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1.
J Org Chem ; 73(17): 6570-9, 2008 Sep 05.
Article in English | MEDLINE | ID: mdl-18681400

ABSTRACT

In this paper, we describe theoretical studies, using gas-phase quantum chemical calculations, on carbocationic rearrangement pathways leading to the sesquiterpenes sativene, cyclosativene, alpha-ylangene, and beta-ylangene. For all four sesquiterpene natural products, viable pathways are presented, and these are compared both to mechanistic proposals found in the literature, and in certain cases to alternative stereochemical and rearrangement possibilities, thus providing a basis for comparison to experimental results. We find that these four sesquiterpenes likely arise from a common bicyclic intermediate and, furthermore, that the computed pathways are mostly in agreement with previous mechanistic proposals, although the few differences that we have uncovered are significant. Additionally, the potential energy profiles of the pathways are found to be very flat, supporting the notion that following the initial ionization of farnesyl diphosphate, minimal enzymatic intervention may be required for the generation of such sesquiterpenes.


Subject(s)
Algorithms , Biological Products , Models, Theoretical , Sesquiterpenes , Biological Products/biosynthesis , Biological Products/chemical synthesis , Carbon/chemistry , Carbon/metabolism , Cations , Cyclization , Polyisoprenyl Phosphates/chemistry , Polyisoprenyl Phosphates/metabolism , Quantum Theory , Sesquiterpenes/chemical synthesis , Sesquiterpenes/chemistry , Sesquiterpenes/metabolism , Stereoisomerism , Thermodynamics
2.
Org Lett ; 9(6): 1069-71, 2007 Mar 15.
Article in English | MEDLINE | ID: mdl-17309272

ABSTRACT

Herein we describe quantum chemical calculations on a two-step proton-transfer pathway that interconverts key intermediates in the biosynthesis of taxa-4,11-diene, a precursor of Taxol, that provides an energetically more favorable alternative to the usually proposed direct proton-transfer pathway. In effect, the bicyclic diterpene skeleton involved in this rearrangement provides a cage of three pi-bonds that surrounds a locally mobile proton. [reaction: see text]


Subject(s)
Algorithms , Alkenes/metabolism , Diterpenes/metabolism , Protons , Molecular Structure , Quantum Theory , Thermodynamics
3.
J Am Chem Soc ; 128(18): 6172-9, 2006 May 10.
Article in English | MEDLINE | ID: mdl-16669687

ABSTRACT

In this article, we describe studies, using quantum chemical computations, on possible polycyclization pathways of the farnesyl cation leading to the complex sesquiterpene pentalenene. Two distinct pathways to pentalenene with similar activation barriers are described, each differing from previous mechanistic proposals, and each involving unusual and unexpected intermediates. Direct deprotonation of intermediates on these pathways leads to sesquiterpene byproducts, such as humulene, protoilludene, and asteriscadiene, supporting the notion that a key function of pentalenene synthase, the enzyme that produces pentalenene in Nature, is to regulate the timing and location of proton removal. The implications of the computational results for experimental studies on pentalenene synthase are discussed.


Subject(s)
Cyclopentanes/chemical synthesis , Polyisoprenyl Phosphates/chemistry , Sesquiterpenes/chemistry , Cations , Cyclization , Models, Molecular , Quantum Theory , Thermodynamics
4.
J Phys Chem A ; 110(10): 3785-9, 2006 Mar 16.
Article in English | MEDLINE | ID: mdl-16526663

ABSTRACT

The nature of the bonding in a set of nonclassical carbocations with tetracoordinated protons "sandwiched" between two C=C pi bonds was scrutinized using the new computational methodology of generalized population analysis. The results of this theoretical analysis strongly corroborate the conclusions of a previous study in which the occurrence of delocalized 5-center 4-electron (5c-4e) bonding in the C-C-H-C-C fragments of such cations was anticipated.

6.
Inorg Chem ; 42(25): 8161-70, 2003 Dec 15.
Article in English | MEDLINE | ID: mdl-14658866

ABSTRACT

A mixed-valence compound (Sn2I3(NPPh3)3) with nonequivalent Sn atoms in characteristic 2+ and 4+ Sn geometries, raised the idea of an average Sn3+ structure. The extended structures of Sr4Sn2Se9 and Sr4Sn2Se10 contain an unusual Sn2Se6 subunit, which has two equal Se-Sn-Se angles close to 160 degrees. This was suggestive of a Sn3+/Sn3+ compound, similar to the putative transition state for the valence state interchange in the molecular compound. These interesting geometrical features of two quite different molecules prompt a series of computations, a detective story of geometries and oxidation states, which concludes tentatively that the Sn with the abnormal angle in the extended structure is still likely to be formally Sn4+.

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