ASYMMETRIC-SYNTHESIS USING TARTRATE ESTER MODIFIED ALLYLBORONATES .1. FACTORS INFLUENCING STEREOSELECTIVITY

A detailed study of the factors that influence the enantio- and diastereoselectivity of the reactions of tartrate allylboronate 1 with chiral and achiral aldehydes is reported. The stereoselectivity of these reactions is sensitive to variables such as reaction temperature (best results invariably are obtained at -78 °C), solvent (toluene is best for aliphatic aldehydes; THF is preferred for aromatic aldehydes), and moisture (use of molecular sieves is recommended to maintain an anhydrous reaction environment), but not on the structure of the tartrate ester. Tartrate allylboronate 1 has been found to be exceptionally reactive compared to other, previously studied allylboronates, and even the reactions of very hindered substrates (e.g., pivalaldehyde) are complete within several hours at -78 °C. An improved method for synthesis of 1 is described that involves the reaction of allylmagnesium bromide with (iPrO)3B followed by aqueous hydrolysis and esterification with DIPT. Yields of 1 are considerably higher (65-76%) by using this new procedure, and the crude reagent so prepared may be used directly in allylboration experiments. A simple method for standardizing solutions of 1 is described. Finally, the absolute stereochemistry of five homoallylic alcohols (5a-e) were assigned by correlation with epoxy alcohols prepared via the Sharpless asymmetric epoxidation. The results of these correlations are in complete agreement with the stereochemical picture presented in our 1985 publication. © 1990, American Chemical Society. All rights reserved.