Synthesis of β-Aryl Substituted N-Tosyl Aza-Baylis-Hillman Adducts: Heck Reaction of N-Tosyl Aza-Baylis-Hillman Adducts

During the last two decades notable improvements in Baylis-Hillman chemistry have been achieved in view of the reaction rate and synthetic applications of Baylis-Hillman or aza-Baylis-Hillman adducts.1 However, the general and efficient synthesis of β-branched aza-Baylis-Hillman ad-ducts has remained unsolved. Although many approaches have been examined, most of the methods suffer from low yields and lack of generality.2,3 Thus, development of an efficient synthetic method of these compounds would be helpful in chemical transformations of Baylis-Hillman ad-ducts.1-4 The most simple and convenient method for the prepa-ration of β-aryl-substituted Baylis-Hillman adducts could be the palladium-mediated Heck reaction with aryl halides. Actually intermolecular Heck type arylation of Baylis-Hillman adducts has been examined by some research groups.5 However, the reaction gave benzyl-substituted β-keto ester (A) as the major product instead of β-aryl-substituted Baylis-Hillman type adduct (B) as shown in Scheme 1.5 The compound (A) was generated via the syn-elimination of HaPdOAc from the intermediate (I) and the following keto-enol tautomerization.5c This unfavorable result might be the principle reason for the lack of any trials on the synthesis of β-aryl aza-Baylis-Hillman adducts via the Heck type arylation strategy. Three types of compounds including 3a, 4a and 5a could be produced from the Heck reaction of N-tosyl aza-Baylis-Hillman adduct 1a as in Scheme 2. However, we expected that the conformation of the intermediate (II, Scheme 2) might be differ with that of the corresponding intermediat