ABSTRACT
The early studies into citrus essential oils revealed that they gave significant amounts of non-volatile residue on distillation. The isolation of pure crystalline compounds, such as bergapten (5-methoxypsoralen) from bergamot oil (Pomerantz, 1891) and limettin (5,7-dimethoxycoumarin) from lime oil (Tilden and Burrows, 1902), provided the first indication that these residues contained oxygen heterocyclic compounds. Further studies over the years have established that coumarins, psoralens (furanocoumarins) and polymethoxyflavones are mainly present. Some 70 of these compounds have been reported to occur in the cold-pressed peel oils of the Citrus species (Gray and Waterman, 1978; Murray et al., 1982). Without exception they are all derivatives of the three basic structures shown in Figure 15.1. The majority of the coumarins and psoralens are substituted in the numbered positions by methoxy and/or isopentenyl or geranyl related alkoxy groups. In certain cases, the terminal double bond of a prenyl substituent has undergone oxidation forming an epoxide which may subsequently react further to yield a side chain containing a diol, enol or carbonyl. Methoxy substitutents predominate in the flavones but hydroxy groups are occasionally present.
