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Synlett 2003(12): 1859-1861  
DOI: 10.1055/s-2003-42032
LETTER
© Georg Thieme Verlag Stuttgart · New York

Ruthenium-Mediated Isomerization and Ring-Closing Metathesis: Versatile Syntheses of 6-, 7- and 8-Membered Benzo-Fused Heterocycles

Willem A. L. van Otterlo*, Rakhi Pathak, Charles B. de Koning*
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits, 2050, Johannesburg, South Africa
e-Mail: willem@aurum.chem.wits.ac.zu; e-Mail: dekoning@aurum.chem.wits.ac.za;
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Publication History

Received 27 June 2003
Publication Date:
19 September 2003 (online)

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Abstract

Ruthenium-mediated isomerization and ring-closing metathesis (RCM) were the key steps used to convert a simple substrate, 2-allyl-3-isopropoxy-4-methoxybenzaldehyde, into a variety of 6-, 7- and 8-membered nitrogen- and oxygen-containing benzo-fused heterocycles.

Key words

ruthenium catalysts - metathesis - ring closure - isomerizations

23

Taken from the ongoing PhD of Rakhi Pathak.

24

To a degassed solution of N-allyl-N-(2-allyl-3-isopropoxy-4-methoxybenzyl)-4-methylbenzenesulfonamide (4b) (120 mg) in toluene (10 cm3) was added [RuClH(CO)(PPh3)3] (0.5 mol%). The reaction mixture was heated at 110 °C for 2 h under a N2 atmosphere. Grubbs’ catalyst 1 (5 mol%) was added and the reaction mixture was stirred for another 3 h at 110 °C under N2. After cooling the mixture, the toluene was evaporated under reduced pressure and the organic residue was then subjected to silica gel column chromatography (5-20% EtOAc-hexane) to afford the desired product, 5-isopropoxy-2-[(4-methylphenyl)sulfonyl]-1,2-dihydro-6-isoquinolinylmethyl ether (7) as a brown oil (0.079 g, 76%). Found: M+, 373.1348. C20H23NSO4 requires M+, 373.1348. IR (CHCl3): νmax = 1625 and 1597 (ArC=C) and 1167 (NSO2) cm-1. 1H NMR (300 MHz, CDCl3, Me4Si): δ = 1.21 [6 H, d, J = 6.2 Hz, CH(CH 3)2], 2.37 (3 H, s, ArCH3), 3.77 (3 H, s, OCH3), 4.35 {1 H, sept, J = 6.2 Hz, [CH(CH3)2]}, 4.48 (2 H, s, NCH2), 6.22 (1 H, d, J = 7.9 Hz, ArCH=CH), 6.64 (2 H, s, 7-ArH and 8-ArH), 6.74 (1 H, d, J = 7.9 Hz, NCH=CHAr), 7.25 (2 H, d, J = 8.1 Hz, 2 × ArH), 7.67 (2 H, d, J = 8.1 Hz, 2 × ArH). 13C NMR (75 MHz, CDCl3): δ = 21.4 (ArCH3), 22.4 [CH(CH3)2], 46.8 (NCH2), 55.7 (OCH3), 75.2 [CH(CH3)2], 106.3 (CH), 110.7 (CH), 120.4 (CH), 120.7 (C), 125.3 (C), 126.1 (CH), 126.9 (C), 127.1 (2 × CH), 129.2 (C), 129.7 (2 × CH), 143.9 (C), 152.4 (C). MS:
m/z (%) = 374 (24), 373 (90) [M+], 218 (18), 176 (100),
175 (89), 161 (35), 144 (45), 132 (30) and 91 (43).