Intramolecular cycloadditions, nitroso compounds

DFT studies of the intramolecular ene-like (or the so-called 1,3-dipolar ene) reaction between nitrile oxides and alkenes show that this reaction is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate. The competitive reactions, either the intramolecular [3 + 2] cycloaddition between nitrile oxides and alkenes or the intermolecular dimerization of nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar ene reaction if the tether joining the nitrile oxide and alkene is elongated, or if substituents such as trimethylsilyl are absent (425). 

Cycloaddition Reactions. Chiral acrylamides derived from pyrrolidines (2) or (3) undergo stereoselective [4 + 2] cycloaddition reactions with a variety of cyclic dienes. Similarly, nitroso compounds derivatized with pyrrolidine (2) and generated in situ give cycloadducts with a high degree of stereoselectivity (eq 6). Intramolecular [2 + 2] cycloadditions involving pyrrolidine-derived keteniminium salts have been shown to produce chiral cyclobutanones. ... 

Intramolecular cycloaddition reactions with nitroso dienophiles have been used in a number of syntheses of alkaloids. For example, cycloaddition of the acyl nitroso compound formed from the hydroxamic acid 31 gave the dihydro-oxazine 32, which was converted to the alkaloid gephyrotoxin 223AB (3.31). ... 

---