Thermal retrocycloaddition

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

The first total synthesis of the cytotoxic agent (+)-euplotin A was completed by the research team of R.L. Funk. The key step of the synthetic effort was the intramolecular hetero DIels-Alder cycloadditlon of a 3-acyl oxadiene (generated from 5-acyl-4H-1,3-dioxins via thermal retrocycloaddition) with a substituted dihydrofuran to afford the tricyclic skeleton of the natural product. The correct relative stereochemistry of the required dihydrofuran substrate was established using the Paterno-Buchi reaction between ethyl glyoxylate and furan. Subsequently, the oxetane ring was opened stereoselectively under Lewis acid catalysis. 

Compound (808) undergoes a thermal retrocycloaddition of N O about 10 times faster than (809), strongly indicative of a concerted reaction for the former. The situation is less clear for (810) and (811), for which intermediate rates were obtained. Electrophilic substitution in the aromatic ring of 2-aryl-gfem-dichlorocyclopropanes takes place mainly in the para-position in the case of nitration or bromination of the... 

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>. 

This suggests that the formation of quadricyclane during the thermal decomposition of 70 exo results from the competing [2s+2s+2s] retrocycloaddition process rather than via intramolecular trapping of the diradical intermediate by a C-C bond. Thus, the thermal decomposition of 70 exo resulted from [2s+2s+2s] cycloreversion reaction rather than from a biradical pathway. If the latter pathway was active, both exo and endo isomers should have furnished quadricyclane 72. In fact, 70 endo is geometrically incapable to participate in the cycloreversion process. 

A theoretical treatment of 1,2-oxathietane indicates planarity with aS-0 bond length of 1.669 A and a C-S-0 angle of 100.6°. The electronic spectrum was calculated. The character of the HOMO is largely that of the sulfur 3p orbital. A CNDO molecular orbital study of the retrocycloaddition of 1,2-oxathietane 2-oxide to sulfur dioxide and ethylene shows that strong heteroatom asymmetry lifts the stereoelectronic requirement that the thermal fragmentation occur by a suprafacial-antarafacial path. ... 

The unusual high thermal stability is observed in highly fluorinated [2.2.2] propellane (38) in contrast to the unstable hydrocarbon analogue (39). The fluorinated propellane 38 is extremely thermally stable andso nochangeis observed evenat 100°Cfor lOh [12]. In contrast, nonfluorinated propellane 39 undergoes retrocycloaddition to yield dimethylenecyclohex-anes (40) and (41) with a half-life of 25 min at 25°C (Scheme 1.40) [13]. 

The greater thermal stability of azoxyalkanes than azoalkanes carries over to compounds that lose N2O or N2 by concerted pericyclic retrocycloaddition. Thus the activation energy for N O... 

A thermal 2 + 2-retrocycloaddition of the strained photocycloadduct (40) of a cyclobutene and a cyclopentene provides a new entry to functionalized 5-8-5 ring systems (41) which feature in some natural products. ... 

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