Coumarins, dimerization

Dimerization. A coumarin dimer is formed by prolonged exposure of coumarin to sunlight or uv radiation. Photodimerization is also catalyzed by boron trifluoride (30). 

In order to determine the sensitivity of coumarin dimerization to heavy-atom perturbation, the direct photolysis was carried out in butyl chloride, propyl bromide, and ethyl iodide. Comparison of these results with those obtained in nonpolar solvents indicated that there was no significant change in the amount of the triplet-derived product (80) formed in the heavy-atom solvents as would be expected if heavy-atom perturbation were important.<90) More recent investigations,<93W however, of the formation of (79) in carbon tetrachloride indicate that halocarbon enhancement of the formation of this isomer exists. A direct measure of the intersystem quantum... 

Scheme 2.155. Reaction of the chloro-coumarin dimer 2-693 with primary amines.

Figure 16.8 Photochemical gate based on coumarin dimerization, (a) in its open and (b) in its closed state.

Acridone alkaloids comprise a relatively small group of alkaloids that are found solely in some Rutaceae genera. Some 100 examples of this alkaloid group have been isolated and these include the monomeric acridones and the acridone-coumarin dimers (acrimarines) isolated from Citrus plants some binary alkaloids have recently been isolated and described (Takamura et al, 1995). 

Kean ZS, Gossweiler GR, Kouznetsova TB, Hewage GB, Craig SL (2015) A coumarin dimer probe of mechanochemical scission efficiency in the sonochemical activation of chain-centered mechanophore polymers. Chem Commun 51 9157... 

Certain plants of the Rutaceae produce a number of different coumarin dimers in which the monomeric units, an isopentadienyl-substituted coumarin, are joined in a Diels-Alder fashion. The structural features resulting from this mode of dimerization are not restricted to coumarins in the Rutaceae but can also be found in two sets of dimeric 2-quinolones, indole alkaloids and related isopentenyl substituted compounds. A remarkable feature of all of these dimers is that they occur as racemates in spite of the fact that they all have two or more asymmetric centers. The racemic nature of these coumarin dimers has raised the possibility that these substances might arise by a non-enzymatic Diels-Alder reaction during biosynthesis. 

The isopentadienyl system can react with either of the two double bonds in the dienophile. Because of the unsymmet-rical nature of the dienophile, there are two possible Diels-Alder adducts for each double bond. This leads to a total of four different possible structures (Figs. 1 and 2). The formation of coumarin dimers in every case published follows Figure 1. Moreover, only one of the two possible adducts has been reported. Since each of the different coumarin adducts contains two asymmetric centers, there are two diastereoisomers possible and with one possible exception only one of these has apparently been isolated in each case. The non-coumarin optically inactive Diels-Alder dimers. 

A recent paper by Schroeder and Stermitz on isopentenyl substituted 2-phenethylamine alkaloids aids greatly in resolving some of these question. These workers found a series of four racemic alkaloids in a Zanthoxylium species. These alkaloids are all formal Diels-Alder adducts in which addition has occurred in a fashion (Fig. 2) entirely different from that found in the coumarin dimers (Fig. 1). 

Synthetic work directed towards the preparation of these 2-phenethylamine alkaloids led to structural analogues of the coumarin dimers. Thus, when the isopentendienyl hordenine (15) was allowed to stand at room temperature, both diastereoisomers 16 and 17 were formed in a 9 1 ratio. 

KIS 14] Kiskan B., Yagci Y., Self-healing of poly(propylene oxide)-polybenzoxazine thermosets by photoinduced coumarine dimerization , Journal... 

Reaction of an anti head-to-tail 4-chlorocoumarin dimer (34) with amines afforded benzopyranocoumarin derivatives (35) in high yields the reaction is different from the expected reaction of coumarin dimer derivatives with amines. On the basis of spectral information, a possible reaction mechanism (Scheme 6) was proposed involving, in sequence, a tandem iactone ring-opening reaction, elimination, cycloreversion of cyclobutene, addition-elimination, and Michael addition. " ... 

Saigo, K., Sekimoto, K., Yonezawa, N., Ishii, F., and Hasegawa, M., Optically active head-to-head coumarin dimer. A new agent for the determination of enantiomeric excess of amines and alcohols. Bull. Chem. Soc. Jpn., 58,1006,1985. 

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