Compounds of the formula (1), or their salts, ##STR1## are prepared as follows: (A) A COMPOUND OF THE FORMULA (II) ##STR2## is reduced; or (B) A COMPOUND OF THE FORMULA (III) ##STR3## is reacted, e.g. after protecting the phenolic hydroxy group attached to position 3 of the ring, with a compound of the formula (IV), ##STR4## or (C) GROUP R.sup.1 of a compound of the formula (V) ##STR5## is removed in an acidic medium, AND, IF DESIRED, THE COMPOUND OF THE FORMULA (I) is converted into salt, or a salt of a compound of the formula (I) is converted into the free acid or another salt thereof.The compounds of the formula (I) and their salts are valuable artificial sweetening agents.In the above formulae"alkyl" is a C.sub.1-4 alkyl group,R is hydrogen or a protecting group capable of splitting off upon hydrogenolysis, preferably benzyl or benzyloxycarbonyl,M+ is a proton or another cation,X is a group of the general formula O.sup.- M.sup.+,Y is halogen, orX and Y together represent an oxygen atom, andR.sup.1 is a protecting group capable of splitting off in acidic media, such as 2-tetrahydropyranyl or 1-ethoxyethyl.