RING-CLOSING METATHESIS STRATEGY TO UNSATURATED γ- AND δ-LACTONES: SYNTHESIS OF HYDROXYETHYLENE ISOSTERE FOR PROTEASE INHIBITORS

Enantioselective Synthesis and Biological Evaluation of Sanglifehrin A and B and Analogs

Sanglifehrin A and B are immunosuppressive macrocyclic natural products endowed with and differentiated by a unique spirocyclic lactam. Herein, we report an enantioselective total synthesis and biological evaluation of sanglifehrin A and B and analogs. Access to the spirocyclic lactam was achieved through convergent assembly of a key pyranone intermediate followed by a stereo-controlled spirocyclization. The 22-membered macrocyclic core was synthesized by ring-closing metathesis in the presence of 2,6-bis(trifluoromethyl) benzeneboronic acid (BFBB). The spirocyclic lactam and macrocycle fragments were united by a Stille coupling to furnish sanglifehrin A and B. Additional sanglifehrin B analogs with variation at the C40 position were additionally prepared. Biological evaluation revealed that the 2-CF₃ analog of sanglifehrin B exhibited higher anti-proliferative activity than the natural products sanglifehrin A and B in Jurkat cells. Both natural products induced higher-order homodimerization of cyclophilin A (CypA), but only sanglifehrin A promoted CypA complexation with inosine-5'-monophosphate dehydrogenase 2 (IMPDH2). The synthesis reported herein will enable further evaluation of the spirolactam and its contribution to sanglifehrin-dependent immunosuppressive activity.

Total Synthesis and Stereochemical Confirmation of (-)-Olivil, (+)-Cycloolivil, (-)-Alashinols F and G, (+)-Cephafortin A, and Their Congeners: Filling in Biosynthetic Gaps

For the first time, we describe the stereocontrolled total syntheses of olivil, cephafortin A, 4-des-O-methyl-4-O-rhamnosyl cephafortin A, and alashinol F from a common precursor using a combination of chemoenzymatic and biomimetic methods for the systematic introduction of functional groups on three vicinal stereogenic carbon atoms. We revised the previously assigned stereochemistry of (+)-cephafortin A, which was reported as the enantiomer. Natural and unnatural congeners provide insights into the biogenetic interrelations of members of this family.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Ring-closing-metathesis-based synthesis of annellated coumarins from 8-allylcoumarins

8-Allylcoumarins are conveniently accessible through a microwave-promoted tandem Claisen rearrangement/Wittig olefination/cyclization sequence. They serve as a versatile platform for the annellation of five- to seven-membered rings using ring-closing olefin metathesis (RCM). Furano-, pyrano-, oxepino- and azepinocoumarins were synthesized from the same set of precursors using Ru-catalyzed double bond isomerizations and RCM in a defined order. One class of products, pyrano[2,3-f]chromene-2,8-diones, were inaccessible through direct RCM of an acrylate, but became available from the analogous allyl ether via an assisted tandem catalytic RCM/allylic oxidation sequence.

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.

Structure and reactivity of sulfonamide- and acetate-chelated ruthenium alkylidene complexes

The formation of chelated ruthenium alkylidenes with the oxygen of sulfonamides and acetates and their complexation with carbon monoxide have been studied.

Synthesis of a library of tricyclic azepinoisoindolinones

Hydrozirconation of 1-hexyne, the addition to in situ prepared N-acyliminium species, and ring-closing metathesis (RCM) were key steps in the preparation of a tricyclic isoindolinone scaffold. An unusual alkene isomerization process during the RCM was identified and studied in some detail. Chemical diversification for library synthesis was achieved by a subsequent alkene epoxidation and zinc-mediated aminolysis reaction. The resulting library products provided selective hits among a large number of high-throughput screens reported in PubChem, thus illustrating the utility of the novel scaffold.

The catalytic performance of Ru-NHC alkylidene complexes: PCy₃versus pyridine as the dissociating ligand

The catalytic performance of NHC-ligated Ru-indenylidene or benzylidene complexes bearing a tricyclohexylphosphine or a pyridine ligand in ring closing metathesis (RCM), cross metathesis, and ring closing enyne metathesis (RCEYM) reactions is compared. While the PCy₃ complexes perform significantly better in RCM and RCEYM reactions than the pyridine complex, all catalysts show similar activity in cross metathesis reactions.

Applications of the Ugi reaction with ketones

A convenient synthesis of highly functionalized, α,α-disubstituted amino acid amide derivatives has been accomplished by using cyclic and acyclic ketones as the carbonyl inputs in the Ugi multicomponent reaction. An application of this extension of the Ugi reaction to the synthesis of α,α-divinyl amino acids that may be cyclized via ring-closing metathesis to provide highly substituted pyrrolidines is described.

Total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide via target oriented beta-C-glycoside formation

The total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide via target oriented beta-C-glycoside formation is described. The two key reactions involved a stereoselective reduction of the appropriate oxocarbenium cation and a highly chemo- and diastereoselective ring-closing metathesis protocol for the formation of the macrocyclic core. In addition to the described chemistry, in vitro screening of the antipode of natural dactylolide against the NCI's 60 cancer cell line helped to illuminate the critical importance of the N-acyl hemiaminal side chain of natural zampanolide for its reported potent nanomolar cytotoxicities. Furthermore, by means of the in vitro screen of (-)-dactylolide, a promising cancer therapeutic lead has now emerged for a variety of carcinomas. More specifically, (-)-dactylolide exhibited GI50 values in the nanomolar (25-99 ng/mL) range against the four cell lines HL-60, K-562, HCC-2998, and SF-539, while displaying modest LC50 values.

Asymmetric multi-component reactions: convenient access to acyclic stereocenters and functionalized cyclopentenoids

Asymmetric multi-component reactions of optically active phenyl dihydrofuran, keto ester or N-tosyl imino ester, and allylsilane provided functionalized phenyl tetrahydrofurans with multiple stereogenic centers diastereoselectively. Cleavage of the resulting substituted tetrahydrofurans readily provided acyclic derivatives with three contiguous asymmetric centers via an acyloxycarbenium ion intermediate. Ring closing olefin metathesis, using Grubbs catalyst, afforded functionalized cyclopentene derivatives in optically active form. A one pot tandem tetrahydrofuran ring cleavage followed by ring closing olefin metathesis also provided functionalized cyclopentenes in good yield.

A Divergent Synthesis of Functionalized Unsaturated δ-Lactones from α-Alkenoyl-α-carboxyl Ketene Dithioacetals

A divergent synthesis of functionalized unsaturated delta-lactones 2, 3, 4, and 5 has been developed starting from the readily available alpha-alkenoyl-alpha-carboxyl ketene dithioacetals 1 in high to excellent yields under mild reaction conditions. Thus, 6-substituted 3-(1,3-dithiolan/dithian-2-ylidene)-3H-pyran-2(6H)-ones 2, obtained from a consecutive reduction with NaBH4 and acidic workup of 1 via a novel vinylogous Pummerer cyclization, can be further transformed into alpha-pyranones 3, 4, and 5 upon a sequential isomerization catalyzed by triethylamine (to give 3), followed by dethioacetalization (to give 4) or a formylation with Vilsmeier reagent (to give 5).

Asymmetric Total Synthesis of (−)-Pironetin Employing the SAMP/RAMP Hydrazone Methodology

A convergent asymmetric total synthesis of pironetin (1), a polyketide with immunosuppressive, antitumor, and plant-growth regulating activities is described. The synthesis was realized by coupling between the C(8)-C(14) 2 and C(7)-C(2) 15 fragments, respectively, by using a Mukaiyama-aldol reaction. The stereogenic centers of each fragment were generated by employing the SAMP/RAMP hydrazone (SAMP=(S)-1-amino-2-methoxymethylpyrrolidine, RAMP=(R)-1-amino-2-methoxymethylpyrrolidine) methodology as a key step. An asymmetric alpha-alkylation of diethyl ketone permitted the introduction of the C(10) stereogenic center of 2, whereas the stereocenters C(4) and C(5) of 15 were installed by an asymmetric aldol reaction. Finally, the formation of the alpha,beta-unsaturated delta-lactone was achieved by ring-closing metathesis in the presence of catalytic amounts of titanium tetraisopropoxide.

Biology-Oriented Synthesis of Stereochemically Diverse Natural-Product-Derived Compound Collections by Iterative Allylations on a Solid Support

A strategy aiming at the introduction of stereocenters into polymer-bound natural-product-derived and -inspired compound collections is presented. Treatment of immobilized aldehydes with Brown's pinene-derived allylboranes results in the stereoselective formation of homoallylic alcohols with up to 89 % ee (ee=enantiomeric excess). Subsequent iterative ozonolysis-allylation sequences with up to three allylations on a solid support give access to 1,3-polyols with different relative configurations. Esterification with acryloyl chloride and final ring-closing metathesis yields alpha,beta-unsaturated delta-lactones with multiply oxygenated side chains, a substructure found in a group of natural products with a broad range of biological activity. The flexibility of the approach is exemplified by the parallel synthesis of all eight diastereomers of cryptocarya diacetate on a solid support. The individual isomers are obtained in overall yields of 40-60 % over 10 steps and with 63-85 % diastereoselectivity for the major isomer.

Efficient Total Syntheses and Structural Verification of Both Diospongins A and B via a Common δ-Lactone Intermediate

The total syntheses of the two aryl C-glycoside natural products diospongins A and B are described. The key reactions involved stereoselective reductions of the appropriate oxocarbenium cations that were derived from a common delta-lactone intermediate.

An expedient total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide

[reaction: see text] A highly convergent and efficient total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide is reported.