Enantioselective organocatalytic intramolecular aza-Michael reaction: a concise synthesis of (+)-sedamine, (+)-allosedamine, and (+)-coniine

The intramolecular aza-Michael reaction of carbamates bearing remote alpha,beta-unsaturated aldehydes under organocatalytic conditions took place with good yields and excellent ee's when J?rgensen catalyst IV was used in the process, giving rise to the enantioselective formation of several five- and six-membered heterocycles. The developed methodology was applied to the synthesis of three piperidine alkaloids.     

Asymmetric synthesis and applications of beta-amino Weinreb amides: asymmetric synthesis of (S)-coniine

Conjugate addition of lithium (S)-N-benzyl-N-alpha-methylbenzylamide to a range of alpha, beta-unsaturated Weinreb amides proceeds with high levels of diastereoselectivity (>95% de). The beta-amino Weinreb amide products may be transformed into beta-amino ketones via reactions with Grignard reagents, while treatment with DIBAL-H furnishes beta-amino aldehydes. Trapping of the aldehyde via Wadsworth-Emmons reaction and subsequent manipulation offers an efficient route to homochiral delta-amino acid derivatives and 2-substituted piperidines. The application of this methodology for the synthesis of (S)-coniine is demonstrated.     

Syntheses of (+)-alismoxide and (+)-4-epi-alismoxide

The first total syntheses of (+)-alismoxide and (+)-4-epi-alismoxide are reported. Formal chemo-, regio-, and stereoselective addition of water to 10alpha-acetoxy-1alphaH,5betaH-guaia-3,6-diene afforded the target compounds after reduction. The absolute stereochemistry of (+)-alismoxide has been established. The low [alpha](D) +8.6 value indicates that significant amounts of alismoxide result from biosynthetic processes. Furthermore, the structure of the natural guaienediol isolated from Silphium perfoliatum has been corrected to (-)-alismoxide.     

Practical highly enantioselective synthesis of terminal propargylamines. An expeditious synthesis of (S)-(+)-coniine

The one-pot three-component addition reaction of trimethylsilylacetylene, aldehydes and dibenzylamine provides in the presence of CuBr/Quinap as catalyst, various enantiomerically enriched propargylamines in good yields (up to 99%) and excellent enantiomeric excess (up to 98%ee) which can be used as a key intermediate in the synthesis of the alkaloid (S)-(+)-coniine.     

Asymmetric synthesis of 6-alkyl- and 6-arylpiperidin-2-ones. Enantioselective synthesis of (S)-(+)-coniine

A variety of diolefinic hydrazides (1) have been assembled in a highly diastereoselective manner by addition of allyllithium to chiral SAMP hydrazones followed by N-acylation with acryloyl chloride. Substrates 1 undergo ring-closing metathesis to give the cyclic enehydrazides (5) which can be easily converted into virtually enantiopure 6-alkyl- or 6-arylpiperidin-2-ones (7). The versatility of this hydrazone addition-RCM protocol has been further exemplified by the conversion of the unsaturated heterocycle 5b into the piperidine alkaloid (S)-(+)-coniine.     

Carbon-branched carbohydrate chirons: synthesis of C-3 and C-4-branched sugar lactones derived from d-erythronolactone

Two carbon chain extensions using a Wittig reaction on both a 1-deoxy ribulose derivative and a C-2-branched erythrose derivative are reported. Subsequent dihydroxylation resulted in the synthesis of C-3 and C-4 methyl-branched sugar lactones, the useful synthetic building blocks. Control of the stereoselectivity of both the Wittig reaction and the dihydroxylation is investigated, and 3-C-methyl and 4-C-methyl d-altrono-1,4-lactones and d-glucono-1,4-lactone and 4-C-hydroxymethyl-d-altrono-1,4-lactone were synthesised.     

Asymmetric synthesis of conformationally constrained 4-hydroxyprolines and their applications to the formal synthesis of (+)-epibatidine

This report describes the synthesis of enantiomerically pure (1S,3S,4R)- and (1S,3R,4R)-3-hydroxy-7-azabicyclo[2.2.1]heptane-1-carboxylic acids, two new conformationally constrained 4-hydroxyprolines, using a straightforward synthetic route and starting from (−)-8-phenylmenthyl 2-acetamidoacrylate. The easy transformation of the pure (1S,3S,4R)-3-hydroxy-7-azabicyclo[2.2.1]heptane-1-carboxylic acid into (1R,4S)-N-Boc-7-azabicyclo[2.2.1]heptan-2-one constitutes a new formal synthesis of (+)-epibatidine.     

Total Syntheses of (+)-Longicamphor and (+)-Longiborneol

Recent total syntheses within the longifolane class of sesquiterpenes have included such elaborately bridged structures as longicyclene,¹ longifolene,^2,3 isolongifolene,⁴ and culmorin.⁵ A tricyclic member of this interesting group of naturally occurring compounds; namely, longicamphor (6) has previously been synthesized from natural (+)-longifolene via (+)-longiborneol (7).⁶ Longiborneol (Juniperol, macrocarpol) was isolated from Cupressus macrocarpa, the famous Monterey cypress.^7,8 We wish to report herein stereoselective total syntheses of (+)-longicamphor (6) and (+)-longiborneol (7).     

Sedum alkaloids—VII : Structure and synthesis of (+)-4-hydroxysedamine and (+)-4-hydroxyallosedamine

The structures of (+)-4-hydroxysedamine ¹ and (+)-⁴-H-hydroxyallosedamine ², two new minor alkaloids from Sedum itacre, are reported 1 and 2 were synthesized, and their absolute configuration established, via the optically pure tetrahydro 1,3-oxazines 17 and 18.     

A stereoconvergent synthesis of (+)-4-demethoxydaunomycin

Sharpless kinetic asymmetric epoxidation on (±)-2-(l-hydroxyethyl)-5,8-dimethoxy-3,4-dihydronaphthalene (8) followed by LAH reduction gave R-2-(S-l-hydroxyethyl)-2hydroxy-5,8-dimethoxy-1,2,3,4-tetrahydronaphthalene and the undesired antipode. The former was converted to R-(-)-2-acetyl-2 hydroxy-5,8-dimethoxy-l,2,3,4-tetrahydronapthalen[R-(-)-5],while the latter was epimerized and recycled. R-(-)-5 has been exploited for the synthesis of(+)-4-demethoxydaunomycin     

Total Syntheses of (+)-Goniotriol and (+)- 8-Acetylgoniotriol

Two styryl-pyrones, (+)-goniotriol (1) and (+)-8-acetylgoniotriol (2), which are cytotoxic to human turnout cells, were isolated from the ethanolic extracts of the stem bark of Goniothalamus giganteus Hook. F., Thomas (Annonaceae), in addition (+)-goniotriol was also isolated from the leaves and twigs of Goniothalamus     

4-Arylvinyl-2,6-di(pyridin-2-yl)pyrimidines: synthesis and optical properties

A series of 4-arylvinyl-2,6-di(pyridin-2-yl)pyrimidines have been efficiently prepared by a double cross-coupling reaction between 2,4-dichloro-6-methylpyrimidine and 2-(tributylstannyl)pyridine, followed by aldol condensation with the appropriate aromatic aldehyde substituted with electron-donating, electron-withdrawing, dendritic, or water-soluble groups. The effect of different protic and aprotic solvents on the optical absorption and emission properties of these systems was studied. Compounds with electron-donating groups display strong emission solvatochromism, suggesting the formation of an intramolecular charge-separated emitting state. The solvatochromic behavior depends not only on the solvent polarity but also on the hydrogen bonding parameters of the solvent. The effect of protonation was also studied, and the abilities of some of these molecules to function as colorimetric and luminescent pH sensors were demonstrated with dramatic color changes and luminescence switching upon the introduction of acid.     

Temporary thio-derivatization in the synthesis of (+)-4-acetylbromoxone

A stereocontrolled synthesis of the marine natural products (+)-bromoxone (1) and (+)-4-acetylbromoxone (2) is reported. The sequence features the enzymatic kinetic resolution of 4-hydroxycyclohexenone (6) via its S-benzyl adduct. Thereafter, a base-mediated elimination-silylation generated an optically active (−)-4S-4-tert-butyldimethylsilyoxycyclohexenone (5), which then underwent diastereoselective epoxidation. Saegusa-Ito oxidation enabled formation of the corresponding α,β-unsaturated ketone 13. Bromination-elimination and subsequent removal of the silicon protecting group afforded (+)-bromoxone (1) which was converted into (+)-(4S,5R,6R)-4-acetoxy-2-bromo-5,6-epoxycyclohex-2-enone (2) [(+)-4-acetylbromoxone]. Using a luciferase gene reporter assay ED₅₀ for NFκB inhibition of 9 μM was determined.     

A new asymmetric synthesis of 2,6-cis-disubstituted 4-methylenepiperidines: total synthesis of (+)-alkaloid 241D and (+)-isosolenopsin A

A highly diastereoselective synthesis of 2,6-cis-disubstituted-4-methylenepiperidines based on a Mannich type intramolecular cyclization of an allylsilane on an iminium ion is described. The synthetic potential of this methodology is demonstrated by the enantioselective synthesis of two natural piperidine alkaloids: (+)-alkaloid 241D and (+)-isosolenopsin A.     

Asymmetric synthesis of (+)-phosphinothricin and (+)-2-amino-4-phosphonobutyric acid

Asymmetric synthesis of (+)-phosphinothricin, (+)-2-amino-4-phosphono-butyric acid, and their enantiomers has been achieved by the Michael addition of chiral glycine Schiff bases to vinyl phosphorus compounds.     

Enantioselective [4 + 2] cycloadditions of o-quinone methides: total synthesis of (+)-mimosifoliol and formal synthesis of (+)-tolterodine

The first example of an enantioselective cycloaddition of an o-quinone methide (o-QM) with a chiral enol ether is described along with the total synthesis of (+)-mimosifoliol and the formal synthesis of (+)-tolterodine. These syntheses exemplify a three-component, one-pot benzopyran approach for the construction of chiral benzylic junctions. Cycloadditions of various enol ethers and o-QMs are examined, and diastereoselectivities >95% are obtained with trans-2-phenyl-1-cyclohexanol and 2,2-diphenylcyclopentanol vinyl ethers.     

A short enantioselective synthesis of (+)-eleutherin, (+)-allo-eleutherin and a formal synthesis of (+)-nocardione B

A short enantioselective synthesis of (+)-eleutherin, (+)-allo-eleutherin and the formal synthesis of (+)-nocardione B is described. The synthesis is completed in six steps in overall yields of 8% for eleutherin and 14% for allo-eleutherin. The synthetic strategy features an efficient combination of the Dötz annulation reaction with a chiral alkyne and an oxa-Pictet Spengler reaction as the keys steps in the stereodivergent synthesis of (+)-eleutherin and (+)-allo-eleutherin. The synthesis of (S)-(+)-2-(2′-hydroxypropyl)-5-methoxy-1,4-naphthoquinone entails the formal synthesis of (+)-nocardione B.     

Communic acids: occurrence, properties and use as chirons for the synthesis of bioactive compounds

Communic acids are diterpenes with labdane skeletons found in many plant species, mainly conifers, predominating in the genus Juniperus (fam. Cupresaceae). In this review we briefly describe their distribution and different biological activities (anti- bacterial, antitumoral, hypolipidemic, relaxing smooth muscle, etc.). This paper also includes a detailed explanation of their use as chiral building blocks for the synthesis of bioactive natural products. Among other uses, communic acids have proven useful as chirons for the synthesis of quassinoids (formal), abietane antioxidants, ambrox and other perfume fixatives, podolactone herbicides, etc., featuring shorter and more efficient processes.     

L-(S)-Erythrulose: The synthesis of (R)-1,2,4-butanetriol and of some related C4 chirons.

L-(S)-Erythrulose can easily be transformed into (R)-1,2,4-butanetriol and related C₄, chiral building blocks. A formal synthesis of (-)GABOB is presented. Also the formation of 3-methylene-1,2,4-butanetriol derivatives is described.