Synthesis and structure of indoline spiropyrans based on benzo[ f ]coumarin
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Russian Chemical Bulletin, International Edition, Vol. 69, No. 7, pp. 1378—1384, July, 2020
Synthesis and structure of indoline spiropyrans based on benzo[f]coumarin* O. G. Nikolaeva,a O. Yu. Karlutova,a A. A. Guseva,a E. B. Gaeva,a A. G. Starikov,a A. D. Dubonosov,b V. A. Bren,a A. V. Metelitsa,a and V. I. Minkina aInstitute
of Physical and Organic Chemistry, Southern Federal University, 194/2 prosp. Stachki, 344090 Rostov-on-Don, Russian Federation. Fax: +7 (863) 243 4667. E-mail: [email protected] bFederal Research Centre the Southern Scientific Centre of the Russian Academy of Sciences, 41 prosp. Chekhova, 344006 Rostov-on-Don, Russian Federation. Fax: +7 (863) 266 5677. E-mail: [email protected] Indoline spiropyrans containing benzo[f]coumarin moiety annulated to the 2H-pyran cycle were synthesized. The obtained compounds exist in solutions as a tautomeric mixture of their spirocyclic and merocyanine forms. The spiro form exhibits an enhanced resistance to UV irradiation and does not rearranges into the merocyanine isomer in the temperature range of 278—303 K. Key words: spiropyrans, benzo[f]coumarin, merocyanine, quantum chemical calculations.
Natural and synthetic coumarins possess diverse biological, pharmacological, and photophysical properties.1—4 They are employed as antibacterial and anti-inflammatory drugs and stimulants of the central nervous system and exhibit various biological activities.5—9 Coumarin derivatives are also applied in photonics for design of molecular switches, laser dyes, light-emitting devices, and chemoand biosensors.2,10,11 It has been revealed earlier that a relative stability of the spirocyclic (S) and merocyanine (M) isomers of coumarin-based spiropyrans (SPPs) and their photochromic properties depend significantly on the type of annelation of the coumarin and 2H-pyran heterocycles.12—15 In particular, we have previously synthesized16 indoline SPPs 1 based on 6-formyl-5-hydroxy-4,7-dimethylcoumarin and exhibiting photochromic properties at room temperature (Scheme 1). Once the UV irradiation of their solutions in toluene was stopped, fast thermal 1-М → 1-S relaxation processes were observed. In the present work, we have obtained a series of SPPs 2 (Scheme 2) containing an expanded π-system based on benzo[f]coumarin and explored their structure and photochromic properties. π-Expanded benzocoumarins are the poorly explored, but promising objects in applied organic chemistry.17—19 Pechmann condensation of 2,7-dihydroxynaphthalene with acetoacetic ester led to 9-hydroxy-1-methylbenzo[f]coumarin 3 that was subsequently formylated with hexamethylenetetramine (HMT) according to the Duff * Based on the materials of the XXI Mendeleev Congress on General and Applied Chemistry (September 9—13, 2019, St. Petersburg, Russia).
Scheme 1
R = H, Me, Cl, NO2
i. UV irradiation.
method to give 8-formyl-9-hydroxy-1-methylbenzo[f]coumarin 4. The latter was converted into spiropyrans 2 upon its condensation with the corresponding 2,3,3-trimethylindolenyl perchlorates in isopropyl alcohol in the
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