A process for preparing a compound of formula (I) where R.sup.4 and R.sup.5 are as defined in the specification; and R.sup.6 is hydrogen or a protecting group, which process comprises cyclisation of a compound of formula (II) where R.sup.4, R.sup.5 and R.sup.6 are as defined in relation to formula (I), and R.sup.7 is nitrogen-protecting group, and removing the group R.sup.7--, and thereafter if desired or necessary, removing any protecting group R.sup.6 to obtain the corresponding carboxylic acid. Novel intermediates and the use of the products in the preparation of pharmaceutical compounds is also described and claimed. ##STR00001##