1314 LETTER Synlett 2001, No. 8, 1314–1316 ISSN 0936-5214 © Thieme Stuttgart · New York Montmorillonite K10 Clay-Microwave Assisted Isomerisation of Acetates of the Baylis-Hillman Adducts: A Facile Method of Stereoselective Synthesis of (E)-Trisubstituted Alkenes Ponnusamy Shanmugam,* Paramasivan Raja Singh Organic Chemistry Division, Regional Research Laboratory (CSIR), Trivandrum-695 019, Kerala, India Fax +0091-471-491712; E-mail: shan@csrrltrd.ren.nic.in Received 14 June 2001 Dedicated to Professor K. Rajagopalan on the occasion of his 60 th Birthday. Abstract: Montmorillonite K10-microwave assisted stereoselec- tive isomerisation of acetates of the Baylis-Hillman adducts fur- nished (E)-trisubstituted alkenes in high yields. A comparative study of this reaction with Mont-K10, ion-exchanged clay (Fe 3+ - Mont-K10) and acid treated regional natural Kaolinite clay and the efficiency of these clays in this reaction is described. Key words: montmorillonite K10 clay, microwave, stereoselective isomerisation, Baylis-Hillman reaction, E-alkenes, ion-exchanged clay, natural kaolinite clay The usefulness of montmorillonite K10 clay in organic synthesis and its application as a catalyst for a number of organic reactions are well documented. 1-4 The montmoril- lonite K10 and its structurally modified clays are known to act as both Bronsted and Lewis acid catalysts for a va- riety of industrially important organic reactions. 1 The clay catalysts are known as eco-friendly acid catalysts which have potential for replacing the conventional mineral ac- ids and are non-pollutant. The advantages of the clay-cat- alyzed reactions are that they are generally mild, solvent free and work-up is easy. The Baylis-Hillman reaction is one of the important Carbon-Carbon bond forming reac- tions and has been used in organic synthesis for the syn- thesis of a variety of compounds having diverse functional groups and has been used as the starting point for a variety of synthetic organic transformations. 5-13 Stereoselective synthesis of (E)-trisubstituted alkenes is one of the diffi- cult tasks in organic synthesis and only a few methods are known in the literature. 8, 9,14 Stereoselective isomerisation of acetates of Baylis-Hillman adducts catalyzed by TM- SOTf, 8,9 trifluoroacetic acid, 10 benzyl-trimethylammoni- um fluoride 11 have appeared in the literature. Montmorillonite K10-microwave combination has been utilized for effecting many organic transformations as a catalyst. 15-17 In continuation of our research work on clay catalysis, 18-20 we herein report the mont- K10-microwave assisted stereoselective isomerisation of acetates of Bayl- is-Hillman adducts into its (E)- trisubstituted olefins. The general isomerisation studies are schematically repre- sented in Scheme 1. The Baylis-Hillman acetate adducts 2 (a-p) were prepared according to the literature procedure. 8 Stirring the acetate 2a with 50% w/w montmorillonite K10 clay in CH 2 Cl 2 at r.t. for 48 h furnished the starting material with 20% deacylated product. Heating the same reaction mixture at reflux for 24 h again furnished only the starting material. However, when a slurry of the acetate 2a with 50% w/w mont-K10 clay without any solvent was ir- radiated in a microwave oven for 6 min., a clean isomer- ised product 3a was obtained in 60% yield (5-10% decomposition) as determined by 1 H NMR. Among the several variations tested to optimize the condition, 21 the following conditions involving acetate 2a with 30% w/w of mont-K10 clay, 70% microwave power level (PL) and 13 min. irradiation time was found to be best and yielded the clean isomerised product 3a (in 9:1, E:Z isomer) 22 in 74% after column purification. The microwave irradiation of the acetate 2a under similar conditions without any clay furnished the starting material quantitatively confirming the necessity of clay catalyst for this reaction. Adduct 2a (deacetylated) under similar conditions furnished 20% of the isomerised product with remaining decomposed prod- ucts. Hence acetate protection of the Baylis-Hillman ad- ducts is necessary for good yields. The results are summarised in Table 1. R H O Z R OH Z R OAc Z R H Z OAc a b c R=Ph, 4-Cl-Ph, 2,4-Cl 2 -Ph, 4-CH 3 -Ph, 4-CH 3 O-Ph, Naphth-1-yl, Naphth-2-yl; Z= CO 2 Et, CN, COCH 3 + 1(a-p) 2-(a-p 3(a-p) 3a E:Z=9:1 Scheme 1 Reagents and Conditions: a) DABCO, Neat; b) Acetyl chloride, Py. 0 °C, 1h; c) 30% w/w mont. K10, MW, 13 min (70% PL), Neat, 57–80%. Isomerisation of Baylis-Hillman adducts catalysed by mont.K10-microwave irradiation