[CH2:1]=O.[CH2:3]([NH:10][CH:11]1[CH2:23][C:22]2[C:14](=[CH:15][C:16]3[CH2:20][CH2:19][O:18][C:17]=3[CH:21]=2)[CH2:13][CH2:12]1)[C:4]1[CH:9]=[CH:8][CH:7]=[CH:6][CH:5]=1>O.C(O)=O.[OH-].[Na+]>[CH2:3]([N:10]([CH:11]1[CH2:23][C:22]2[C:14](=[CH:15][C:16]3[CH2:20][CH2:19][O:18][C:17]=3[CH:21]=2)[CH2:13][CH2:12]1)[CH3:1])[C:4]1[CH:5]=[CH:6][CH:7]=[CH:8][CH:9]=144 ml of 37% strength formaldehyde solution in water are added to a solution of 21 g of the compound of Example 7 in 26 ml of concentrated formic acid maintained at 0° C. After 2 hours under reflux, the cooled reaction medium is diluted with 40% strength sodium hydroxide solution in water and extracted with methylene chloride. The organic phase is washed, dried and then evaporated under vacuum. The residue, purified by passage through a silica column, leads to the production of an oil. The elution solvent used is a mixture of methylene chloride and ethyl acetate (85:15 V/V)
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