LnY3 · nLiY, wherein Ln is chosen from lanthanide ions and Y is chosen from halogenide or hydroxide ions, and n is 0, 1, 2 or 3, preferably n = 0, 2, wherein the reaction is carried out in an ether as solvent.