Facile Iodine-Catalyzed Michael Addition of Indoles to ,-Bis(arylmethylene)cyclopentanones: An Efficient Synthesis of E-2-(3-Indolylphenylmethyl)-5-phenylmethylenecyclopentanones

Iodine-catalyzed reaction of indoles with , -bis(arylmethylene)cyclopentanones afforded one diastereomer of the corresponding Michael adducts, namely, E-2-(3-indolylphenylmethyl)-5-phenylmethylenecyclopentanones, in a good yield. The products form a new group of indole derivatives. 1. Introduction The Michael reaction has attracted much attention as one of the most important C–C bond-forming reactions [1, 2]. Indole is a very good Michael donor through its 3-position, and the 3-indolyl system is one of the important building blocks for many biologically active compounds, both natural and unnatural [3]. Traditionally, the Michael addition of indole to α,β-unsaturated ketones as well as addition reactions of indole to other electron deficient olefins is known to be catalyzed by strong bases and Bronsted acids [4–8]. In recent years, it has been reported that these reactions can also be promoted by Lewis acids [9–20]. Following a recent report of iodine-catalyzed Michael addition of indole to some α,β-unsaturated ketones in ethanol [13], we studied the reaction between indole and α,α′-bis(arylmethylene)cyclohexanones under the same condition when we got bis(3-indolyl)methylarenes instead of any conjugate addition product [21]. A survey of the literature revealed that the Michael reaction between indoles and α,α′-bis(arylmethylene)cyclopentanones has not been studied so far. We, therefore, carried out this work under the iodine-catalyzed condition and obtained Michael adducts in good yields (Scheme 1). The results of our study are presented herein. Scheme 1: Iodine-catalyzed Michael addition of indoles to α, α′-bis(arylmethylene) cyclopentanones. 2. Results and Discussion In a preliminary study, α,α′-bis(phenylmethylene)cyclopentanone (1a) was treated with indole (2a) (mole ratio 1？:？1) in the presence of 10？mol% iodine as catalyst in dry dichloromethane at room temperature. The reaction was found to be complete within 4？h affording the conjugate addition product (3a) in 60% yield. The reaction using double the amount of indole also gave the same product in a comparable yield (62%). Encouraged by this result, we carried out the reaction by a variation of conditions, the results of which are given in Table 1. Table 1: Screening for optimum reaction conditions a. It is evident from the results presented in Table 1 that the reaction gave the best yield of the product when dichloromethane was used as solvent and catalyst iodine was used to the extent of 10？mol% (Table 1, Entry 1). When ethanol was used as solvent, the solubility of the substrates at room