Tropane is a bicyclic amine, characterized by a two-ringed structure with a pyrrolidine and a piperidine ring sharing a single nitrogen atom and two carbon atoms. Thus, the common structural element of the tropane alkaloids is the bicyclic azabicyclo-octane skeleton (Lounasmaa and Tamminen, 1993).

Fig. Tropane (all equivalent)

The wide range of tropane alkaloids occurring in the Solanaceae family arises from the esterification of acids, such as acetic acid, propanoic acid, isobutyric acid, isovaleric acid, 2-methylbutyric acid, tiglic acid, (+)-α-hydroxy-β-phenylpropionic acid, tropic acid, and atropic acid with various hydroxytropanes (α-tropanol, α− tropane-diol or α-tropane-triol).

Fig: Various types of carboxylic acids found in tropane alkaloids

The most important natural tropane alkaloids hyoscyamine and scopolamine (Figure 1) are esters of tropane-3α-ol (and the 6-7 epoxide of tropane-3α-ol) and tropic acid. The asymmetric α-carbon of tropic acid allows the formation of two stereoisomers.

Fig. Some natural tropane alkaloid

Almost all of the tropane based pharmaceuticals are natural or semi-synthetic esters. There are also alkylated or arylated tropane-compounds known as phenyltropanes.

Fig. Arylated or alkylated tropane (left) and semi-synthetic esters (right)