A process for the preparation of a compound of formula in the form of any one of its stereoisomers or mixtures thereof, and wherein R represents a C 2 -C 10 group of formula COR a wherein R a is an alkyl or alkenyl group optionally comprising one or two ether functional groups or is a phenyl or benzyl group optionally substituted by one to three alkyl, alkoxyl, carboxyl, acyl, amino or nitro groups or halogen atoms; by reacting together a compound of formula in the form of any one of its stereoisomers or mixtures thereof, and wherein R 1 represents a hydrogen atom or a Si(R 2) 3 or B(OR 2') 2 group, R 2 representing a C 1-4 alkyl or alkoxyl group and R 2' representing, taken separately, a C 1-4 alkyl group or a or a phenyl group optionally substituted by one to three C 1-3 alkyl or alkoxy groups, or said R 2', taken together, representing a C 2-6 alkanediyl group or a diphenyl or dinaphthyl group optionally substituted by one to three C 1-3 alkyl or alkoxy groups; with a compound of formula in the form of any one of its stereoisomers or mixtures thereof, and wherein R has the meaning defined in formula and L represents a halogen atom or an OR group; in the presence of: 1) at least one acid selected amongst I. a Lewis acid selected from the group consisting of: i) a metal salt of a an element of the group 2, 3, 4, 13 or of a 3d element or of tin; ii) an alkyl aluminium chloride of formula Al(R 4) a Cl 3-4, a representing 1 or 2 and R 4 representing C 1-10 alkyl or alkoxide group; and iii) a boron derivative of formula BZ 3, wherein Z represents a fluoride or a phenyl group optionally substituted, and any one of its adduct with a C 2 -C 10 ether or a C 1 -C 8 carboxylic acid; and/or II. a protic acid having a pk a comprised between 2.5 and -20; and 2) optionally an additive selected amongst the group consisting of alkaline-earth hydroxide or oxide and of the compounds of formula R b COCl, ClSi(R b) 3, R b COOR c or (R b COO) 2 R d, R b representing a C 1-12 alkyl group or a phenyl group optionally substituted by one or two C 1-4 alkyl or alkoxyl group, and R c representing a alkaline metal cation or a R b CO acyl group, and R d representing a alkaline-earth metal cation. 1) at least one acid selected amongst I. a Lewis acid selected from the group consisting of: i) a metal salt of a an element of the group 2, 3, 4, 13 or of a 3d element or of tin; ii) an alkyl aluminium chloride of formula Al(R 4) a Cl 3-4, a representing 1 or 2 and R 4 representing C 1-10 alkyl or alkoxide group; and iii) a boron derivative of formula BZ 3, wherein Z represents a fluoride or a phenyl group optionally substituted, and any one of its adduct with a C 2 -C 10 ether or a C 1 -C 8 carboxylic acid; and/or II. a protic acid having a pk a comprised between 2.5 and -20; and I. a Lewis acid selected from the group consisting of: i) a metal salt of a an element of the group 2, 3, 4, 13 or of a 3d element or of tin; ii) an alkyl aluminium chloride of formula Al(R 4) a Cl 3-4, a representing 1 or 2 and R 4 representing C 1-10 alkyl or alkoxide group; and iii) a boron derivative of formula BZ 3, wherein Z represents a fluoride or a phenyl group optionally substituted, and any one of its adduct with a C 2 -C 10 ether or a C 1 -C 8 carboxylic acid; and/or i) a metal salt of a an element of the group 2, 3, 4, 13 or of a 3d element or of tin; ii) an alkyl aluminium chloride of formula Al(R 4) a Cl 3-4, a representing 1 or 2 and R 4 representing C 1-10 alkyl or alkoxide group; and iii) a boron derivative of formula BZ 3, wherein Z represents a fluoride or a phenyl group optionally substituted, and any one of its adduct with a C 2 -C 10 ether or a C 1 -C 8 carboxylic acid; II. a protic acid having a pk a comprised between 2.5 and -20; and 2) optionally an additive selected amongst the group consisting of alkaline-earth hydroxide or oxide and of the compounds of formula R b COCl, ClSi(R b) 3, R b COOR c or (R b COO) 2 R d, R b representing a C 1-12 alkyl group or a phenyl group optionally substituted by one or two C 1-4 alkyl or alkoxyl group, and R c representing a alkaline metal cation or a R b CO acyl group, and R d representing a alkaline-earth metal cation.