Dextromethorphan, chirality

The alkaloid dextromethorphan is an antitussive drug manufactured by Lonza in enantiomerically pure form. While early synthesis involved the tedious resolution of an octahydroi-soquinoline intermediate with mandelic acid, a more recent process takes advantage of the catalytic reduction of a C-N double bond promoted by a chiral Ir/ferrocenylphosphine complex [70]. 

Figure 6.67 Dextromethorphan hydrobromide provides an example of a compound in which the stereochemistry of the three chiral centers of morphinan is inverted.

Most commonly, drugs which are administered as racemates have only one chiral centre. Levorphan and dextrorphan (based on the morphine structure) are unusual in that they have the opposite configurations at three chiral centres. Levorphan ([-] isomer 9 R, 13 R 14 R) is a narcotic analgesic which is potentially addictive whereas dextrorphan ([-L] isomer 9S, 13 S, 14S) is used as an antitussive drug in the form of its 0-methylether dextromethorphan. 

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