Pictet–Spengler reaction

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The Pictet–Spengler reaction is a chemical reaction in which a β-arylethylamine such as tryptamine undergoes ring closure after condensation with an aldehyde or ketone. Usually an acidic catalyst is employed and the reaction mixture heated,[1][2] but some reactive compounds give good yields even at physiologic conditions.[3] The Pictet–Spengler reaction can be considered a special case of the Mannich reaction.

The reaction was discovered in 1911 by Amé Pictet and Theodor Spengler. It has remained an important reaction in the fields of alkaloid and pharmaceutical synthesis. The Pictet–Spengler reaction product of tryptophan and aldoses can be identified in foodstuffs such as soy sauce and ketchup.

Nucleophilic aromatic rings such as indole or pyrrole give products with good yields and mild conditions, while less nucleophilic aromatic rings such as phenyl give poor yields despite high temperatures and strong acid. The original Pictet–Spengler reaction was the reaction of β-phenethylamine with the dimethyl acetal of formaldehyde and hydrochloric acid forming a tetrahydroisoquinoline.

Like the Mannich reaction, aldehydes give good yields while ketones tend to give lower conversion.[citation needed]

The Pictet–Spengler reaction has been applied to solid-phase combinatorial chemistry with great success.[4][5]

An analogous reaction with an aryl-β-ethanol is called Oxa-Pictet–Spengler reaction[6]

Reaction mechanism

It is the electrophilicity of the imine double bond that is the driving force of the cyclization.[7] The reaction mechanism occurs by initial formation of an iminium ion (4) followed by electrophilic substitution at the 2-position. After deprotonation, the desired product (6) is formed. The reaction shown is an example of a 6-endo-trig reaction, which is favoured by Baldwin's ring closure rules. The reaction is driven by the nucleophilicity of the 2' position on the indole.

Variations

Pictet–Spengler tetrahydroisoquinoline synthesis

Replacing an indole with a 3,4-dimethoxyphenyl group give the reaction named the Pictet–Spengler tetrahydroisoquinoline synthesis. Reaction conditions are generally harsher than the indole variant, and require refluxing conditions with strong acids like hydrochloric acid, trifluoroacetic acid or superacids.[8][9]

N-acyliminium Pictet–Spengler reaction

Instead of catalyzing the Pictet-Spengler cyclization with strong acid, one can acylate the iminium ion forming the intermediate N-acyliminium ion. The N-acyliminium ion is a very powerful electrophile and most aromatic ring systems will cyclize under mild conditions with good yields.[10]

Tadalafil is synthesized via the N-acyliminium Pictet–Spengler reaction.[11] This reaction can also be catalyzed by AuCl3 and AgOTf.[12]

Asymmetric Pictet–Spengler reaction

When the Pictet–Spengler reaction is performed with an aldehyde other than formaldehyde, a new chiral center is created. Several substrate- or auxiliary-controlled diastereoselective Pictet–Spengler reactions have been developed.[13][14] Additionally, List et al. have published a chiral Lewis acid that catalyzes asymmetric Pictet–Spengler reactions.[15]

Tryptophans: diastereocontrolled reaction
The reaction of enantiopure tryptophan or its short-chain alkylesters leads to 1,2,3,4-tetrahydro-β-carbolines in which a new chiral center at C-1 adopts either a cis or trans configuration towards the C-3 carboxyl group. The cis conduction is kinetically controlled, i.e. it is performed at lower temperatures. At higher temperatures the reaction becomes reversible and usually favours racemisation. 1,3-trans dominated products can be obtained with Nb-benzylated tryptophans, which are accessible by reductive amination. The benzyl group can be removed hydrogenolytically afterwards. As a rough rule, 13C NMR signals for C1 and C3 are downfield shifted in cis products relative to trans products (see steric compression effect).[7][16]

See also

References

  1. ↑ Pictet, A.; Spengler, T. (1911). "Über die Bildung von Isochinolin-derivaten durch Einwirkung von Methylal auf Phenyl-äthylamin, Phenyl-alanin und Tyrosin". Berichte der deutschen chemischen Gesellschaft 44 (3): 2030–2036. doi:10.1002/cber.19110440309. 
  2. ↑ Whaley, W. M.; Govindachari, T. R. (1951). "The Pictet-Spengler synthesis of tetrahydroisoquinolines and related compounds". Org. React. 6: 74. 
  3. ↑ Hahn G., Ludewig H. (1934). "Synthese von Tetrahydro-harman-Derivaten unter physiologischen Bedingungen". Chemische Berichte 67 (12): 2033. doi:10.1002/cber.19340671221. 
  4. ↑ Nielsen, T. E.; Diness, F.; Meldal, M. (2003). "Solid-Phase Synthesis of Pyrroloisoquinolines via the Intramolecular N-Acyliminium Pictet-Spengler Reaction". Curr. Opin. Drug Discov. Devel. 6 (6): 801–814. PMID 14758752. 
  5. ↑ Nielsen, T. E.; Meldal, M. (2005). "Solid-Phase Synthesis of Pyrroloisoquinolines via the Intramolecular N-Acyliminium Pictet-Spengler Reaction". J. Comb. Chem. 7 (4): 599–610. doi:10.1021/cc050008a. 
  6. ↑ Larghi, E. L.; Kaufman, T. S. (2006). "The Oxa-Pictet-Spengler Cyclization. Synthesis of Isochromanes and Related Pyran-Type Heterocycles". Synthesis (2): 187–210. doi:10.1055/s-2005-918502. 
  7. ↑ 7.0 7.1 Cox, E. D.; Cook, J. M. (1995). "The Pictet-Spengler condensation: a new direction for an old reaction". Chemical Reviews 95 (6): 1797–1842. doi:10.1021/cr00038a004. 
  8. ↑ Yokoyama, Akihiro; Ohwada, Tomohiko; Shudo, Koichi (1999). "Prototype Pictet−Spengler Reactions Catalyzed by Superacids. Involvement of Dicationic Superelectrophiles". J. Org. Chem. 64 (2): 611–617. doi:10.1021/jo982019e. 
  9. ↑ Quevedo, R.; Baquero, E.; Rodriguez, M. Regioselectivity in isoquinoline alkaloid Synthesis, (2010), Tetrahedron Letters, 51 (13), 1774–1778. http://dx.doi.org/10.1016/j.tetlet.2010.01.115
  10. ↑ B. E. Maryanoff, H.-C. Zhang, J. H. Cohen, I. J. Turchi, and C. A. Maryanoff (2004). "Cyclizations of N-acyliminium ions". Chem. Rev. 104 (3): 1431–1628. doi:10.1021/cr0306182. 
  11. ↑ Bonnet, D.; Ganesan, A. (2002). "Solid-Phase Synthesis of Tetrahydro-β-carbolinehydantoins via the N-Acyliminium Pictet-Spengler Reaction and Cyclative Cleavage". J. Comb. Chem. 4 (6): 546–548. doi:10.1021/cc020026h. 
  12. ↑ Youn, S. W. (2006). "Development of the Pictet-Spengler Reaction Catalyzed by AuCl3/AgOTf". J. Org. Chem. 71 (6): 2521–2523. doi:10.1021/jo0524775. 
  13. ↑ Gremmen, C.; Willemse, B.; Wanner, M. J.; Koomen, G.-J. (2000). "Enantiopure Tetrahydro-β-carbolines via Pictet-Spengler Reactions with N-Sulfinyl Tryptamines". Org. Lett. 2 (13): 1955–1958. doi:10.1021/ol006034t. 
  14. ↑ a) The intermolecular Pictet-Spengler condensation with chiral carbonyl derivatives in the stereoselective syntheses of optically-active isoquinoline and indole alkaloids Enrique L. Larghi, Marcela Amongero, Andrea B. J. Bracca, and Teodoro S. Kaufman Arkivoc (RL-1554K) pp 98–153 2005 (Online Review); b) Teodoro S. Kaufman “Synthesis of Optically-Active Isoquinoline and Indole Alkaloids Employing the Pictet-Spengler Condensation with Removable Chiral Auxiliaries Bound to Nitrogen”. in “New Methods for the Asymmetric Synthesis of Nitrogen Heterocycles”; Ed.: J. L. Vicario. ISBN 81-7736-278-X. Research SignPost, Trivandrum, India. 2005. Chapter 4, pp. 99–147.
  15. ↑ Seayad, J.; Seayad, A. M.; List, B. (2006). "Catalytic Asymmetric Pictet-Spengler Reaction". J. Am. Chem. Soc. 128 (4): 1086–1087. doi:10.1021/ja057444l. 
  16. ↑ Ungemach, F.; Soerens, D.; Weber, R.; Dipierro, M.; Campos, O.; Mokry, P.; Cook, J. M.; Silverton, J. V. (1980). "General method for the assignment of stereochemistry of 1,3-disubstituted 1,2,3,4-tetrahydro-β-carbolines by carbon-13 spectroscopy". J. Am. Chem. Soc. 102 (23): 6976. doi:10.1021/ja00543a012. 
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