Solvent Trapping of Photochemically Generated Pyran-4-one-Derived Oxyallyls: A Convenient Cyclopentannulation Method

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

References and Notes

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• Notably, intermolecular trapping of non-photochemically generated oxyallyl cations seems to favor attack at the less substituted terminus:
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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 D₂O exchange. In acetone-d ₆, the hydroxyl protons of both cis-8 and trans-9 all appeared downfield of 4 ppm.

15

Higher concentrations of H₂SO₄ 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): λ_max 252 nm (e = 12.6 × 10³ cm^-1M^-1); [5] 2b (MeOH): λ_max 252 nm (e = 13.6 × 10³ cm^-1M^-1); [5] 2b (EtOH): λ_max 252 (e = 5.6 × 10³ cm^-1M^-1). [17] The data for 2c-f are: 2c (MeOH): λ_max 256 nm (e = 10.2 × 10³ cm^-1M^-1); 2d (MeOH): λ_max 256 nm
(10^-5 M; e = 11.4 × 10³ cm^-1M^-1); 2e (MeOH): λ_max 256 nm (10^-5 M; e = 11.1 × 10³ cm^-1M^-1); 2f (EtOH): λ_max 262 nm (e = 7.5 × 10³ cm^-1M^-1.