Thermisches Verhalten von Cyclopropa[c]chromenen

The Cyclopropa[c]chromenes 14, trans‐and cis‐15, trans‐and cis‐16 and 17 rearrange on heating > 200° in N, N‐diethylaniline to give 2‐alkyl‐2H‐chromenes 7, 8, 21, 22. The rate determining step of this rearrangement is the ‘homoelectrocyclic’ ring opening of the cyclopro‐pa[c]chromenes to give ω‐allyl‐quinomethanes of type 4. These intermediates show fast [1,5s] and [1,7a] H‐shifts, followed by electrocyclic ring closure. Deuterium labelling experiments are in agreement with this mechanism. The remarkable dependence of the rates of rearrangement with respect to the stereochemistry of the cyclopropa[c]chromenes (cf. table 2) suggests that in the first step only one of the two possible disrotatory modes of ring opening is involved.