Synthetic routes toward a B-alkylthiolanthionine derivative, as found in the quinomycin depsipeptide antibiotics, have been studied through a sequence involving as the key intermediates and steps (a) (S) - Z-[(benzyloxycarbonyl)amino]-3,3-dimethoxy-l-propanol (2), prepared from N-benzyloxycarbonyl-L-serine in 3 or 4 steps, (b) N-benzyloxycarbonyl- 0 - tetrahydropyranyl-B,B-(ethylsulfinylethylthio)-alaninol (4a), converted from 2, and (c) attempted Lewis acid catalyzed replacement of alkylsulfinyl function by thiol moiety of cysteine, which gave undesired products. Stability of protecting groups used in this study, which are N,0-isopropylidenyl and tetrahydropyranyl functions in N-protected-L-serinol 20 or 26, under acidic conditions was found to play an important role in determining the optical purity obtained in 2. Jone's oxidation and methylation of 2 led to the formation of N-benzyloxycarbonyl-B,B-(dimethoxy)alanine methyl ester (3) in low yield. Swern oxidation or Moffat oxidation of Z-L-Ser-Val-OMe dipeptide (5), which was expected to be converted to aldehyde 34, were carried out with unsuccessful results. Swern oxidation was applied to various N-protected amino alcohols derived from a-amino acids to give the corresponding aldehydes with excellent yields and optical purities.