Thermisches Verhalten von Cyclopropa[c]chromenen

Abstract 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‐2 H ‐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 [1,5 s ] and [1,7 a ] 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.