Studies in organic mass spectrometry—XII: Mass spectra of chalcones and flavanones. The isomerisation of 2′‐hydroxy‐chalcone and flavanone

Abstract

Abstract-The mass spectra of isomeric 2'-hydroxy-chalcones and flavanones (i.e. 2-phenyl-chroman-4-ones) are essentially identical. Thermal isomerisation prior to ionisation was ruled out. A study of metastable transitions (first and second field free region of a double focusing instrument), as well as low energy spectra of both isomers, pointed towards completely identical fragmentation pathways for the two compounds. Five deuterated compounds were then investigated. The heats of formation of the most important ions were determined. It was concluded that an intramolecular equilibrium exists between a chalcone type and a flavanone type molecular ion. I N T R O D U C T I O NSEVERAL reports on the mass spectral investigation of chalcones and flavanones have already been pub1ished.l to The mass spectrum of chalcone (I) was first recorded by Beynon :l the major fragmentations of this compound were rationalised and found to be characteristic. Pelter and c o -w o r k e r~~.~ studied the mass spectral behaviour of flavanones. It was not until 1966 that AudieS examined some 2'-hydroxy-~halcones. Some intense peaks in the spectra did not fit into the general breakdown pattern of chalcones. He also noticed that the spectra of chalcones bearing a 2'-hydroxyl function were nearly identical to those of the isomeric flavanones. He proposed an equilibrium between a chalcone type and a flavanone type of molecular ion.The mass spectra of 2'-hydroxy-chalcones and flavanones provide a striking analogy between behaviour under electron-impact and solution chemistry. Indeed, these isomers are known to give the same equilibrium mixture in either basic or acidic solution. ' The mechanism of the isomerisation in aqueous solution involves a proton from the solvent. This of course is impossible in mass spectral rearrangements so that the proton must be one of the molecule itself; the isomerisation has to be intramolecular. In order to check this intramolecular hydrogen shift and to determine the equilibration mechanism, the following compounds were synthesised: 6,977 (1972).

Gas-phase nazarov cyclization of protonated 2-methoxy and 2-hydroxychalcone: An example of intramolecular proton-transport catalysis

“…Chalcones 1 to 7 were synthesized by standard procedures [9, 29, 30] with focus on various substituents at the 2- and 4- position of the ‘a’ ring. The benzhydrols used for generating product ions for comparison were synthesized by procedures already reported [31].…”

“…The formation and structures of the [M - H] + ion and [M − H - CO] + ions from the M + , were one focus [9-14], including an ion-structure study with an ion-trap mass spectrometer [15]. Recently, the mechanisms for elimination of C 6 H 6 and CO from the APCI-generated [M + H] + ions of chalcones were established [16].…”

Electrospray tandem mass spectrometry analysis of methylenedioxy chalcones, flavanones and flavones

“…The cyclization of 2'-hydroxychalcones to the corresponding isomeric flavanones has been observed in the gas phase and justifies their similar behavior. [15] Hence, the discussion of chalcones and flavanones (5-12) will be presented together based on the similar fragmentation patterns observed while those of flavones (13-16) will be discussed separately.…”

“…The main product ions [M+H] + observed in the ESI-MS/MS spectra of flavones (13)(14)(15)(16) and the corresponding relative abundance (RA) are summarized in Table 2. The common fragmentation pathways for all these compounds will be discussed followed by the interesting differences which help in differentiating between the two monomethoxy (13 and 14; see Fig.…”

Synthesis of α, β‐Unsaturated Ortho‐Hydroxy Aromatic Ketones by Fries Rearrangement of α, β‐Unsaturated Para‐Cresylesters

“…I n connection with mass s p e c t r a l s t u d i e s ( 9 ) , w e were i n t e r e s t e d i n t h e ortho-hydroxy ketones I , of which Ia, Ic and Id were r e p o r t e d by Sven and a 1 ( 2 f 5 ) '…”