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DOI: 10.1055/s-2001-15067
Solvent Trapping of Photochemically Generated Pyran-4-one-Derived Oxyallyls: A Convenient Cyclopentannulation Method
Publication History
Publication Date:
24 September 2004 (online)
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Abstract
Fused bicyclic pyran-4-ones 2a-f are readily available via one-step annulation with diketene or diketene equivalents. Irradiation of 2b-e in methanol furnished ring-contracted solvent adducts 4 in moderate yields, whereas irradiation in aqueous H2SO4 gave the corresponding glycols 8 and 9. The unusual predominance of cis-fused product 8b appears to result from in situ acid-catalyzed equilibration. Substrate 2f could also be trapped in aqueous acid, and displayed a modest stereoselectivity that may derive from conformational control by the methyl group on the starting material’s stereogenic center.
Key Words
annulations - bicyclic compounds - diols - heterocycles - photochemistry - ring contractions
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References and Notes
Molecular mechanics calculations (AM1)indicate that cis-isomer 8b is more stable than trans-isomer 9b by as much as 12 kcal/mol.
12In contrast, trans-isomers 9c and 9d did not undergo any isomerization under the reaction conditions.
13X-ray diffraction data for 4e, 8d and 8f have been deposited with the Cambridge Crystallographic Data Centre (deposition numbers CCDC 158715, CCDC 158907 and CCDC 158908, respectively).
14Hydroxyl proton assignments were confirmed by D2O exchange. In acetone-d 6, the hydroxyl protons of both cis-8 and trans-9 all appeared downfield of 4 ppm.
15Higher concentrations of H2SO4 led to variable product ratios, presumably via acid-catalyzed isomerization as noted with 8b/9b.
16The UV absorbance data for known compounds 2a,b have been previously reported: 2a (MeOH): λmax 252 nm (e = 12.6 × 103 cm-1M-1);
[5]
2b (MeOH): λmax 252 nm (e = 13.6 × 103 cm-1M-1);
[5]
2b (EtOH): λmax 252 (e = 5.6 × 103 cm-1M-1).
[17]
The data for 2c-f are: 2c (MeOH): λmax 256 nm (e = 10.2 × 103 cm-1M-1); 2d (MeOH): λmax 256 nm
(10-5 M; e = 11.4 × 103 cm-1M-1); 2e (MeOH): λmax 256 nm (10-5 M; e = 11.1 × 103 cm-1M-1); 2f (EtOH): λmax 262 nm (e = 7.5 × 103 cm-1M-1.