<p>A compound of the formula [I] and pharmaceutically acceptable salts thereof; and a synthetic intermediate represented by the fomula [II] and pharmaceutically acceptable salts thereof, <CHEM> <CHEM> wherein R 1 is hydrogen or lower alkyl; R 2 is hydrogen or a protective group of thiol; R 3 is hydroxy, a remainder of ester group or a remainder of amido group; A is straight or branched lower alkylene, and the sulfur atom in the side chain may be directly join with -CO- to conform a thiolactone ring. The compounds of this invention have excellent thymulin-like activities and are expected to be useful for treatment of various immune disorders such as immunodeficiency and autoimmune diseases.</p>
