8 SYSTEMATIC SYNTHESES AND CHARACTERIZATION OF DODECADIEN-1-OLS WITHCONJUGATED DOUBLE BOND, LEPIDOPTEROUS SEX PHEROMONES
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Four geometical isomers of 5,7-, 6,8-, 7,9- and 8,10-dodecadien-1-ols were systematically synthesized by two routes. One involved the Wittig reaction between (E)-2-alkenal and a phosphorane with an appropriate carbon chain, and yielded a mixture of (E,Z)- and (E,E)-isomers in a ratio of ca. 3:2. The other route comprised of the Wittig reaction of a 2-alkynal and the stereoselective reduction of the triple bond to a (Z)-double bond via hydroboration with dicyclohexylborane to give a mixture of (Z,Z)- and (Z,E)-isomers in a ratio of 6:1〜10:1. Both the mixtures were separately chromatographed on a silica gel column impregnated with silver nitrate to give four geometically pure isomers. With 9,11-diene they were analysed by GC on a capillary column, HPLC, silver nitrate impregnated TLC, ^1H and ^<13>C NMR, and GC-MS. By the ^<13>C NMR analysis of all the isomers a new method for assigning ^<13>C signals of a conjugated system was proposed. The method is based on two empirical rules. The first involves the ^<13>C shift differences of olefinic carbons induced by a substitutional change at an end of the conjugated diene system. The second rule concerns the chemical shift changes of allylic and olefinic carbons by converting the geometry of conjugated diene systems. On the electron impact mass spectrometry, every dienic compound showed typical series of C_nH_<2n-2>^+〜C_nH_<2n_5>^+ with abundance maxima around C_4, C_5, C_6 or C_7. Each double bond positional isomer characteristically yielded different ion peaks in the series, which were useful for its distinction from other isomers. The process of formation of some of the ions can be speculated by the comparison with the mass spectra of deuterated compounds.
