Chemoenzymatic and enzymatic cascade reactions enable the synthesis of complex stereocomplementary 1,3,4‐trisubstituted tetrahydroisoquinolines (THIQs) with three chiral centers in a step‐efficient and selective manner without intermediate purification. The cascade employs inexpensive substrates (3‐hydroxybenzaldehyde and pyruvate), and involves a carboligation step, a subsequent transamination, and finally a Pictet–Spengler reaction with a carbonyl cosubstrate. Appropriate selection of the carboligase and transaminase enzymes enabled the biocatalytic formation of (1R,2S)‐metaraminol. Subsequent cyclization catalyzed either enzymatically by a norcoclaurine synthase or chemically by phosphate resulted in opposite stereoselectivities in the products at the C1 position, thus providing access to both orientations of the THIQ C1 substituent. This highlights the importance of selecting from both chemo‐ and biocatalysts for optimal results.
Due to its function as a vasopressor, the vicinal amino alcohol methoxamine is a potential candidate for the treatment of hypotension and incontinence or has applications in ophthalmology. In this study, a biocatalytic method was developed to produce each of the four stereoisomers of methoxamine in a sequential 1-pot 2-step cascade reaction, starting from readily available pyruvate and 2,5-dimethoxybenzaldehyde without intermediate isolation. All four isomers are accessible with high conversions and very good stereoselectivities through the modular combination of carboligases and transaminases with high stereoselectivities. The development of these cascades was made possible, among other factors, by the integration of a recently engineered triple variant of the pyruvate decarboxylase from Acetobacter pasteurianus (ApPDC-E469G-I468A-W543F), which provided access to the intermediate (S)-1-hydroxy-1-(2,5-dimethoxyphenyl)propan-2-one with a high enantiomeric excess of 98%. For the amination of these sterically demanding 2-hydroxy ketones, Bacillus megaterium transaminase in particular has proven to be a highly active and selective catalyst. All four methoxamine stereoisomers were achieved with isomeric contents between 94% and 99% and total conversions of both steps between 59% and 80%. Separation of each isomer without derivatizing was possible using supercritical fluid chomratographie with two columns connected in a row. A preparative scale (75 mL) of the 1-pot 2-step cascade to (1S,2R)-methoxamine including nonoptimized product isolation showed 85 mg HCl salt (46% isolated yield) with 94% purity (NMR) and an isomeric content of 98%.
Chemoenzymatic and enzymatic cascade reactions enable the synthesis of complex stereocomplementary 1,3,4-trisubstituted tetrahydroisoquinolines (THIQs) with three chiral centers in as tep-efficient and selective manner without intermediate purification. The cascade employs inexpensive substrates (3-hydroxybenzaldehyde and pyruvate), and involves ac arboligation step,asubsequent transamination, and finally aP ictet-Spengler reaction with ac arbonyl cosubstrate.A ppropriate selection of the carboligase and transaminase enzymes enabled the biocatalytic formation of (1R,2S)-metaraminol. Subsequent cyclization catalyzed either enzymatically by an orcoclaurine synthase or chemically by phosphate resulted in opposite stereoselectivities in the products at the C1 position, thus providing access to both orientations of the THIQ C1 substituent. This highlights the importance of selecting from both chemo-and biocatalysts for optimal results.The tetrahydroisoquinoline (THIQ) moiety is a" privileged scaffold", [1] and as such, it is found in numerous bioactive natural products (e.g., noscapine, [2] dioncophyllines [3] )a nd synthetic pharmaceutical drugs (e.g., solifenacin [4] ). THIQcontaining compounds demonstrate aw ide range of bioactivities,i ncluding antitumor, [2] antiparasitic, [3] and anticholinergic [4] properties. Naturally derived THIQs may be obtained by extraction or fermentation, [5] but in many cases,only small quantities are available,typically as one component of amixture.Chemical syntheses are af requent source of these compounds,b ut complex multistep asymmetric routes that give high stereoselectivities can be challenging,i ncluding at scale,a nd are often dependent on the use of toxic or environmentally harmful chemicals.[6] Therefore,n ovel synthetic routes towards THIQs are of significant interest.In vitro biocatalysis provides av iable method of producing complex THIQs in high stereoselectivities and under mild conditions. [7][8][9] Previous studies have shown norcoclaurine synthase (NCS), the Pictet-Spenglerase from plant benzylisoquinoline alkaloid biosynthesis, [10,11] to be av ersatile biocatalyst. NCS can convert 3-hydroxyphenethylamines (e.g.,d opamine) and ac arbonyl cosubstrate into a( 1 S)-THIQ with high stereoselectivities. [12][13][14][15] Thee nzyme has demonstrated ar emarkably wide carbonyl cosubstrate tolerance,a ccepting av ariety of aldehydes and ketones. [13][14][15][16][17][18][19][20] NCS has also been successfully employed in chemoenzymatic cascades. [12,15,16,21] As an alternative to NCS,i norganic phosphate can be used to catalyze the Pictet-Spengler reaction (PSR), typically forming racemic THIQs under aqueous conditions. [22] This can be successfully combined with enzyme steps. [21] Modular biocatalytic cascades [23] for the stereoselective production of pharmaceutical compounds are exemplified by the work conducted on phenylpropanolamines. [24][25][26] All four possible isomers of nor(pseudo)ephedrine are available from simple starting materials by one-pot two-enzym...
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