Rhodium carbenoid mediated cyclisations. Part 3. Synthesis of cyclic ethers from lactones

Ethyl lithiodiazoacetate ring opens lactones, N-Boc lactams, and cyclic anhydrides and carbonates to give ω-functionalised α-diazo-β-keto esters in varying yield (Tables 1 and 2). Thus simple 5-, 6-, 7-, and 8-membered lactones react to give α,ω-diazo alcohols, and benzo-fused lactones give diazo phenols. The corresponding reaction with N-Boc lactams gives Boc-amino diazo compounds, and cyclic anhydrides give α,ω-diazo carboxylic acids. Treatment of the diazo compounds derived from 5-, 6-, and 7-membered lactones with a catalytic amount of rhodium(II) acetate resulted in rhodium carbenoid mediated cyclisation to give 6-, 7-, and 8-membered cyclic ethers (Table 3). Attempted preparation of larger rings, however, resulted in formation of cyclopentanones by competing C–H insertion reactions. Similarly, the Boc-protected amino diazo compounds only gave the products of C–H insertion on treatment with rhodium(II) acetate.