The synthesis of methyl N-(1-aza-6-oxaspiro[2.5]oct-1-en-2-yl)-L-prolinate (1e) has been performed by consecutive treatment of methyl N-[(tetrahydro-2H-pyran-4-yl)thiocarbonyl]-L-prolinate (5) with COCl2, 1,4- diazabicyclo[2.2.2]octane (DABCO), and NaN3 (Scheme 1). As the first example of a novel class of dipeptide synthons, 1e has been shown to undergo the expected reactions with carboxylic acids and thioacids (Scheme 2). The successful preparation of the nonapeptide 16, which is an analogue of the C-terminal nonapeptide of the antibiotic Trichovirin I 1B, proved that 1e can be used in peptide synthesis as a dipeptide building block (Scheme 3). The structure of 7 has been established by X-ray crystal-structure analysis (Figs. 1 and 2).