Comparison of solid-phase and solution-phase chiral auxiliaries in the alkylation/iodolactonization sequence to gamma-butyrolactones

1,3-Diaza-Claisen Rearrangements of Vinyl Pyrrolidines Tethered to In Situ Generated Carbodiimides Afford Ring-Expanded [9,5]- and [9,6]-Bicyclic Guanidines

Vinyl pyrrolidines tethered to isothioureas are activated to the corresponding carbodiimides with AgOTf and Et₃N. Intramolecular addition of the vinyl pyrrolidine amine to the carbodiimide followed by a 1,3-diaza-Claisen rearrangement affords [9,5]- and [9,6]-bicyclic guanidines depending on the tether length in good to excellent yields. Density functional theory calculations reveal that the pathway involving intramolecular addition of the amine to carbodiimide to afford a zwitterionic intermediate followed by the zwitterionic 1,3-diaza-Claisen rearrangement was consistently a higher energy pathway than the path involving protonation of the zwitterionic intermediate followed by the cationic 1,3-diaza-Claisen rearrangement.

Synthesis of a New Chiral Auxiliary: Non-Cross-Linked Polystyrene Bound (4S)-Oxazolidine-2-Thione

A new chiral auxiliary, non-cross-linked polystyrene bound (4 S)-oxazolidine-2-thione, was synthesised from L-tyrosine in 24.2% by a seven step method.

Non-cross-linked polystyrene-supported 2-imidazolidinone chiral auxiliary: synthesis and application in asymmetric alkylation reactions

Asymmetric alkylation reactions using non-cross-linked polystyrene (NCPS)-supported 2-imidazolidinone chiral auxiliaries were successfully investigated with excellent diastereocontrol (>99% de). The recovery and the recycling of this soluble polymer-supported chiral auxiliary were achieved in order to produce highly optical pure carboxylic acids.

Chiral distinction between the enantiomers of bicyclic alcohols by UDP-glucuronosyltransferases 2B7 and 2B17

The stereoselective binding and transformation of optically pure bicyclic alcohols by human UDP-glucuronosyltransferases from subfamily 2B were investigated. The enantiomers of 1-indanol, 1-tetralol, and 1-benzosuberol were synthesized by asymmetric Corey-Bakshi-Shibata reduction and subjected to glucuronidation assays. The alcohols studied were primarily glucuronidated by UGT2B7 and UGT2B17. The catalytic transformation by UGT2B17 was highly stereoselective, favoring conjugation of the (R)-enantiomers. UGT2B7, on the other hand, did not exhibit stereoselectivity toward 1-benzosuberol, the best substrate in this series. To assess binding affinities to the enzymes, the six different compounds were tested for their efficiency as inhibitors of either UGT2B7 or UGT2B17. The results of the latter analyses indicated that the affinities of both enantiomers of each pair towards UGT2B7 and UGT2B17 were of the same order of magnitude. Therefore, the findings of this study suggest that the spatial arrangement of the hydroxy group plays an important role in the glucuronic acid transfer reaction, but not necessarily in substrate binding to the UGTs.

Stereoselective alpha-fluoroamide and alpha-fluoro-gamma-lactone synthesis by an asymmetric zwitterionic aza-Claisen rearrangement

BACKGROUND

Asymmetric introduction of fluorine alpha-to a carbonyl has become popular recently, largely because the direct fluorination of enolates by asymmetric electrophilic fluorinating reagents has improved, and as a result such compounds are becoming attractive synthons. We have sought an alternative but straightforward asymmetric method to this class of compounds, utilising the zwitterionic aza-Claisen rearrangement by reacting alpha-fluoroacid chlorides and homochiral N-allylpyrrolidines as starting materials.

RESULTS

Treatment of N-allylmorpholine with 2-fluoropropionyl chloride under Yb(OTf)3 catalysis generated the zwitterionic aza-Claisen rearrangement product in good yield and demonstrated the chemical feasibility of the approach. For the asymmetric reaction, N-allyl-(S)-2-(methoxymethyl)pyrrolidine was treated with either 2-fluoropropionyl chloride or 2-fluorophenylacetic acid chloride under similar conditions and resulted in N-(alpha-fluoro-gamma-vinylamide)pyrrolidine products as homochiral materials in 99% de. These products were readily converted to their corresponding alpha-fluoro-gamma-lactones by iodolactonisation and in good diastereoselectivity.

CONCLUSION

Molecules which have fluorine at a stereogeneic centre are finding increasing utility in pharmaceutical, fine chemicals and materials research. The zwitterionic aza-Claisen rearrangement proved to be an effective and competitive complement to asymmetric electrophilic fluorination strategies and provides access to versatile synthetic intermediates with fluorine at the stereogenic centre.

Unprecedented, fully recyclable, solid-supported reagent for the kinetic resolution of racemic amines through enantioselective N-acetylation

The high-yielding synthesis and application of the first polymer supported reagent for the kinetic resolution (KR) of amines through enantioselective acetylation is described; this new supported chiral reagent allows the KR of primary amines with excellent selectivities at room temperature; moreover, this supported approach is highly efficient as the Merrifield-supported chiral scaffold can be quantitatively recovered and recycled.