A method for preparing a compound of formula wherein is a single or a double bond, wherein formula comprises the compound of the formula (Ia) the compound of the formula (Ib) the compound of the formula (Ic) and stereoisomers thereof; whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; comprising at least the steps of: a) providing a compound of formula (IIa), whereby m is 0; R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3; R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; b) subjecting the compound of the formula (IIa) to a compound selected from peroxyacids and peroxides to obtain a compound of formula (IIb), c) reacting the compound of the formula (IIb) with a compound of formula (IIc), R 1 CH 2 MgX formula (IIc), wherein R 1 is selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; X is Br, Cl or I, to obtain a compound of formula (IId), whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; d) subjecting the compound of formula (IId) to a dehydration reaction to obtain the compound of the formula. the compound of the formula (Ia) the compound of the formula (Ib) the compound of the formula (Ic) and stereoisomers thereof; whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; comprising at least the steps of: a) providing a compound of formula (IIa), whereby m is 0; R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3; R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; b) subjecting the compound of the formula (IIa) to a compound selected from peroxyacids and peroxides to obtain a compound of formula (IIb), c) reacting the compound of the formula (IIb) with a compound of formula (IIc), R 1 CH 2 MgX formula (IIc), wherein R 1 is selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; X is Br, Cl or I, to obtain a compound of formula (IId), whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; d) subjecting the compound of formula (IId) to a dehydration reaction to obtain the compound of the formula. a) providing a compound of formula (IIa), whereby m is 0; R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3; R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 0; R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3; R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; b) subjecting the compound of the formula (IIa) to a compound selected from peroxyacids and peroxides to obtain a compound of formula (IIb), c) reacting the compound of the formula (IIb) with a compound of formula (IIc), R 1 CH 2 MgX formula (IIc), wherein R 1 is selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; X is Br, Cl or I, to obtain a compound of formula (IId), whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 0; R 1, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; whereby m is 1, 2 or 3 and R 1, R 2, R 3, R 4 and R 5, identical or different, are selected from the group consisting of H, C 1 -C 4 -alkyl and C 3 -C 4 -cycloalkyl; d) subjecting the compound of formula (IId) to a dehydration reaction to obtain the compound of the formula.