Characterization of Vinyl-Substituted, Carbon-Carbon Double Bonds by GC/FT-IR Analysis

摘要

Vapor-phase infrared spectra.allow the determination of the stereochemistry of carbon-carbon double bonds conjugated with a vinyl group. Cis and trans isomers of unsubstituted 1,3-alkadienes can be differentiated on the basis of the differences observed in the 900-1000 cm↑(-1) region (spectra of cis isomers show two bands at 993 and 906 cm↑(-1), while those of trans compounds show three absorptions at 998, 949, and 902 cm↑(-1)) and the 1590-1650 cm↑(-1) region (the C=C stretch bands are observed at 1595 and 1642 cm↑(-1) for cis compounds and at 1604 and 1650 cm↑(-1) for trans compounds). Compounds bearing CH↓(2)=CHC(CH↓(3)=CHCH↓(2) and CH↓(2)=CHC(=CH↓(2))-CH↓(2)- structural moieties, referred to as a- and fi-type compounds, are frequently encountered as natural products. For compounds bearing a-type groups, the cis/trans configuration of the trisubstituted, double bond can be determined unambiguously. An absorption at 3095-3091 cm↑(-1), for the =CH↓(2) stretch vibration, is common to both of these groups; however, due to the presence of two =CH↓(2) groups, the relative intensity of the band is much higher for ,βB-type compounds. For a-type compounds, a cis configuration at the C-3 carbon atom is characterized by a =CH↓(2) wag absorption at 907-906 cm↑(-1). For fi-type compounds and 3E-α-type compounds,band appears at 899-897 cm↑(-1). In addition, a wavy "fingerprint" pattern with two minima at 1632 (low intensity) and 1595-1594 cm↑-1 (high intensity) is characteristic for β-type compounds. Our generalizations are based on spectra of cis and trans ocimene, myrcene, and dehydration products of many 3-methyl- 1-alken-3-ols. Six isomers of farnesene can be characterized by GC/FT-IR. Furthermore, gas-phase IR allows the determination of configuration of the trisubstituted double bond at C-3 in α-type farnesene congeners. For example, the homo-and bishomofarnesene isomers from Myrmica ants were shown to include a 3Z bond.