Synthesis of Enantiomerically Pure Lignin Dimer Models for Catalytic Selectivity Studies

Abstract

A series of highly enantioselective transformations, such as the Sharpless asymmetric epoxidation and Jacobsen hydrolytic kinetic resolution, were utilized to achieve the complete stereoselective synthesis of β-O-4 lignin dimer models containing the S, G, and H subunits with excellent ee (>99%) and moderate to high yields. This unprecedented synthetic method can be exploited for enzymatic, microbial, and chemical investigations into lignin's degradation and depolymerization as related to its stereochemical constitution. Preliminary degradation studies using enantiopure Co(salen) catalysts are also reported.

Enantioselective Syntheses of Lignin Models: An Efficient Synthesis of β‐O‐4 Dimers and Trimers by Using the Evans Chiral Auxiliary

“…The crude compound was purified by column chromatography (hexane/CH 2 Cl 2 /acetone, 7:1.5:1.5) to afford the diol intermediate (1.79 g, 95 %) as a white solid. The spectroscopic data for 8 b , as well as its complimentary enantiomer satisfactorily matched all previously reported data …”

“…The filtrate was concentrated under vacuum to provide the crude lignin dimer, which was purified by column chromatography (CH 2 Cl 2 /acetone, 7:3) to afford the enantiomerically pure lignin dimer (1.2 g, 96 %) as a clear viscous oil. The spectroscopic data for 9 b , as well as its complimentary enantiomer satisfactorily matched all previously reported data using a different methodology …”

“…Synthesis of enantiomerically pure β‐O‐4 dimers has been reported but requires either a tedious resolution of a racemic mixture or separation of diastereomeric derivatives of each enantiomer, which makes it impossible to obtain enantiopure lignin dimer models in multigram quantities . Recently, we reported the synthesis of enantiomerically pure β‐O‐4 dimer models incorporating each of the primary H, G, and S subunits (Figure ) …”

Enantioselective Synthesis of Dilignol Model Compounds and Their Stereodiscrimination Study with a Dye-Decolorizing Peroxidase

“…32 However, attempts to scale up this 9-step reaction resulted in a low yield. 33 Later, the same syn compounds were obtained by a 5-step synthesis similar to the method B (Scheme 1) described in this report.…”

“…Thus, the stereospecific anti -dilignols were obtained in just four steps with an overall 42% yield, which was a significant improvement over the other asymmetric methods reported so far. 32, 33 …”