Chemoenzymatic total synthesis of cryptocaryalactone natural products

A new chemoenzymatic route for the preparation of cryptocaryalactones natural products using a kinetic resolution process as the key step is described. Novozyme-435 catalyzed hydrolysis of the prochiral (±)-monoester 7 afforded the precursors of cryptocaryalactones with high enantiomeric excess and excellent yields. The compounds (4S,6S)-7 and (4R,6R)-7 were converted to (+)-(6R,2′S)-cryptocaryalactone (1) and (?)-(6S,2′R)-cryptocaryalactone (2), respectively by employing Wittig-olefination, lactonization and acylation reactions.     

Chemoenzymatic formal total synthesis of (−)-bestatin

A highly stereoselective, enzymatic reduction of an α-chloro-β-keto ester provided the key intermediate for a total synthesis of the α-hydroxy-β-amino acid moiety of (−)-bestatin. The reduction product was cyclized to a glycidic ester that was opened in a Ritter reaction with benzonitrile, affording a trans-oxazoline, which was hydrolyzed under acidic conditions to the target molecule.     

Chemoenzymatic total synthesis of the phytotoxic lactone herbarumin III

Asymmetric total synthesis of phytotoxic nonenolide herbarumin III was accomplished by a chemoenzymatic approach. The main highlight of the synthesis was to fix the hydroxyl stereocenters (C7 and C9) by lipase catalyzed irreversible transesterification.     

Chemoenzymatic total synthesis of paecilocin A and 3-butyl-7-hydroxyphthalide

A highly enantioselective total synthesis of paecilocin A and 3-butyl-7-hydroxyphthalide is described. The key steps involved in this synthesis are enzymatic kinetic resolution and Alder–Rickert reaction.     

Chemoenzymatic synthesis of isogalactofagomine

A new chemoenzymatic synthesis of optically pure isogalactofagomine 2 starting from achiral starting materials is presented. Dimethyl 4-hydroxypyridine-3,5-dicarboxylate (7) was synthesized and converted to the corresponding saturated piperidine 8. Then the key step of the synthesis was carried out: Lipase M catalyzed hydrolysis of the prochiral diester 8 to cause formation of an asymmetric monoacid with at least 98% enantiomeric excess. Reduction of the acid, saponification of the remaining ester, and radical iododecarboxylation gave an iodide that after substitution with silver trifluoroacetate and hydrolysis gave 2.     

Chemoenzymatic approaches to the montanine alkaloids: a total synthesis of (+)-nangustine

The synthesis of (+)-nangustine [(+)-2] has been achieved, for the first time, using the enantiomerically pure cis-1,2-dihydrocatechol 3 as starting material. The latter compound is available, in multi-gram quantities, through a whole-cell-mediated biotransformation of chlorobenzene using genetically engineered organisms that over-express the responsible enzyme, namely toluene dioxygenase. Since the enantiomer of compound 3 is available by related means, the present work also represents a formal total synthesis of the montanine alkaloid (−)-nangustine [(−)-2]. Single-crystal X-ray analyses of compounds 13 and (+)-2 are reported.     

Chemoenzymatic approaches to the montanine alkaloids: a total synthesis of (+)-brunsvigine

The readily available and enzymatically derived cis-1,2-dihydrocatechols 3a and 3b have been elaborated over 17 steps, including a novel radical addition/elimination sequence, into the enantiomer, (+)-1, of the montanine alkaloid brunsvigine [(-)-1].     

Chemoenzymatic synthesis of conduritol analogues

Several conduritol and conduramine analogues have been synthesized from β-substituted naphthalenes via a chemoenzymatic approach, in a high regio- and stereocontrolled way.     

Chemoenzymatic synthesis of phosphocarnitine enantiomers

Racemic phosphocarnitine 3 has been synthesized starting from diethyl 3-chloro-2-oxopropanephosphonate 4 in three steps involving reduction of 4 to the corresponding 2-hydroxyphosphonate 5, conversion of the latter to phosphonic acid 6, and final reaction with trimethylamine, affording the trimethylammonium salt of 3. Baker's yeast reduction of 4 and enzymatic kinetic resolution of (+/-)-5 afforded the enantiomerically pure precursors of phosphocarnitine, (R)-(+)-5 and (S)-(-)-5, which were converted to (S)-(-)- and (R)-(+)-phosphocarnitine 3, respectively.     

Chemoenzymatic synthesis of sialyl-trimeric-Lewis x

The decasaccharide sialyl-trimeric-Lewis x is a component of glycoproteins and glycolipids that serve as E- and P-selectin ligands. The synthesis of this target structure was accomplished by utilizing a combination of chemical and enzymatic methods. Highlights of the chemical synthesis include minimal use of protecting groups and regioselective glycosylations to arrive at a linear tri-lactosamine structure. Glycosyltransferase-catalyzed reactions were then employed for the addition of the terminal sialic acid and branch-point fucose residues. Notably, fucosyltransferases V and VI showed different specificities for the sialyl-tri-lactosamine core structure.     

Chemoenzymatic synthesis of enantiomerically enriched kavalactones

Lipase-mediated kinetic resolution of methyl-3-hydroxy-5-phenylpentanoate and (6E)-ethyl 5-hydroxy-3-oxo-7-phenylhept-6-enoate is described in high enantiomeric excess and good yields. The effect of different lipases in different solvents has been screened using different acylating agents. This protocol has been extended for the preparation of enantiomerically pure biologically important kavalactones.     

Chemoenzymatic synthesis of carbasugars from iodobenzene

The versatile enantiopure cis-dihydrodiol metabolite 1, formed by bacterial metabolism of iodobenzene, has been used for the synthesis of the pyranose carbasugars (pseudosugars) carba-beta-D-altropyranose 2, carba-alpha-L-galactopyranose 3, carba-beta-D-idopyranose 4 and carba-beta-L-glucopyranose 5. Substitution of the iodine atom by a carbomethoxy group, stereoselective catalytic hydrogenation of an alpha,beta-unsaturated ester, and regioselective inversion of one or two allylic chiral centres are the key steps used in the synthesis of carbasugars 2-5. The relative and absolute configurations of compounds 2-5 were established by a combination of stereochemical correlation, X-ray crystallography and 1H-NMR spectroscopy.     

Chemoenzymatic approaches to glycoprotein synthesis

The construction of homogeneous glycoproteins presents a formidable challenge to the synthetic chemist. Over the past few years there has been an explosion in the number of methods developed to address this problem. These methods include the development of novel ligation technologies for the synthesis of the protein backbone, as well chemical and enzymatic approaches for introducing complex glycans into the peptide backbone. This tutorial review discusses the application of these techniques to the synthesis of peptides and proteins possessing well defined glycans.     

Chemoenzymatic synthesis of discotic liquid crystals

Abstract

Acylations at the 6-position of alkyl glycosides were obtained using the esterification potential of lipases. Thus, sugars with two alkyl chains result showing columnar discotic phases. The correlation between the configuration of the sugar moieties and the different clearing points can be understood on the basis of simple stereochemical assumptions.     

Chemoenzymatic synthesis of azacycloalkan-3-ols

Optically active ω-bromocyanohydrins are easily synthesized through an enantioselective (R)-oxynitrilase-catalyzed reaction from their corresponding ω-bromoaldehydes. These cyanohydrins are starting materials for the preparation of medium size nitrogen heterocycles. The reduction of (R)-(+)-5-bromo-2-hydroxypentanenitrile affords, in one-pot, piperidin-3-ol. Azepan-3-ol and azocan-3-ol are readily obtained from their corresponding cyanohydrins in high enantiomeric excesses.     

Chemoenzymatic synthesis of ketomethylene tripeptide isosteres

A chemoenzymatic method is described for the synthesis of a desired ketomethylene tripeptide isostere. The key step is an enzymatic hydrolysis, which removes the C-terminal ester-protecting group under mild conditions without epimerizing the existing stereogenic center and produces the desired stereoisomer in high enantiomeric excess (98% de, 97% ee). The method is short with overall 15–20% yields after seven synthetic steps from readily available starting materials being obtained.     

Chemoenzymatic synthesis of discotic liquid crystals

Acylations at the 6-position of alkyl glycosides were obtained using the esterification potential of lipases. Thus, sugars with two alkyl chains result showing columnar discotic phases. The correlation between the configuration of the sugar moieties and the different clearing points can be understood on the basis of simple stereochemical assumptions.     

Chemoenzymatic total synthesis of potent HIV RNase H inhibitor (−)-1,3,4,5-tetragalloylapiitol

Starting from racemic dimethyl 2-acetoxy-3-methylenesuccinate, the chemoenzymatic facile total synthesis of (−)-1,3,4,5-tetragalloylapiitol has been demonstrated via an efficient lipase catalyzed resolution followed by a DIBAL reduction-double gallyolation, osmium tetroxide dihydroxylation-double gallyolation, and reductive global O-benzyl deprotection pathway.     

Chemoenzymatic synthesis and synthetic application of enantiopure aminocyclopentenols: total synthesis of carbocyclic (+)-uracil polyoxin C and its alpha-epimer

Carbocyclic uracil polyoxin C (+)-2 and its alpha-epimer (-)-3 were synthesized in an efficient fashion from cis-4-(N-tert-butylcarbamoyl)cyclopent-2-en-1-ol (+/-)-7. The synthesis incorporates a concise, inexpensive chemoenzymatic synthesis of enantiopure aminocyclopentenols, a Pd(0)-catalyzed substitution reaction, and a mild reduction of an alpha-nitro ester by TiCl(3)/sodium borohydride. Significantly, this process demonstrates the synthetic utility of the versatile enantiopure aminocyclopentenol building block (-)-4.