Heterocyclic photochemistry. 1. Phototranspositions in hydroxypyrylium cations. Permutation pattern analysis and mechanistic studies

Abstract

A number of dialkyl-4-pyrones have been irradiated (as the corresponding 4-hydroxypyrylium cations) in sulfuric acid and the transposed 2-pyrones have been identified. A permutation pattern analysis of these and literature data shows that the photorearrangement follows a P4 permutation pattern, thus defining the fate of all the ring atoms. This conclusion is supported by a total-labeling investigation: a study of the rearrangement of a l4C-labeled 4-pyrone confirmed that the alkyl migrations are intramolecular, the alkyl groups remaining bonded to the ring atoms during the rearrangement. The irradiation of 3,5-dimethyl-4-pyrone in sulfuric acid led to the isolation of a new intermediate, the cyclic sulfate 22, which in sulfuric acid is converted thermally or photochemically to the 2-pyrone 8. In aqueous sulfuric acid, 2-acylfurans are also found as irradiation products. It is concluded that the excited 4-hydroxypyrylium cations relax to hydroxyoxabicyclohexenyl cations (30) as primary ground-state intermediates. A minor pathway of isomerization via a P8 permutation pattern is also discussed. The photointcrconversion of 2-hydroxypyrylium cations (e.g., eq 8) is shown to occur by a P8 permutation pattern.

Sex pheromone for the brownbanded cockroach is an unusual dialkyl-substituted alpha-pyrone.

“…2. Chemical shift positions of the ring protons favored the 3,5-disubstituted structures (18), and long-range coupling data favored the 3-alkyl-5-methyl isomer. Because the NMR data alone could not distinguish among the four possibilities, we undertook the synthesis (Fig.…”

“…Additional purification on a polar capillary column revealed that behavioral and electrophysiological activity coincided with a single major peak whose Kovat tively. This spectrum coupled with the presence of a major peak at m/z 95 in the El-MS spectrum suggested (among other possibilities) a derivative of 2H-pyran-2-one (18). The 2H-pyran-2-one ring system requires all four unsaturation equivalents as well as the two oxygen atoms from our formula of C15H2402, thus leaving one or more saturated hydrocarbon side chains to account for the remaining CloH21.…”

“…To our knowledge, this synthesis represents the first practical and flexible synthesis of 3,5-dialkyl-2H-pyran-2-ones. The only other reported synthesis of 3,5-dimethyl-2H-pyran-2-one (18) involved a lowyield (=1%) photochemical decomposition. Careful analysis of the 'H NMR spectrum, the nuclear Overhauser effect difference spectrum (23), and the twodimensional correlation spectroscopy via long-range coupling (COLOC) spectrum (24) of 3,5-dimethyl-2H-pyran-2-one confirmed that our original assignment of the pheromone as an alkyl-substituted a-pyrone was correct.…”

The Concept of Transient Chirality in the Stereoselective Synthesis of Functionalized Cycloalkenes Applying the Retro-Diels−Alder Methodology

“…These strained highly functionalized five-membered ring compounds are not attainable at all, applying the conventional wet chemistry as they rapidly react with both nucleophilic and electrophilic reagents. The many attempts to isolate or synthesize cyclopentadienone epoxides in solution have always been in vain 15

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“…The many attempts to isolate or synthesize cyclopentadienone epoxides in solution have always been in vain. 24 The first example of the synthetic applicability of cyclopentadienone epoxides in natural product chemistry is the stereoselective synthesis of rac-terrein 73, a mold metabolite from Aspergillus terreus. 25 Although already isolated in 1935, its sensitivity to acid and base precluded an efficient synthesis.…”