On the mechanism of the unexpected facile formation of meso-diacetate products in enzymatic acetylation of alkanediols

Chemoenzymatic asymmetric total synthesis of naturally occurring resorcylic acid lactones hamigeromycins F and G

Asymmetric total synthesis of two naturally occurring resorcylic acid lactones (RALs), hamigeromycins F and G, was achieved for the first time. The synthetic strategy involved a late-stage intramolecular macrolactonization achieved with a transesterification reaction. Stereocenters (C10' and C6') in both the molecules were accessed via an enzymatic kinetic resolution reaction. The penta-substituted aromatic core present in hamigeromycins F and G was accessed using vanillin as a precursor. The crucial E-olefinic unsaturated bond at C1'-C2' was constructed with a stereoselective Julia-Kocienski olefination reaction.

Influence of delta-functional groups on the enantiorecognition of secondary alcohols by Candida antarctica lipase B

The selectivity of acetylation of delta-functionalized secondary alcohols catalyzed by Candida antarctica lipase B has been examined by molecular dynamics. The results from the simulation show that a delta-alcohol functionality forms a hydrogen bond with the carbonyl group of Thr 40. This interaction stabilizes the tetrahedral intermediate and thus leads to selective acetylation of the R enantiomer. A stabilizing interaction of the delta-(R)-acetoxy group with the peptide NH of alanine 282 was also observed. No stabilizing interaction could be found for the delta-keto functionality, and it is proposed that this is the reason for the experimentally observed decrease in enantioselectivity. From these results, it was hypothesized that the enantioselectivity could be restored by mutating the alanine in position 281 for serine. The mutation was made experimentally, and the results show that the E value increased from 9 to 120.

Highly Efficient Synthesis of Enantiopure DiacetylatedC2-Symmetric Diols by Ruthenium- and Enzyme-Catalyzed Dynamic Kinetic Asymmetric Transformation (DYKAT)

Highly efficient synthesis of enantiopure diacetates of 2,4-pentanediol and 2,5-hexanediol starting from commercially available mixtures of the diols (dl/meso approximately 1:1) has been realized by combining a fast ruthenium-catalyzed epimerization with an enzymatic transesterification. The in situ coupling of these two processes produces the diacetates in high yield in >99 % enantiomeric excess.

Regio- and stereo-selectively kinetic resolution of four stereoisomers of 1-phenylbutan-1,3-diol by Candida rugosa lipase

Regio- and stereo-selective Candida rugosa lipase (CRL)-catalyzed acetylations of (±)-syn-1-phenylbutan-1,3-diols ((±)-syn-1) generated two monoacetates, (+)-syn-2 and (–)-syn-3, whereas that of (±)-anti-1 produced a monoacetate, (+)-anti-3, and an unreacted (–)-anti-1.Key words: Candida rugosa lipase, kinetic resolution, 1,3-diols.

Enzymatic Resolution, Desymmetrization, and Dynamic Kinetic Asymmetric Transformation of 1,3-Cycloalkanediols

An efficient desymmetrization of cis-1,3-cyclohexanediol to (1S,3R)-3-(acetoxy)-1-cyclohexanol ((R,S)-2a) was performed via Candida antarctica lipase B (CALB)-catalyzed transesterification, in high yield (up to 93%) and excellent enantioselectivity (ee's up to >99.5%). (R,R)-Diacetate ((R,R)-3a) was obtained in a DYKAT process at room temperature from (1S,3R)-3-acetoxy-1-cyclohexanol ((R,S)-2a), in a high trans/cis ratio (91:9) and in excellent enantioselectivity of >99%. Metal- and enzyme-catalyzed dynamic transformation of cis/trans-1,3-cyclohexanediol using PS-C gave a high diastereoselectivity for cis-diacetate (cis/trans = 97:3). The (1R,3S)-3-acetoxy-1-cyclohexanol (ent-(R,S)-2a) was obtained from cis-diacetate by CALB-catalyzed hydrolysis in an excellent yield (97%) and selectivity (>99% ee). By deuterium labeling it was shown that intramolecular acyl migration does not occur in the transformation of cis-monoacetate to the cis-diacetate.

One-pot synthesis of enantiopure syn-1,3-diacetates from racemic syn/anti mixtures of 1,3-diols by dynamic kinetic asymmetric transformation

A one-pot synthesis of enantiomerically pure syn-1,3-diacetates starting from readily accessible racemic diastereomeric mixtures of 1,3-diols has been realized by combining (i) enzymatic transesterification, (ii) ruthenium-catalyzed epimerization of a secondary alcohol in a diol or diol monoacetate, and (iii) intramolecular acyl migration in a syn-1,3-diol monoacetate. The in situ coupling of these three processes results in an efficient enantioselective synthesis of acyclic syn-1,3-diacetates via combined deracemization-deepimerization and constitutes a dynamic kinetic asymmetric transformation concept. Several differently substituted unsymmetrical, acyclic syn-1,3-diacetates were obtained in yields up to 73% with excellent enantioselectivities (>99%) and good diastereomeric ratios (>90% syn).

Chemoenzymatic dynamic kinetic resolution

During the past decade a new concept has appeared in asymmetric catalysis involving the combination of a biocatalyst and a chemocatalyst in one 'pot' leading to efficient deracemization via dynamic kinetic resolution (DKR). Here, we outline the different strategies that have been developed for efficient chemoenzymatic DKR, in particular the powerful combination of an enzyme and a metal catalyst.

Aminocyclopentadienyl Ruthenium Complexes as Racemization Catalysts for Dynamic Kinetic Resolution of Secondary Alcohols at Ambient Temperature

Aminocyclopentadienyl ruthenium complexes, which can be used as room-temperature racemization catalysts with lipases in the dynamic kinetic resolution (DKR) of secondary alcohols, were synthesized from cyclopenta-2,4-dienimines, Ru(3)(CO)(12), and CHCl(3): [2,3,4,5-Ph(4)(eta(5)-C(4)CNHR)]Ru(CO)(2)Cl (4: R = i-Pr; 5: R = n-Pr; 6: R = t-Bu), [2,5-Me(2)-3,4-Ph(2)(eta(5)-C(4)CNHR)]Ru(CO)(2)Cl (7: R = i-Pr; 8: R = Ph), and [2,3,4,5-Ph(4)(eta(5)-C(4)CNHAr)]Ru(CO)(2)Cl (9: Ar = p-NO(2)C(6)H(4); 10: Ar = p-ClC(6)H(4); 11: Ar = Ph; 12: Ar = p-OMeC(6)H(4); 13: Ar = p-NMe(2)C(6)H(4)). The tests in the racemization of (S)-4-phenyl-2-butanol showed that 7 is the most active catalyst, although the difference decreased in the DKR. Complex 4 was used in the DKR of various alcohols; at room temperature, not only simple alcohols but also functionalized ones such as allylic alcohols, alkynyl alcohols, diols, hydroxyl esters, and chlorohydrins were successfully transformed to chiral acetates. In mechanistic studies for the catalytic racemization, ruthenium hydride 14 appeared to be a key species. It was the major organometallic species in the racemization of (S)-1-phenylethanol with 4 and potassium tert-butoxide. In a separate experiment, (S)-1-phenylethanol was racemized catalytically by 14 in the presence of acetophenone.

Controlling chirality

New strategies are continually being developed for using enzymes to efficiently catalyse the enantioselective synthesis of chiral non-racemic compounds. Alternatives to asymmetric synthesis or kinetic resolution include dynamic kinetic resolution, deracemisation and enantioconvergent transformations. Moreover, a much greater understanding is being developed of the parameters that can affect the stereochemical outcome of the reaction (e.g. solvent, substrate design, immobilization and directed evolution).

Combined metal catalysis and biocatalysis for an efficient deracemization process

Driven by the demand to enhance the economic balance of chemical processes, the transformation of a racemate into a single stereoisomer (the deracemization process) is one of the most important areas of research. The recent development of the combined use of enzyme and metal catalysis has led to efficient deracemization processes. Dynamic kinetic resolution and cyclic deracemization processes have shown particular promise in this area.