No abstract

No abstract

No abstract

The number of natural and synthetic coumarin (2H-chromen-2-one) derivatives 1) have been reported to exert notably antimicrobial 2,3) as well as antifungal, 4,5) tuberculostatic 6) and anti-human immunodeficiency virus (HIV) 7,8) activity. Moreover, the antibiotic novobiocin belongs to the hydroxycoumarin series. The importance of these compounds have motivated many workers to synthesize a number of them, and numerous methods have been developed for their preparation.9-11) On the other hand, catechols are a promising group of compounds worthwhile for further investigation, which may lead to the discovery of selective acting, biodegradable agrochemicals having high human, animal and plant compatibility. 12,13) A literature survey reveals that, in contrast to the widely studied, coumarin derivatives, no paper has reported the synthesis of 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives. Following our experiences in electrochemical oxidation of catechols in the presence of nucleophiles, [14][15][16][17][18][19][20][21] we envisaged that synthesis of organic compounds with both structures of catechol and coumarin might cause an enhancement of pharmaceutical properties and medicinal activities. This idea prompted us to investigate the electrochemical oxidation of catechols in the presence of 4-mercaptocoumarin as the nucleophile and we have discovered an easy and one-pot electrochemical method for the synthesis of 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives (4a-d) in high yield and purity, using this environmentally friendly method with high atom economy. In order to study the feasibility of the electrochemical synthesis of 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives (4a-d) and mechanistic aspects of this conversions, electrochemical oxidation of catechols in the absence and presence of 4-mercaptocoumarin was studied using cyclic voltammetry. As well, in this work computational studies were used for quantitative answer to the question concerning the difference in electrochemical oxidation of catechol in the presence of 4-hydroxycoumarin and 4-mercaptocoumarin. Results and DiscussionElectrochemical Studies Figure 1, curve a, shows the voltammetric curve obtained for the oxidation of catechol (1 mM) in water/acetonitrile (50/50) solution containing sodium acetate (cϭ0.2 M) on a glassy carbon electrode. In the studied potential-range, a well defined voltammetric curve is obtained that has an anodic (A 1 ) and the corresponding cathodic (C 1 ) peaks. These peaks are correspond to the oxidation of catechol (1a) to o-benzoquinone (2a) and vice versa within a quasi-reversible two electron process.14-21) The oxidation of catechol (1a) in the presence of 4-mercaptocoumarin (3) as a nucleophile was studied in some details. The 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives have been synthesized by direct electrochemical oxidation of catechols in the presence of 4-mercaptocoumarin as a nucleophile in water/acetonitrile (50/50) solution, in a one-pot process, at carbon rod electrode, in an undivided...

The B3LYP/6-31G à method was used to investigate the configurational properties of allene (,9-decanonaene (9). The calculations at the B3LYP/6-31G à level of theory showed that the mutual interconversion energy barrier in compounds 1-8 are: 209.73, 131.77, 120.34, 85.00, 80.91, 62.19, 55.56, and 46.83 kJ mol À1 , respectively. The results showed that the difference between the average C --C double bond lengths (d) values in cumulene compounds 1 and 2, is larger than those between 7 and 8, which suggest that with large n (number of carbon atoms in cumulene chain), the d values approach a limiting value. Accordingly, based on the plotted data, the extrapolation to n ¼ 1, gives nearly the same limiting d(i. e., d lim ). Also, NBO results revealed that the sum of p-bond occupancies, p occupancy , decrease from 1 to 8, and inversely, the sum of p-antibonding orbital occupancies, p à occupancy , increase from compound 1 to compound 8. The decrease of Dðp occupancy À p à occupancy Þ values for compounds 1-8, is found to follow the same trend as the barrier heights of mutual interconversion in compounds 1-8, while the decrease of the barrier height of mutual interconversion in compounds 1-8 is found to follow the opposite trend as the increase in the number of carbon atom. Accordingly, besides the previously reported allylic resonant stabilization effect in the transition state structures, the results reveal that the d values, Dðp occupancy À p à occupancy Þ, D(E HOMO À E LUMO ), and the C atom number could be considered as significant criteria for the mutual interconversion in cumulene compounds 1-8. This work reports also useful predictive linear relationships between mutual interconversion energy barriers (DE a 0 ) in cumulene compounds and the following four parameters: d, Dðp occupancy À p Ã

No abstract

No abstract

No abstract