Novel Sesquiterpene Skeletons by Multiple Wagner-Meerwein Rearrangements of a Longipinane-1,9-diol Derivative.

The tricyclic sesquiterpene (1R,3R,4S,5S,7S,8S,9S,10R,11R)-7,8-diangeloyloxylongipinan-1,9-diol, or rasteviol (7), underwent multiple Wagner-Meerwein molecular rearrangements and several hydride shifts when treated with Et2O-BF3 to generate the six new compounds (1R,3R,4S,5R,7S,8S,9S,10R,11S)-7,8-diangeloyloxy-1,9-epoxyjiquilpane (8), (1R,3R,4S,5R,7R,8S,9S,11S)-8-angeloyloxy-1,7-epoxyzamor-10(14)-ene (11), (2S,3R,4R,5R,6R,7R,8S,9S,10S)-7,8-diangeloyloxy-6,9-epoxyjanitziane (14), (4R,5R,7S,8S,9S,10S,11S)-7,8-diangeloyloxy-9-hydroxyjiquilp-3(15)-ene (16), (2S,3S,5R,7S,8R,10S,11R)-7,8-diangeloyloxyiratzian-9-one (18), and (2S,3S,5R,10S,11R)-8-angeloyloxyiratzi-7-en-9-one (22), of which 8, 11, 14, and 18 possess new hydrocarbon skeletons. Their structures were determined by 1D and 2D NMR in combination with single-crystal X-ray diffraction analyses of derivatives 10, 15, 20, and 21, which allowed confirmation of their absolute configurations by means of the Flack and Hooft parameters. In addition, some reaction mechanism information was gained from deuterium labeling experiments.

Gas-Phase Acidities and Basicities of Alanines and N-Benzylalanines by the Extended Kinetic Method.

This paper reports an experimental determination of the gas-phase acidities and basicities of N-benzylalanines, in both their α and ß forms, by means of the extended kinetic method (EKM). The experimental gas-phase acidity of ß-alanine was also determined. Standard ab initio molecular orbital calculations at the G3 level were performed for alanines, and at the G3(MP2)//B3LYP level for N-benzylalanines. There is a very good agreement between the experimental and the calculated values. The more branched α-amino acids are more acidic and less basic than the linear ß-amino acids.

Experimental and computational thermochemical study of N-benzylalanines.

Calorimetric measurements are expected to provide useful data regarding the relative stability of α- versus ß-amino acid isomers, which, in turn, may help us to understand why nature chose α- instead of ß-amino acids for the formation of the biomolecules that are essential constituents of life on earth. The present study is a combination of the experimental determination of the enthalpy of formation of N-benzyl-ß-alanine, and high-level ab initio calculations of its molecular structure. The experimentally determined standard molar enthalpy of formation of N-benzyl-ß-alanine in gaseous phase at T = 298.15 K is -(298.8 ± 4.8) kJ·mol(-1), whereas its G3(MP2)//B3LYP-calculated enthalpy of formation is -303.7 kJ·mol(-1). This value is in very good agreement with the experimental one. Although the combustion experiments of N-benzyl-α-alanine were unsuccessful, its calculated enthalpy of formation is -310.7 kJ·mol(-1); thus, comparison with the corresponding experimental enthalpy of formation of N-benzyl-ß-alanine, -(298.8 ± 4.8) kJ/mol, is in line with the concept that the more branched amino acid (α-alanine) is intrinsically more stable than the linear ß-amino acid, ß-alanine.

Experimental and computational thermochemical study of α-alanine (DL) and ß-alanine.

This paper reports an experimental and theoretical study of the gas phase standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of α-alanine (DL) and ß-alanine. The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline α-alanine (DL) and ß-alanine were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g), and H2O(l), measured by static-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of both amino acids were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation at T = 298.15 K was derived from the Clausius−Clapeyron equation. The experimental values were used to calculate the standard (p° = 0.1 MPa) enthalpy of formation of α-alanine (DL) and ß-alanine in the gaseous phase, Δ(f)H(m)°(g), as −426.3 ± 2.9 and −421.2 ± 1.9 kJ·mol(−1), respectively. Standard ab initio molecular orbital calculations at the G3 level were performed. Enthalpies of formation, using atomization reactions, were calculated and compared with experimental data. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out.

Preparation of chiral derivatives of beta-Ala containing the alpha-phenylethyl group: useful starting materials for the asymmetric synthesis of beta-amino acids.

The use of microwave (MW) irradiation for the condensation reaction between acetophenone and alpha-phenylethylamine to prepare (R,R)-bis[alpha-phenylethyl]amine results in significantly reduced reaction times relative to the use of conventional heating. In this protocol, a secondary amine, (R,R)-bis(alpha-phenylethyl)amine is treated with acryloyl chloride to afford conjugated amide N,N-bis[(R)-alpha-phenylethyl]prop-2-enamide, (R,R)-3. 1,4-Addition of alpha-phenylethylamine to unsaturated (R,R)-3 affords propanamide N,N-Bis[(R)-alpha-phenylethyl]-3-N-[(S)-alpha-phenylethyl]amino-propanamide, (R,R,S)-4, which can be alkylated with high diastereoselectivity to give derivative N,N-Bis[(R)-alpha-phenylethyl]-3-N'-[(S)-alpha-phenylethyl]amino-propanamide, (R,R,S,S)-5. Hydrogenolysis of (R,R,S,S)-5 catalyzed by palladium hydroxide and final hydrolysis (4 N HCl) resulting in the formation of (S)-alpha-benzyl-beta-alanine, (S)-7, is facilitated by MW irradiation. The use of MW irradiation in this step prevents racemization of the desired amino acid. The present protocol constitutes one of the simplest strategies for the asymmetric synthesis of biologically relevant alpha-substituted-beta-amino acids since it takes advantage of inexpensive, commercially available beta-Ala and either (R)- or (S)-alpha-phenylethylamine as chiral auxiliary. The required time for this protocol is approximately 90 h, which can be carried out in 5 d.

Calorimetric and computational study of 1,3- and 1,4-oxathiane sulfones.

The enthalpies of formation in the condensed and gas states, DeltafH degrees m(cd) and DeltafH degrees m(g), of 1,3- and 1,4-oxathiane sulfones were derived from their respective enthalpies of combustion in oxygen, measured by a rotating bomb calorimeter and the variation of vapor pressures with temperatures determined by the Knudsen effusion technique. Standard ab initio molecular orbital calculations at the G2(MP2) and G3 levels were performed, and a theoretical study on molecular and electronic structure of the compounds has been carried out. Calculated DeltafH degrees m(g) values at the G3 level using atomization reactions agree well with the experimental ones. These experimental and theoretical studies support that the destabilization found in 1,3-oxathiane sulfone, 11.2 kJ mol-1 respecting to 1,4-oxathiane sulfone, is due to the electrostatic repulsion between the negative charges of the axial oxygen of the sulfone and the oxygen of the ring and apparently masks any stabilization originating from the hyperconjugative nO --> sigma*C-SO2 stereoelectronic interaction.

Calorimetric and computational study of 1,4-dithiacyclohexane 1,1-dioxide (1,4-dithiane sulfone).

This work reports the enthalpies of formation in the condensed and gas state of 1,4-dithiacyclohexane 1,1-dioxide (1,4-dithiane sulfone, 5), derived from the enthalpy of combustion in oxygen, measured by a rotating bomb calorimeter and the variation of vapor pressures with temperatures determined by the Knudsen effusion technique. The theoretically estimated enthalpy of formation was calculated from high-level ab initio molecular orbital calculations at the G2(MP2) level. The theoretical calculations appear to be in very good agreement with experiment. A comparison of the conversion of thiane sulfone 3 to 1,3-dithiane sulfone 4 and 1,4-dithiane sulfone 5 clearly shows the 1,3 isomer to be 6.7 kJ mol(-1) less stable, probably owing to diminished electrostatic repulsion between the sulfur heteroatoms in 1,4-sulfone 5.

Thermochemistry of 1,3-dithiacyclohexane 1-oxide (1,3-dithiane sulfoxide): calorimetric and computational study.

The enthalpies of combustion and sublimation of 1,3-dithiacyclohexane 1-oxide (1,3-dithiane sulfoxide, 2) were measured by a rotating-bomb combustion calorimeter and the Knudsen effusion technique, and the gas-phase enthalpy of formation was determined, DeltafH degrees m(g) = -98.0 +/- 1.9 kJ mol(-1). This value is not as large (negative) as could have been expected from comparison with thermochemical data available for the thiane/thiane oxide reference system. High-level ab initio molecular orbital calculations at the MP2(FULL)/6-31G(3df,2p) level were performed, and the optimized molecular and electronic structures of 2 afforded valuable information on (1) the relative conformational energies of 2-axial and 2-equatorial--the latter being 7.1 kJ mol(-1) more stable than 2-axial, (2) the possible involvement of nS --> sigma*(C-S(O)) hyperconjugation in 2-equatorial, (3) the lack of computational evidence for sigma(S-C) --> sigma*(S-O) stereoelectronic interaction in 2-equatorial, and (4) the relevance of a repulsive electrostatic interaction between sulfur atoms in 1,3-dithiane sulfoxide, which apparently counterbalances any nS --> sigma*(C-S(O)) stabilizing hyperconjugative interaction and accounts for the lower than expected enthalpy of formation for sulfoxide 2.

Calorimetric and computational study of 1,3-dithiacyclohexane 1,1-dioxide (1,3-dithiane sulfone).

The enthalpies of combustion and sublimation of 1,3-dithiacyclohexane 1,1-dioxide (1,3-dithiane sulfone) were measured by a rotating-bomb combustion calorimeter and the Knudsen effusion technique, and the gas-phase enthalpy of formation was determined, Delta(f)H(m)*(g) = -326.3 +/- 2.0 kJ mol(-1). Standard ab initio molecular orbital calculations at the G2(MP2) level were performed, and a theoretical study on molecular and electronic structure of the compound has been carried out. Calculated Delta(f)H(m)*(g) values agree very well with the experimental one. These experimental and theoretical studies support the relevance of the repulsive electrostatic interaction between sulfur atoms in 1,3-dithiane sulfone, that apparently counterbalances any n(S) --> rho(C-SO2)* stabilizing hyperconjugative interaction.

Calorimetric and computational study of thiacyclohexane 1-oxide and thiacyclohexane 1,1-dioxide (thiane sulfoxide and thiane sulfone). Enthalpies of formation and the energy of the S=O bond.

A rotating-bomb combustion calorimeter specifically designed for the study of sulfur-containing compounds [J. Chem. Thermodyn. 1999, 31, 635] has been used for the determination of the enthalpy of formation of thiane sulfone, 4, Delta(f)H(o) m(g) = -394.8 +/- 1.5 kJ x mol(-1). This value stands in stark contrast with the enthalpy of formation reported for thiane itself, Delta(f)H(o) m(g) = -63.5 +/- 1.0 kJ x mol(-1), and gives evidence of the increased electronegativity of the sulfur atom in the sulfonyl group, which leads to significantly stronger C-SO2 bonds. Given the known enthalpy of formation of atomic oxygen in the gas phase, Delta(f)H(o) m(O,g) = +249.18 kJ x mol(-1), and the reported bond dissociation energy for the S=O bond in alkyl sulfones, BDE(S=O) = +470.0 kJ x mol(-1), it was possible to estimate the enthalpy of formation of thiane sulfoxide, 5, a hygroscopic compound not easy to use in experimental calorimetric measurements, Delta(f)H(o) m(5) = -174.0 kJ x mol(-1). The experimental enthalpy of formation of both 4 and 5 were closely reproduced by theoretical calculations at the G2(MP2)+ level, Delta(f)H(o) m(4) = -395.0 kJ x mol(-1) and Delta(f)H(o) m(5) = -178.0 kJ x mol(-1). Finally, calculated G2(MP2)+ values for the bond dissociation energy of the S=O bond in cyclic sulfoxide 5 and sulfone 4 are +363.7 and +466.2 kJ x mol(-1), respectively.