Pennyroyal (Mentha pulegium) contains the tocic monoterpenoid Pulgenone, which derives its name from the botanical name of Pennyroyal. (Pulgenone should not be confused with
Pulegone which is found in some other Mint genus plants).
Pulgenone is used as a flavouring agent, its mild toxicity being of no concern at the low concentrations required, being less than 1/10,000th of the toxic dose. In the body it is metabolized to Menthofuran, shown below, which is 10 times as toxic as Pulgenone, but still of no safety concern. Pulgenone occurs in greater concentration in Pennyroyal, and has been implicated in human Pennyroyal poisoning in those un-able to metabolize Pulgenone into Menthofuran. The only difference between Pulgenone and Menthol below is the C=C double bond in Pulegenone which is a single bond in Menthone.
The main constituents of the essential oil of Pennyroyal are Menthone (at 31%). Others include Pulegone (14%),
NeoMenthol (14%) (although the percentages vary wildly from the various sources to be found on the internet), and Caryophyllene Oxide, according to one source. Other volatile compounds are
Pulegone Acetate, Caryophyllene, γ-Terpinene and another 35 or so unspecified but more minor constituents.
NeoMenthol is the simplest of these three, an alcohol. It has a mentholic type odour and flavour and is found in many foods, such as Broccoli, Sweet Orange, Yellow Bell Pepper, etc. Speaking personally, your Author has never detected any menthol-type aroma emanating from brocolli, either when cooking or upon eating. It is used as both a flavouring and as a fragrance.
Piperitone is a ketone and is found in nature in both its' possible stereoisometric forms: D- and L- forms. D-Piperitone has a peppermint aroma and is the form found in some plants of the Mentha genus. L-Piperitone, on the other hand, is found in
Sitka Spruce and the Eucalyptus dives tree but probably smells dissimilar.
Piperitenone is found in the citrus fruits such as Grapefruit, Lemon, Orange and also in Spearmint and Peppermint. It is used as a flavouring agent. It is a
p-Menthane monoterpenoid (as are the others).