AbstractÐMethyl a-[di-tert-butyl(propargyloxy)silyl]-a-diazoacetate (6) undergoes a thermal intramolecular 1,3-dipolar cycloaddition reaction to form bicyclic pyrazole 8. Structurally similar, but Si±iPr 2 -rather than Si±tBu 2 -linked diazoacetates 12a,b, which react thermally to form cyclooligomers by inter-/intramolecular cycloaddition sequences, undergo the intramolecular [312] cycloaddition in the presence of silver(I) oxide. Silver pyrazolide 13a could be isolated and was transformed into NH-pyrazole 14a under acidic conditions. (Alkenyloxy)silyl and (alkynyloxy)silyl diazoacetates 15a±h were prepared from a silyl bis(tri¯ate), tert-butyl diazoacetate, and an unsaturated alcohol. A thermally induced intramolecular [312] cycloaddition leading to bicyclic pyrazoles 16 was observed for 15b and 15c, while all other diazoacetates 15 underwent unspeci®c decomposition. Only in the case of 15f, the tetracyclic 1,5-diazabicyclo[3.3.0]octane derivative 20 could be isolated in 2% yield and was structurally characterized by X-ray diffraction analysis.