You can use this method to use the the mannich reaction to get N-phenethyl-4-piperidone if you change the benzylamine for phenethylamine and the 3-methyl-2-butanone for acetone.
Personally I would use methylethylketone.
From Patent US20030232833 (http://l2.espacenet.com/dips/viewer?PN=US20030232833&CY=gb&LG=en&DB=EPD)
Preparation of N-Benzyl-3,3-dimethyl-4-piperidone. 26
In a 1 liter 3-neck flask equipped with a mechanical stirrer, an addition funnel and a calcium chloride drying tube was added a 37% weight solution of formaldehyde (168.5 mL, 2.25 mole) dissolved in 500 mL of absolute ethanol. The resulting solution was cooled in an ice-water bath to 10.degree. C., and benzylamine (109 mL, 1 mole) was added dropwise over a one hour period. In a separate 3-liter 3-neck flask equipped with a mechanical stirrer, an addition funnel and two condensers was added 3-methyl-2-butanone (113 mL, 1.06 mole) dissolved in 500 ml of absolute ethanol and concentrated hydrogen chloride (92 mL, 1.11 mole). The resulting solution was brought to reflux and the formaldehyde/benzylamine solution is added dropwise over a 2 hour period. This solution was heated at reflux overnight, and then cooled to ambient temperature. Diisopropylethylamine (142.2 g, 1.1 mole) and formaldehyde (22.46 mL, 0.3 mole) were added and the resulting solution was heated to reflux for six hours, and then cooled to ambient temperature. The solution was quenched with potassium hydroxide (61.6 g, 1.1 mole) in 200 mL of water, and then extracted 3 times with 500 ml ethyl acetate. The organic layers were concentrated under vacuum to give 225 g of a red oil. The crude oil was dissolved in 1 liter of methylene chloride. This solution was carefully poured over 1 kg of silica gel on a sintered glass filter. The silica gel was washed with 4 L of methylene chloride. The methylene chloride was concentrated under vacuum to provide 142 g of a yellow oil which was crystallized in a freezer overnight. Yield=65.4%. MS(ion spray)=218.3(M+1)
There is another route to piperid-4-ones using divinyl ketone. This reagent is quite useful for other stuff as well and if someone could take some time to search for an easy preparation of it, I would appreciate it very much.
I only came across two non practical preparations:
Patent GB459537 (http://l2.espacenet.com/dips/viewer?PN=GB459537&CY=gb&LG=en&DB=EPD)
Patent GB544188 (http://l2.espacenet.com/dips/viewer?PN=GB544188&CY=gb&LG=en&DB=EPD)
But there must be easier ways. Maybe simply a dry distillation of barium or calcium salt of acrilyc acid would yield divinyl ketone?
Check this paper for more on this way toward piperid-4-ones:
Britten-Kelly M., Willis B.J, Barton D.H.R.; Michael additions to alkyl substituted divinyl ketones. Synthesis (1980) 1, 27-29.
(and Post 475654 (https://www.thevespiary.org/talk/index.php?topic=11492.msg47565400#msg47565400)
(Nicodem: "Bad conscience at work!", Novel Discourse) for a proposal of using it)
Beilstein essentially provides two general routes to divinyl ketone:
Mannich alkylation of acetone followed by elimination in the presence of a polymerization inhibitor
Reactants: propan-2-one, formaldehyde
Product: penta-1,4-dien-3-one
Reagents: diethylamine, hydroquinone
Reference: J.Appl.Chem.USSR (Engl.Transl.) 40, 1710-1714 (1967) // Zh.Prikl.Khim.(Leningrad) 40, 1777-1782 (1967)
Elimination of 3-pentanones substituted in either 2,4- or 1,5-position with chlorine, bromine or methoxy groups
Reactants: 2,4-dibromo-pentan-3-one
Product: penta-1,4-dien-3-one
Reagents: KOBu-t, DMSO
Reference: Tetrahedron Lett. 40(16), 3247-3250 (1999)
(No synthetic details given, only 3-Pentanone --Br2/MeOH--> 2,4-dibromo-3-pentanone --KOtBu/DMSO--> Divinyl ketone)
A detailed prep of the dibromoketone can be found in Organic Syntheses, CV 6, 520 (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0520)
(http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0520) and in Organic Syntheses, CV 6, 512 (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0512)
(http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0512)
Reactants: 1,5-dibromo-2-pentanone
Product: penta-1,4-dien-3-one
Reagents: Na2CO3
Conditions: Heating, 65-80 Torr, 78%
Reference: J.Org.Chem.USSR (Engl.Transl.) 27; 7.1, 1251-1253 (1991) // Zh.Org.Khim.; 27; 7; 1431-1433 (1991)
Reactants: 1,5-dichloro-pentan-3-one
Product: penta-1,4-dien-3-one
Reagents: diethylaniline
Conditions: 180°C
Reference: Patent US2105792 (http://l2.espacenet.com/dips/viewer?PN=US2105792&CY=gb&LG=en&DB=EPD)
Patent GB459537 (http://l2.espacenet.com/dips/viewer?PN=GB459537&CY=gb&LG=en&DB=EPD)
Reactants: 1,5-dimethoxy-pentan-3-one
Product: penta-1,4-dien-3-one
Conditions: Distillation with p-toluenesulfonic acid
Reference: Izv.Akad.Nauk SSSR Ser.Khim.; 1947; 495, 499; Chem.Abstr.; 1948; 7735.
Here is the article mentioned in the posts Post 481803 (https://www.thevespiary.org/talk/index.php?topic=10423.msg48180300#msg48180300)
(Megatherium: "Thank you for this information :-) .", Newbee Forum) and Post 344538 (https://www.thevespiary.org/talk/index.php?topic=8925.msg34453800#msg34453800)
(Ritter: "2,5 dimethyl benzyl fentanyl!", Methods Discourse)
4-Anilidopiperidine Analgesics I: Synthesis and Analgesic Activity of Certain Ring-Methylated 1-Substituted 4-Propananilidopiperidines
T.N. Riley, D.B. Hale, and M.C. Wilson
Journal of Pharmaceutical Sciences Vol. 62, No. 6, 983-986 (1973) (https://www.thevespiary.org/rhodium/Rhodium/pdf/fentanyl/4-anilinopiperidine.analgesics-1.pdf)
(https://www.thevespiary.org/rhodium/Rhodium/pdf/fentanyl/4-anilinopiperidine.analgesics-1.pdf)
Abstract
In view of the potency-enhancing effect of methyl substitution of the piperidine ring of the 4-phenylpiperidine analgesics and the alkylene chain of the acyclic basic anilide analgesics, the 1-methyl, 1-benzyl, and 1-phenylethyl derivatives of 2-methyl-, 3-methyl-, and 2,5-dimethyl-4-propananilidopiperidine were prepared. The analgesic activity of these compounds indicates that 3-methylation has the greatest effect in enhancing analgesic potency whereas 2-methyl and 2,5-dimethyl substitution is detrimental to analgesic activity.