A process for the manufacture of the compound of formula VI below, wherein both of R1 and R2 are either methyl or ethyl, said process comprising the following step, a) reacting a compound of formula I, with an anhydrous acid and a methyl alcohol or ethyl alcohol to obtain a compound of formula II, wherein R2 is methyl when methyl alcohol is applied in the reaction, and ethyl when ethyl alcohol is applied in the reaction, b) reacting the compound of formula II with a base and methyl- or ethyl glyoxylate to obtain a compound of formula III, c) converting the compound of formula III to a compound of formula IV by hydrogenation d) reacting the compound of formula IV with methyl- or ethyl chloroformate to obtain the compound of formula V wherein R1 is methyl when methyl chloroformate is applied in the reaction, or ethyl when ethyl chloroformate is applied in the reaction, and wherein R2 is methyl when methyl alcohol is applied in the reaction, or ethyl when ethyl alcohol is applied in the reaction, and wherein R3 is methyl when methyl glyoxylate is applied in the reaction, or ethyl when ethyl glyoxylate is applied in the reaction, e) reacting the compound of formula V with sodium methoxide in methanol or sodium ethoxide in ethanol to obtain the compound of formula VI, wherein R1, R2 and R3 of the compound of formula V independently represents methyl or ethyl, and R1 and R2 of the compound of formula VI are both methyl when sodium methoxide in methanol is applied in the reaction, or R1 and R2 of the compound of formula VI are both ethyl when sodium ethoxide in ethanol is applied in the reaction. a) reacting a compound of formula I, with an anhydrous acid and a methyl alcohol or ethyl alcohol to obtain a compound of formula II, wherein R2 is methyl when methyl alcohol is applied in the reaction, and ethyl when ethyl alcohol is applied in the reaction, b) reacting the compound of formula II with a base and methyl- or ethyl glyoxylate to obtain a compound of formula III, c) converting the compound of formula III to a compound of formula IV by hydrogenation d) reacting the compound of formula IV with methyl- or ethyl chloroformate to obtain the compound of formula V wherein R1 is methyl when methyl chloroformate is applied in the reaction, or ethyl when ethyl chloroformate is applied in the reaction, and wherein R2 is methyl when methyl alcohol is applied in the reaction, or ethyl when ethyl alcohol is applied in the reaction, and wherein R3 is methyl when methyl glyoxylate is applied in the reaction, or ethyl when ethyl glyoxylate is applied in the reaction, wherein R1 is methyl when methyl chloroformate is applied in the reaction, or ethyl when ethyl chloroformate is applied in the reaction, and wherein R2 is methyl when methyl alcohol is applied in the reaction, or ethyl when ethyl alcohol is applied in the reaction, and wherein R3 is methyl when methyl glyoxylate is applied in the reaction, or ethyl when ethyl glyoxylate is applied in the reaction, e) reacting the compound of formula V wherein R1, R2 and R3 of the compound of formula V independently represents methyl or ethyl, and R1 and R2 of the compound of formula VI are both methyl when sodium methoxide in methanol is applied in the reaction, or R1 and R2 of the compound of formula VI are both ethyl when sodium ethoxide in ethanol is applied in the reaction.