Sulfamic acid: a novel and efficient catalyst for the synthesis of aryl-14H-dibenzo[a.j]xanthenes under conventional heating and microwave irradiation
Graphical abstract
Introduction
The synthesis of xanthenes, especially benzoxanthenes, has emerged as a powerful tool in organic synthesis due to their wide range of biological and therapeutic properties such as antibacterial,1 antiviral2 and anti-inflammatory activities,3 as well as in photodynamic therapy4 and for antagonism of the paralyzing action of zoxazolamine.5 Furthermore, due to their useful spectroscopic properties, they are used as dyes,6 in laser technologies,7 and in fluorescent materials for visualization of biomolecules.8 Many procedures describe the synthesis of xanthenes and benzoxanthenes including cyclodehydrations,9 alkylations γ to the heteroatoms,10 trapping of benzynes by phenols,11 cyclocondensation between 2-hydroxyaromatic aldehydes and 2-tetralone,12 the reaction of β-naphthol with aldehydes or acetals under acidic conditions and intramolecular phenyl carbonyl coupling reactions of benzaldehydes and acetophenones.13 In addition, 14H-dibenzo[a.j]xanthenes and related products are prepared by reaction of β-naphthol with formamide,14 2-naphthol-1-methanol15 and carbon monoxide.16
Even though various procedures are reported, disadvantages including low yields, prolonged reaction times, use of an excess of reagents/catalysts and use of toxic organic solvents necessitate the development of an alternative route for the synthesis of xanthene derivatives. Recently, sulfamic acid has emerged as a promising solid acid catalyst for acid catalyzed reactions, such as functional group protections and deprotections17 and the synthesis of isoamyl acetate18 and polymeric ethers.19 Moreover, some important organic transformations, including the Beckmann rearrangement,20 and Pechmann21 and Bignelli condensations,22 have been performed successfully in the presence of sulfamic acid. The reported route is an efficient, convenient and novel method for condensation of aldehydes with β-naphthol in the presence of sulfamic acid as catalyst (Scheme 1).
In the conventional method (Method A), β-naphthol was heated with different aromatic aldehydes at 125 °C to afford the products in 6–12 h. As part of our ongoing work with microwave irradiation,23 β-naphthol was heated under solvent-free conditions with different aromatic aldehydes in the presence of sulfamic acid in a micrcowave oven (Method B) for the appropriate time (Table 1) to yield the desired products.
Section snippets
Conventional method (method A)
A mixture of the aldehyde (1 mmol), β-naphthol (2 mmol) and sulfamic acid (0.1 mmol) was stirred at 125 °C for the appropriate time according to Table 1. Completion of the reaction was indicated by TLC. The reaction was cooled to 25 °C, water was added and the mixture stirred for 5 min. The solid obtained was removed by filtration and recrystallized from ethyl alcohol.
Microwave irradiation method (method B)
To a mixture of aldehyde (1 mmol) and β-naphthol (2 mmol), sulfamic acid (0.04 mmol) was added and the mixture was inserted in a
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