1. N-(2-phenylethyl)-propionamide (10 g, 56.4 mmol) was mixed with phosphorus pentoxide (80 g, 563 mmol) in tetrahydronaphthalene (500 ml), heated to reflux and maintained for 15 minutes.
2. After cooling the reaction mixture, additional phosphorus pentoxide (80 g) was added and heated to reflux again and maintained for 15 minutes.
3. The reaction mixture was allowed to stand overnight and tetrahydronaphthalene was removed by decantation. 4.
4. The residue was decomposed with ice water and the resulting suspension was filtered through diatomaceous earth and washed with two parts of ether.
5. The aqueous solution is alkalized with concentrated ammonium hydroxide and extracted with three parts of ether.
6. The ether extracts were combined, dried over magnesium sulfate and subsequently evaporated to remove the solvent to afford the red solid product 1-ethyl-3,4-dihydroisoquinoline (5.0 g, 55.7% yield).
7. The product was confirmed by mass spectrometry as MS [M + H]+ 160.
[1] Organic Letters, 2015, vol. 17, # 10, p. 2478 - 2481
[2] Tetrahedron Letters, 2009, vol. 50, # 26, p. 3698 - 3701
[3] Organic Letters, 2017, vol. 19, # 12, p. 3151 - 3154
[4] Patent: US5929085, 1999, A
[5] Journal of the American Chemical Society, 1952, vol. 74, p. 666
