Triphenylphosphine Mediated Synthesis of Functionalized Benzo-Fused Coumarins from Some OH Acids and Dialkyl Acetylene Dicarboxylate

Coumarins comprise a very large class of compounds found throughout the plant kingdom [1]. The bioactivity of coumarin andmore complex related derivatives appears to be based on the coumarin nucleus [2, 3]. Coumarin compounds can display anticancer, anticoagulant, antimicrobial, antiinflammatory, and antioxidant activities [4–8]. In addition, as an important class of organic heterocyclic dyes, coumarin derivatives exhibit unique photochemical and photophysical properties, which render them useful in a variety of applications such as optical brighteners, laser dyes, nonlinear optical chromophores, solar energy collectors, fluorescent labels and probes in biology and medicine, and two-photon absorption (TPA) materials [9–12]. Coumarins have been synthesized by several methods [13–18]. In the interest of synthesizing new coumarin ring systems for possible evaluation as biologically active compounds, we have described a synthesis of carboxymethyl coumarins from 3-hydroxyl pyridine [19], carboxylic systems [19–22], we wish to report here the synthesis of some benzofused coumarins.The preparations of coumarins are depicted in Schemes 1, 2, and 3. 2. Experimental

In addition, as an important class of organic heterocyclic dyes, coumarin derivatives exhibit unique photochemical and photophysical properties, which render them useful in a variety of applications such as optical brighteners, laser dyes, nonlinear optical chromophores, solar energy collectors, fluorescent labels and probes in biology and medicine, and two-photon absorption (TPA) materials [9][10][11][12].

General Procedure for the Preparation of Compound 3.
To a stirred solution of 0.52 g of Ph 3 P (2 mmoL) and 0.28 g 1 (2 mmoL) in toluene (10 mL) was added dropwise a mixture of 2 (2 mmoL) in toluene (2 mL) at room temperature over 5 min.The reaction mixture was heated under reflux for 24 h.The solvent was removed under reduced pressure, and the viscous residue was purified by column chromatography (SiO 2 ; hexane/AcOEt) to afford the pure adducts.
The mass spectra of these compounds displayed molecular ion peaks at appropriate m/z values.Its 1 H NMR spectrum showed 3H singlet at  = 4.08 for only one methoxy group and a singlet (1H) at  = 7.09 for methine proton.The 13 C NMR spectrum one carbon appeared for the coumarin carbonyl ( = 158.6 ppm) and one carbon for the carbonyl ester moiety ( = 163.5 ppm).The 1 H and 13 C NMR spectra of 4b and 4c are similar to those of 4a except for the alkoxy moieties, which exhibited characteristic resonances with appropriate chemical shifts.Treatment of 3-isoquinolinol 5 with dialkyl acetylene dicarboxylate 2 and PPh 3 in refluxing toluene for 24 h and separation of the reaction mixture by column chromatography gave methyl 3-oxo-3H-pyrano[2,3-c]isoquinoline-1carboxylate 6 in 76% yield (Scheme 3), obviously via a reaction sequence similar to that depicted in Scheme 1.The reaction with di-tert-butyl acetylene dicarboxylate was not detected because of the bulky group (Scheme 2).
Mechanistically [13,19] it is conceivable that the reaction involves the initial formation of a zwitterionic intermediate between Ph 3 P and the acetylenic compound and subsequent protonation of reactive 1 : 1 adduct followed by electrophilic attack of vinyltriphenylphosphonium cation on the aromatic ring at the ortho position relative to the strong activation group.The product is presumablyproduced by intramolecular lactonization of the unsaturated diester 10 (Scheme 4).

Conclusion
From the above results, we conclude that treatment of oxygen atom of the pyran ring belongs to the starting phenol, while the periselectivity in the construction of this new ring depends on the higher reactivity of the free o-positions of the phenol during the aromatic electrophilic substitution sequence.These functionalised coumarins may be considered as potentially useful synthetic intermediates because they possess atoms with different oxidation states.The advantages of present method are the fact that it performed under neutral condition and substances are utilized in their basic form without any modification.The simplicity of the present procedure makes it an interesting alternative to other approaches.