13 c4 atoms lie in a single plane. 14 Both transition-state geometries are orbital symmetry allowed for diazo compounds. Formation from 5 of a vinylcyclopropene can now be understood in terms of (1) base-catalyzed isomerization of 5 to its less stable tautomer 9, 15 (2) 1, 3-~ ycloelimination to produce acyclic diazo compound 10, and (3) nitrogen loss to yield a vinylcarbene which cyclizes to 6 (eq 4). 16 The stereochemical assignments for the ...