<unk>  (INN; L-Polamidon, L-Polamivet, Levadone, Levothyl), or<unk> , is a synthetic opioid analgesic and antitussive which is marketed in Europe and is used for pain management and in opioid maintenance therapy. In addition to being used as a pharmaceutical drug itself,<unk> , or R-(−)-methadone, is the active enantiomer of methadone, having approximately 50x the potency of the S-(+)-enantiomer as well as greater μ-opioid receptor selectivity. Accordingly, it is about twice as potent as methadone by weight and its effects are virtually identical in comparison. In addition to its activity at the opioid receptors,<unk>  has been found to act as a weak competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor complex and as a potent noncompetitive antagonist of the α3β4 nicotinic acetylcholine (nACh) receptor.
There is now an asymmetric synthesis available to prepare both<unk>  [R-(−)-methadone] and dextromethadone [S-(+)-methadone].
The separation of the stereoisomers is one of the easier in organic chemistry and is described in the original patent.  It involves "treatment of racemic methadone base with d-(+)-tartaric acid in an acetone/water mixture [which] precipitates almost solely the dextro-methadone levo-tartrate, and the more potent<unk>  can easily be retrieved from the mother liquor in a high state of optical purity"
Levomethadone
Methadone 
Morphine 
Buprenorphine 
Levacetylmethadol
Morpholine
Acetylmethadol
Dexmedetomidine