Eight membered ethers via diene metathesis: Synthetic approach to laureatin natural products

Enantioselective Synthesis of Medium-Sized-Ring Lactones via Iridium-Catalyzed Z-Retentive Asymmetric Allylic Substitution Reaction

Medium-sized rings are important structural units, but their synthesis, especially in a highly enantioselective manner, has been a great challenge. Herein we report an enantioselective synthesis of medium-sized-ring lactones by an iridium-catalyzed Z-retentive asymmetric allylic substitution reaction. The reaction features mild conditions and a broad substrate scope. Various eight- to 11-membered-ring lactones can be afforded in moderate to excellent yields (up to 88%) and excellent enantioselectivity (up to 99% ee). The utilization of both Z-allyl precursors and an Ir catalyst is critical for the medium-sized-ring formation.

The efficient desymmetrization of glycerol using scaffolding catalysis

Glycerol is an ideal building block for the synthesis of complex molecules, because it is inexpensive and highly functionalized. We report the desymmetrization of glycerol through silyl transfer, using a chiral organic catalyst in high yield and enantioselectivity.

About the activity and selectivity of less well-known metathesis catalysts during ADMET polymerizations

We report on the catalytic activity of commercially available Ru-indenylidene and "boomerang" complexes C1, C2 and C3 in acyclic diene metathesis (ADMET) polymerization of a fully renewable α,ω-diene. A high activity of these catalysts was observed for the synthesis of the desired renewable polyesters with molecular weights of up to 17000 Da, which is considerably higher than molecular weights obtained using the same monomer with previously studied catalysts. Moreover, olefin isomerization side reactions that occur during the ADMET polymerizations were studied in detail. The isomerization reactions were investigated by degradation of the prepared polyesters via transesterification with methanol, yielding diesters. These diesters, representing the repeat units of the polyesters, were then quantified by GC-MS.

Construction of eight-membered carbocycles with trisubstituted double bonds using the ring closing metathesis reaction

Medium sized carbocycles are particularly difficult to synthesize. Ring closing metathesis reactions (RCM) have recently been applied to construct eight-membered carbocycles, but trisubstituted double bonds in the eight-membered rings are more difficult to produce using RCM reactions. In this review, model examples and our own results are cited and the importance of the preparation of suitably designed precursors is discussed. Examples of RCM reactions used in the total synthesis of natural products are also outlined.

Total synthesis of the indolizidine alkaloid tashiromine

BACKGROUND

Tashiromine 1 is a naturally occurring indolizidine alkaloid. It has been the subject of thirteen successful total syntheses to date. Our own approach centres on the stereoselective construction of the indolizidine core by capture of an electrophilic acyliminium species by a pendant allylsilane. The key cyclisation precursor is constructed using olefin cross-metathesis chemistry, which has the potential to facilitate both racemic and asymmetric approaches, depending upon the choice of the allylsilane metathesis partner.

RESULTS

The use of the allyltrimethylsilane cross-metathesis approach enables the rapid construction of the key cyclisation precursor 3 (3 steps from commercial materials), which undergoes acid-induced cyclisation to give the desired bicyclic indolizidine skeleton as a 96:4 mixture of diastereomers. Simple functional group interconversions allowed the completion of the total synthesis of racemic tashiromine in six steps (19% overall yield). Three chiral alpha-alkoxyallylsilanes (12,14 and 15) were prepared in enantioenriched form and their cross-metathesis reactions studied as part of a putative asymmetric approach to tashiromine. In the event, alpha-hydroxysilane 12 underwent isomerisation under the reaction conditions to acylsilane 17, while silanes 14 and 15 were unreactive towards metathesis.

CONCLUSION

A concise, stereoselective total synthesis of racemic tashiromine has been developed. Attempts to translate this into an asymmetric synthesis have thus far been unsuccessful.

Asymmetric Total Syntheses of (+)-3-(Z)-Laureatin and (+)-3-(Z)-Isolaureatin by “Lone Pair−Lone Pair Interaction-Controlled” Isomerization

The first asymmetric total syntheses of the dihalogenated medium-sized dioxabicyclic marine natural products (+)-3-(Z)-isolaureatin (1) and (+)-3-(Z)-laureatin (2) have been accomplished. Notable features of the highly stereo-, regio-, and chemoselective syntheses of these alpha,alpha'-trans-oxocene natural products include an intramolecular amide enolate alkylation to construct the alpha,alpha'-cis-oxocene, novel "lone pair-lone pair interaction-controlled" epimerizations to the alpha,alpha'-trans-oxocenes, various strategies for stereoselective introduction of halogen atoms, and novel olefin cross-metatheses for construction of the (Z)-enyne systems.

Selective Construction of Carbocyclic Eight-Membered Rings by Ring-Closing Metathesis of Acyclic Precursors

Although the efficient preparation of cyclooctanoids has remained a long-standing objective, a real breakthrough was possible only relatively recently with the development of more specific reagents and by the development of new reactions. The direct construction of carbocyclic eight-membered rings from acyclic precursors is still a challenge, mainly because of unfavorable entropic and enthalpic factors that preclude ring formation. This Minireview describes the utilization of ring-closing metathesis (RCM) as a new approach to synthesize cyclooctanoids.

Towards novel difluorinated sugar mimetics; syntheses and conformational analyses of highly-functionalised difluorinated cyclooctenones

Highly-functionalised difluorinated cyclooctenones were synthesised from trifluoroethanol using either metallated difluoroenol acetal or carbamate chemistry, followed by a [2,3]-Wittig rearrangement or aldol reaction. Efficient RCM reactions afforded the title compounds which showed rather restricted fluxional behaviour by VT (19)F NMR. Topological characterisation by molecular modelling and NOESY/ROESY experiments offered a number of challenges, but allowed the identification of two favoured boat-chair conformers which interconverted by pseudorotation with relatively large activation barriers.

Synthesis of muscothiazoles A and B: Critical role of methyl group substitution in RCM-based syntheses of macrocycles

[reaction: see text] Muscothiazoles A (2b) and B (2c) have been prepared by two approaches that differ in the order of assembly of the rings. Comparative studies show that substitution of the carbon chains in substrate 5 or 12 (respective precursors to 13-membered and 14-membered rings by RCM), even by a single methyl group, can have a profound effect on increasing the efficiency of the macrocyclization.