Regio‐ and stereochemistry of the photo‐ring contraction of 4‐pyrones

Abstract

Irradiation of 2‐ethyl‐3‐methyl‐4‐pyrone 3 in aqueous solution led to the formation of trans‐4‐ethyl‐5‐methy‐4,5‐dihydroxycyclopent‐2‐enone 4 and trans‐2‐methyl‐3‐ethyl‐4,5‐dihydroxycyclopent‐2‐enone 5. In the latter case 1H nmr analysis confirmed the trans‐configuration at C4 and C5. These results are consistent with trapping of a photochemically generated oxabicyclohexenyl zwitterion by nucleophilic attack of both sides of the oxyally system along a path anti to the epoxide ring.

Synthesis of Bridged Bicyclic Ethers and Fused Oxetanes from Pyran-4-ones via Tandem Solvent Trapping and Norrish Type II Cyclization1

“…With unsymmetrically substituted pyran-4-ones, regiochemistry was also at issue. We , and others had previously observed a marked preference for solvent trapping at the more substituted terminus of the oxyallyl system, presumably resulting from greater charge density at this position. To better understand the scope of this reaction, we examined the solvent trapping process with several polyalkyl pyran-4-ones and alcohols.…”

The photochemistry of some pyranopyrazoles

“…Thus, as shown in Scheme 6, irradiation of 2 at 254 nm in acetonitrile solvent led to the formation of a single photoproduct 13, shown by mass spectroscopy, elemental analysis, and 1 H and 13 C NMR spectroscopy to be the cis-head-to tail dimer 13 formed in 58% yield. …”

“…All melting points were determined using a MEL-TEMP apparatus and are uncorrected. 1 H NMR spectra (400 MHz) and 13 C NMR spectra (100 MHz) were recorded on a Bruker FT-NMR system. 1 H and 13 C chemical shifts were measured relative to internal TMS (0 ppm) and CDCl 3 (77.0 ppm) respectively.…”

“…These include the three methyl groups absorbing at δ14.6, 14.8, and 33.3, and the methylene carbon of the ethoxy group absorbing at δ60.7. Significantly, the 13 C NMR spectrum also exhibits a signal for a quaternary carbon at δ167.2, consistent with an ester carbonyl carbon, and signals for quaternary carbon atoms at δ157.7, 147.8, and 92.7, consistent with the C3, C5, and C4 atoms respectively of a pyrazole ring. Taken together, the spectroscopic data suggests that photoproduct 10 is ethyl 5-hydroxy-1, 3-dimethyl-1H-pyrazole-4-carboxylate.…”