A novel strategy for the asymmetric synthesis of chiral cyclopropane carboxaldehydes

SuperQuat chiral auxiliaries: design, synthesis, and utility

The design, synthesis and outline of some of the most common synthetic applications of the SuperQuat (4-substituted 5,5-dimethyloxazolidine-2-one) family of chiral auxiliaries, developed to address the shortcomings of the Evans (4-substituted oxazolidin-2-one) family of chiral auxiliaries, are presented.

Synthesis and cytotoxic activities of goniothalamins and derivatives

Substituted goniothalamins containing cyclopropane-groups were efficiently prepared in high yields and good selectivity. Antiproliferative activity was measured on three human cancer cell lines (A549, MCF-7, HBL-100), to show which of the structural elements of goniothalamins is mandatory for cytotoxicity. We found that the configuration of the stereogenic centre of the δ-lactone plays an important role for cytotoxicity. In our studies only (R)-configured goniothalamins showed antiproliferative activity, whereby (R)-configuration accords to natural goniothalamin (R)-1. Additionally, the δ-lactone needs to be unsaturated whereas our results show that the vinylic double bond is not mandatory for cytotoxicity. Furthermore, with a two-fold in vitro and in vivo strategy, we determined the inhibitory effect of the compounds to the yeast protein Pdr5. Here, we clearly demonstrate that the configuration seems to be of minor influence, only, while the nature of the substituent of the phenyl ring is of prime importance.

Direct asymmetric syntheses of chiral aldehydes and ketones via N-acyl chiral auxiliary derivatives including chiral Weinreb amide equivalents

This article covers N-acyl chiral auxiliary-based approaches to the asymmetric synthesis of enantiopure aldehydes and ketones. The use of diastereoisomerically pure N-acyl derivatives of chiral auxiliaries (including chiral Weinreb amide equivalents) and their conversion to the corresponding enantiopure aldehydes and ketones in a single synthetic operation by treatment with a hydride reducing agent or an organometallic reagent, respectively, are highlighted.

Solid-phase asymmetric synthesis using a polymer-supported chiral Evans'-type oxazolidin-2-one

This protocol describes the synthesis of (S)-4-(4-hydroxybenzyl)-oxazolidin-2-one, its attachment to a Merrifield-Cl resin and its use for asymmetric synthesis. The chiral auxiliary is prepared in four steps from N-Boc-L-tyrosine on a multigram scale in high yield and attached to Merrifield-Cl resin via its phenolic group to afford a solid-supported chiral auxiliary for asymmetric synthesis that takes ∼7 d to prepare. A procedure for its N-acylation is reported and a method for carrying out diastereoselective solid-supported Evans' syn-aldol reactions is described, with aldol products being cleaved from the polymer by either hydrolysis or reduction. The use of the supported auxiliary for two sequential 'on-bead' reactions has been demonstrated for the synthesis of a chiral cyclopropane aldol and a γ-lactone in a >95:5 diastereomeric ratio. These polymer-supported reactions are carried out in IRORI Kan resin capsules to protect the polymer support from mechanical degradation, thus allowing multiple on-bead reactions to be performed.

A novel class of tunable cyclopropanation reagents (RXZnCH2Y) and their synthetic applications

The Simmons-Smith cyclopropanation is a widely used method to synthesize cyclopropanes from alkenes using methylene iodide and a zinc reagent. A novel class of organozinc species, RXZnCH(2)Y, has been found to efficiently cyclopropanate alkenes, including traditionally unreactive unfunctionalized alkenes. The reactivity and selectivity of this class of organozinc reagents can be regulated by tuning the electronic and/or steric nature of the RX group attached to Zn. During recent years, this class of organozinc reagent has been widely used in organic synthesis as a reagent for cyclopropanation and other useful synthetic transformations. Catalytic, asymmetric versions of this reaction have been developed providing high enantiomeric excess for unfunctionalized olefins.

A Total Synthesis of Tarchonanthuslactone Exploiting N-Pyrrole Carbinols as Efficient Stereocontrolling Elements

[reaction: see text] A short stereoselective total synthesis of the polyketide natural product, tarchonanthuslactone, has been achieved. The key sequence involves the first reported catalytic enantioselective reduction of an N-acyl pyrrole and subsequent use of this stereocenter in a diastereoselective reductive cascade. This proceeded with unprecedentedly high stereocontrol and offered an elegant method of generating the desired syn stereochemistry present in the final target in one step.

Stereoselective synthesis of (E)-trisubstituted α,β-unsaturated amides and acids

Potassium alkoxides of N-acyl-oxazolidin-2-one-syn-aldols undergo stereoselective elimination reactions to afford a range of trisubstituted (E)-alpha,beta-unsaturated amides in >95% de, that may be subsequently converted into their corresponding (E)-alpha,beta-unsaturated acids or (E)-alpha,beta-unsaturated oxazolines in good yield. syn-Aldols derived from alpha,beta-unsaturated aldehydes gave their corresponding trisubstituted (E)-alpha,beta-unsaturated-amides with poorer levels of diastereocontrol, whilst there was a similar loss in (E)-selectivity during elimination of syn-aldols derived from chiral aldehydes. These elimination reactions proceed via rearrangement of the potassium alkoxide of the syn-aldol to a 1,3-oxazinane-2,4-dione enolate intermediate that subsequently eliminates carbon dioxide to afford a trisubstituted (E)-alpha,beta-unsaturated amide. The (E)-selectivity observed during the E1cB-type elimination step has been rationalised using a simple conformational model that employs a chair-like transition state to explain the observed stereocontrol.