Synthesis of a-Amino Acid Esters

Very high enantio- and diastereoselective (up to 96% ee) synthesis of some amino acid esters have been achieved by the reaction of 9-allyl- and 9-but-2-enyl-9-BBN with a-imino esters (Eq. 9.6) [17]. 

The reaction of allyl-9-BBN with a-iminoesters in which (S)-(-)-a-methyl-benzylamine is utilized as the chiral source produced the corresponding amino acid in high chemical (92%) and optical (92% ee) yield. These are increased to 96% when (-)-l-cyclohexylethylamine is employed. The reaction of allylzinc or magnesium or titanium reagents, on the other hand, has resulted in lower or negligible ee s. 

As depicted in Chart 9.3, the transition state [A] resulting from allyl-9-BBN and chiral iminoester (a) leads to aminoester with SS configurations. However, transition state [B] resulting from methallyl-9-BBN and chiral iminoester (a) possesses three 1,3-diaxial interactions as compared with only one 1,3-diaxial interaction in [A]. Thus, [B] is highly destabilized and thus methallylboration 

The reaction is extended with crotyl-9-BBN [19], The chiral iminoester reacts with crotyl-9-BBN (Eq. 9.7) in essentially quantitative yield to give Cram-erythro amino acid ester. 

Even acrolein, which reacts with remarkable speed with hindered trialkylboranes, fails to react with B-R-9-BBN. See Brown HC, Rogid MM, Rathke MW, Kabalka GW (1969) J Am Chem Soc 91 2150 

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TABEE 5.14 SYNTHESIS OF a-AMINO ACID ESTERS EROM 5,5-DIALKOXY-2-OXAZOLIDIN-... 

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