A stereoselective elimination in the mass spectra of hexahydrofluorene derivatives related to gibberellins

Abstract

The mass spectra of methyl gibberate and methyl allogibberate and their 9- and 6-epimers were examined. Fragmentation pathways are proposed on the basis of specific deuterium labelling and measurements at high resolution. In particular, when the C-9 hydrogen and C-6 methoxycarbonyl substituents were cis to each other, elimination of methyl formate was much preferred over simple methoxycarbonyl cleavage, which became of comparable or greater importance only when the substituents were trans. Knowledge of these stereoselective mass spectral fragmentations has been applied to the solution of the relative stereochemistry of two isomers of 1,2,3,4,4a,9a-hexahydrofluorene-9-carboxylic acid.