A facile transesterification of b-ketoesters using catalytic iodine is described.The transesterification of b-ketoesters has been found to be a very useful tool in organic synthesis having wide applications in academic as well as industrial research. A number of methods have been reported to effect transesterification of b-ketoesters. Although uncatalysed transesterification of b-ketoesters is reported in the literature 1-3 they require the use of either an excess of ketoester or longer reaction times and employ high boiling alcohols or utilise unconventional energy sources thus limiting their usage. Similarly uncatalysed transesterification of tertbutyl acetoacetate is also reported in the literature. 4 However, this method is confined to tert-butyl acetoacetate which itself is not readily available thus limiting its usage.Transesterification of b-ketoesters has been reported using reagents like DMAP, 5 DBU, 6 titanium tetraalkooxide, 7 tetrabutyl distannoxanes, 8,9 p-TSA. 10 However, most of these reagents are toxic, expensive or difficult to prepare. We have already described the utility of heterogeneous catalysts like S-SnO 2 11 and Amberlyst-15 12 as efficient catalysts for transesterification of b-ketoesters.Several other catalysts have also been shown to be effective for transesterification of b-ketoesters. 13,14 In recent years reagents like indium triiodide, 15 iodotrimethylsilane/I 2 16 have been used for transesterification. A recent report on the use of iodine 17 for transesterification of esters shows that iodine acts as an efficient catalyst to effect the transesterification. It has limitations, as benzyl alcohol fails to undergo transesterification with normal esters. Additionally the iodine-catalysed protocol involves the use of excess alcohol to push the equilibrium driven reaction in the desired direction and requires longer reaction times. Recently we have demonstrated that zinc can be activated by iodine 18 and this reagent was shown to effect transesterification with normal alcohols as well as phenols to furnish coumarins. In this communication we wish to report that even iodine acts as an efficient catalyst to effect transesterification (Scheme 1). The results of this protocol are summarised in Table 1.
Scheme 1The noteworthy feature of our protocol is that b-ketoesters like ethyl acetoacetate and ethyl cyclopentanone-2-carboxylate underwent transesterification with benzyl alcohol to furnish the transesterified products in good yields (83% and 88% respectively). In most of the cases only 1.2 equivalents of alcohols were required for efficient conversions. However, in the case of volatile alcohols like 2-propanol, 1-propanol and propargyl alcohol, 2 equivalents of alcohol are required to obtain good yields of the corresponding ester. The other important feature of this protocol is that transesterification of b-ketoesters by various alcohols like benzyl, allylic, propargyl alcohols has been effectively catalysed by iodine giving products with moderate to high yields in comparatively reduced re...
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