The chemistry of 1,6-dioxapyrenes part 3 : Scope and limitations of an acid catalyzed ring-closing reaction

A new donor for charge-transfer systems: synthesis and properties of 2,4,7,9-tetramethyl-1,6-dithiapyrene (TMDTP) and structure of (TMDTP)3(PF6)2·2THF and TMDTP–TCNQ

The heterocyclic donor molecule 2,4,7,9-tetramethyl-1,6-dithiapyrene (TMDTP) has been synthesized in five steps. Oxidation of TMDTP is facile (E₁^1/2 = 0.27 V and E₂^1/2 = 0.79 V vs. SCE). The charge-transfer complex, TMDTP–TCNQ, has been prepared and the salt, (TMDTP)₃(PF₆)₂·2THF, obtained by electrocrystallization. The structure of TMDTP, TMDTP–TCNQ and (TMDTP)₃(PF₆)₂·2THF has been characterized by X-ray crystallography and computationally.

The heterocyclic donor molecule 2,4,7,9-tetramethyl-1,6-dithiapyrene (TMDTP) has been synthesized in five steps. The charge-transfer complex, TMDTP–TCNQ, has been prepared and the salt, (TMDTP)₃(PF6)₂·2THF, obtained by electrocrystallization.

Low-energy collision-induced dissociation tandem mass spectrometry of 7-acetonyloxycoumarins

RATIONALE

Coumarins are naturally occurring, oxygen-containing heterocycles with considerable pharmaceutical potential. For structural elucidation of natural or synthetic coumarins, tandem mass spectrometry (MS(n)) represents an essential tool. In this study, fragmentation characteristics of twenty-two 7-acetonyloxycoumarins, having promising anti-inflammatory properties, were investigated with low-energy collision-induced dissociation (CID).

METHODS

Accurate mass measurements were performed on a 12-T Fourier transform ion cyclotron resonance (FT-ICR) instrument. Most CID-MS(n) measurements were performed on a quadrupole ion trap (QIT) instrument, except some additional CID-MS(2) measurements performed on the FT-ICR instrument for further confirmation of some fragment ions. Positive-ion electrospray ionization (ESI) was employed throughout. Density functional theory (DFT) calculations (B3LYP) were carried out to analyze putative ion structures/fragmentation channels.

RESULTS

The most favourable dissociation channel for [M + H](+) ions of 7-acetonyloxycoumarins was the elimination of a C3H5O(●) radical (57 Da) from the 7-acetonyloxy group via homolytic bond cleavage. The resulting phenolic radical ion was the primary fragment ion for the most compounds studied. Losses of even-electron neutrals, C3H4O and C3H6O (56 and 58 Da), were also observed. These primary eliminations were accompanied with other characteristic neutral losses from the coumarin skeleton, including H2O, CO, CO2, and C2H2O (ketene). In addition, propene (C3H6) loss was also observed for 4-propyl or 3-ethyl-4-methyl-substituted compounds.

CONCLUSIONS

The studied coumarins showed interesting characteristics in low-energy CID due to the presence of a 7-acetonyloxy group, leading to both even- and odd-electron product ions. The main dissociation channels observed for each compound were highly dependent on the substituents in the benzopyranone ring. The present results will advance our knowledge on the dissociation characteristics of both synthetic and natural coumarins.

Rhodium-catalyzed cascade oxidative annulation leading to substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp2)-H/C(sp3)-H and C(sp2)-H/O-H bonds

The cascade oxidative annulation reactions of benzoylacetonitrile with internal alkynes proceed efficiently in the presence of a rhodium catalyst and a copper oxidant to give substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp(2))-H/C(sp(3))-H and C(sp(2))-H/O-H bonds. These cascade reactions are highly regioselective with unsymmetrical alkynes. Experiments reveal that the first-step reaction proceeds by sequential cleavage of C(sp(2))-H/C(sp(3))-H bonds and annulation with alkynes, leading to 1-naphthols as the intermediate products. Subsequently, 1-naphthols react with alkynes by cleavage of C(sp(2))-H/O-H bonds, affording the 1:2 coupling products. Moreover, some of the naphtho[1,8-bc]pyran products exhibit intense fluorescence in the solid state.