One Pot Synthesis of Spirocycles and Cyclopropa[b]pyrans by Alkenylation-Rearrangement of Cyclopropenes.

A one pot alkenylation followed by a stereoselective Alder-ene cycloisomerization of cyclopropenes give (aza)spiro[2.4]heptanes and spiro[2.5]octanes in high yields. Total trans diastereoselectivity is achieved for spiro[2.4]heptanes if the cyclopropene is monosubstituted in C3. When an α,ß-unsaturatedcarbonyl-containing bromide is used, an alternative cyclization takes place giving cyclopenta[c]cyclopropa[b]pyrans.