Matthias Brewer, The University of Vermont, Burlington Vermont, 05405 A novel ring fragmentation reaction: Methods for preparing polycyclic heterocycles and large rings Matthias Brewer, The University of Vermont, Burlington Vermont, 05405 With the generous financial support provided by the ACS- PRF we have developed the novel ring fragmentation reaction shown below. This fragmentation is quite general and provides synthetically useful tethered aldehyde yonoate (and ynone) products. We have also shown that these products are excellent substrates for intramolecular azomethine ylide 1,3-dipolar cycloaddition reactions, which provide an efficient route to polycyclic nitrogen-containing heterocycles. We have also discovered that bicyclic γ-silyloxy-β-hydroxy-α-diazo carbonyl compounds in which the β-γ bond is the ring fusion bond of the bicyclic system, productively fragment to provide medium sized cyclo-2-ynone products. Synthetic routes to medium and large ring systems are lacking, and this fragmentation provides a novel route to these ring system.