Photochemical transformations of chalcone-podands into cyclobutane-containing benzocrown ethers

Abstract

A method for the synthesis of cyclobutane containing benzocrown ethers by the template photocycloaddition of the corresponding chalcone podands in solutions was developed. It was shown by X ray diffraction analysis, 1 H and 13 C NMR, and UV spectroscopy that the intra molecular [2π+2π] photocycloaddition (PCA) is stereoselective with the predominant forma tion of structures of the β and γ truxinic type. The PCA rate was shown to depend on the size of the oxyethylene spacer and stereoorientation of the chalcone fragments in the podands. The selectivity of the PCA process can be controlled by a change in the concentration of the template (alkaline metal ions) and variation of the wavelength range of irradiation.

The First Example of Cascade Synthesis of Alkaloid-like Subunit Incorporated into Crown Ethers

“…The formation of 7 takes place in the presence of sodium, potassium or cesium hydroxides, thus suggesting that the template assistance of these ions [11][12][13] might be a driving force to push a domino sequence of intramolecular transformations. It is well-known that the conformationally mobile podands are able to manifest various types of complexes with alkali ions.…”

“…A similar effect has been observed earlier in photoinduced transformation of chalcone podands. [13] Thus, the secondary C-addition results in the formation of the cyclic ether 9. Orientation of the C=O groups in the intermediate 9 provides their vicinity to N-and C-nucleophilic centers, thus facilitating subsequent intramolecular condensations, leading to 10, and finally to the compound 7.…”

“…The following procedure for the synthesis of 7 and 8 has been used: a solution of 0.25 g (0.48 mmol) of chalcone podand 5, [13,14] 0.095 g (0.96 mmol) of 4-aminopent-3-en-2-one and 0.054 g (0.96 mmol) of КОН (or NaOH, Cs 2 CO 3 , 1,8-diazabicyclo[5.4.0]undec-7-ene, 2-tert-butyl-1,1,3,3-tetramethylguanine, triethylamine and tetramethylammonium hydroxide) in DMF (10 ml) was heated up to 80 °С, and kept at this temperature for 35 h. The reaction mixture was evaporated and a residue obtained was washed out with several portions of water on a filter. The subsequent chromatography of dry residue on a column with silica gel (TLC, benzene/ethyl acetate 1:1, R f = 0.63) yielded 0.13 g (48%, 60% according to HPLC) 7 as colorless crystals.…”

“…13 С NMR (100 MHz, DMSO-d 6 ) δ C ppm: 38.19 (C(18')); 43.82 (C (18) X-Ray crystallography. Crystals of 7 and 8 were obtained by the slow evaporation of DMF solutions.…”

“…[10] In supramolecular chemistry the oligooxyethylene fragment of noncyclic crown ethers in combination with the template effect of metal ions plays a similar role in the synthesis of various macrocycles. [11,12] For instance, recently we have reported [13] on photoinduced [2+2]-cycloaddition of chalcone podands, such as 5 [14] (Scheme 2), proceeding in the solutions in the presence of alkaline metal ions and leading directly to the formation of the corresponding Scheme 1. Synthesis of (±)-methyl homosecondaphniphyllate.…”