Diaminomethylenemalononitrile organocatalysts promote the asymmetric chlorination of simple cyclic β-keto esters such as methyl, ethyl, and benzyl esters of 1-oxo-2,3-dihydro-1H-indene-2-carboxylic acid. This affords the corresponding chiral α-chlorinated carbonyl products in excellent yields with high enantioselectivities.Key words organocatalyst; asymmetric chlorination; β-keto ester Chiral α-chlorinated carbonyl compounds are valuable building blocks in the production of various biologically active compounds. [1][2][3][4][5] To synthesize enantiomerically enriched α-chlorinated carbonyl compounds, direct chlorination of carbonyl compounds using a chlorine source, such as N-chlorosuccinimide (NCS), is a reliable method. Several research groups have reported on the asymmetric chlorination of β-keto esters using metal catalysts.6-17) Organocatalysis, meanwhile, has received considerable attention in the area of organic synthesis because, in comparison to reactions involving metal catalysts, it is an environmentally benign method. Synthetic methods for producing α-chlorinated compounds from β-keto esters using organocatalysts have also been developed. [18][19][20][21][22][23][24] Although chiral α-chlorinated β-keto esters are valuable synthetic intermediates, their preparation using organocatalysts and NCS with high enantioselectivity (>80% enantiomeric excess (ee)) has rarely been reported upon.19-21) Several methods for the synthesis of chiral α-chlorinated β-keto esters require high catalyst loading (>5 mol%), a complex chlorine source, and bulky ester units such as the t-butyl and adamantyl groups. The highly stereoselective (>80% ee) α-chlorination of simple esters of 1-oxo-2,3-dihydro-1H-indene-2-carboxylic acid, such as methyl, ethyl, and benzyl esters (6a-c), is very rare.12,18) Among the various chlorine sources, NCS is one of the best reagents because of its simple structure and low cost. The α-chlorination of a simple cyclic β-keto ester with NCS, in the presence of an organocatalyst (ca. 1 mol%), is therefore highly desirable, and it is one of the most challenging research topics.Recently, we reported on some asymmetric reactions using organocatalysts bearing diaminomethylenemalononitrile (DMM) or diaminomethyleneindenedione (DMI) motifs. [25][26][27][28][29][30][31][32] In order to further demonstrate the value of DMM organocatalysts, we attempted to apply them to the asymmetric α-chlorination of simple cyclic β-keto esters.
Results and DiscussionWe examined organocatalysts 1-5 (Fig. 1) for an enantioselective α-chlorination of β-keto ester 6a, as shown in Table 1. DMM organocatalysts 1-3 with tertiary or secondary amine groups provided the desired product 7a in excellent yields with low enantioselectivities (entries 1-3). Organocatalyst 4, which bears a chiral primary amine group, accelerates the α-chlorination of 6a and gives 7a with good enantioselectivity