Discrimination in resolving systems. III: Pseudoephedrine-mandelic acid

Efficient resolution of venlafaxine and mechanism study via X-ray crystallography

Numbers of resolving factors were investigated to improve resolution of venlafaxine 1. An effective resolving agent, O,O'-di-p-toluoyl-(R, R)-tartaric acid 2, was screened using similar method of 'Dutch resolution' from tartaric acid derivatives. The resolution efficiency was up to 88.4%, when the ratio of rac-1 and 2 was 1:0.8 in THF with little water (10:1 v/v). Enantiomerically pure venlafaxine was prepared with 99.1% ee in 82.2% yield. The chiral resolution mechanism was first explained through X-ray crystallographic study. One diastereomeric salt with well solubility forms a columnar supramolecular structure as the acidic salt (R)-1·2, while the other diastereomeric salt with less solubility forms a multilayered sandwich supramolecular structure by enantio-differentiation self-assembly as the neutral salt 2(S)-1·2. The water molecules play a key role in the optical resolution, as indicated by the special structures of the diastereomeric salts.

Deoxyephedrine – mandelic acid diastereomers 1

Resolution of mandelic acid with (–)(R)-deoxyephedrine in 95% ethanol produces essentially non-discriminating unsolvated binary mandelate salts. (R)-deoxyephedrinium (R)-mandelate (I) is orthorhombic, P212121, a = 8.076(2) Å, b = 8.926(3) Å, c = 23.242(15) Å, V = 1675.3(14) Å3, Z = 4. (R)-deoxyephedrinium (S)-mandelate (II) is monoclinic, P21, a = 16.061(14) Å, b = 8.902(8) Å, c = 19.585(20) Å, β = 111.76(8)°, V = 2600(4) Å3, Z = 6 with three ion-pairs comprising the asymmetric unit. The principle interionic interactions are ribbon-like chains of salt-bridge hydrogen bonds which associate protonated secondary ammonium ions with carboxylates in I and II along crystallographic screw axes with a six-atom repeating unit H–N+–H···O–C−–O [C2 2(6)]. The (R)-mandelate additionally shows intraion hydrogen bonding and the (S)-mandelate shows interanion hydrogen bonding. Solubilities (in 95% ethanol), fusion points and heats of fusion indicate essentially no diastereometric discrimination. From acetone, the (S)-mandelate salt forms a efflorescent hemiacetone solvated binary phase (III), triclinic, P1, a = 10.869(9) Å, b = 10.838(13) Å, c = 17.138(18) Å, α = 90.32(9)°, β = 91.34(8)°, γ = 108.20(8)°, V = 1917(4) Å3, Z = 4. Ions form hydrogen bonded chains similar to those in II; acetones are unassociated. In I, columns form with the polar hydrogen bonded chains at the core and nonpolar aryl rings radiating and interdigitating adjacent columns. Packing in II and III produces bilayers of alternating polar and nonpolar regions, with acetones of III in the nonpolar regions.

Discrimination in resolving systems. V. Pseudoephedrine - fluoromandelic acid diastereomers

Resolution of the isomeric 2'-, 3'-, and 4'-fluoromandelic acids with (+)-(1S;2S)-pseudoephedrine in 95% ethanol produces both well and poorly discriminating, hydrated and unsolvated binary salts. Seven observed diastereomeric phases are represented by five crystal structure types including three of the four types observed in the pseudoephedrine mandelates. Type a: monoclinic hemihydrate less-soluble (L) (R)-3'-fluoromandelate and more-soluble (M) (R)-4'-fluoromandelate (I); type b: orthorhombic unsolvated M (S)-2'-fluoromandelate; type c: orthorhombic unsolvated L (R)-2'-fluoromandelate; type d: orthorhombic dihydrate M (S)-3'-fluoromandelate and L (S)-4'-fluoromandelate; type e: monoclinic unsolvated M (R)-4'-fluoromandelate (II). Largest (15-fold) discriminating solubilities in 95% ethanol are found between the diastereomers with 2'-fluoromandelic acid, 50% more than in the corresponding ephedrine system. Principle interionic interactions are hydrogen-bonds between protonated secondary ammonium ions and carboxylates. Infinite chains of these are found in type c, with a four-atom repeating unit H-N(+)-H.O(-C(-)-O) [C(2)(1)(4)], and in types b and d, with a six-atom repeating unit H-N(+)-H.O-C(-)-O [C(2)(2)(6)]. Water of crystallization intervenes in the chains of type a but not of type d hydrated salts, according with higher average dehydration temperatures in the former. Hydrated salts in general are excessively soluble in 95% ethanol.