Christopher J. Samuel

Stereochemistry of the 4-hydroxyisoleucine from Trigonella foenum-graecum

Abstract The stereochemistry of the 4-hydroxyisoleucine from fenugreek ( Trigonella foenum-graecum ) has been reinvestigated. The absolute configuration was shown to be (2 S ,3 R ,4 S by a combination of chemical, spectroscopic and X-ray crystallographic techniques.

Glycosidase-catalysed oligosaccharide synthesis: preparation of N-acetylchitooligosaccharides using the β-N-acetylhexosaminidase of Aspergillus oryzae

The beta-N-acetylhexosaminidase of Aspergillus oryzae catalyses the formation of 2-acetamido-4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-deoxy-D- glucopyranose (di-N-acetylchitobiose) and 2-acetamido-6-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-deoxy-D- glucopyranose from p-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside and 2-acetamido-2-deoxy-D-glucopyranose. The ratio of the two disaccharides is time-dependent. The ratio of (1-->4)- to (1-->6)-isomers is a maximum (approximately 9:1) at the point of disappearance of the glycosyl donor. If left to evolve, the ratio changes to 92:8 in favour of the (1-->6)-isomer. Either the (1-->4)- or the (1-->6)-isomer can be isolated by treating the appropriately enriched dissaccharide mixture with the beta-N-acetylhexosaminidase of Jack bean (Canavalia ensiformis) or the beta-N-acetylhexosaminidase of A. oryzae, respectively. Di-N-acetylchitobiose [GlcNAc(beta 1-4)GlcNAc] is an efficient donor of 2-acetamido-2-deoxy-D-glucopyranosyl units in reactions catalysed by the N-acetylhexosaminidase of A. oryzae. Di-N-acetylchitobiose itself acts as acceptor to give tri-N-acetylchitotriose [GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc]. As the trisaccharide accumulates it, in turn, acts as acceptor giving tetra-N-acetylchitotetraose [GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc]. The product mixture consisting of mono-, di-, tri-, and tetrasaccharides is conveniently separated by charcoal-Celite chromatography.

Long-range σ-inductive interactions through saturated C–C bonds in polymethylene chains

Analysis of the NMR shifts of unsaturated carbon in monounsaturated linear long-chain esters, alcohols and acids, including data from jojoba oil, shows a highly linear dependence of the logarithm of the shift separation in the CC bond upon the number of bonds separating this from the ester moiety. Both the linearity of this dependence, and the observation that it does not change sign upon reversal of the ester grouping, rule out the ‘electric field effect’ as an explanation, but are semi-quantitatively consistent with a remarkably simple, ‘σ-inductive’ through-bond mechanism. Semi-empirical MO calculations support this conclusion. The attenuation factor is 1.75 per saturated C–C bond in each class of compound, although the actual shift separations, for a given chain length, vary up to twofold with changes in the polarizing group and with (Z)–(E) isomerism at the CC bond. Other series of compounds show the same attenuation factor.

Turmerones: isolation from turmeric and their structure determination

The presence of two turmerones in turmeric is demonstrated and their structures are defined as 2-methyl-6-(4-methylcyclohexa-2,4-dien-1-yl)hept-2-en-4-one (5, ‘α-turmerone’) and 2-methyl-6-(4-methylenecyclohex-2-en-1-yl)hept-2-en-4-one (2, ‘β-turmerone’).

σ-Inductive interaction through up to fourteen saturated C–C bonds

Analysis of the NMR shifts of unsaturated carbon atoms in monounsaturated linear long-chain esters and acids, including new data from jojoba oil, rules out any explanation in terms of an ‘electric field effect’, but is semiquantitatively consistent with a ‘σ-inductive’ mechanism, having an attenuation factor of 1.75 per saturated C–C bond.

The σ-inductive effects of CC and CC bonds: predictability of NMR shifts at sp2 carbon in non-conjugated polyenoic acids, esters and glycerides

The 13C NMR shift separations in a wide range of non-conjugated polyenoic acids can be reliably predicted from the σ-inductive theory developed previously for monoenes, with only one further adjustable parameter, the dipolar effect of one double bond upon another. Further improvements can be made by allowing the enhanced transmission of charge by an intervening double bond. Shifts in poly-ynes are also predicted, though in this case both the dipolar effect and the transmission gain approximately double. The theory also semi-quantitatively explains the small differences in carbonyl shift, used in the quantitative analysis of glycerides.

A novel computational approach to the estimation of steric parameters application to the Menschutkin reaction

Abstract A new purely computational approach to the estimation of steric parameters, suitable for use in QSAR studies, is described and tested by application to the Menschutkin reaction of substituted pyridines.

A novel computational approach to the estimation of steric parameters III. Extension to aliphatic amines and application to the adrenergic blocking activity of β-haloalkylamines

Abstract We have extended our new purely computational approach to the estimation of steric parameters, &, intended for QSAR studies, to the consideration of aliphatic amines and correlated kinetic data for the Menschutkin reaction of 26 amines with six alkyl iodides in LFERs. We have also used our calculated steric factors in QSARs to correlate the adrenergic blocking activity of β-haloalkylamines

Photochemistry of 2-cyanofuran

Abstract Irradiation of 2-cyanofuran in acetonitrile containing furan leads to two stereoisomeric products, arising from Diels—Alder addition of 1-cyanocyclopropene-3-carboxaldehyde (1) to furan. Under no conditions were we able to find any evidence for formation of the expected 3-cyanofuran.

A novel computational approach to the estimation of steric parameters II. Extension to thiazoles

Abstract We have extended our new purely computational approach to the estimation of steric parameters, &, intended for QSAR studies, to the consideration of thiazoles, we have combined data for hindered 2- and 4-substituted thiazoles and 2-substituted pyridines to correlate kinetic data for the Menschutkin reaction of all three groups in a single LFER, and we have offered an interpretation of the nature of these parameters which permits this success.