New asymmetrical per-substituted cyclodextrins (2-O-methyl-3-O-ethyl- and 2-O-ethyl-3-O-methyl-6-O-t-butyldimethylsilyl-β-derivatives) as chiral selectors for enantioselective gas chromatography in the flavour and fragrance field

Asymmetrically substituted 6I-VII-O-t-butyldimethylsilyl(TBDMS)-3I-VII-O-ethyl-2I-VII-O-methyl-β-cyclodextrin (MeEt-CD) and 6I-VII-O-TBDMS-2I-VII-O-ethyl-3I-VII-O-methyl-β-cyclodextrin (EtMe-CD) were synthesised to evaluate the role of the substitution pattern in positions 2 and 3 on the enantioselectivity, in particular in view of their application to routine analysis in fast enantioselective gas chromatography (Es-GC). The chromatographic properties and enantioselectivities of the new derivatives were tested by separating the enantiomers of a series of medium-to-high volatility racemates in the flavour and fragrance field, and compared to those of the corresponding symmetrically substituted 6I-VII-O-TBDMS-2I-VII,3I-VII-O-methyl-β-CD (MeMe-CD) and 6I-VII-O-TBDMS-2I-VII,3I-VII-O-ethyl-β-CD (EtEt-CD), and were then applied to analysis of real-world essential oil (e.o.) samples. A new synthetic process including the sonochemical approach to obtain synthetic reproducibility and significant yields of the per-substituted derivatives with acceptable reaction times was developed. The results show that asymmetrically substituted methyl/ethyl CDs compared to the methyl or ethyl symmetrical derivatives in general provide better enantioselectivity in terms of both enantiomer resolution and number of separated chiral compounds, and show how the substitution pattern in positions 2 and 3 of the CD ring can influence the separation. Moreover, these new CD derivatives with better enantioselectivity are also shown to be very useful in routine analysis for the exhaustive control of samples containing several chiral characterizing markers in a single run.