A method for stereospecifically preparing a substantially pure diastereomeric compound of formula (I): or a salt thereof, wherein R 1 is -OH, -OC(O)-R 1a or, -OC(O)R 1b COOH and R 4 is -OH, -OC(O)-R 4a or, - OC(O)R 4b COOH, wherein R 1a and R 4a are independently hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl or C 3 -C 6 cycloalkyl, and R 1b and R 4b are independently -C 1 -C 6 alkyl- or -C 2 -C 6 alkenyl-; R 2 and R 5 are -C=CH; R 6 and R 7 are independently C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl or C 3 -C 6 cycloalkyl; R 8 and R 9 are independently hydrogen, -OH, -NH 2 , -NO 3 , halogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 6 cycloalkyl, C 6 -C 14 aryl, 5-6-membered heteroaryl or 3-6 membered heterocyclyl, comprising: (a) reacting a di-ketone compound of formula (II) wherein R 6 , R 7 , R 8 and R 9 are defined as for formula (I), with a silylacetylene of formula (III) wherein R a , R b and R c are independently selected from the group consisting of hydrogen, -OH, C 1 -C 20 alkyl optionally substituted by -OH, halogen or C 1 -C 6 alkyl, C 1 -C 6 alkoxy optionally substituted by -OH, halogen or C 1 -C 6 alkyl, C 3 -C 6 cycloalkyl, C 6 -C 14 aryl, 5-6-membered heteroaryl or 3-6 membered heterocyclyl, in the presence of an organometallic reagent R 10 -M, wherein M is Li, Na or K, and R 10 is C 1 -C 20 alkyl, optionally substituted by -OH, halogen or C 1 -C 6 alkyl. R 1 is -OH, -OC(O)-R 1a or, -OC(O)R 1b COOH and R 4 is -OH, -OC(O)-R 4a or, - OC(O)R 4b COOH, wherein R 1a and R 4a are independently hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl or C 3 -C 6 cycloalkyl, and R 1b and R 4b are independently -C 1 -C 6 alkyl- or -C 2 -C 6 alkenyl-; R 2 and R 5 are -C=CH; R 6 and R 7 are independently C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl or C 3 -C 6 cycloalkyl; R 8 and R 9 are independently hydrogen, -OH, -NH 2 , -NO 3 , halogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 6 cycloalkyl, C 6 -C 14 aryl, 5-6-membered heteroaryl or 3-6 membered heterocyclyl, (a) reacting a di-ketone compound of formula (II) wherein R 6 , R 7 , R 8 and R 9 are defined as for formula (I), with a silylacetylene of formula (III) wherein R a , R b and R c are independently selected from the group consisting of hydrogen, -OH, C 1 -C 20 alkyl optionally substituted by -OH, halogen or C 1 -C 6 alkyl, C 1 -C 6 alkoxy optionally substituted by -OH, halogen or C 1 -C 6 alkyl, C 3 -C 6 cycloalkyl, C 6 -C 14 aryl, 5-6-membered heteroaryl or 3-6 membered heterocyclyl, in the presence of an organometallic reagent R 10 -M, wherein M is Li, Na or K, and R 10 is C 1 -C 20 alkyl, optionally substituted by -OH, halogen or C 1 -C 6 alkyl. wherein R 6 , R 7 , R 8 and R 9 are defined as for formula (I), with a silylacetylene of formula (III) wherein R a , R b and R c are independently selected from the group consisting of hydrogen, -OH, C 1 -C 20 alkyl optionally substituted by -OH, halogen or C 1 -C 6 alkyl, C 1 -C 6 alkoxy optionally substituted by -OH, halogen or C 1 -C 6 alkyl, C 3 -C 6 cycloalkyl, C 6 -C 14 aryl, 5-6-membered heteroaryl or 3-6 membered heterocyclyl, in the presence of an organometallic reagent R 10 -M, wherein M is Li, Na or K, and R 10 is C 1 -C 20 alkyl, optionally substituted by -OH, halogen or C 1 -C 6 alkyl. <EOS>