Nan Ibrahim

The absolute configuration of 6,8-dioxabicyclo[3.2.1]octane and several methyl substituted derivatives

Abstract The enantiomers of 6,8-dioxabicyclo[3.2.1]octane and the alkyl substituted derivatives, exo- and endo-7-methyl, exo- and endo-5,7-dimethyl, 7,7-dimethyl, and exo- and endo-7-ethyl-5-methyl (exo- and endo-brevicomin), were synthesized stereoselectively with known configuration by standard synthetic methods, or with bakers yeast, or both. The correlation between the absolute configuration of the bicyclic rings and the optical rotation was established by means of chiral complexation gas chromatography for two molecules for which this correlation was not known previously. In all cases, the (1R) enantiomer exhibits positive rotation. The absolute configurations were established for the yeast products which were always produced in high optical purity.

Determining the absolute configuration of exo-7 derivatives of 5-methyl-6,8-dioxabicyclo[3.2.1]octane by VCD

Abstract The VCD spectra in the region of 1100–1400 cm −1 of four derivatives of 5-methyl-6,8-dioxabicyclco[3.2.1]octane reveal common features which indicate the absolute configuration of the bicyclic ketal ring system. One of these in particular can serve as a more general configurational marker for the chiral unit-C★H(CH 2 R)X-, X = O and S, when contained in different size ring molecules.

The oxidation of 4-aza-5-prregnene-3,20-dione with benzeneseleninic anhydride. Formation and decomposition of a selenoxide intermediate

Abstract The enamide moeity of the title azasteroid reacted with benzeneseleninic anhydride to afford products derived chiefly from a Pummerer-type reaction at C-6. The intermediate selenoxide was also prepared by oxidation of the corresponding selenide.

VCD Spectrum Of 6,8-Dioxabicyclo[3.2.1]Octane: Comparison Of Experimental And Model Spectra

We report the measurement of the vibrational circular dichroism (VCD)1 and infrared absorption spectra of 6,8-dioxabicyclo[3.2.1]octane in the region 800 to 1500 cm-1. The observed bands are assigned with the guidance of a scaled ab initio 3-21G force field. VCD and absorption intensities were calculated using the fixed partial charge (FPC), charge flow (CF), and atomic polar tensor (APT) models.1 FPC yields correct signs but low absolute intensities for most of the observed VCD bands. Introduction of one CF parameter which allows charge flow along the C-0 bonds improves the FPC results remarkably and brings them close to the corresponding observed and APT intensities.

The VCD Spectrum Of 2-Methyl Oxetan

The VCD and absorbance spectra of 2-methyl oxetan are reported for the region of 800-1500 cm-1. A force field consistent with experimental vibrational spectra is derived by an ab initio calculation at the STO-3G level, and is used to compute the VCD signs and intensities using the fixed partial charge (FPC) method. The VCD signs are predicted correctly for all observable bands save one.

The Vibrational Spectra Of Selected Bicyclic Compounds

A valence force field and the vibrational wavenumbers are determined for bicyclo[2.2.1]- heptane (I), bicyclo[2.2.1]hepta-2,5-diene (II), and 7-oxabicyclo[2.2.1]heptane (III) by ab initio calculations at the STO-3G level. The calculated wavenumbers are compared to ob-served infrared absorptions and Raman emissions. New and in some cases revised assignments are derived.