Cooperative Al(salen)-pyridinium catalysts for the asymmetric synthesis of trans-configured β-lactones by [2+2]-cyclocondensation of acylbromides and aldehydes: Investigation of pyridinium substituent effects

Molecules

Volumn 17, Issue 6, 2012, Pages 7121-7150

  Meier, Patrick ^a   Broghammer, Florian ^a   Latendorf, Katja ^a   Rauhut, Guntram ^a   Peters, René ^a  

^a UNIVERSITY OF STUTTGART   (Germany)

Author keywords

2 Oxetanone; Aluminum; Cooperative catalysis; Ion pair catalysis; Ketene; Pyridinium

Indexed keywords

ALDEHYDE; BROMINATED HYDROCARBON; ETHYLENEDIAMINE DERIVATIVE; LACTONE; LEWIS ACID; LIGAND; ORGANOMETALLIC COMPOUND; PYRIDINIUM DERIVATIVE; SALEN ALUMINUM COMPLEX; SALEN-ALUMINUM COMPLEX;

ARTICLE; CATALYSIS; CHEMISTRY; NUCLEAR MAGNETIC RESONANCE; SYNTHESIS;

ALDEHYDES; CATALYSIS; ETHYLENEDIAMINES; HYDROCARBONS, BROMINATED; LACTONES; LEWIS ACIDS; LIGANDS; NUCLEAR MAGNETIC RESONANCE, BIOMOLECULAR; ORGANOMETALLIC COMPOUNDS; PYRIDINIUM COMPOUNDS;

EID: 84863009768     PISSN: None     EISSN: 14203049     Source Type: Journal    
DOI: 10.3390/molecules17067121     Document Type: Article

References (90)

1. Lowe, C.; Vederas, J. C. Naturally occurring β-lactones: Occurrence, syntheses and properties. A review. Org. Prep. Proceed. Int. 1995, 27, 305-346.
2. Pommier, A.; Pons, J.-M. The Synthesis of Natural 2-Oxetanones. Synthesis 1995, 729-744.
3. Feling, R. H.; Buchanan, G. O.; Mincer, T. J.; Kauffman, C. A.; Jensen, P. R.; Fenical, W. Salinosporamide A: A Highly Cytotoxic Proteasome Inhibitor from a Novel Microbial Source, a Marine Bacterium of the New Genus Salinospora. Angew. Chem. Int. Ed. Engl. 2003, 42, 355-357. (Pubitemid 36176323)
4. Masse, C.; Morgan, J.; Adams, J.; Panek, J. Syntheses and Biological Evaluation of (+)-Lactacystin and Analogs. Eur. J. Org. Chem. 2000, 2513-2528.
5. Kridel, S. J.; Axelrod, F.; Rosenkrantz, N.; Smith, J. W. Orlistat Is a Novel Inhibitor of Fatty Acid Synthase with Antitumor Activity. Cancer Res. 2004, 64, 2070-2075 (Pubitemid 38339455)
6. Knowles, L. M.; Axelrod, F.; Browne, C. D.; Smith, J. W. A Fatty Acid Synthase Blockade Induces Tumor Cell-cycle Arrest by Down-regulating Skp J. Biol. Chem. 2004, 279, 30540-30545. (Pubitemid 38937985)
7. Yang, P.-Y.; Liu, K.; Hong Ngai, M.; Lear, M. J.; Wenk, M. R.; Yao, S. Q. Activity-Based Proteome Profiling of Potential Cellular Targets of Orlistat-An FDA-Approved Drug with Anti-Tumor Activities. J. Am. Chem. Soc. 2010, 132, 656-666.
8. Greenberg, A.; Liebman, J. F. Strained Organic Molecules; Academic Press: New York, NY, USA, 1978.
9. Yang, H. W.; Romo, D. Methods for the synthesis of optically active β-lactones (2-oxetanones). Tetrahedron 1999, 55, 6403-6434. (Pubitemid 29236439)
10. Wang, Y.; Tennyson, R. L.; Romo, D. β-Lactones as Intermediates for Natural Product Total Synthesis and New Transformations. Heterocycles 2004, 64, 605-658.
11. Nelson, S. G.; Wan, Z. Catalytic Asymmetric Propionate Aldol Reactions via Acyl Halide-Aldehyde Cyclocondensations. Org. Lett. 2000, 2, 1883-1886.
12. Zipp, G. G.; Hilfiker, M. A.; Nelson, S. G. Enantioenriched Dihydropyrones from β-Lactone Templates. Org. Lett. 2002, 4, 1823-1826.
13. Nelson, S. G.; Cheung, W. S.; Kassick, A. J.; Hilfiker, M. A. A de Novo Enantioselective Total Synthesis of (.)-Laulimalide. J. Am. Chem. Soc. 2002, 124, 13654-13655. (Pubitemid 35378962)
14. Palomo, C.; Oiarbide, M.; Garcia, J. M. Current progress in the asymmetric aldol addition reaction. Chem. Soc. Rev. 2004, 33, 65-75. (Pubitemid 38388005)
15. Tidwell, T. T. Ketenes; Wiley: Hoboken, NJ, USA, 2006.
16. Tidwell, T. T. Ketene Chemistry after 100 Years: Ready for a New Century. Eur. J. Org. Chem. 2006, 563-576.
17. Tidwell, T. T. The First Century of Ketenes (1905-2005): The Birth of a Versatile Family of Reactive Intermediates. Angew. Chem. Int. Ed. Engl. 2005, 44, 5778-5785. (Pubitemid 41319691)
18. Paull, D. H.; Weatherwax, A.; Lectka, T. Catalytic, asymmetric reactions of ketenes and ketene enolates. Tetrahedron 2009, 65, 6771-6803.
19. Orr, R. K.; Calter, M. A. Asymmetric synthesis using ketenes. Tetrahedron 2003, 59, 3545-3565. (Pubitemid 36555844)
20. Tamai, Y.; Yoshiwara, H.; Someya, M.; Fukumoto, J.; Miyano, S. Asymmetric [2+2] cycloaddition of ketene with aldehydes catalysed by chiral bissulfonamide-trialkylaluminium complexes. J. Chem. Soc. Chem. Commun. 1994, 2281-2282.
21. Dymock, B. W.; Kocienski, P. J.; Pons, J.-M. 3-(Trimethylsilyl) oxetan-2-ones via enantioselective [2+2] cycloaddition of (trimethylsilyl) ketene to aldehydes catalysed by methylaluminiomidazolines. Chem. Commun. 1996, 1053-1054. (Pubitemid 126425195)
22. Yang, H. W.; Romo, D. Studies of the asymmetric [2+2] cycloaddition of silylketenes and aldehydes employing Ti-TADDOL catalysts. Tetrahedron Lett. 1998, 39, 2877-2880. (Pubitemid 28184048)
23. Nelson, S. G.; Peelen, T. J.; Wan, Z. Catalytic Asymmetric Acyl Halide-Aldehyde Cyclocondensations. A Strategy for Enantioselective Catalyzed Cross Aldol Reactions. J. Am. Chem. Soc. 1999, 121, 9742-9743. (Pubitemid 29498506)
24. Evans, D. A.; Janey, J. M. C2-Symmetric Cu (II) Complexes as Chiral Lewis Acids. Catalytic, Enantioselective Cycloadditions of Silyl Ketenes. Org. Lett. 2001, 3, 2125-2128. (Pubitemid 33631730)
25. Nelson, S. G.; Wan, Z. Catalytic Asymmetric Propionate Aldol Reactions via Acyl Halide. Aldehyde Cyclocondensations. Org. Lett. 2000, 2, 1883-1886.
26. Nelson, S. G.; Zhu, C.; Shen, X. Catalytic Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reactions of Substituted Ketenes. J. Am. Chem. Soc. 2004, 126, 14-15. (Pubitemid 38090316)
27. Forslund, R. E.; Cain, J.; Colyer, J.; Doyle, M. P. Chiral Dirhodium (II) Carboxamidate-Catalyzed [2+2]-Cycloaddition of TMS-Ketene and Ethyl Glyoxylate. Adv. Synth. Catal. 2005, 347, 87-92. (Pubitemid 40312178)
28. Kull, T.; Peters, R. Practical Enantioselective Synthesis of β-Lactones Catalyzed by Aluminum Bissulfonamide Complexes. Adv. Synth. Catal. 2007, 349, 1647-1652.
29. Schneider, C. Catalytic, Enantioselective Syntheses of β-Lactones-Versatile Synthetic Building Blocks in Organic Chemistry. Angew. Chem. Int. Ed. 2002, 41, 744-746. (Pubitemid 34218511)
30. Borrmann, D.; Wegler, R. Über die Umsetzung von Carbonylverbindungen mit Säurechloriden in Gegenwart tertiärer Amine, II. Optisch aktive β-Lactone. Chem. Ber. 1967, 100, 1575-1575.
31. Wynberg, H.; Staring, E. G. J. Asymmetric synthesis of (S) - and (R)-malic acid from ketene and chloral. J. Am. Chem. Soc. 1982, 104, 166-168.
32. Wynberg, H.; Staring, E. G. J. Catalytic asymmetric synthesis of chiral 4-substituted 2-oxetanones. J. Org. Chem. 1985, 50, 1977-1979.
33. Cortez, G. S.; Tennyson, R. L.; Romo, D. Intramolecular, Nucleophile-Catalyzed Aldol-Lactonization (NCAL) Reactions: Catalytic, Asymmetric Synthesis of Bicyclic β-Lactones. J. Am. Chem. Soc. 2001, 123, 7945-7946. (Pubitemid 32899375)
34. Wilson, J. E.; Fu, G. C. Asymmetric Synthesis of Highly Substituted β-Lactones by Nucleophile-Catalyzed [2+2] Cycloadditions of Disubstituted Ketenes with Aldehydes. Angew. Chem. Int. Ed. 2004, 43, 6358-6360. (Pubitemid 40029471)
35. Oh, S. H.; Cortez, G. S.; Romo, D. Asymmetric Synthesis of Bicyclic β-Lactones via the Intramolecular, Nucleophile-Catalyzed Aldol Lactonization: Improved Efficiency and Expanded Scope. J. Org. Chem. 2005, 70, 2835-2838.
36. Henry-Riyad, H.; Lee, C.; Purohit, V. C.; Romo, D. Bicyclic-and Tricyclic-β-lactones via Organonucleophile-Promoted Bis-Cyclizations of Keto Acids: Enantioselective Synthesis of (+) - Dihydroplakevulin. Org. Lett. 2006, 8, 4363-4366. (Pubitemid 44449982)
37. Mondal, M.; Ibrahim, A. A.; Wheeler, K. A.; Kerrigan, N. J. Phosphine-Catalyzed Asymmetric Synthesis of β-Lactones from Arylketoketenes and Aromatic Aldehydes. Org. Lett. 2010, 12, 1664-1667.
38. Zhu, C.; Shen, X.; Nelson, S. G. Cinchona Alkaloid-Lewis Acid Catalyst Systems for Enantioselective Ketene-Aldehyde Cycloadditions. J. Am. Chem. Soc. 2004, 126, 5352-5353. (Pubitemid 38596335)
39. Calter, M. A.; Tretyak, O. A.; Flaschenriem, C. Formation of Disubstituted β-Lactones Using Bifunctional Catalysis. Org. Lett. 2005, 7, 1809-1812. (Pubitemid 40685370)
40. Gnanadesikan, V.; Corey, E. J. Enantioselective β-Lactone Formation from Ketene and Aldehydes Catalyzed by a Chiral Oxazaborolidine. Org. Lett. 2006, 8, 4943-4945. (Pubitemid 44629499)
41. Lin, Y.-M.; Boucau, J.; Li, Z.; Casarotto, V.; Lin, J.; Nguyen, A. N.; Ehrmantraut, J. A Lewis Acid-Lewis Base Bifunctional Catalyst from a New Mixed Ligand. Org. Lett. 2007, 9, 567-570.
42. Chidara, S.; Lin, Y.-M. Reaction rate acceleration enabled by tethered Lewis acid-Lewis base bifunctional catalysis: A catalytic, enantioselective [2+2] ketene aldehyde cycloaddition reaction. Synlett 2007, 9, 1675-1679.
43. Denmark, S. E.; Wynn, T.; Beutner, G. L. Lewis Base Activation of Lewis Acids. Addition of Silyl Ketene Acetals to Aldehydes. J. Am. Chem. Soc. 2002, 124, 13405-13407. (Pubitemid 35285916)
44. Ishitani, H.; Yamashita, Y.; Shimizu, H.; Kobayashi, S. Highly anti-Selective Catalytic Asymmetric Aldol Reactions. J. Am. Chem. Soc. 2000, 122, 5403-5404. (Pubitemid 30395280)
45. Denmark, S. E.; Chung, W.-J. Lewis Base Activation of Lewis Acids: Catalytic Enantioselective Glycolate Aldol Reactions. Angew. Chem. Int. Ed. 2008, 47, 1890-1892.
46. Yoshikawa, N.; Kumagai, N.; Matsunaga, S.; Moll, G.; Ohshima, T.; Suzuki, T.; Shibasaki, M. Direct Catalytic Asymmetric Aldol Reaction: Synthesis of Either syn-or anti-a, β-Dihydroxy Ketones. J. Am. Chem. Soc. 2001, 123, 2466-2467. (Pubitemid 32904797)
47. Lecea, B.; Arrieta, A.; Lopez, X.; Ugalde, J. M.; Cossio, F. P. On the Stereochemical Outcome of the Catalyzed and Uncatalyzed Cycloaddition Reaction between Activated Ketenes and Aldehydes to form cis-and trans-2-Oxetanones. An ab Initio Study. J. Am. Chem. Soc. 1995, 117, 12314-12321.. (Pubitemid 126635344)
48. Pons, J.-M.; Oblin, M.; Pommier, A.; Rajzmann, M.; Liotard, D. Formation of β-Lactones through Lewis Acid-Promoted [2+2] Cycloaddition Reaction. A Theoretical Study. J. Am. Chem. Soc. 1997, 119, 3333-3338. (Pubitemid 27213835)
49. Singleton, D. A.; Wang, Y.; Yang, H. W.; Romo, D. Mechanism and Origin of Stereoselectivity in Lewis Acid Catalyzed [2+2] Cycloadditions of Ketenes with Aldehydes. Angew. Chem. Int. Ed. Engl. 2002, 41, 1572-1575. (Pubitemid 34534404)
50. For a trans-diastereoselective Lewis acid-Lewis base catalyzed procedure using chiral enantiopure aldehydes, see: Shen, X.; Wasmuth, A. S.; Zhao, J.; Zhu, C.; Nelson, S. G. Catalytic Asymmetric Assembly of Stereodefined Propionate Units: An Enantioselective Total Synthesis of (-)-Pironetin. J. Am. Chem. Soc. 2006, 128, 7438-7439. (Pubitemid 43877507)
51. For trans-selective alkylation of β-lactones, see e.g., Mulzer, J.; Kerkmann, T. α Deprotonation of β-lactones-An example of a forbidden β elimination. J. Am. Chem. Soc. 1980, 102, 3620-3622.
52. Mulzer, J.; Chucholowski, A.; Lammer, O.; Ibrill, I.; Huttner, G. Diastereoselective Michael additions of β-lactone enolates to dimethyl maleate. J. Chem. Soc. Chem. Commun. 1983, 869-871.
53. Kull, T.; Peters, R. Contact Ion Pair Directed Lewis Acid Catalysis: Asymmetric Synthesis of trans-Configured β-Lactones. Angew. Chem. Int. Ed. Engl. 2008, 47, 5461-5464.
54. Kull, T.; Cabrera, J.; Peters, R. Catalytic Asymmetric Synthesis of trans-Configured β-Lactones: Cooperation of Lewis Acid and Ion Pair Catalysis. Chem. Eur. J. 2010, 16, 9132-9139.
55. IV-catalyzed reactions providing racemic dioxanones acyl halide enolates were suggested as reactive intermediates: Nelson, S. G.; Peelen, T. J.; Wan, Z. Mechanistic alternatives in Lewis acid-catalyzed acyl halide-aldehyde cyclocondensations. Tetrahedron Lett. 1999, 40, 6541-6543.
56. -Cl proved computationally unstable relative to ketene and chloride: Rosenberg, R. E. Enols and Enolates of Carboxylic Acid Derivatives. A G2 (MP2) ab Initio Study. J. Org. Chem. 1998, 63, 5562-5567 (Pubitemid 128500841)
57. Ammonium salts of acid chloride enolates can be isolated, if the electron density of the enolate C atom is stabilized by an electron withdrawing group such as a Cl atom. Studies by Brady and Scherubel have shown that the enolate forming step is reversible. In contrast, in the absence of an electron withdrawing substituent, the enolate salts could neither be isolated nor detected spectroscopically as a result of their instability: Brady, W. T.; Scherubel, G. A. Halogenated ketenes. XXIII. Mechanism of the dehydrohalogenation of acid halides. J. Am. Chem. Soc. 1973, 95, 7447-7449.
58. Brady, W. T.; Scherubel, G. A. Halogenated ketenes. XXVII. Mechanism of the dehydrohalogenation of α-halo acid halides. J. Org. Chem. 1974, 39, 3790-3791.
59. Yoon, T. P.; Jacobsen, E. N. Privileged Chiral Catalysts. Science 2003, 299, 1691-1693. (Pubitemid 36337187)
60. Matsumoto, K.; Saito, B.; Katsuki, T. Asymmetric catalysis of metal complexes with non-planar ONNO ligands: Salen, salalen and salan. Chem. Commun. 2007, 35, 3619-3627. (Pubitemid 47346658)
61. Myers, J. K.; Jacobsen, E. N. Asymmetric Synthesis of β-Amino Acid Derivatives via Catalytic Conjugate Addition of Hydrazoic Acid to Unsaturated Imides. J. Am. Chem. Soc. 1999, 121, 8959-8960. (Pubitemid 29461553)
62. Cabrera, J.; Hellmuth, T.; Peters, R. Synthesis of Densely Substituted Trans-Configured 4-Acylated Piperidine-2, 4-diones as 3:1 adducts of Imines and Ketenes. J. Org. Chem. 2010, 75, 4326-4329.
63. Tiseni, P. S.; Peters, R. Catalytic Asymmetric Formation of δ-Lactones from Unsaturated Acyl Halides. Chem. Eur. J. 2010, 16, 2503-2517.
64. Tiseni, P. S.; Peters, R. Lewis Acid. Lewis Base Catalyzed Enantioselective Hetero-Diels-Alder Reaction for Direct Access to α,β-Lactones. Org. Lett. 2008, 10, 2019.2022.
65. Tiseni, P. S.; Peters, R. Catalytic Asymmetric Formation of β-Lactones by [4+2]-Cycloaddition of Zwitterionic Dienolates Generated from α, β-Unsaturated Acid Chlorides. Angew. Chem. Int. Ed. 2007, 46, 5325-5328. (Pubitemid 47067625)
66. Koch, F. M.; Peters, R. Lewis-Acid/Lewis-Base Catalyzed [2+2]-Cycloaddition of Sulfenes and Aldehydes: A Versatile Entry to Chiral Sulfonyl and Sulfinyl Derivatives. Chem. Eur. J. 2011, 17, 3679-3692.
67. Zajac, M.; Peters, R. Catalytic Asymmetric Synthesis of β-Sultams as Precursors for Taurine Derivatives. Chem. Eur. J. 2009, 15, 8204-8222.
68. Peters, R.; Fischer, D. F.; Jautze, S.; Koch, F. M.; Kull, T.; Tiseni, P. S.; Xin, Z.-Q.; Zajac, M. Catalytic Methods for Direct Access to Chiral High-Added-Value Products. Chimia 2008, 62, 497-505.
69. Koch, F. M.; Peters, R. Rapid Asymmetric Access to β-Hydroxysulfinic Acids and Allylsulfonic Acids by Chemoselective Reduction of β-Sultones. Synlett 2008, 1505-1509. (Pubitemid 351930806)
70. Zajac, M.; Peters, R. Catalytic Asymmetric Formation of β-Sultams. Org. Lett. 2007, 9, 2007.2010.
71. Koch, F. M.; Peters, R. First Catalytic Enantio-and Diastereoselective Formation of β-Sultones: Ring-Strained Precursors for Enantioenriched β-Hydroxysulfonyl Derivatives. Angew. Chem. Int. Ed. 2007, 46, 2685.2689.
72. Bhatt, S.; Nayak, S. K. Reductive deoxygenation of ortho-hydroxyaromatic aldehydes to 1, 2-bis (hydroxyaryl) ethanes: Application to the synthesis of ethylene bridged calixareneanalogous metacyclophanes. Tetrahedron Lett. 2009, 50, 5823.
73. Hofsløkken, N. U.; Skattebøl, L. Convenient Method for the ortho-Formylation of Phenols. Acta Chem. Scand. 1999, 53, 258.
74. Wang, Q.; Wilson, C.; Blake, A. J.; Collinson, S. R.; Tasker, P. A.; Schröder, M. The one-pot halomethylation of 5-substituted salicylaldehydes as convenient precursors for the preparation of heteroditopic ligands for the binding of metal salts. Tetrahedron Lett. 2006, 47, 8983-8986.
75. Larrow, J. F.; Jacobsen, E. N. Asymmetric processes catalyzed by chiral (salen) metal complexes. Top. Organomet. Chem. 2004, 6, 123-152.
76. Pommier, A.; Pons, J. M. Recent Advances in β-Lactone Chemistry. Synthesis 1993, 441-459.
77. Konsler, R. G.; Karl, J.; Jacobsen, E. N. Cooperative Asymmetric Catalysis with Dimeric Salen Complexes. J. Am. Chem. Soc. 1998, 120, 10780-10781. (Pubitemid 28498834)
78. Weber, M.; Jautze, S.; Frey, W.; Peters, R. Bis-Palladacycle Catalyzed Brønsted-Acid/-Base Promoted Asymmetric Tandem Azlactone Formation-Michael Addition. J. Am. Chem. Soc. 2010, 132, 12222-12225.
79. Jautze, S.; Peters, R. Bimetallic Enantioselective Catalysis of Michael Additions Forming Quaternary Stereocenters. Angew. Chem. Int. Ed. Engl. 2008, 47, 9284-9288.
80. Jautze, S.; Seiler, P.; Peters, R. Synthesis of nearly Enantiopure Allylic Amines by Aza-Claisen Rearrangement of Z-Configured Allylic Trifluoroacetimidates Catalyzed by Highly Active Ferrocenylbispalladacycles. Chem. Eur. J. 2008, 14, 1430.1444.
81. Jautze, S.; Seiler, P.; Peters, R. Macrocyclic Ferrocenyl Bisimidazoline Palladacycle Dimers (FBIPs) as Highly Active and Enantioselective Catalysts for the Aza-Claisen Rearrangement of (Z)-Configured N-para-Methoxyphenyl Trifluoroacetimidates. Angew. Chem. Int. Ed. Engl. 2007, 46, 1260.1264.
82. Satyanarayana, T.; Abraham, S.; Kagan, H. B. Nonlinear Effects in Asymmetric Catalysis. Angew. Chem. Int. Ed. Engl. 2009, 48, 456.494.
83. Werner, H.-J.; Knowles, P. J.; Manby, F. R.; Schütz, M.; Celani, P.; Knizia, G.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G.; et al. MOLPRO, version 2010.1; a package of ab initio programs, 2010.
84. Wang, X.-L.; Wan, H.; Guan, G.-F. Structure and Interaction of Ion-Pairs of [EPy]Cl and [EPy]Br in Gas and Liquid Phases. Acta Phys.-Chim. Sin. 2008, 24, 2077-2082.
85. Katsuki, T. Some recent advances in metallosalen chemistry. Synlett 2003, 281.297.
86. Katsuki, T. Unique asymmetric catalysis of cis-β metal complexes of salen and its related Schiff-base ligands. Chem. Soc. Rev. 2004, 33, 437.444.
87. Jacobsen, H. Computational Studies of the Hydrolytic Kinetic Resolution of Terminal Epoxides Catalysed by Co (salen) Complexes: Evaluation of Suitable Model Systems and Appropriate Density Functional Approaches. In Frontiers in Organometallic Chemistry; Cato, M. A., Ed.; Nova Science Publishers: New York, NY, USA, 2006; pp. 1-16.
88. 2 and Cyclohexene Oxide. Inorg. Chem. 2005, 44, 1433-1442. (Pubitemid 40333467)
89. Yu, X.; Scheller, D.; Rademacher, O.; Wolff, T. Selectivity in the Photodimerization of 6-Alkylcoumarins. J. Org. Chem. 2003, 68, 7386-7399. (Pubitemid 37122490)
90. Aldoshin, S. M.; Sanina, N. A.; Minkin, V. I.; Voloshin, N. A.; Ikorskii, V. N.; Oveharenko, V. I.; Smirnov, V. A.; Nagaeva, N. K. Molecular photochromic ferromagnetic based on the layered polymeric tris-oxalate of Cr (III), Mn (II) and 1-[(1', 3', 3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2, 2'-indoline]-8-yl) methyl]pyridinium. J. Mol. Struct. 2007, 826, 69-74. (Pubitemid 46075124)