Studies of the thermal rearrangements of dispiro-1,2,4-Trioxanes
Taylor, Benjamin J.
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A series of α-alkoxy-3,6-dispiro-1,2,4-trioxanes have been synthesised by acid-catalysed perhydrolyses of α-alkoxy methylenecyclohexane oxides (provides ring A) to give selectively the corresponding 1-hydroperoxy-1-(hydroxymethyl)cyclohexanes followed by acid-catalysed condensation with an appropriate cycloalkanone (provides ring C). Analogous perhydrolyses catalysed by MoO2(acac)2 afforded mixtures of regioisomeric β-hydroxy hydroperoxides, albeit in overall increased yields. The resulting 1- (hydroperoxymethyl)-1-hydroxycyclohexanes allowed entry to the isomeric 3,5-dispiro- 1,2,4-trioxanes. X-ray crystallographic analysis of the isomeric dispiro-1,2,4-trioxanes revealed that (a) they originate from different diastereoisomers of the epoxide substrates, and (b) the 1,2,4-trioxane rings of the 3,5-isomers adopt distorted half-chair rather than chair conformations as a consequence of intramolecular 1,3-diaxial steric interactions. Modelling studies of the perhydrolysis process are in broad agreement with the regioselectivity of the acid-catalysed reactions, but suggest that the α-alkoxysubstituted epoxides can act as bidentate ligands which can adopt different binding modes to the Mo catalyst and hence provide alternative reaction pathways. Thermolysis of dilute solutions of the α-alkoxy-3,6-dispiro-1,2,4-trioxanes in decane afforded a variety of 13-, 14-, 15- and 20-membered fully ring-expanded keto lactones in high yield via stepwise, β-scission/radical recombination reactions in contrast to the partially ring-expanded oxalactones obtained previously from other 3,6-dispiro-1,2,4- trioxane derivatives. An investigation of substituent effects on the thermal rearrangement mechanisms of 3,6- dispiro-1,2,4-trioxanes using DFT calculations indicated that, after the initial O-O bond homolysis to form the corresponding oxy biradical, ring C generally opens significantly faster than the unsubstituted ring A because of the greater delocalisation of radical character into ring C. In these cases, the lowest rearrangement energy barrier links directly to the partially ring-expanded oxalactone product as observed experimentally. Methyl or methoxy substituents at the α-position of ring A render its ring opening by β- scission increasingly more competitive to that of ring C due to increased delocalisation of radical character onto the α-substituent, consistent with the ‘α-effect’. Methoxysubstituents are also noted to engage in close range interactions with the 1,2,4-trioxane ring. Since the energy barrier for ring A opening falls below that of ring C in the methoxy model, formation of the fully ring-expanded keto lactone becomes favoured.