Sorry Sedrick, this is not really addressing your point about 4,4-diphenylpiperidine analogues, but it's slightly related -- it has to do with synthesizing intermediates for very potent fentanyl analogs such as carfentanil. (I hope this is an acceptable thread to post the reference):
An optimized synthesis of a key pharmaceutical intermediate methyl 4-[(1-oxopropyl)phenylamino]piperidine-4-carboxylate
V. D. Kiricojevi, M. D. Ivanovi, I. V. Micovic, J. B. Djordjevic, G.M. Roglic
J.Serb.Chem.Soc. 67(12), 793–802 (2002)
http://www.shd.org.yu/htdocs/shd/Vol67/No12/V67-No12-01.pdf (http://www.shd.org.yu/htdocs/shd/Vol67/No12/V67-No12-01.pdf)
Abstract:
An efficient synthesis of methyl 4-[(1-oxopropyl)phenylamino]piperidine-4-carboxylate(7) has been developed,starting from 1-benzylpiperidin-4-one (1). The compound is a key intermediate in the synthesis of new generation, highly active narcotic analgesics, such as remifintanil, as well as the novel classes of fentanyl analogues. An optimized Strecker-type condensation of piperidone 1 with aniline and HCN yielded the anilino-nitrile 2 (~90 %) which, upon selective hydrolysis with conc.H2SO4, gave the anilino-amide 3. After vigorous basic hydrolysis of 3, followed by acidification and successive treatment with SOCl2 and MeOH, the anilino-ester 5 was obtained (40–45 %, in 3 steps). N-Acylation of 5 with propionyl chloride yielded the anilido-ester 6 (70–80 %). [[ is this carfentanil? ]] In the final step, the catalytic N-debenzylation of 6 was examined under various conditions and optimized to yield 7 in near quantitative yields.
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Getting back to your question, I guess the structure of the compounds Shulgin mentions would look something similar to this, but I could be completely wrong:
Molecule: (https://www.the-hive.ws/forum/faq.pl?Cat=#applet)
4,4-diphenyl-6-(piperidin-1-yl)heptan-3-one ("CCC(C(C2=CC=CC=C2)(C3=CC=CC=C3)CC(N1CCCCC1)C)=O")
The reference given in the Shulgin article for these analogs is:
P. A. J. Janssen and C. A. M. van der Eycken, "The Chemical Anatomy of Potent Morphine-like Analgesics." Drugs Affecting the Central Nervous System, (A. Burger, ed.), Dekker, New York 1968, pp. 25-60.