Pergamon Tetrahedron Letters 40 (1999) 6061-6064 TETRAHEDRON LETTERS MgBr2-Directed endo-Selective Conjugate Addition of Organometallic Reagents to Enones Anchored on Cr(CO)a Vishwanath M. Swamy, Sunil K. Mandai and Amitabha Sarkar* Division of Organic Chemistry (Synthesis) National Chemical Laboratory, Pune - 411008, India. e-mail : sarkar@ems.nd.res.in Received 12 April 1999; accepted 29 June 1999 Abstract Additionof RMgXor RLi/MgBr2to 2-arylidene-l-tetralone-Cr(COhcomplexestakes place in a conjugatemanner with complete endo selectivity. © 1999Elsevier ScienceLtd. All rights reserved. Nucleophilic addition at a benzylic or homobenzylic site of an arene tricarbonylchromium complex occurs commonly from the exo face (opposite to the metal earbonyl). 1 For stereochemically defined enone substrates, the presence of a Lewis acid in the medium can alter such steric preference. 2 For instance, conjugate addition of a strong nucleophile, such as organolithium reagent, to enones in the presence of TiCI4 has been shown to occur from the endo face. 2b In the absence of a Lewis acid, such reagents add exclusively in an exo-selecfive manner to the earbonyl functionality (1,2-attack) instead of the double bond (1,4-attack). In absence of additives, Grignard reagents were also expected to provide exo-selective 1,2-addition products from the same set of substrates. However, the results were dramatically different. Contrary to expectation, a series of Grignard reagents added to the tricarbonyl-chromium anchored enones 2c la-b in a conjugate manner from the endo-faee of the molecule exclusively (Scheme-I, Table-l, Entry 1-4). 0040-4039/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. I'll." S0040-4039(99)01257-5