Eiji Osawa

Theoretical description of the preference of vicinal alkyl/phenyl gauche conformation by molecular mechanics. an alternative interpretation for the ch ⋯π attractive interaction

Abstract Molecular mechanics calculations on alkyl benzyl carbinols, ketones, sulphides and sulphoxides were carried out as a function of the torsional angle about the benzyl C-O, C-S, or C-SO bond by use of the MM2 program. Theoretical consequences generally agree with the experimental results showing that the phenyl/alkyl gauche conformer is the most preferred for all 1-phenylethyl compounds and also for some of benzyl compounds investigated. Detailed examination on the energy terms revealed the contribution of attractive non-bond interaction between the alkyl and phenyl groups vicinally located in stabilizing the phenyl-alkyl gauche conformer. Thus the preference of CH ⋯π conformation can be interpreted by the contribution of the dispersive force. Possible participation of other attractive forces is discussed briefly.

Allinger's gauche hydrogen hypothesis as tested by an alternative force field model

Abstract Allingers hypothesis that gauche hydrogen, rather than gauche methyl interactions are mainly responsible for conformational equilibria was tested by molecular mechanics calculations using the Engler force field model. For the gauche-anti conformational energy of n-butane, effective cancellation among non-bonded in teractions involving the methyl groups results in the gauche hydrogen interactions across the central C-C bond as the single largest contribution to the energy difference. However, the contribution of this interaction is only one third of the 0.9 kcal difference. For 2,3-dimethylbutane, strain analysis of the gauche-anti conformational energy contributions reveals that geminal and gauche CH 3 /CH 3 interactions dominate over the gauche hydrogen interaction Similar analyses for several monosubstituted cyclohexanes confirm that “across-the-ring” interactions. between axial substituents and syn -axial hydrogen atoms are still largely responsible for the instability of the axial relative to the equatorial conformer. In disagreement with Allingers proposal, the “equatorial hydrogen effect” is found to contribute only a minor amount to the conformational energy difference. Allingers hypothesis is concluded to be force field dependent, and not to have general validity.

Strain energy considerations in the photochemical [π2 + π2] cage cyclisations. A case study en route to [6]-prismane

Abstract Six novel cases of (2+2) photocycloaddition of strained cage dienes, aiming at producing [6]-prismane and congeners, are reported and the steric criteria for the success of reaction are presented based on molecular mechanics.

Theoretical and experimental study of [2+2]-photoclosure approachtoward [7]-prismane analogs

Abstract All 39 possible intramolecular [2+2]-photocycloaddition pathways toward [7]-prismane and its homo, seco, homo-seco, bishomo and bishomo-seco analogs have been explored by MM2 calculations. Fourteen of these are predicted to offer reasonable chance of success. Experimental support for a few of these is provided.

A simple procedure for quantitative predictions of the CC framework bond distances and angles in n-hydrocarbons

Abstract The structures of four conformations of n-pentane, and of ten conformations of n-hexane were determined by ab-initio gradient geometry refinements on the 4–21G level. The results are used to demonstrate the efficacy of a simple empirical procedure to predict the CC bond distances and CCC angles of long-chain n-hydrocarbons from small molecules. Specifically, the CC bond distances of n-pentane and n-hexane are predicted from those of n-butane (published previously) with a most frequent error of 0.001 A, and a maximum error of 0.003 A, while the CCC angles of n-hexane are predicted from n-pentane with a typical deviation of 0.1° in unstrained conformational states, and a maximum deviation of 0.5° in strained conformations. It is expected that the procedure is generally effective in quantitative structural predictions of the CC framework of the low energy conformational states of n-hydrocarbons regardless of chain length.

Calculations of structures of biphenyl and alkylbiphenyls by molecular mechanics

Abstract MM2′ parameters for “mechanical” calculation of biphenyl are revised on the basis of Bastiansens gas phase electron diffraction data. These parameters well reproduce the structures of energy minimum conformer of biphenyl and of alkylbiphenyls.

Molecular mechanics analysis of restricted rotation about pivot bond in substituted bicyclohexyls and phenylcyclohexanes. Importance of successive gauche and progauche sequences in conformational dynamics

Abstract Rotation of pivot bond in bicyclohexyls and phenylcyclohexanes carrying methyl groups vicinal to the rotating bond have been simulated by the second derivative molecular mechanics calculations. Barriers are characterized by long-range nonbonded interaction types occurring across the pivot bond such as gg, gp (p=progauche), po (o=ortho), ge (e=eclipse), ggg and gpo with alternating signs regarding gauche and progauche. Very high barriers are expected to appear when at least one 1,b interaction type, ggg or gpo, occurs simultaneously with another interaction type. Experimental examples including known atropisomers have been interpreted in the light of the present results.

Conformational analysis of serricornin : application of molecular mechanics calculations to stereochemical assignment of serricornin, sex pheromone of cigarette beetle (Lasioderma serricorne F.)

Conformational analysis using molecular mechanics (MM) was performed for a determination of the stereochemistry of serricornin, the sex pheromone of the cigarette beetle (Lasioderma serricorne F.). An exhaustive conformational analysis using MM2 calculations with algorithms for covering torsional energy surfaces of flexible molecules furnishes coordinates and steric energies of all local energy minimum conformers of serricornin, both acyclic and the corresponding cyclic forms. These coordinates gave angles required for the calculation of vicinal H/H coupling constants (3JHHs) of each energy minima by Altonas modified Karplus equation. The Boltzmann distributions of all local energy minima were calculated from their steric energies to furnish populations of each energy minimum conformer. Populationweighted averaged3JHHs of four enantiomeric pairs, (S*,S*,S*)-, (S*,S*,R*)-, (S*,R*,S*)-, and (R*,S*,S*)-serricornins were calculated from the data above. The observed3JHHs of the naturally occurring serricornin, both acyclic and cyclic forms, are fitted best to calcd.3JHHs of (4S*, 6S*, 7S*)-acyclic and (3S*, 5S*, 6S*)-cyclic serricornin, respectively, among those of four enantiomeric pairs of serricornin.

Stereochemistry of some diastereoisomeric 2-substituted borneols

Abstract The absolute configurations of the benzylic chiral centers in the four diastereomeric (2-hydroxy-born-2-yl)phenylacetic acids are determined on the basis of the sign of the Cotton effect at about 225 nm of the circular dichroism curves. The favoured conformers are calculated using Allingers 1977 force field. The exo -OH forms are computed to be favoured by about 1 to 1.2 kcal mol −1 with respect to the endo -OH forms.

Remarkably fast [2+2]cycloreversion in methoxy substituted Cookson's cage ketones assisted by the capto-dative substituent effect and by the through-bond interaction

1,7-Dimethoxy groups in Cooksons pentacyclic cage ketone derivatives (2 and 5) accelerate the thermal [2+2]cycloreversion, possibly by means of the unusual lengthening of C1C7 bond and the synergetic capto-dative stabilization of 1,4-diradical (7 and 8).