This thesis describes the work performed developing synthetic approaches to the guanidine containing marine natural products cylindrospermopsin and batzelladine F, and the bicyclic lactone containing compounds, the plakortones. (i) A reductive guanidine addition-cyclisation reaction was employed to prepare the left hand portion of batzelladine F from a bis-a,j3-unsaturated ketone which was prepared from succinaldehyde using Wittig methodology. (ii) Attempts were made to prepare a 6,6-bicyclic analogue of cylindrospermopsin in four steps from 5-hexen-1-ol utilising as the key step a tandem 1,4-addition/epoxide opening methodology. This was found to lead to a 7,6 product via a competitive process. (iii) Intramolecular epoxide opening reactions involving guanidines were also studied providing access to several glycomimetic analogues. These included a simple five and seven membered ring system, together with a more complex substituted five and six membered ring system as well as a dimeric 7, 7 system. (iv) A bicyclic 5,5-furanofuran was prepared via the reaction of n-butylmagnesium bromide with an a-hydroxy-y-ketocarboxylic acid which leads to the formation of a tetrasubstituted y-butyrolactone which in turn was converted to the required system, an analogue of the bicyclic lactones found in the marine natural products, the plakortones.