Precursor-related structural studies

The observation79 in an earlier more extensive study that norpluviine (100) is a precursor for lycorenine (102) has been confirmed.80 Incorporations were also recorded with both norpluviine (100) and pluviine (101) for other alkaloids of related structure to norpluviine, including galanthine (103). Although radio-active narciclasine (96) was also isolated in the experiment with norpluviine, its known derivation76,81 by way of a different pathway through vittatine (93) indicates that this is not significant. 

Several metabolites previously studied as possible precursors are in fact not directly involved in purine synthesis, even though these compounds are related structurally to parts of the purine ring. Thus, labeled... 

The biosynthesis of the fungal tetronic acids penicillic acid (185) and carolic acid (76 a), which are closely related structurally to patulin and multicolic acid, have also been studied extensively. Early work with " C-labelled precursors has clearly demonstrated the polyketide origin of penicillic acid, and also the intermediacy of orsellinic acid... 

New methods for the elaboration of the gibbane carbon skeleton and related Structures continue to be developed. Chiral gibbane derivatives can be prepared by asymmetric aldol condensations using L-proline as catalyst, or by building up the carbon skeleton from a chiral precursor. Examples have already been quoted which bear witness to the considerable synthetic potential of acid-catalysed intramolecular cyclizations of olehnic diazoketones. Mander etal. have given full details of their studies in this area with anisole derivatives such as... 

Application of NMR spectroscopy to heterocyclic chemistry has developed very rapidly during the past 15 years, and the technique is now used almost as routinely as H NMR spectroscopy. There are four main areas of application of interest to the heterocyclic chemist (i) elucidation of structure, where the method can be particularly valuable for complex natural products such as alkaloids and carbohydrate antibiotics (ii) stereochemical studies, especially conformational analysis of saturated heterocyclic systems (iii) the correlation of various theoretical aspects of structure and electronic distribution with chemical shifts, coupling constants and other NMR derived parameters and (iv) the unravelling of biosynthetic pathways to natural products, where, in contrast to related studies with " C-labelled precursors, stepwise degradation of the secondary metabolite is usually unnecessary. 

In a related study, 161 was reported from a dimerization of a 3-vinylindole precursor (61JOC4254). Previously, a compound with the structure 162 had been suggested as a product originating from a dimerization of an alcohol precursor under acidic conditions (63G238). It has recently been demonstrated that the alcohol 163, when treated with aluminum trichloride in the presence of acetic anhydride, produced the indolocarbazole 160, which could be isolated in 41% yield as an 83 17 mixture of cis and trans isomers (98JOC2909). 

It has been shown that ab initio total energy DFT approach is a suitable tool for studies of phase equilibria at low temperatures and high pressures even when small energy differences of the order of 0.01 eV/mol are involved. The constant pressure optimization algorithm that has been developed here allows for the calculation of the equation of state for complex structures and for the study of precursor effects related to phase transitions. 

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