Novel compounds of general formula (I) (where the two R<1> groups are identical lower alkyl groups or together form a lower alkylene group; R<2> and R<3> are both different and are selected from hydrogen atoms and organic groups; X and X', which may be the same or different, are selected from O, S and NR, where R represents an organic group; and the asterisk denotes that the configurations of R<2> and R<3> are such that the compound (I) is in substantially enantiomerically pure 4R- or 4S-form). The compounds are useful chiral auxiliaries to which a wide range of e.g. acyl groups containing prochiral centres may be readily and reversibly coupled to the 3-position amino group.
