Retro Diels−Alder Reactions of 5,6-Disubstituted-7-oxabicyclo[2.2.1]hept-2-enes:  Experimental and Density Functional Theory Studies

Abstract

Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven.

Modulation of Photoswitching Profiles by 10,11‐Dialkoxymethyl Substituents in C2‐Symmetric Dibenzosuberane‐Based Helicenes

“…Mo-sons ether synthesis in 83-99 % yields by treatment with methyl iodide or n-heptyl bromide in THF in the presence of NaH. The C5-methylene unit in the DBS template in 2 b and c was readily oxidized at ambient temperature to the corresponding ketones 3 b and c in 93-99 % yields by KMnO 4 solvated in benzene by using [18]crown-6 as a phase-transfer reagent. (10R,11R)-Dialkoxymethyl-DBS-5thiones 4 b and c were generated by thiation of the corresponding ketones 3 b and c with Lawessons reagent.…”

“…Compound 2 b: [18] 60 % NaH (0.16 g, 6.8 mmol) was added to a 25 mL, two-necked, round-bottomed flask equipped with an N 2 inlet and septum. The NaH was washed with anhydrous hexane (3 6 mL) and dried under vacuum and then suspended in anhydrous THF (10 mL).…”

“…[20] KMnO 4 (0.30 g, 1.9 mmol) in benzene (3 mL) was added to a 25 mL, two-necked, round-bottomed flask equipped with an N 2 inlet and septum. A solution of dicyclohexano [18]crown-6 (0.50 g, 1.9 mmol) in benzene (2 mL) was added. The solution became homogeneous and purple.…”

Application of Furan as a Diene: Preparation of Condensed 1,3‐Oxazines by Retro‐Diels−Alder Reactions

“…Di-exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride [9,10] (1) was prepared from furan and maleic anhydride by DielsϪAlder addition. After ammonolysis of 1, the amide was transformed by Hoffmann degradation through treatment with hypochlorite into di-exo-3-amino-7-oxabicyclo[2.2.1]hept-5-ene-2-carboxylic acid (2) (obtained by another route in the literature, [11] but with no m.p.…”

Monomer sequencing and microstructural analysis on polymers of dimethyl norbornene dicarboxylate and 7‐oxanorbornene dicarboxylic derivatives employing ruthenium catalysts by ring‐opening metathesis polymerization

“…was measured in the absolute value mode, and two-dimensional phase-sensitive proton Nuclear Overhauser Effect spectroscopy (2D-proton NOESY) as supplied in the VNMRJ software, using either 200, 300, or 400 ms mixing times were used. Carbon and proton chemical shift assignments were made by comparison to the literature values, 5,6,19,[29][30][31][32][33][34][35][36][37] and confirmed by 2D-13C-1H-Heterocorrelation, 2D-1H-1H-COSY, and 2D-1H-1H-NOESY experiments. Monomer sequencing was obtained from 2D-13C-1H-HSQC-TOCSY experiments utilizing 30-80 ms spin-lock mixing times using a Clean-MLEV-17 pulse-sequence.…”