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 H2 SO4 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
References and Notes
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11 Molecular mechanics calculations (AM1)indicate that cis -isomer 8b is more stable than trans -isomer 9b by as much as 12 kcal/mol.
12 In contrast, trans -isomers 9c and 9d did not undergo any isomerization under the reaction conditions.
13 X-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).
14 Hydroxyl proton assignments were confirmed by D2 O exchange. In acetone-d
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15 Higher concentrations of H2 SO4 led to variable product ratios, presumably via acid-catalyzed isomerization as noted with 8b /9b .
16 The UV absorbance data for known compounds 2a ,b have been previously reported: 2a (MeOH): λmax 252 nm (e = 12.6 × 103 cm-1 M-1 );
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[5 2b (EtOH): λmax 252 (e = 5.6 × 103 cm-1 M-1 ).
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