Steric acceleration of some pericyclic reactions.

Steric effects are important in synthesis. Whilst steric hindrance is well known in hindering reactions, steric effects can also be employed to accelerate reactions, in particular cycloaddition reactions, and even to promote reactions that otherwise do not occur. A survey is included in previous work on steric effects in chemical reactions, principally cycloadditions. This includes a brief discussion on the importance of orientation and solvent effects on Diels-Alder cyclisations and ene reactions. The effect of substituents on the cyclisation of N-allyl furfurylamines has been studied. It was shown that bulky N-protecting groups enhance cyclisation, an effective buttress being the trityl (triphenylmethyl) group. The latter has the added advantage of being particularly easy to remove. A study of some ene reactions has also been carried out and steric acceleration of these processes has also been observed. A novel reaction involving an intermolecular cycloaddition followed by a sterically accelerated ene reaction has also been uncovered. Some attempts have also been made at carrying out these sterically accelerated reactions on a solid support as required in combinatorial chemistry. This involved the preparation of a new type of substituted support, with a view to utilising this in a combinatorial approach to synthesis. The structures have been supported by using a molecular modelling package, Alchemy 2000 from Tripos Associates Inc