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Synlett 2011(5): 639-642  
DOI: 10.1055/s-0030-1259695
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

A Rapid Assembly of Furo[3,4-b]- and Pyrrolo[3,4-b]carbazolones by Domino Wittig Diels-Alder Reaction

Prachi Torney, Rupesh Patre, Santosh Tilve*
Department of Chemistry, Goa University, Goa 403206, India
Fax: +91(832)2452886; e-Mail: stilve@unigoa.ac.in;
Further Information

Publication History

Received 21 December 2010
Publication Date:
25 February 2011 (online)

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Abstract

Regioisomeric hexahydrofuro[3,4-b]carbazol-1-ones, hexahydropyrrolo[3,4-b]carbazol-1-ones, hexahydrofuro[3,4-b]carbazol-3-ones and hexahydropyrrolo[3,4-b]carbazol-3-ones were synthesized in 59% to 62% yields by domino Wittig Diels-­Alder reactions from indole-3-carboxaldehyde and indole-2-carboxaldehyde and Wittig reagents. Further the corresponding carbazolelactones and carbazolelactams were obtained by oxidation with DDQ.

Key words

domino Wittig - Diels-Alder - furocarbazoles - pyrrolocarbazoles

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General Procedure for the Tandem Wittig-Diels-Alder Reaction for Preparation of Tetrahydrocarbazole Lactones (5a,b/10a,b) and Tetrahydrocarbazole Lactams (5c,d/10c,d): A solution of indole carboxaldehyde 1/7 (1 mmol) and phosphorane 2a-d (1.5 mmol) in diphenyl ether (10 mL) was refluxed under nitrogen atmosphere for 2-8 h. The crude mixture was subjected to column chromatography over silica gel and diphenyl ether was removed using hexanes as eluent. Further elution with
30-40% EtOAc and hexanes afforded the corresponding
γ-lactones 5a,b/10a,b and γ-lactams 5c,d/10c,d.
General Procedure for Aromatization Using DDQ: A mixture of tetrahydrocarbazoles 5a-d/10a-d (1 mmol) and DDQ (3 mmol) in dioxane (10 mL) was refluxed for 8 h. The reaction mixture was allowed to cool to ambient temperature and filtered. The filtrate was then concentrated under reduced pressure. The resulting residue was dissolved in EtOAc (20 mL) and washed with 2 N NaOH (20 mL) and H2O (20 mL). The organic phase was dried over anhyd Na2SO4 and concentrated under reduced pressure. The resulting residue on purification using flash chromatography with hexanes-EtOAc (70:30) gave the oxidized products 6a-d/11a-d.
3,5-Dihydro-1 H -furo[3,4- b ]carbazol-1-one (6a): ¹H NMR (300 MHz, DMSO): δ = 5.46 (s, 2 H), 7.21 (t, J = 7.8 Hz, 1 H), 7.49 (t, J = 7.8 Hz, 1 H), 7.53 (d, J = 8.1 Hz, 1 H), 7.62 (s, 1 H), 8.30 (d, J = 7.8 Hz, 1 H), 8.66 (s, 1 H), 11.78 (s, 1 H). ¹³C NMR (300 MHz, DMSO): δ = 69.85, 104.29, 111.83, 116.03, 118.12, 120.10, 121.47, 122.56, 124.50, 127.28, 141.27, 144.40, 144.85, 171.69. HRMS: m/z [M + Na] calcd for C14H9O2N: 246.0531; found: 246.0524.