This study compared the analytical properties of 12 indole aldehydes.
The compounds in this study were the six regioisomeric 2-, 3-, 4-, 5-, 6- and 7-formyl indoles and the corresponding six regioisomeric N-n-pentylindole aldehydes. These compounds can serve as precursor chemicals and synthetic intermediates for several synthetic cannabinoid drugs. These two sets (the six regioisomeric indole aldehydes as well as the six regioisomeric pentylindole aldehydes) each have identical elemental compositions and differ in the position of attachment of the aldehyde group on the indole ring. The electron ionization mass spectra for the indole aldehydes were essentially identical. However, the vapor phase infrared spectra showed differences in the absorption frequencies for the Nsingle bondH and carbonyl groups based on intramolecular interactions. The associated Nsingle bondH absorption band occurs as low as 3467cm-1 while the free band is as high as 3517-cm-1. The aldehyde carbonyl band for the indole aldehydes varies from 1713-cm-1 to 1686-cm-1. Substitution of the aldehyde group on the pyrrole ring for the N-n-pentylindole aldehydes yields lower carbonyl absorption bands. The EI mass spectra for the pentylindole aldehydes are identical with little information for differentiation among these six regioisomeric compounds. The six compounds were separated on a capillary column using gas chromatography and the elution order appears to be related to the steric crowding of the indole ring substituents. (publisher abstract modified)