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ABSTRACT
A set of heptyl di-O-isopropylidene-D-mannofuranoside structures, which could find applications as biocompatible surfactant precursors in fields like pharmaceuticals and biotechnology, were analyzed using semi-empirical quantum chemical methods. Cations derived from them by removing one methyl group from the heptyl residue were constructed and conformationally optimized. Strong interactions between the sugar moiety oxygen atoms and the alkyl positively charged centers were detected in some cases, providing new cyclic enclosures which offer enhanced stability. The results obtained in this paper concern mainly the mass spectrometry analysis of side-chain glycoderivatives, but similarities can also be found for other oxygenated classes of compounds.
Keywords: glycoderivative, computational chemistry, RM1 semi-empirical method, heat of formation, carbocation, anchimeric assistance, dative bond, mass spectrometry.
1. INTRODUCTION
Sixty glycoderivatives possessing the 2,3:5,6-di-0-isopropylidene-D-mannofuranoside base structure, connected at the anomeric position to different alkyl side chains, were investigated using quantum chemical calculations [1,2]. These compounds, after simple hydrolysis of the isopropylidene acetal units [3], can function as biocompatible and biodegradable surfactants in many fields, particularly in pharmaceuticals, veterinary medicine and biotechnologies, but also as household nontoxic detergents [4].
The side chains include the following branched heptyl isomers: 1,1,2-trimethylbutyl (R/S), 1,1,3-trimethylbutyl, 1,2,2-trimethylbutyl (R/S), 1,2,3-trimethylbutyl (R,R/R,S/S,R/S,S), 2,2,3-trimethylbutyl, 1,3,3-trimethylbutyl (R/S), 2,3,3-trimethylbutyl (R/S), 1-ethyl-1methylbutyl (R/S), 1-ethyl-2-methylbutyl (R,R/R,S/S,R/S,S), 1-ethyl-3-methylbutyl (R/S), 2ethyl-1-methylbutyl (R/S), 2-ethyl-2-methylbutyl, 2-ethyl-3-methylbutyl (R/S), 1-propylbutyl and 1-isopropylbutyl (R/S). The general structures for the studied glycoderivatives are given in Figure 1. As can be seen, all the lateral chains are composed of a butyl main chain on which three other carbon atoms are linked as smaller chains (methyl, ethyl, propyl or isopropyl). The four carbon atoms linear core chain was specifically chosen to test for the possible formation of an anchimerically stabilized nine member ring, as explained further in this paper (in "Results and discussion" section). The study consists of removing one methyl radical from the side chain in all possible variants and calculating the heat of formation of the carbocation thus obtained with the help of the RM1 semi-empirical method [5].
Such carbocations [6-9] could be obtained in molecular fragmentation processes involved in the electron ionization mass spectrometric (EI-MS) analysis of this type of compounds, as seen in Figure 2 [4,10]. When some peaks found in the mass spectrum have an unexpected high-intensity, one can propose the hypothesis...