Michael addition of nitromethane to non-racemic chiral Cr(CO)3 complexes of ethyl cinnamate derivatives: stereoselective synthesis of (R)-(−)-baclofen

We carried out a highly stereoselective Michael addition (96% de) of nitromethane to enantiomerically pure tricarbonyl(ethyl-4-chloro-2-trimethylsilylcinnamate)chromium(0). This reaction is the key step in the synthesis of (R)-(−)-baclofen, a potent antispastic drug.     

Stereoselective synthesis of (R)-10-methyltridecan-2-one,the sex pheromone of the southern corn rootworm,using (4S)-benzylthiazolidinethione as a chiral auxiliary

The stereoselective synthesis of (R)-10-methyltridecan-2-one, the sex pheromone of the southern corn rootworm, was carried out in 20.7% overall yield based on (4S)-benzylthiazolidinethione (five steps). In the crucial step, the stereogenic center was generated by an asymmetric Michael addition using enantiomerically pure (4S)-benzylthiazolidinethione as a chiral auxiliary.     

Total synthesis of (+)-negamycin and its 5-epi-derivative

(+)-Negamycin was prepared in 13 steps in an overall yield of 31% from commercially available ethyl (R)-(+)-4-chloro-3-hydroxybutanoate. The key step in the reaction sequence was a highly stereoselective asymmetric Michael addition of chiral amine (−)-21 [(1S,2R)-(−)-2-methoxybornyl-10-benzylamine] into the α,β-unsaturated carbonyl moiety of key intermediate 8, thus establishing the second chiral center in (+)-negamycin. 5-epi-Negamycin was also prepared in a similar fashion.     

Expedient synthesis of bisabolenol stink bug pheromones via stereodefined cyclohex-2-enones

We recently synthesized all stereoisomers of 1,10-bisaboladien-3-ol and 10,11-epoxy-1-bisabolen-3-ol, including three stink bug pheromones, via a rhodium-catalyzed asymmetric addition of trimethylaluminum to diastereomeric mixtures of cyclohex-2-enones. However, yields of trans isomers were low, and scaling reactions using expensive catalysts were cumbersome. Now we describe a new synthesis of bisabolenol stink bug pheromones via (S)- and (R)-4-((R)-6-methylhept-5-en-2-yl)cyclohex-2-enones prepared by enantioselective Michael additions of methyl vinyl ketone to (S)- and (R)-citronellals and lithium hydroxide monohydrate-catalyzed stereoselective cyclizations of intermediate ketoaldehydes. Addition of methyllithium to these enones provided cis- and trans-1,10-bisaboladien-3-ols, which were separated by chromatography on silica and further converted to 10,11-epoxy-1-bisabolen-3-ols. Thus, we developed more convenient syntheses of pheromones of the rice stink bug, the harlequin bug, and brown marmorated stink bug.     

Stereoselective synthesis of both enantiomers of trans-2-(diphenylmethylideneamino)cyclopropanecarboxylic acid using a chiral pool approach and their incorporation in dipeptides

The stereoselective synthesis of (1R,2R)- and (1S,2S)-trans-2-(diphenylmethylideneamino)cyclopropanecarboxylic acid has been accomplished in six steps starting from (2S)- and (2R)-β-benzyl N-(tert-butoxycarbonyl)aspartate, respectively. The key-step in the reaction sequence is a stereoselective base-induced ring closure with a good trans diastereoselectivity. These novel trans-β-ACC derivatives could be incorporated in dipeptides employing a standard peptide coupling technique.     

An enantioselective total synthesis of pinnaic acid

An enantioselective and convergent total synthesis of marine natural product pinnaic acid has been achieved. Our general synthetic approach is featured with an asymmetric hydrogenation of racemic γ-keto ester 3, a diastereoselective methylation on the α-methylene of the (1R,5R)-lactone 4, and a diastereoselective Michael addition of the tertiary nitro cyclopentane. The central azaspiro[4.5]decane was constructed utilizing reductive cyclization of the δ-nitroketone followed by highly stereoselective reduction of the cyclic imine with NaBH₄. Ultimately, successive use of triethyl-2-phosphonopropionate and Heathcock's phosphorane 18 to elaborate C5 and C13 side chains completed the total synthesis of pinnaic acid.     

Preliminary studies on a novel synthesis of β-amino acids: stereocontrolled transformation of d- and l-glyceraldehyde into 3-amino-2-(2′,2′-dimethyl-1′,3′-dioxolan-4′-yl)propanoic acids

A stereocontrolled synthesis of the methyl ester of (2S)-3-amino-2-((4′S)-2′,2′-dimethyl-1′,3′-dioxolan-4′-yl)propanoic acid from d-glyceraldehyde is described for the first time. This method involves the stereoselective Michael addition of the lithium salt of tris(phenylthio)methane to (S)-2,2-dimethyl-4-((E)-2-nitrovinyl)-1,3-dioxolane followed by hydrolysis of the resulting (4S)-2,2-dimethyl-4-((2′S)-3′-nitro-1′,1′,1′-tris(phenylthio)propan-2′-yl)-1,3-dioxolane to (2S)-methyl 2-((4′S)-2′,2′-dimethyl-1′,3′-dioxolan-4′-yl)-3-nitropropanoate, which was finally reduced to the target compound. A similarly stereocontrolled transformation of l-glyceraldehyde into (2R)-methyl 3-amino-2-((4′R)-2′,2′-dimethyl-1′,3′-dioxolan-4′-yl)propanoate is also described.     

Chemoenzymatic synthesis of 1α,24(R)-dihydroxycholesterol

1α,24(R)-Dihydroxycholesterol, which is the key intermediate for the synthesis of 1α,24(R)-dihydroxyvitamin D₃, was effectively synthesized via stereoselective esterification of the 24(R)-hydroxy group using a lipase in combination with inversion of configuration of the 24(S)-hydroxy group using the Mitsunobu reaction (R:S=99:1).     

Stereoselective synthesis of the both enantiomers of disparlure,the pheromone of the gypsy moth

The both enantiomers of disparlure [(7R, 8S)-(+)-epoxy-2-methyloctadecane and its (7S,8R)-(?)-isomer] were synthesized from (2R, 3R)-(+)-tartaric acid in a stereoselective manner. (+)-Disparlure was found to be biologically active.     

Asymmetric syntheses of N-acetyl-(R)-coniine and N-Boc-(2R,6R)-solenopsin A via ring-closing metathesis

Asymmetric syntheses of piperidine alkaloids N-acetyl-(R)-coniine and (2R,6R)-trans-solenopsin were achieved utilizing stereoselective additions of allylphenylsulfone to chiral alkyl-N-sulfinylimines and ring-closing olefin metathesis.     

Stereoselective synthesis of the sex pheromone of the yellow mealworm using (<Emphasis Type="Italic">S</Emphasis>)-4-benzyloxazolidinone as chiral auxiliary

(R)-4-Methyl-1-nonanol, the sex pheromone of the yellow mealworn (Tenebrio molitor L.), was synthesized with high stereoselectivity using (S)-4-benzyloxazolidinone as chiral auxiliary. The stereoselective synthesis was achieved by asymmetric Michael addition of organocopper reagent to N-crotoyloxazolidinone, and the target product was obtained in an overall yield of 41.8% over 7 steps.     

Hydroxynitrile lyase catalyzed enantioselective HCN addition to O-protected α-hydroxyaldehydes

Various O-protected glycol- and racemic lactaldehydes 3 and 6 as well as O-allyl protected racemic α-hydroxyaldehydes 7 (R¹=Et, Pr, Bu) have been prepared to investigate and perform a stereoselective Kiliani–Fischer synthesis by hydroxynitrile lyase (HNL) catalyzed addition of HCN. From all protecting groups investigated the allyl moiety was most suitable. (R)-PaHNL from bitter almonds (Prunus amygdalus), yielding the (2S)-cyanohydrins 8–10, was found to be a more stereoselective catalyst than (S)-MeHNL from maniok (Manihot esculenta). While (R)-PaHNL led to enantiomeric excesses ≥93%, with (S)-MeHNL the (2R)-cyanohydrins 8–10 were obtained with enantiomeric excesses ≤78%.     

First stereoselective total synthesis of (6R)-6-[(4R,6R)-4,6-dihydroxy-10-phenyldec-1-enyl]-5,6-dihydro-2H-pyran-2-one

A stereoselective total synthesis of (6R)-6-[(4R,6R)-4,6-dihydroxy-10-phenyldec-1-enyl]-5,6-dihydro-2H-pyran-2-one is reported. The strategy utilizes an iterative Jacobsen hydrolytic kinetic resolution, ring opening with a chiral propargylic synthon and a preferential (Z)-Wittig olefination reaction and lactonization as the key steps.     

Isolation and partial synthesis of 3(R)- and 3(S)-deoxypumiloside; structural revision of the key metabolite from the camptothecin producing plant,Ophiorrhiza pumila

Both the C-3 epimeric pair of 3(R)- and 3(S)-deoxypumiloside were found in Ophiorrhiza pumila (Rubiaceae), a source plant of camptothecinoid metabolites. These structures were confirmed by spectroscopic analysis and partial stereoselective syntheses. The configuration at C-3 of the previously reported “deoxypumiloside” is revised to 3(R) from 3(S).     

Stereoselective synthesis of (+)-(1R,2S,5S,7R)-2-hydroxy-exo-brevicomin

A stereoselective approach for the synthesis of (+)-(1R,2S,5S,7R)-2-hydroxy-exo-brevicomin from l-ascorbic acid has been described. The key steps are highly stereoselective nucleophilic addition reaction on aldehyde 8 and also a single pot transformation of 15 to (+)-(1R,2S,5S,7R)-2-hydroxy-exo-brevicomin. The later tandem reaction which involves the hydrogenation of double bond, debenzylation, MOM deprotection and bicyclic ketal formation was carried under Pd/C, H₂ followed by acid treatment.     

Bifunctional thiophosphinamide catalyzed highly enantioselective Michael addition of acetone to (E)-2-azido β-nitrostyrenes and the subsequent reductive cyclization

We have proven that primary amine/thiophosphinamide incorporating (1R,2R)-1,2-diphenylethane-1,2-diamine is an efficient catalyst for the asymmetric Michael addition of acetone to (E)-2-azido β-nitrostyrenes. Under the optimal reaction conditions, the corresponding Michael addition products were obtained in excellent yields with almost perfect stereocontrol. Upon treatment with Et₃SiH/InCl₃, the Michael addition products could be successfully converted to the related 2-methyltetrahydroquinolines in acceptable yields with moderate to excellent diastereoselectivity and without appreciable loss in optical purity. This process provides a highly enantioselective pathway for the synthesis of biologically important 2-methyltetrahydroquinoline derivatives.     

Stereoselective synthesis of (2R,3R) and (2R,3S)-3-hydroxyleucines

A stereoselective synthesis of (2R,3R) and (2R,3S)-3-hydroxyleucine is disclosed. The key step of the reaction sequence involves, stereo- and regioselective bromohydration of 7, using a brominating agent derived in situ from N-bromosuccinimide and 2,6-lutidine, via intramolecular sulfinyl group participation.     

Asymmetric total synthesis of (20S)-Camptothecin using a chiral auxiliary strategy

An asymmetric eight-step total synthesis of (20S)-camptothecin, starting from the known compound tert-butyl (2-chloroquinolin-3-yl)methylcarbamate, is described. A Heck reaction followed by an intramolecular Michael addition to form the C-ring provides the first key step in this synthesis. The construction of the 20(S) chiral center relies on a chiral auxiliary-mediated Michael addition using (2R,5R)-2-tert-butyl-5-ethyl-1,3-dioxolan-4-one as the auxiliary.     

An efficient synthesis of optically active (2R,3R)-2-methyl-3-[(1R)-1-methylprop-2-enyl]cyclopentanone, a useful chiral building block for synthesis of vitamin D and steroids

An efficient synthesis of optically active (2R,3R)-2-methyl-3-[(1R)-1-methylprop-2-enyl]cyclopentanone, a useful chiral building block for synthesis of vitamin D and steroids, has been developed starting from readily accessible optically active secondary propargyl phosphate (R)-2, where the asymmetric Michael addition of a chiral allenyltitanium to alkylidenemalonate 3 is a key reaction.     

Stereoselective total synthesis of (+)-strictifolione and (6R)-6[(E,4R,6R)-4,6-dihydro-10-phenyl-1-decenyl)-5,6-dihydro-2H-2-pyrone

The stereoselective total synthesis of two naturally occurring α-pyrone derivatives, (+)-strictifolione and (6R)-6[(E,4R,6R)-4,6-dihydroxy-10-phenyl-1-decanyl]-5,6-dihydro-2H-2-pyrone has been accomplished involving the Sharpless kinetic resolution and olefin cross-metathesis as the key reactions.