Tetrahedron Letters. Vo1.32. No.36, pp 461946221991 printed in Great Britain oo404039/91 $3.00 + .@I Pergamon Press plc THE ADDITION OF ALLYLBORONATES TO ALPHA-OXOCARBOXYLIC ACIDS Zhe Wang, Xian-Jun Meng and George W. Kabalka* Departments of Chemistry and Radiology The University of Tennessee, Knoxville, TN 379961600 Abstract: Allylboronates react with alpha-oxocarboxylic acids to yield tertiary homoallylic alpha- hydroxycarboxylic acids in a highly chemoselective manner. The reaction presumably proceeds via a bicyclic transition state. The alpha-carboxylic substituent enhances the rate of the reaction. Allylmetallic reagents have been of synthetic, mechanistic, and biochemical importance for more than sixty years because of their well-defined stereo- and regiochemistry.’ Over the past few years, the reactions of allyl- and crotylmetal reagents with carbonyl compounds have proven to be important in acyclic diastereoselective synthesis, 2*3 allyl- and crotylboron reagents are particularly attractive for the synthesis of natural products of propiogeniclacetogenic biosynthetic origin. 4 In recent years, several chiral allyboron reagents have been developed which provide enantio- and diastereoselectivities approaching 100% in reactions with aldehydes.’ In contrast to the allylboranes,’ allylboronates react with ketones only under catalytic or high pressure conditions;’ the reaction of allyboronates with alpha-oxocarboxylic acids has not been reported. In a previous study,s it was discovered that an alpha-hydroxy substituent exerts a remarkable effect on the rate of the reaction of allylboronates with ketones. We now wish to report that allylboronate 1 reacts with alpha-oxocarboxylic acids 2 chemoselectively to yield tertiary homoallylic, alpha-hydroxycarboxylic acids 3. The resultant tertiary homoallylic, alpha-hydroxycarboxylic acid is a particularly important precursor to biologically active natural products, such as pyrrolizidine macrolide alkaloids,’ camptothecin,” and prostaglandins.” The reaction presumably proceeds through a bicyclic transition state such as 4 (Scheme-l) to yield products with chemoselectivities approaching 100%. A plausible mechanism for the triethylamine initiated ahylboration may involve the formation of an alpha-oxocarboxylic triethylammonium salt, followed by a ligand exchange, **to form a mixed boronate which then reacts via transition state 4. 4619