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Synlett 2008(7): 979-982  
DOI: 10.1055/s-2008-1072510
LETTER
© Georg Thieme Verlag Stuttgart · New York

Novel Synthesis of Nine-Membered Oxa-Heterocycles by Pd(0)-Catalyzed Intramolecular Heck Reaction via Unusual 9-endo-trig-Mode Cyclization

K. C. Majumdar*, B. Chattopadhyay
Department of Chemistry, University of Kalyani, Kalyani 741235, West Bengal, India
e-Mail: kcm_ku@yahoo.co.in;
Further Information

Publication History

Received 23 October 2007
Publication Date:
17 March 2008 (online)

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Abstract

Syntheses of nine-membered oxa-heterocyclic compounds by the application of the intramolecular Heck reaction have been difficult to develop. Herein, we describe the synthesis of this class of compounds through the 9-endo-trig cyclization and 8-endo-trig cyclization, respectively - a rare mode of cyclization in the literature.

Key words

Claisen rearrangement - intramolecular Heck reaction - 9-endo-trig - Pd(OAc)2 catalyst - oxa-heterocycles - medium-sized ring compounds

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General Procedure for the Synthesis of Compounds 5a-f and 6a
A mixture of the compound 4a (100 mg, 0.265 mmol), TBAB (1.2 equiv), dry KOAc (2.75 equiv) was taken in dry DMF (10 mL) under nitrogen atmosphere. Then, the catalyst Pd(OAc)2 (10 mmol%, 5.92 mg) was added and the mixture was stirred in an oil bath at 100 °C for about 2-3.5 h. The reaction mixture was cooled, and H2O (3 mL) was added. It was extracted with EtOAc (3 × 10 mL) and washed with H2O (2 ×10 mL), followed by brine (15 mL). The organic layer was dried (Na2SO4) and evaporation of EtOAc furnished the crude mass, which was purified by column chromatography over silica gel. Elution of the column with 10% EtOAc-PE afforded the product 5a. Similarly, the other substrates 4b-f were subjected to the reaction under the same conditions to give products 5b-f and 6a.

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Synthesis of the Precursors 4a-f; Typcial Procedure A mixture of compound 3 and different 2-bromobenzylbromides and dry K2CO3 (2.0 mg) in dry acetone (75 mL) in the presence of NaI was refluxed for a period of 2-3 h. After cooling the reaction mixture was filtered and the solvent was removed. The residual mass was extracted with CHCl3, washed with H2O, followed by brine-H2O, and dried (Na2SO4). Removal of CHCl3 gave a crude product, which was chromatographed over silica gel (60-120 mesh). Elution of the column with PE-EtOAc (2:1) gave compounds 4a-f. Compound 4a: yield 93%, white solid, mp 101-102 °C. IR (KBr): νmax = 1649, 1701 cm-1. 1H NMR (400 MHz, CDCl3): δ = 1.24 (d, 3 H, =CHCH 3, J = 7.2 Hz), 3.32 (s, 3 H, NCH3), 3.39 (s, 3 H, NCH3), 4.28-4.31 (m, 1 H, =CHCH3), 5.02 (dd, 1 H, =CHCH aHb, J = 2.1, 17.4 Hz), 5.11 (dd, 1 H, =CHCHa H b, J = 2.1, 12.2 Hz), 5.12 (s, 2 H, OCH2), 5.72-5.80 (m, 1 H, CH2=CH), 7.18 (dt, 1 H, ArH, J = 1.6, 7.7 Hz), 7.31 (dt, 1 H, ArH, J = 1.0, 7.4 Hz), 7.51-7.56 (m, 2 H, ArH). 13C NMR (125 MHz, CDCl3): δ = 16.1, 28.1, 33.2, 34.0, 73.0, 115.6, 123.5, 127.4, 129.6, 130.0, 130.8, 132.5, 135.9, 137.7, 147.7, 151.3, 157.6. MS: m/z = 378 [M+], 380 [M+ + 2]. Anal. Calcd (%) for C17H19BrN2O3: C, 53.84; H, 5.05; N, 7.39. Found: C, 53.88; H, 5.09; N, 7.23.

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Synthesis of Compounds 5a-f and 6a; General Procedure A mixture of the compound 4a (100 mg, 0.265 mmol), TBAB (1.2 equiv), dry KOAc (2.75 equiv) was taken in dry DMF (10 mL) under nitrogen atmosphere. Then, the catalyst Pd(OAc)2 (10 mmol%, 5.92 mg) was added and the mixture was stirred in an oil bath at 100 °C for about 2-3.5 h. The reaction mixture was cooled, and H2O (3 mL) was added. It was extracted with EtOAc (3 × 10 mL), washed with H2O (2 × 10 ml), and followed by brine (15 mL). The organic layer was dried (Na2SO4), evaporation of EtOAc furnished the crude mass, which was purified by column chromatography over silica gel. Elution of the column with 10% EtOAc-PE afforded the product 5a. Similarly, the other substrates 4b-f were subjected to the reaction under the same conditions to give products 5b-f and 6a.
Compound 5a: yield 90%, white solid, mp 193-194 °C. IR (KBr): νmax = 1648, 1700 cm-1. 1H NMR (400 MHz, CDCl3): δ = 1.84 (s, 3 H, =CCH3), 3.27 (d, 2 H, =CHCH 2, J = 8.6 Hz), 3.35 (s, 3 H, NCH3), 3.43 (s, 3 H, NCH3), 4.65 (d, 1 H, OCH aHb, J = 12.6 Hz), 5.40 (d, 1 H, OCH aHb, J = 12.6 Hz), 5.87 (t, 1 H, J = 8.6 Hz, CH2CH), 7.24-7.27 (m, 2 H, ArH), 7.32-7.36 (m, 2 H, ArH). 13C NMR (135 mode, CDCl3): δ = 21.9, 28.6, 34.4, 36.7, 77.2, 123.3, 127.3, 129.5, 130.3, 130.6, 132.0, 133.9, 137.1, 138.5, 143.5, 151.9, 161.1. DEPT (135 mode, CDCl3): δ = 21.9, 28.6, 34.4, 36.7 (CH2), 77.2 (OCH2), 127.3, 129.5, 130.3, 130.6, 133.9. HRMS: m/z calcd: 299.1406 [M + H], 321.1250 [Na + H]. Found: 299.1448 [M + H], 321.1224 [Na + H]. Anal. Calcd (%) for C17H18N2O3: C, 68.44; H, 6.08; N, 9.39. Found: C, 68.59; H, 6.07; N, 9.55.
The benzylic CH2 protons appear at δ = 4.65 and 5.40 ppm perhaps due to the slow exchange of the two nine-membered ring conformers in NMR time scale. However, DEPT (135 mode) experiment showed that the CH2 carbon is present in benzylic CH2O moiety. This was again confirmed by HMBC and COSY experiments.

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Compound 6a: yield 12%, viscous liquid. IR (neat): νmax = 1648, 1701 cm-1. 1H NMR (400 MHz, CDCl3): δ = 2.34 (s, 3 H, =CHCH 3), 3.21 (s, 3 H, NCH3), 3.27 (s, 3 H, NCH3), 5.24 (s, 2 H, OCH2), 6.02 (s, 1 H, =CH), 7.19-7.25 (m, 2H, ArH), 7.26-7.33 (m, 2 H, ArH). MS: m/z = 284 [M+]. Anal. Calcd (%) for C16H16N2O3: C, 67.59; H, 5.67; N, 9.85. Found: C, 67.71; H, 5.66; N, 9.88.