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The absolute stereostructure of (−)1,6 desoxypipoxide ( 2 ), a new plant metabolite of biosynthetic significance was assigned by spectral interpretation and chemical correlation with (+)pipoxide ( 1 ).

The ketene-anthracene adduct 1 serves as a good precursor in the synthesis of substituted acetylenes, in which the key step is the retro Diels–Alder reaction.

The isolation of benzyl benzoates and o -hydroxybenzyl flavanones from Uvaria ferruginea , in addition to the previously reported cyclohexene oxides, provides evidence in support of the proposed biogenetic route to these compounds via the common benzyl benzoate precursor.

The [3,2] and [1,2] sigmatropic rearrangements of the bicyclic ammonium ylides (8), (11), and (14) are inhibited by the bicyclic framework which prevents correct orbital alignment for π-bonding in the transition state for a concerted [3,2] sigmatropic rearrangement or in the vinylogous nitroxide component of the radical pair required as an intermediate in a [1,2] sigmatropic rearrangement. The ylides (14a) and (14d), based upon the 1-azabicyclo-[3.3.1]nonane system, are the only bicyclic allylic ylides which undergo a clean [3,2] rearrangement, which involves the exo-transition state either exclusively (14a) or predominantly (14d).

The propynylammonium salt (5) reacts with aqueous sodium hydroxide to give the allene (7) and a new ylide (11). The simultaneous formation of (7) and (11) suggests that the apparent [3,2] sigmatropic rearrangement of the propynyl ylide (6) to the allene (7) may involve a two-stage mechanism. The reactions of the ylide (11) are described, as are those of the allene (7) and a number of similar allenes derived from the propynylammonium salts (23) and (27). Three types of reaction are recognised for the allenes including a novel [1,3] rearrangement involving the migration of a dimethylamino-group. The base catalysed rearrangements of the bicyclic propynylammonium salts (52) and (56) do not show the inhibition expected for concerted [3,2] sigmatropic rearrangements in such bicyclic systems. This observation provides further evidence for a two-stage mechanism for the apparent [3,2] sigmatropic rearrangement of propynylammonium ylides.

2-Oxidoanilinium ylides (22) may be prepared from the corresponding quaternary salts (21) by reaction with aqueous sodium hydroxide. The ylides (22) rearrange on heating to give the products (23) of [1,4] rearrangements which, on the basis of mixing experiments, are intramolecular processes. The N-allyl ylide (22c) rearranges to give both the ether (23c) and the allylphenol (30); a study of the rearrangements of specifically deuteriated ylides (41) shows that these reactions involve competing concerted and radical pair processes. The observation of CIDNP during the formation of the allylphenol (30) is shown to be associated with the minor radical pair pathway for the formation of this product.

The carbonyl-stabilised ammonium ylides (3), (12a–n), and (14a–c) were obtained from the reaction of the corresponding ammonium halides (1), (13a–n), and (15a–c) with sodium hydroxide in water or aqueous methanol. The ylides, which were characterised by their molecular formulae and spectroscopic properties, regenerated quaternary ammonium bromides on treatment with hydrobromic acid. In general the reactions of the ammonium ylides resemble those of the corresponding sulphonium ylides. Thus ylides having a suitable migrating group (ArCH2) undergo a Stevens [1,2] rearrangement on heating, and other ylides fragment to give a tertiary amine and products containing the PhCOCH grouping. A number of ylides reacted with dimethyl acetylenedicarboxylate to give the furans (32). The ylides (14b and c) with phenyl isocyanate gave the phenylcarbamoyl-substituted ylides (35).

Instead of undergoing a cyclopentenone annulation, the allylmalonate anion derived from the dicarboxylate (7) rearranges to vinylsuccinate (5), an application of which has led to a convenient synthesis of vinylnaphthoquinones (10).

The [1,4] sigmatropic rearrangement of the anilinium ylides derived from the salts (1) is an intramolecular process, (1)→(2); the N-allyl ylide (8) reacts, in addition, by a sequence of [2,3] and [3,3] sigmatropic rearrangements to give the allyl phenol (10).

The ortho-ylides (4) are isolable compounds which on heating give a variety of products, including the ethers (5) formed by a [1,4] sigmatropic rearrangement; the para-ylides (11) are less stable, and above 0° the ylides (11a and b) undergo consecutive [2,3] and [3,3] sigmatropic rearrangements whereas the ylides (11c–e) apparently react by homolytic fission and recombination.

The thermal rearrangement of the 2-oxyanilinium ylide (1) is shown to involve competing, but distinct, concerted and radical-pair processes.

The rearrangement reactions of acetylenic ammonium ylides are not inhibited by the steric requirements of the bicyclic systems of (7a) and (10a); accordingly a non-concerted mechanism analogous to (1)→(2)→(3) is proposed for these reactions, rather than the [2,3] concerted sigmatropic mechanism.