A new convergent route to aldohexoses from a common chiral building block

[reaction in text] A diastereocontrolled route to the eight aldohexoses has been developed starting from a common cyclohexanoid chiral building block.     

A new convergent route to conduritols A-F from a common chiral building block

A diastereocontrolled route to conduritols A-F has been developed starting from a common chiral building block containing an oxabicyclo[3.2.1]octane framework.     

An expedient route to the Calabar bean alkaloids (-)-physovenine and (-)-physostigmine

An expedient route to the Calabar bean alkaloids (-)-physovenine and (-)-physostigmine has been devised using a chiral building block which was originally designed for diastereocontrolled construction of aldohexoses.     

A diastereocontrolled synthesis of (+)-febrifugine: a potent antimalarial piperidine alkaloid

A diastereocontrolled synthesis of (+)-febrifugine, a potent antimalarial piperidine alkaloid, has been achieved using a chiral block having a bicyclo[3.2.1]octane framework which exhibits inherent convex-face selectivity.     

Diastereoselective synthesis of 4,5'-bis-proline compounds via reductive dimerization of N-acyloxyiminium ions

A short, practical synthesis of novel, unsymmetrical 4,5'-bis-proline compounds has been achieved, highlighted by the application of an unprecedented samarium diiodide-driven regio- and diastereocontrolled reductive dimerization of N-acyloxyiminium ions generated from readily available 2-methoxy-5-silyloxymethyl-N-Boc pyrrolidines. The title proline dimers proved to be pertinent metal-free catalysts in aldol and Mannich reactions.     

De novo asymmetric synthesis of 8a-epi-swainsonine

An enantioselective and diastereocontrolled approach to 8a-epi-d-swainsonine has been developed from achiral furfural. The key step to this synthesis was a one-pot procedure for the hydrogenolytic removal of two protecting groups and two intramolecular reductive amination reactions. The absolute stereochemistry was introduced by asymmetric Noyori reduction of furfuryl ketone. This route relies on diastereoselective palladium-catalyzed glycosylation to install the anomeric bond, and Luche reduction, diastereoselective dihydroxylation to set up the manno-stereochemistry of the indolizidine precursor.     

Enantio- and diastereocontrolled synthesis of (-)-iridolactone and (+)-pedicularis-lactone

[structure: see text]. Two iridoid lactones, (-)-iridolactone and (+)-pedicularis-lactone, have been synthesized in an enantio- and diastereocontrolled manner starting from a tricyclic chiral building block serving as a synthetic equivalent of chiral 3-(hydroxymethyl)cyclopenta-2,4-dien-1-ol.     

Stereoselective approaches to 2,3,6-trisubstituted piperidines. An enantiospecific synthesis of quinolizidine (-)-217A

The enantiospecific and diastereocontrolled total synthesis of alkaloid (-)-217A is described that employs a stepwise [3+3] annelation strategy and a piperidine 2,3-cyclopropanation-ring opening reaction as the key steps.     

Total synthesis of eupomatilones 4 and 6: structurally rearranged and atropisomerically fluxional lignan natural products

[structure: see text] A convergent and diastereocontrolled total synthesis of eupomatilones 4 and 6 is reported and was based on a diastereoselective hydroboration/oxidation sequence and a convergent Lipshutz biarylcuprate cross-coupling reaction. The structure of eupomatilone 6 is revised.     

An asymmetric synthesis of sulfobacin A

A facile synthesis of sulfobacin A has been developed starting from (R)-cyclohexylideneglyceraldehyde (11). The key steps in the synthesis are the highly diastereocontrolled allylation of 11 and syn-selective reduction of a ketone derived from 11. The other attractive features are the operational simplicity and the use of inexpensive compounds/reagents.     

A general strategy for the diastereoselective synthesis of 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans

A strategy for the stereoselective synthesis of all the possible diastereoisomers of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane (furofuran) lignans from a single dihydrofuran precursor is described. The key steps involve a diastereocontrolled templated cationic cyclization followed by stereoselective reduction of the resulting methyl glycoside.     

Enantiospecific total synthesis of the sarpagine related indole alkaloids talpinine and talcarpine as well as the improved total synthesis of alstonerine and anhydromacrosalhine-methine via the asymmetric Pictet-Spengler reaction

The enantiospecific total synthesis of talpinine 1 and talcarpine 2 has been accomplished from D-(+)-tryptophan in 13 steps (11 reaction vessels) in 10% and 9.5% overall yields, respectively. Moreover, this synthetic approach has been employed for the improved synthesis of alstonerine 3and anhydromacrosalhine-methine 4 in 12% and 14% overall yield, respectively. A convenient synthetic route for the enantiospecific, stereospecific preparation of the key intermediate (-)-N(a)-H, N(b)-benzyl tetracyclic ketone 15a via the asymmetric Pictet-Spengler reaction on a multihundred-gram scale has been developed. A diastereocontrolled (>30:1) anionic oxy-Cope rearrangement and the intramolecular rearrangement to form ring-E and an N(b)-benzyl/N(b)-methyl transfer reaction also served as key steps. This general approach can now be utilized for the synthesis of macroline/sarpagine related indole alkaloids and their antipodes for biological screening.     

Studies towards complex bridged alkaloids: regio- and stereocontrolled enolate chemistry of 2,5-diketopiperazines

The substitution of symmetrical N-protected diketopiperazines (DKPs) via enolate intermediates has been studied. The enolate reactions were highly diastereocontrolled, leading to enantiopure DKP products if chiral amino acid precursors were employed, and giving racemic products, starting with centrosymmetric DKPs, even when a chiral lithium amide base was used to generate the lithium enolate. With unsymmetrical DKPs derived from proline and either alanine, phenylalanine or valine, the enolate substitution occurred with high regio- and stereoselectivity on the proline residue. This enabled the synthesis of substituted DKPs that could be cyclised via cationic processes to give the bicyclo[2.2.2]diazaoctane core structure present in paraherquamide and stephacidin natural products.     

Stereocontrolled synthesis of piperidine alkaloids, (−)-241D and (−)-isosolenopsin

Highly diastereocontrolled synthesis of alkaloids, (−)-241D and (−)-isosolenopsin was achieved in 7.7% and 5.3% yields, respectively, using a Barbier-type allylation of a chiral imine and d-proline catalyzed aldol addition reaction of a β-amino aldehyde with acetone as the key steps. The synthesis involves a nine-step sequence using (S)-valinate imine in a Barbier-type allylation for the first time.     

Mitomycin synthetic studies: stereocontrolled and convergent synthesis of a fully elaborated aziridinomitosane

Full details of a stereocontrolled and convergent synthetic route to 9a-desmethoxymitomycin A (1) are reported. The target molecule possesses the parent tetrahydropyrrolo[1,2-a]indole ring system characteristic of the mitomycin family of antitumor agents. The synthesis was based on the diastereocontrolled addition of a fully elaborated cinnamylstannane to a pyrrolidine-based N-acyliminium ion as the key convergent step, which resulted in the installation of the C9 and C9a stereogenic centers.     

A concise, stereocontrolled total synthesis of rippertenol

The first total synthesis of the unique terpene rippertenol, a molecule with dense stereochemical complexity arrayed on a compact framework largely devoid of functional groups, is described. Key elements include orchestrated and unique applications of aldol condensations, Diels-Alder chemistry, and a ring expansion to advance a chiral starting material containing a single chiral center into the final target in a concise and diastereocontrolled manner.     

The total synthesis of eupomatilones 2 and 5

Efficient and diastereocontrolled syntheses of natural lignans eupomatilone 2 (1) and 5 (2) are described. Biaryl coupling was achieved using Suzuki chemistry. The lactone moiety was constructed using allylmetal reagents, which were synthesized from Baylis-Hillman adducts. Allylindium reagents were found to be most effective.     

Lewis acid promoted intramolecular (3 + 2) 'cycloadditions' of methyleneaziridines with alkene and alkyne acceptors

2-Methyleneaziridines can be tethered to a variety of alkene or alkyne acceptors through the saturated carbon of the heterocyclic ring by application of a simple lithiation/alkylation sequence (8 examples, 31-59%). Treatment of the resultant alkene bearing substrates with BF(3)·OEt(2) leads to cis-octahydrocyclopenta[c]pyrroles in which up to four contiguous stereocentres are created in a diastereocontrolled manner after reductive work-up. Using an alkyne based substrate, a 2,4,5,6-tetrahydrocyclopenta[c]pyrrole is produced by rapid tautomerisation of the initially formed bisenamine. Evidence that these (3 + 2) 'cycloadditions' proceed in a stepwise manner via a 2-aminoallyl cation is presented.     

Total synthesis of (+/-)-kainic Acid with an aza-[2,3]-Wittig sigmatropic rearrangement as the key stereochemical determining step

A flexible route to the kainoid skeleton is exemplified by the synthesis of (+/-)-kainic acid from 3-butyn-1-ol. The route relies on the aza-[2,3]-Wittig sigmatropic rearrangement to efficiently install the relative stereochemistry between C2-C3. The C4 stereocenter was derived from a diastereocontrolled iodolactonization. The aza-[2,3]-Wittig rearrangement potentially allows structural diversity at C3 and the displacement of the tosyloxy group with retention of stereochemistry allows structural diversity at C4. The trans-C2 carboxylic acid functional group was found to be the most important for retention of stereochemistry at C4 upon treatment with a higher order cyano cuprate reagent.     

Nucleophilic ring-opening of epoxide and aziridine acetates for the stereodivergent synthesis of β-hydroxy and β-amino γ-lactams

A highly regio- and stereoselective synthesis of novel β,γ-disubstituted γ-lactams with either an anti or syn relative configuration was developed from readily available epoxide and aziridine acetates. The key steps include the regio- and diastereocontrolled nucleophilic ring-opening of these three-membered heterocycles followed by mild reductive cyclization of the γ-azido ester intermediate. The method was also extended to an asymmetric synthesis of (4R,5S)-4-hydroxy-5-phenylpyrrolidin-2-one from a chiral epoxide acetate. The main features of this versatile synthesis of functionalized γ-lactams include the involvement of inexpensive reagents and mild conditions together with high chemical efficiency.