Heterocyclic compounds 3,3 -sigmatropic rearrangements

The most frequent rearrangement of substituted hydroxylamines is the thermal [3,3]-sigmatropic rearrangement, the 3-aza-4-oxa-Cope rearrangement also called 3-aza-4-oxy-Cope rearrangement, that proceeds by a cyclic six-membered transition state (equation 1). Albeit the difficulty to prepare hydroxylamine derivatives, this rearrangement proceeds in mild conditions and is useful in the synthesis of several heterocyclic compounds. 

Charge-accelerated suhbnium [3,3]-sigmatropic rearrangements 12S175. Chemistry of heterocyclic diazo compounds 13MI4. 

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems. > Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain/tautomer-ism, a sigmatropic reaction, has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(di-tert-butylcyclohexadien-one)perimidine 44a and its analogues. The occurrence of a proton transfer step is in accord with the fact that the N,N -dimethyl derivative of 44a exhibits no photochromic properties. 

---