Paspalinine,a tremorgenic metabolite from Claviceps paspali Stevens et Hall

The structure of paspalinine, a tremorgenic fungal metabolite, is presented.     

VLADIMIR PRELOG

Vladimir Prelog, one of the truly influential organic chemists of the past century, died in Zurich after a short illness on 7 January 1998. For his contributions to stereochemistry, he shared the 1975 Nobel Prize in Chemistry with J.W. Cornforth. In his 1991 autobiographical sketch (53) 1 Prelog told what he wanted to tell about himself and his work. The present memoir borrows heavily on this, but we have also drawn on our own perceptions gathered over many years of companionship and communion.

1 Numbers in this form refer to the bibliography at the end of the text.

     

Zum stereochemischen Verlauf der Biosynthese von 2-Oxo-pantolacton: Synthese von stereospezifisch indiziertem Pantolacton aus Äpfelsäure

(+)-Pantolactone¹ ( 13 ) has been synthesized from (?)-(S)-dimethyl malate (7) in 40% yield in a short sequence involving double alkylation (7 → 10 → 11 ), selective hydrolysis ( 11 → 12 ) and subsequent reduction ( 12 → 13 ). Through variation of the alkylating agents and preparation of the two diastereomeric 3-ethyl-3-methyl malates 14 and 15 it was possible to show that the diastereoselectivity of the second alkylation step is brought about by preferential attack from the Re-face of the critical enolate (9, see also Scheme 1). This knowledge, in turn, has been exploited for the synthesis of a sample of pantolactone specifically enriched with ¹³C in its Si-methyl group. Analysis of the ¹³C-NMR. spectrum of this sample together with the results of biosynthetic experiments previously reported by Aberhart demonstrates that the biological hydroxymethylation of 2-oxoisovaleric acid ( 3 ) to 2-oxopantoic acid ( 4 ), an important step in the biosynthesis of pantothenic acid, takes place in a retention mode (of. Scheme 2).     

On the Early Steps of Cineol Biosynthesis in Eucalyptus globulus

Samples of [4‐²H₁]‐1‐deoxyxylulose ( 17a ) and [2‐¹³C, 4‐²H₁]‐1‐deoxyxylulose ( 17b ), have been prepared by modification of known procedures and fed in aqueous solution to twiglets of Eucalyptus globulus. The probes of cineol ( 6 ) isolated from these experiments were analyzed by GC/MS, ²H‐ and ¹³C‐NMR techniques. In the experiments with 17b , the formation of five isotopomers of 6 could be detected. Their structure and relative abundance demonstrate that the ¹³C‐label is incorporated to the same extent into the two C₅‐units of 6 , and that the ²H label is retained to an extent of 57% in the starter dimethylallyl‐diphosphate unit (DMAPP; 12 ), but completely or almost completely lost in the unit derived from isopentenyl diphosphate (IPP; 11 ), in the elongation step which leads to geranyl diphosphate (GPP; 1 ). These results confirm that the recently discovered mevalonate‐independent pathway to IPP and DMAPP is operative in the biosynthesis of cineol, and indicate, together with previous finding, that, within this pathway, formation of IPP and DMAPP occurs in independent rather than in sequential steps. In addition, the demonstration of different metabolic origins for the olefinic H‐atoms of GPP ( 1 ), the aliphatic C₁₀‐precursor of 6 , paves the way for a realistic interpretation of the strikingly consistent but hitherto unexplained anomalies detected in the natural‐abundance ²H‐NMR spectra of (+)‐ and (−)‐α‐pinene and of (+)‐limonene.     

Penicillin biosynthesis the immediate origin of the sulphur atom

A mixture of tripeptide isotopomers δ-(L-α-aminoadipyl)-L-cystelnyl-D [2-²H]-valine and δ-(L-α-aminoadipyl)-L-(³⁴S-cysteinyl)-D-valine were converted by the enzyme isopenicillin-N-synthetase into isopenicillin N. The distribution of the ²H and ³⁴S in this product, determined by mass spectrometry, showed there was no transfer of the sulphur between the precursor molecules during conversion to penicillin.     

Biosynthesis of terpenes. Preparation of (E)-1-hydroxy-2-methyl-but-2-enyl 4-diphosphate,an intermediate of the deoxyxylulose phosphate pathway

(E)-1-hydroxy-2-methyl-but-2-enyl 4-diphosphate (E-6) was synthesized in six reaction steps from hydroxyacetone (9) and (ethoxycarbonylmethenyl)-triphenylphosphorane (11) with an overall yield of 38%. The compound was shown to be identical with the product of IspG protein, which serves as an intermediate in the nonmevalonate terpene biosynthetic pathway.     

Detection of 1,2-hydride shifts in the formation of euph-7-ene by the squalene-tetrahymanol cyclase of Tetrahymena pyriformis

Incubation of samples of 2,3-dihydrosqualene, specifically labeled with deuterium at either carbon position 7 or 11, with an enzyme extract from Tetrahymena pyriformis, containing a squalene-tetrahymanol cyclase, provided specimens of euph-7-enes displaying deuterium patterns consistent with the biosynthetic operation of two consecutive 1,2-hydride shifts.     

Mathematical developments arising from ‘semantical implication’ and the evaluation of membership characteristic functions

In this paper, the actuation of a semantical ordering in a property set is exposed. Such an ordering follows from the concept of semantical implication: the relation that takes place between two properties, when the values of the details forming one of these are the same as of the other excepting those which, in the implied property, are irrelevant. Mainly, the argumentation deals with the analysis of the operations which, due to the relations in subject, are allowed in the property set considered. This, with the direct purpose of attaining to the structure of the set itself.