Donald Cameron

Application of 3-oxabicyclo[3.2.0]hept-6-ene-2,4-dione (cyclobut-3-ene-1,2-dicarboxylic anhydride) as an acetylene equivalent in cycloadditions

The effectiveness and limitations of dioxo-3-oxabicyclo[3.2.0]hept-6-ene-2,4-dione (cyclobut-3ene-1,2-dicarboxylic anhydride) as an acetylene equivalent in both 1,3-dipolar and Diels–Alder cycloadditions is reported; it reacted readily with a variety of reagents, including N-benzylideneaniline N-oxide, nitrile oxides, diazomethane, cyclopentadiene, tetracyclone, anthracene, 1,2,5-triphenylphosphole 1-oxide and 1,3-diphenylisobenzofuran. The structures and stereochemistry of the adducts were deduced from their NMR data; in all cases, the sterically favoured anti-isomers are formed exclusively. The configuration of the Diels–Alder adducts are assigned as endo with the exception of that from tetracyclone (and possibly 1,3-diphenylisobenzofuran) for which an exo-structure is assumed on the basis of steric arguments. Adducts were not obtained with several other reagents; possible reasons for this lack of reactivity are discussed. When subjected to flash vacuum pyrolysis, the adducts underwent thermal fragmentation, either by a retro-cleavage, or by loss of maleic anhydride to form products that are derived formally from reaction of acetylene in the cycloaddition step. A concerted pathway is proposed for the pyrolytic conversion into the ‘formal acetylene cycloadduct’ rather than a stepwise radical mechanism.

Case clustering in pityriasis rosea: support for role of an infective agent

better than that of a deputising service because of accessibility to records, and long acquaintance with patients, etc, it would seem logical to pay more to the doctor who undertakes his own out-of-hours calls than to the doctor who is delegating his care to the deputising service. In this way there would be a financial incentive for doctors to care for their own patients (a process which most of us look on as optimum), although at the same time it would be possible for those who do not wish to undertake out-of-hours work to delegate their work. My own practice (for the record) is to care for my own patients in a limited rota of four partners. R M RIDSDILL SMITH

Intramolecular nucleophilic interception by the sulfonyl group of reaction intermediates arising from electrophilic addition to unsaturated non-conjugated bicyclic sulfones

Electrophilic addition of molecular bromine and related reactions to unsaturated bicyclic sulfones resulting in atypical addition is described. Bromination of the bicyclic sulfone 3 results in a ca. 1 : 1 mixture of exo,exo- and exo,endo-dibromides 4 and 5, respectively. Similarly, acid-catalysed cleavage of epoxide 8 produces a 37 : 63 mixture of exo,exo- and exo,endo-bromohydrins 9 and 10, respectively. In both cases, formation of exo,exo-products is explained in terms of a favourable stabilising effect, which can be quenched by the presence of water in the reaction media, between the polar SO2 group and the incipient carbocation centre as depicted in 6. Also described is formal nucleophilic neighbouring group participation by the endo-sited sulfonyl group in norbornylene systems 12 which leads exclusively to the exo,exo-dibromides 13 upon ionic bromination. In the case of unsaturated sulfone 27, bromination to form the exo,exo-dibromide 30 occurs via. decomposition of an isolable, albeit metastable, tribromide salt 29. Owing to a similar effect, treatment of the related exo-epoxide 16 with hydrogen bromide in acetic acid results in the regio- and stereo-specific formation of exo-bromohydrin 17 as the sole product.

Divergent thermal behaviour of phenoxymethyl phenyl sulphoxide under gas and condensed-phase conditions

A comparison is made between the behaviour of phenoxymethyl phenyl sulphoxide (2) under different thermolytic conditions. Passage of (2) through a quartz tube in the gas phase at 600 °C results in products which can be rationalised by assuming homolytic cleavage of the S–C bond and the generation of both the phenylsulphinyl (4) and phenoxymethyl (6) radicals which decompose by separate pathways to yield S-phenyl benzenethiosulphonate (3) and benzaldehyde, respectively. On the other hand, solution thermolysis of (2) in [2H8]toluene at 110 °C leads to a facile S→O 1,2-shift and the formation of thermally stable phenoxymethyl benzenesulphenate (9) in quantitative yield. A bimolecular oxygen-transfer mechanism is proposed for the conversion of (2) into (9) which is rationalised in terms of a double (stabilising) anomeric effect brought about by the acetal group in (9). Cross-over experiments support the mechanism.

The sulphone group as a nucleophile: intramolecular formation of a novel thiadecanylium tribromide via addition of bromine to a double bond in an overcrowded environment

Instead of furnishing a normal bromide addition product, bromination of 4-thiatricyclo[5.2.0.02,6]non-8-ene 4-4-dioxide (1) gives 5-bromo-1-oxo-10-oxa-1λ6-thiatetracyclo[4.3.1.03,8.04,7]decan-1-ylium tribromide (3) through intramolecular reaction of the bromonium intermediate with the adjacent weakly nucleophilic sulphone group.

cis-Bromination of a non-conjugated cyclic alkene. Unprecedented electrophilic stereoselection by means of a remote SO2 group

Contrary to expectation, addition of molecular bromine to 3-thiabicyclo[3.2.0]hept-6-ene 3,3-dioxide (1) yields substantial amounts of the cis-1,2-dibromide (confirmed by X-ray diffraction), suggesting that the remote SO2 group exerts an extraordinary directive influence by means of a long-range Coulomb interaction that stabilises an open carbocation intermediate at the expense of the usually favoured bridged bromonium ion.