High-Yield Synthesis of Enantiopure 1,2-Amino Alcohols from l-Phenylalanine via Linear and Divergent Enzymatic Cascades

Abstract

Enantiomerically pure 1,2-amino alcohols are important compounds due to their biological activities and wide applications in chemical synthesis. In this work, we present two multienzyme pathways for the conversion of l-phenylalanine into either 2-phenylglycinol or phenylethanolamine in the enantiomerically pure form. Both pathways start with the two-pot sequential four-step conversion of l-phenylalanine into styrene via subsequent deamination, decarboxylation, enantioselective epoxidation, and enantioselective hydrolysis. For instance, after optimization, the multienzyme process could convert 507 mg of l-phenylalanine into (R)-1-phenyl-1,2-diol in an overall isolated yield of 75% and >99% ee. The opposite enantiomer, (S)-1-phenyl-1,2-diol, was also obtained in a 70% yield and 98-99% ee following the same approach. At this stage, two divergent routes were developed to convert the chiral diols into either 2-phenylglycinol or phenylethanolamine. The former route consisted of a one-pot concurrent interconnected two-step cascade in which the diol intermediate was oxidized to 2-hydroxy-acetophenone by an alcohol dehydrogenase and then aminated by a transaminase to give enantiomerically pure 2-phenylglycinol. Notably, the addition of an alanine dehydrogenase enabled the connection of the two steps and made the overall process redox-self-sufficient. Thus, (S)-phenylglycinol was isolated in an 81% yield and >99.4% ee starting from ca. 100 mg of the diol intermediate. The second route consisted of a one-pot concurrent two-step cascade in which the oxidative and reductive steps were not interconnected. In this case, the diol intermediate was oxidized to either (S)- or (R)-2-hydroxy-2-phenylacetaldehyde by an alcohol oxidase and then aminated by an amine dehydrogenase to give the enantiomerically pure phenylethanolamine. The addition of a formate dehydrogenase and sodium formate was required to provide the reducing equivalents for the reductive amination step. Thus, (R)-phenylethanolamine was isolated in a 92% yield and >99.9% ee starting from ca. 100 mg of the diol intermediate. In summary, l-phenylalanine was converted into enantiomerically pure 2-phenylglycinol and phenylethanolamine in overall yields of 61% and 69%, respectively. This work exemplifies how linear and divergent enzyme cascades can enable the synthesis of high-value chiral molecules such as amino alcohols from a renewable material such as l-phenylalanine with high atom economy and improved sustainability.

Figure 1

Examples of biologically active compounds and chiral auxiliaries bearing phenylethanolamine or 2-phenylglycinol moieties.

Scheme 1. Two-Pot Four-Step Sequential Biocatalytic Cascades for the Conversion of l-Phenylalanine (l-1) into (R)- or (S)-1-Phenylethane-1,2-diol ((R)- or (S)-5)

There is only one intermediate extraction work-up after step 2.

Scheme 2. One-Pot Concurrent Interconnected Two-Step Biocatalytic Cascade for the Conversion of (R)-1-Phenylethane-1,2-diol ((R)-5)) into (R)- or (S)-Phenylglycinol ((R)- or (S)-7))

Scheme 3. One-Pot Concurrent Disconnected Two-Step Biocatalytic Cascade for the Conversion of (R)- or (S)-1-Phenylethane-1,2-diol ((R)-5 or (S)-5) into (R)- or (S)-Phenylethanolamine ((R)- or (S)-9)

Figure 2

Summary of the biocatalytic pathways developed in this work and related synthetic strategies.