The cation triphenylmethylium and the cation radical tris-(p-bromophenyl)ammoniumyl are effective catalysts for oxygenation of ergosteryl acetate (1 ; R = Ac) to the peroxide (2; R = Ac) ; the triphenylmethylium-catalysed reaction is a photo-oxygenation, whereas the cation radical reaction is thermal. Several other ' forbidden ' additions of triplet oxygen to cisoid dienes have been demonstrated. A number of cation radicals have been examined for catalytic oxygenation ability. Ergosteryl acetate can also be converted into its peroxide by triplet oxygen in the presence of various Lewis acids.Two mechanisms for overcoming the spin barrier for triplet oxygen addition have been defined by experimental work. A mechanistic interpretation of the facts is proposed.

No abstract

No abstract

1 a-Adrenoceptor stimulant activity was determined for noradrenaline (NA), clonidine and a series of para-substituted derivatives of clonidine on rat aortic strips, a rat brain synaptosome preparation, and anaesthetized pithed rats. The effects on the blood pressure of intraventricular (i.c.v.) injections of para-aminoclonidine were also determined in anaesthetized rats. 2 Para-substituted derivatives of clonidine (amino-, diethylamino-, ethylamino-, acetamido-, bromoacetamido-, N-chloroethyl-N-methyl-amino and N-3-chloroethyl-N-methylaminomethyl-) retain a-adrenoceptor stimulant activity.3 pD2 values determined on rat aortic strips were 11.2, 7.67 and 9.05 respectively for paraaminoclonidine, clonidine and noradrenaline. The K; values of these agents, determined on a rat brain synaptosomal preparation with a radioreceptor assay using [3H]-clonidine as ligand, were 1.3, 8.0 and 23 nm respectively for para-aminoclonidine, clonidine and NA. When given by i.c.v. injection in rats, para-aminoclonidine lowered the blood pressure.4 N-p-chloroethyl-N-methylaminomethylclonidine is an alkylating agent with an unusual agonist activity. It elicits contractions of the rat aorta that persist despite repeated washing. 5 cx-Adrenoceptor affinities are discussed in relation to their structural features.