B.
N-Methyl-2-phenyl-Δ2-tetrahydropyridine. A
500-ml., nitrogen-purged flask equipped with serum cap, reflux condenser with nitrogen inlet connection, thermometer, and stirring bar, is charged with
100 ml. of dry tetrahydrofuran (Note
9) and
21.0 ml. (0.155 mole) of diisopropylamine (Note
10). The solution is cooled to −30° with an
acetone bath to which dry ice was added as needed, and
72.4 ml. (0.155 mole) of 2.14 M n-butyllithium in hexane (Note
11) is added while keeping the temperature below 0°. After cooling the mixture to −40°,
20.6 g. (0.155 mole) of the imine (from part A) is added
via syringe over a period of about 2 minutes. The resulting yellow solution is maintained at −40° to −30° for 15 minutes then cooled to −60°. To the cold solution is added
25.2 g. (16.5 ml., 0.160 mole) of 1-bromo-3-chloropropane (Note
12) in one portion
via syringe while the temperature is maintained below −40°. The reaction mixture is maintained between −60° and −50° for 5 minutes, the bath is removed, and the mixture is allowed to warm to room temperature. The reaction mixture is refluxed 3 hours to effect ring closure (Note
13). After the addition of
150 ml. of 10% aqueous potassium carbonate to the cooled solution, the reaction mixture is stirred several minutes and transferred to a nitrogen-purged
separatory funnel. The reaction flask is rinsed with
100 ml. of 1 : 1 benzene–ether which is added to the
separatory funnel, and the entire mixture is diluted with 150 ml. of water. After shaking, the aqueous layer is removed, the organic layer is washed with
100 ml. of brine, shaken with anhydrous granular
sodium sulfate, and filtered into a
1-l., round-bottomed flask. The solvents are removed on a
rotary evaporator, and the residue is transferred to a
100-ml., round-bottomed flask. Short-path distillation under high vacuum yields
18.8–21.7 g. (
70–81%) of pale yellow enamine, b.p.
87–88° (4 mm.) (Note
14) and (Note
15).