71082-30-9Relevant academic research and scientific papers
Rapid and selective spiro-cyclisations of O-centred radicals onto aromatic acceptors
McBurney, Roy T.,Eisenschmidt, Annika,Slawin, Alexandra M. Z.,Walton, John C.
, p. 2028 - 2035 (2013/05/09)
Substituted benzyloxycarbonyloxyl radicals were generated by sensitised photolyses of benzyl oxime carbonates. EPR spectroscopy showed they ring closed exclusively by spiro-cyclisation onto the ipso-C-atoms of the aromatic rings. β-Scission of the alkoxycarbonyloxyls to CO2 and benzyloxyl radicals increasingly competed and became dominant above 270 K. The first rate parameters for spiro-cyclisations of O-centred radicals onto aromatics were obtained by the steady-state kinetic EPR method. Pentafluoro-substitution of the ring substantially reduced the spiro-cyclisation rate. Activation barriers of the spiro-cyclisations were computed by DFT to be about half those of the alternative ortho-cyclisations. Consideration of the TS structures suggested the preference for spiro- over ortho-cyclisation resulted from better overlap of the oxyl SOMO with the aromatic π-system during spiro closure.
Radical 4-exo cyclizations onto O-alkyloxime acceptors: Towards the synthesis of penicillin-containing antibiotics
Scanlan, Eoin M.,Walton, John C.
, p. 2133 - 2143 (2007/10/03)
The 4-exo cyclizations of two types of carbamoyl radicals onto O-alkyloxime acceptor groups were studied as potential routes to 3-amino-substituted azetidinones and hence to penicillins. A general synthetic route to 'benzaldehyde oxime oxalate amides' (=2
Photolytic and radical induced decompositions of O-alkyl aldoxime ethers
McCarroll, Andrew J.,Walton, John C.
, p. 1868 - 1875 (2007/10/03)
Direct photolytic radical induced homolyses of O-alkyl arylaldoxime ethers (ArCH=NOR) were studied by EPR spectroscopy and by end product analyses. Initiating radicals (X·), including t-BuO·, t-BuS· , alkyl and Me3Sn·, added rapidly to the C=N double bond to give adduct oxyaminyl radicals (ArCHXN·OR) that could be observed and characterised by EPR spectroscopy. For O-alkyl arylaldoxime ethers containing H-atoms attached to the carbon adjacent to the ether oxygen (OR = OCHR12), t-BuO· radicals also abstracted this hydrogen to yield oxyalky radicals that underwent rapid β-scission to afford iminyl radicals (ArCHN·) and an aldehyde or ketone (R12CO). judged by the relative importance of ROH and ArCN amongst the products, abstraction of the iminyl hydrogen atom also took place to yield oximidoyl radicals (ArC·=NOR), although this could not be confirmed by EPR spectroscopic observation of these radicals. Thus, homolysis induced by t-BuO· radicals took place comparatively unselectively. Addition of the t-BuO· radical to the C=N double bond of oxime ethers was very fast, the rate constant being comparable to that for addition of the same radical to nitrones. Direct and photosensitised UV photolysis of O-alkyl arylaldoxime ethers gave alkoxyl and aryliminyl radicals in very low yields. Although traces of 2-methyl-tetrahydrofuran were detected from cyclisation of the pent-4-enyloxyl radical generated by direct photolysis of O-pent-4-enyl benzaldoxime, yields were too low for preparative purposes.
Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles
McCarroll, Andrew J.,Walton, John C.
, p. 2399 - 2409 (2007/10/03)
Photolyses of aldoxime esters, containing a considerable range of alkyl groups, lead to cleavage of their N-O bonds and formation of aryliminyl and alkyl radicals. The process was found to be favoured by 4-methoxyacetophenone as a photosensitiser and by methoxy substituents in the aryl rings. 4-Nitro- and pentafluoro-substitutions of the aryl rings were, on the other hand, deleterious. The intermediate iminyl radicals, together with primary, secondary and tertiary alkyl radicals were characterised by 9 GHz EPR spectroscopy. Cyclopropyl, CF3, and CCl3 radicals were probably also formed, but were too reactive for direct EPR spectroscopic detection. Photosensitised reaction of benzophenone oxime O-nonanoyl ester produced the diphenylmethaniminoxyl, as well as the expected n-octyl and iminyl radicals. This indicated that O-C bond scission accompanied O-N scission for this ketoxime ester. At higher temperatures the C-centred radicals added to the starting oxime esters to produce alkoxyaminyl radicals that were also spectroscopically detected in some cases. No evidence for abstraction of the iminyl hydrogen by tertbutoxyl radicals was obtained. Instead, the t-BuO radicals added to the C=N double bonds of the oxime esters. Similarly, chlorine abstraction from alkylbenzohydroximoyl chlorides by trimethyltin radicals did not take place. Preparative scale experiments with oxime esters containing suitably unsaturated alkyl groups showed that good yields of cyclised products could be obtained in the presence of the photosensitiser. This process constitutes a general method by which carboxylic acids or acid chlorides can be converted into alkyl radicals and hence to cyclised derivatives.
