Efficient synthesis of (−)-(R)- and (+)-(S)-rolipram

A novel, efficient and protecting group free enantioselective synthetic approach of (?)-(R)-1 and (+)-(S)-rolipram 2 is described employing the organocatalyzed asymmetric Michael addition, Henry condensation, Wittig olefination and reductive lactamization reactions as key steps.     

Diastereoselective α-iminoamine rearrangement: asymmetric synthesis of (R)-(−)- and (S)-(+)-2-benzyl-2-hydroxycyclohexanone

A convenient asymmetric synthesis of both (R)-(−)- and (S)-(+)-2-benzyl-2-hydroxycyclohexanones starting from racemic 2-benzyloxycyclohexanone and the chiral auxiliary 1-phenylethylamine is reported. The route involves a [1,3]-sigmatropic shift and a new diastereoselective α-iminoamine rearrangement of a 2-benzyl-2-iminocyclohexanamine substrate.     

Synthesis of (S)-(+)-sotalol and (R)-(−)-isoproterenol via a catalytic enantioselective Henry reaction

A unified approach for the synthesis of (S)-(+)-sotalol and (R)-(?)-isoproterenol has been developed. The enantioselective Henry reaction of the appropriate aldehyde in the presence of a camphor-derived amino pyridine–Cu(II) complex was the key step of the synthesis. The reduction of the nitro group to give the corresponding amino alcohols followed by reductive alkylation of the amine provided the target products with high enantiomeric excesses.     

Stereoselective total synthesis of (−)-zeylenol,a key intermediate for the synthesis of (+)-pipoxide, (−)-uvarigranol G and (−)-tonkinenin A

Total synthesis of (?)-zeylenol, a key intermediate for the synthesis of (+)-pipoxide, (?)-uvarigranol G and (?)-tonkinenin A was achieved from commercially available starting material d-mannose. The key steps are mixed aldol condensation, Grignard reaction, ring closing metathesis and regioselective benzoylation.     

Asymmetric total synthesis of (+)-isocryptotanshinone and formal synthesis of (−)-cryptotanshinone

The first asymmetric total synthesis of (+)-isocryptotanshinone was achieved in 12 linear steps with 12% overall yield from commercially available dihydrobenzopyrone. The key step was a base-mediated cyclization reaction. In addition, the synthetic strategy included the formal synthesis of (−)-cryptotanshinone.     

A simple synthesis of (−)-(R)-ipsdienol and (−)-(S)-ipsenol

A short and efficient synthesis of the unnatural enantiomer of (+)-(S)-ipsdienol 1 and (−)-(S)-ipsenol 2 is presented via an asymmetric allylboration. The synthesis was achieved by using a one-pot reduction–methylenation of an exo-methylene lactone intermediate.     

Efficient synthesis of (S)-(−)- and (R)-(+)-enantiomers of 15-deoxyspergualin (15-DSG)

An efficient synthesis of the (S)-(−)- and (R)-(+)-enantiomers of 15-deoxyspergualin (15-DSG) is reported. The synthesis involves preparative HPLC separation and subsequent hydrogenolysis of two diastereoisomers 10a and 10b of fully protected 15-DSG penultimates. Alternatively, diastereomerically pure amide 10b (97% de) was also prepared from acid 8b (97% de), which was obtained via crystallization of a 1:1 diastereomeric mixture of 8a and 8b.     

A simple asymmetric synthesis of (+)- and (−)-2,6-diaminopimelic acids

The asymmetric synthesis of both the enantiomers of 2,6-diaminopimelic acid (2,6-DAP) has been accomplished starting from the chiral synthon 1.     

First enantioselective total synthesis of (S)-(−)-longianone

An efficient stereoselective synthesis of (S)-(?)-longianone was achieved from sugar derived 1,2-cyclopropanecarboxylate involving bromonium ion mediated solvolytic ring opening and a one-pot dehydrohalogenation, stereoselective intramolecular hetero Michael addition (IHMA) and ester hydrolysis as key steps.     

Stereoselective synthesis of spiro[pyrrolidin-3,3′-oxindoles] via organocatalyzed asymmetric Mannich-type reaction

The organocatalyzed asymmetric Mannich-type spirocyclization was employed to access the spiro[pyrrolidin-3,3′-oxindoles], a biologically privileged ring structure, from readily available 2-oxotryptaphan methyl ester and aromatic aldehydes. The stereoselectivity of this cyclization are closely correlated with the organocatalyst and solvent used for the reaction. The best diastereoselectivity (d.r. ≥99%) was achieved using only 5% loading of the bifunctional organocatalyst, Takemoto’s thiourea, and dioxane as solvent, resulting in the formation of the the spirocycle with the (2′R, 3R) configuration. The yield could be increased up to 97% when excess of benzaldehyde was used to overcome its trimerization.     

Novel asymmetric total syntheses of (R)-(−)-pyridindolol, (R)-(−)-pyridindolol K1, and (R)-(−)-pyridindolol K2 via a mild one-pot aromatization of N-tosyl-tetrahydro-β-carboline with (S)-2,3-O-isopropylidene-l-glyceraldehyde as the source of chirality

Novel total syntheses of (R)-(?)-pyridindolol 1, (R)-(?)-pyridindolol K1 2, and (R)-(?)-pyridindolol K2 3 are described. By using l-tryptophan methyl ester and (S)-2,3-O-isopropylidene-l-glyceraldehyde as the starting materials, (R)-(?)-pyridindolol 1, (R)-(?)-pyridindolol K1 2, and (R)-(?)-pyridindolol K2 3 were synthesized in 5–7 steps in 66%, 41%, and 55% overall yields, respectively. The characteristic step of the total syntheses is a mild one-pot aromatization of N-tosyl-1,2,3,4-tetrahydro-β-carboline (N-Ts-THBC), which was obtained via Pictet–Spengler reaction of l-tryptophan methyl ester with (S)-2,3-O-isopropylidene-l-glyceraldehyde, and subsequent N-tosylation.     

A convenient synthesis of (+)-albicanol based on enzymatic function: total syntheses of (+)-albicanyl acetate, (−)-albicanyl 3,4-dihydroxycinnamate, (−)-drimenol, (−)-drimenin and (−)-ambrox

By using lipase ‘PL-266’ from Alcaligenes sp. enantioselective acetylation of (±)-albicanol 4 with isopropenyl acetate gave the enantiomerically pure (+)-albicanyl acetate 3 and (+)-albicanol 4. Deprotection of (+)-3 afforded the natural (+)-albicanol 4 which was converted to the natural products (−)-albicanyl 3,4-dihydroxycinnamate 7, (−)-drimenol 8, (−)-drimenin 9 and (−)-ambrox 10.     

Total synthesis of (+)-varitriol and (+)-6′-epi-varitriol

Total synthesis of (+)-varitriol and its C6′-epimer have been achieved starting from commercially available d-(?)-ribose and o-anisic acid. The key steps involved are Corey Chaykovsky reaction, triethylamine mediated epimerization, and an olefin cross-metathesis.     

Enantioselective preparation of (2R,3R)-(+)- and (2S,3S)-(−)-2,3-epoxy-2-methylbutanoic acids and some derivatives

(2R,3R)-(+)- and (2S,3S)-(−)-2,3-epoxy-2-methylbutanoic acids (epoxyangelic acids) were prepared from (Z)-2-methyl-2-butenoic acid using the Sharpless asymmetric epoxidation method in combination with the use of (−)- and (+)-menthol as chiral auxiliaries. Both substances, obtained in high enantiomeric excess, were characterized by spectroscopic and optical activity data. Their absolute configuration was determined by correlation with (R)-(+)-2-methyl-1,2-butanediol.     

Stereocontrolled synthesis of substituted chiral piperidines via one-pot asymmetric 6π-azaelectrocyclization: asymmetric syntheses of (-)-dendroprimine, (+)-7-epidendroprimine, (+)-5-epidendroprimine, and (+)-5,7-epidendroprimine

The asymmetric one-pot 6π-azaelectrocyclization of alkenyl vinyl stannane, ethyl (Z)-2-iodo-4-oxobutenoate, and (-)-7-isopropyl-cis-aminoindanol in the presence of a Pd(0) catalyst stereoselectively produced the tetracyclic aminoacetal compounds, resulting from the four-bond formation accompanying by controlling the stereochemistry at the two asymmetric centers. The produced cyclic aminoacetals can be regarded as synthetic precursors of substituted chiral piperidines, and the syntheses of 2,4- and 2,4,6-substituted piperidines were realized from the obtained aminoacetals by the stereoselective hydrogenation of the double bond conjugated with the C-4 ester group and alkylation at the aminoacetal moiety. In addition, the stereoselective synthesis of an indolizidine alkaloid, (-)-dendroprimine, and its three stereoisomers, (+)-7-epidendroprimine, (+)-5-epidendroprimine, and (+)-5,7-epidendroprimine, were achieved.     

Enantioselective synthesis of (2S,2′R)-erythro-methylphenidate

A new approach towards the enantioselective synthesis of (2S,2′R)-erythro-methylphenidate (1) is described. The key step in the synthesis utilizes Evans' 4-substituted-2-oxazolidinone chiral auxiliary to control the diastereofacial selectivity in the hydrogenation of enamine intermediate 6, yielding the hydrogenated product 7 with excellent diastereoselectivity. Methanolysis of 7 afforded 1 with excellent enantiopurity.     

An improved synthesis of (S)-(+)- and (R)-(−)-4-ethenyl[2.2]paracyclophane

The resolution of the 4-acetyl[2.2]paracyclophane by the SAMP-hydrazone method is described. A new, short, high yield synthesis of both enantiomers (S)-(+)- and (R)-(−)-4-ethenyl[2.2]paracyclophane is reported.