A new approach to synthesis of neoflavonoids, based on a high yielding Montmorillonite K-10 catalyzed lactone ring forming cyclization process, is described. The utility of this methodology is exemplified by its employment in the preparation of the substituted 4-phenylneoflavonoids 1-8. The free radical scavenging properties of these substances were evaluated. The neoflavonoids 1 and 5, which mimic esculetin-type antioxidants, were observed to quench hydrazyl free radicals.As part of a recent search 1 to prepare new biologically active substances, we were attracted to the coumarin family whose members are widely distributed in biologically important natural products and pharmaceutical agents. 2 Coumarins exist in free or glycoside-conjugated forms in a wide variety of plants. 3 Indirect analgesic effects (abolition of oedema and inflammation) of coumarin in rabbit have been observed. 4 Coumarin and 7-hydroxycoumarin also inhibit the biosynthesis of prostaglandins and leucotrienes to about the same degree as acetylsalicylic acid. 5 Moreover, this substance has anti-oedematous, lymphokinetic and haemorheological properties. 6 Naturally occurring neoflavonoid glycosides are known to serve as inhibitors of energy transfer in spinach chloroplasts. 7 Inspite of these effects, Zobel and Brown 8 have concluded that the biologcal roles of coumarins are not well understood.In our previous studies, we have demonstrated that esculetin (6,7-dihydroxycoumarin) possesses potent antioxidant and antitumor promotion properties. 9 The data indicate that this compound induces apoptosis in human leukemia cells by increasing cytosolic translocation of cytochrome c and activating the cysteine protease 32 kDa proenzyme (CPP32). 4-Phenylcoumarins are also of synthetic interest owing to the fact that this ring system is present in a number of naturally occurring compounds, such as serratin, 10a dalbergin, 10b-10d calophyllolide, 10e calomelanols B and C 11 and vismiaguianones D and E. 3dAs a consequence of the biological and structural interest in these substances, we initiated a study aimed at developing a general and high yielding method to prepare coumarins. The commercial availability of cinnamic acids and phloroglucinol prompted us to probe a route to the neoflavonoids, which is based on an esterification-cyclization sequence. It is known that regioselective esterification of the a,b-unsaturated carboxylic acids with polyphenols often is problematic due to competition between cyclization by Friedel-Crafts acylation and Fries rearrangement of the initially formed ester intermediate. 12 Although the synthesis of some coumarins has been successfully approached by using this strategy, 13 harsh condition and high substrate concentrations are commonly required. In addition, Shamsuddin has reported a one-pot procedure for the synthesis of 4-phenylcoumarins. 14 However, the scope of this process appears to be limited. For instance, by following Shamsuddin's procedure, we were unable to efficiently prepare 5,7-dihydroxy-4-phenylcoumarin f...
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