The configuration of the double bond in pseudomonic acid A (1 a) is shown to be € by comparison of the spectroscopic properties of its methyl ester (1 b) with those of methyl isopseudomonate A (2) obtained from (1 b) by photolysis. Ozonolysis of methyl pseudomonate A (1 b) afforded the crystalline ketone (3a). An X-ray analysis of the o-bromophenylhydrazone derivative of ketone (3a) confirmed the structural assignment and provided the absolute stereochemistry a t each of the eight chiral centres. Pseudomonic acid A. therefore, may be formulated as (4). 9-(4-[5S-(2S,3S-epoxy-5Shydroxy-4S-methylhexyl)-3R,4R-dihydroxytetrahydropyran-2S-yl] -3met h y I b u t -2 (€ )en o y I oxy} no n a no i c a c i d .

Several mono-substituted fluorenes have been diacetylated under Friedel-Crafts conditions, and the nature and position of some substituents were shown to have a profound influence on the resulting substitution pattern. Unambiguous structural assignment was facilitated by n.m.r. spectroscopy using lanthanide shift reagents.

The mass spectra of a series of &unsaturated cyclic and acyclic carboxylic acids are reported. Metastable ion studies and precise mass measurements have been employed in demonstrating that the ten compounds studied conform to one or other of two generalised fragmentation pathways.MASS spectral studies of carboxylic acids by electron-impact are relatively few, since they are more conveniently handled as the derived esters. Consequently the fragmentation behaviour of By-unsaturated acids has not been reported. The few studies of aliphatic carboxylic acids which have been carried out are of formic acid1S2 and higher homol~gues.~ to These show predominant formation of the McLafferty rearrangement product (a) at m/e 60 (where the chain length permits) and of a-and y-cleavage products (ions at m/e 45 (b) and m/e 73 (c) respectively)

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