Retrocycloadditions

If a reaction is deemed allowed, its reverse is also allowed. In principle, any allowed cycloaddition we have discussed can also be run in the reverse direction, producing two 

---

The chemical potentials and free energies of the 2-isoxazolines have also been studied and the electron impact and chemical ionization mass spectra determined (77MI41614). Fragmentation pathways and retrocycloadditions of various derivatives were discussed in these reports. 

A large fraction of the chemical reactions known are used to form heterocyclic compounds. Displacement reactions and cycloadditions are particularly important, and their rates are therefore of great practical interest. The same is true for the rates of reverse reactions — ring opening by displacements or retrocycloadditions. It was realized over the last 40 years that... 

Stannanethiones and stannaneselones can be generated by the thermal retrocycloaddition of trichalcogenastannolanes or the dechalcogenation of tetrachalcogenastannolanes in the presence of triphenylphosphine... 

The 5-triflate of 1,3-dioxin undergoes a variety of facile Pd-catalysed cross coupling reactions affording 5-substituted 1,3-dioxins from which acrolein can be thermally generated in high yield by a retrocycloaddition. In particular, this approach leads to a new class of 2-acylacroleins (Scheme 61) <00T10275>. 

A trihydroxyindolizidine lactone is obtained by elaboration of an isoxazolidine synthesized by a domino stereoselective retrocycloaddition/intramolecular cycloaddition process of an enantiopure pyrroline-iV-oxide <2006TA292>. 

Glycine acts as an acid-base catalyst in this reaction. C8 and Cl 1 are very acidic, and once deprotonated they are very nucleophilic, so they can attack C2 and C3 in an aldol reaction. Dehydration gives a key cyclopentadienone intermediate. (The mechanism of these steps is not written out below.) Cyclopentadienones are antiaromatic, so they are very prone to undergo Diels-Alder reactions. Such a reaction occurs here with norbomadiene. A retro-Diels-Alder reaction followed by a [4 + 1] retrocycloaddition affords the product. 

The mechanism begins with a-hydride elimination to give a benzylidenetitanium complex. A [2 + 2] cycloaddition gives the titanaoxetane, and [2 + 2] retrocycloaddition affords the product and the byproduct. 

The mechanism again consists of a series of [2 + 2] cycloadditions and retrocycloadditions. 

The fragmentation strategy is based on a similar concept to traceless solid-phase chemistry, like retrocycloaddition cleavage, cycloelimination or cyclofragmentation. These are synthetically useful reactions with wide scope for the construction of rigid templates of different ring sizes. Up to now only one example for the attachment of heteroatoms via addition/elimination strategy has been developed, by Kurth et al. (Scheme 3.7) [188, 189]. 

Thermolysis of ) -sultines leads via retrocycloaddition to the fragmentation products SO2 and olefin. The fragmentation is stereospecific in accordance with orbital symmetry principles. 

Reactions which formally can be classified as cycloadditions or retrocycloadditions have been observed in homogeneous solution with single electron oxidative initiation, so the observation of parallel activity on irradiated semiconductor suspensions is certainly reasonable. The first example of such a reaction involved the photocatalysis by ZnO or CdS of the ring-opening of a strained hydrocarbon which could also be opened in the dark by a single electron oxidant, ceric ammonium nitrate, Eq. (38)... 

---