The Synthesis of 2,2,8-Trimethyltricyclo [6,2,2,01,6] Dodec-5-Ene and Related Compounds

Abstract

Introductory paragraphs: Man's chemical knowledge has been greatly advanced by his investigations into the nature, synthesis, and reactions of the sesquiterpenes1. These C15 natural products encompass a wide range of acyclic, monocyclic, bicyclic, tricyclic, and tetracyclic systems. The problems of skeletal construction and stereochemical control posed by this variety of natural product have often provided the stimuli for the development of new synthetic methods with wider application. Similarly, our motivation for probing certain types of reaction mechanisms, particularly in the field of carbonium ion chemistry, has stemmed from the many rearrangement reactions of sesquiterpenes. A rearrangement may be defined as a change in the atomic disposition in the molecule with concomitant a or it bond cleavage and subsequent bond reformation. This definition includes processes such as cyclisations, Wagner-Meerwein rearrangements, hydride shifts, etc. and can take place under a myriad of reaction conditions. Rearrangements involving cationic intermediates, usually generated under acidic conditions, can shed some light on the biogenesis of the sesquiterpenes in that they constitute 3 a working model for enzyme-catalysed rearrangements. The acid-catalysed rearrangements of some sesquiterpenes will be examined as a prelude to the discussion of the synthesis of the title compound of this thesis which, as will be shown, is derived from the sesquiterpene thujopsene. These rearrangements are specific to each structure and will, therefore, be discussed individually. Caryophyllene, whose structure was established by Barton in 1951, is the main constituent of oil of cloves. Examination of this structure reveals a highly strained cyclobutane ring trans-fused to a nine-membered ring in a bicyclic [7,2,0] system with an exo- and an endocyclic double bond. The endocyclic double bond has the less-favoured trans geometry. The nine-membered ring allows a fair degree of mobility within the molecule in that the double bonds can be brought into fairly close proximity.