Special Issue for OMCOS 20 in Heidelberg.

In this Editorial, Professors Hashmi and de Meijere talk about a brief history of OMCOS and the current and emerging roles that organometallic chemistry plays in organic synthesis. All of these lead to OMCOS 20 to be held on July 21-25, 2019 in Heidelberg, Germany.

Propellanes-From a Chemical Curiosity to "Explosive" Materials and Natural Products.

Propellanes are a unique class of compounds currently consisting of well over 10 000 representatives, all featuring two more or less inverted tetrahedral carbon atoms that are common to three bridging rings. The central single bond between the two bridgeheads is significantly weakened in the smaller entities, which leads to unusual reactivities of these structurally interesting propeller-like molecules. This Review highlights the synthesis of such propellanes and their occurrence in material sciences, natural products, and medicinal chemistry. The conversion of [1.1.1]propellane into bridgehead derivatives of bicyclo[1.1.1]pentane, including oligomers and polymers with bicyclo[1.1.1]penta-1,3-diyl repeat units, is also featured. A selection of natural products with larger propellane subunits are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is touched in brief to show the pioneering work of David Ginsburg, Günther Snatzke, Kenneth B. Wiberg, Günter Szeimies, and others.

Synthesis of benzannelated sultams by intramolecular Pd-catalyzed arylation of tertiary sulfonamides.

A new and efficient approach to five- and six-membered benzannelated sultams by intramolecular C-arylation of tertiary 1-(methoxycarbonyl)methanesulfonamides under palladium catalysis is described. In case of the α-toluenesulfonamide derivative, an unexpected formation of a 2,3-diarylindole was observed under the same conditions.

Short Route to the Total Synthesis of Natural Muricadienin and Investigation of Its Cytotoxic Properties.

An original synthesis of the acetogenin muricadienin, the bioprecursor of solamin, has been developed. The key step in the five-step 41% overall yield synthesis is the catalytic cross-cyclomagnesiation reaction of functionally substituted 1,2-dienes with EtMgBr in the presence of Cp2TiCl2 and magnesium metal. It has been demonstrated for the first time that muricadienin exhibits a moderate in vitro inhibitory activity against topoisomerases I and IIα, key cell cycle enzymes. Using flow cytometry, muricadienin was shown to have high cytotoxicity toward the HEK293 kidney cancer cells (IC50 0.39 µM).

A Minimal ß-Lactone Fragment for Selective ß5c or ß5i Proteasome Inhibitors.

Broad-spectrum proteasome inhibitors are applied as anticancer drugs, whereas selective blockage of the immunoproteasome represents a promising therapeutic rationale for autoimmune diseases. We here aimed at identifying minimal structural elements that confer ß5c or ß5i selectivity on proteasome inhibitors. Based on the natural product belactosin C, we synthesized two ß-lactones featuring a dimethoxybenzyl moiety and either a methylpropyl (pseudo-isoleucin) or an isopropyl (pseudo-valine) P1 side chain. Although the two compounds differ only by one methyl group, the isoleucine analogue is six times more potent for ß5i (IC50=14 nM) than the valine counterpart. Cell culture experiments demonstrate the cell-permeability of the compounds and X-ray crystallography data highlight them as minimal fragments that occupy primed and non-primed pockets of the active sites of the proteasome. Together, these results qualify ß-lactones as a promising lead-structure motif for potent nonpeptidic proteasome inhibitors with diverse pharmaceutical applications.

(2R,1'S,2'R)- and (2S,1'S,2'R)-3-[2-Mono(di,tri)fluoromethylcyclopropyl]alanines and their incorporation into hormaomycin analogues.

Efficient and scalable syntheses of enantiomerically pure (2R,1'S,2'R)- and (2S,1'S,2'R)-3-[2-mono(di,tri)fluoromethylcyclopropyl]alanines 9a-c, as well as allo-D-threonine (4) and (2S,3R)-ß-methylphenylalanine (3), using the Belokon' approach with (S)- and (R)-2-[(N-benzylprolyl)amino]benzophenone [(S)- and (R)-10] as reusable chiral auxiliaries have been developed. Three new fluoromethyl analogues of the naturally occurring octadepsipeptide hormaomycin (1) with (fluoromethylcyclopropyl)alanine moieties have been synthesized and subjected to preliminary tests of their antibiotic activity.

Synthesis and biological activity of simplified belactosin C analogues.

Successful biochemical studies of the natural products belactosin A and C and their acylated congeners have shown a ß-lactonecarboxamide moiety to be a possible core structure of powerful proteasome inhibitors. As a part of further investigations, variously decorated simplified ß-lactonecarboxamides have been synthesized in order to understand structure-biological activity relations in detail, to find ways of improving their biological activity and stability and to reduce the complexity of their preparation. Biological tests showed that the best compounds possess a high potential against phytopathogenic fungi in the greenhouse.

Chemistry of polyhalogenated nitrobutadienes, 10: Synthesis of highly functionalized heterocycles with a rigid 6-amino-3-azabicyclo[3.1.0]hexane moiety.

The nitropolychlorobutadienes 3, 4 are valuable building blocks for various amination and successive heterocyclization products. Nucleophilic substitution reactions of the partially protected, bioactive amines 1, 2 with either vinyl, imidoyl or carbonyl chlorides result in the formation of the enamines 11, 12, 13, 16, 25, the amidine 6, and the amides 20, 21, respectively. In the following, cyclization to the highly functionalized pyrazoles 27, 28, pyrimidine 26 and pyridopyrimidine 24 succeeded. Deprotection of 21, 12 and 28 proved to be only partially feasible.

Discovery of a Cryptic Nitro Intermediate in the Biosynthesis of the 3-(trans-2'-Aminocyclopropyl)alanine Moiety of Belactosin A.

Belactosin A, a ß-lactone proteasome inhibitor, contains a unique 3-(trans-2'-aminocyclopropyl)alanine moiety. We recently identified the biosynthetic gene cluster of the belactosin series from Streptomyces sp. UCK14. To shed light on the formation of the aminocyclopropylalanine, we established a heterologous pathway expression, constructed a set of gene deletion mutants, and performed feeding studies for a chemical complementation that include the incorporation of stable isotope-labeled precursors. We thereby show that, in the biosynthesis of this building block, a cryptic nitrocyclopropylalanine intermediate is generated from l-lysine. The subsequent reduction of the N-oxygenated precursor to the corresponding amine is mediated by the molybdopterin-dependent enzyme BelN.

Synthesis and biological activity of optimized belactosin C congeners.

Successful biochemical studies of the natural products belactosin A and C as well as their more stable acylated derivatives have proved them to be powerful proteasome inhibitors and thereby potential candidates as pharmacologically relevant active compounds. In order to understand their structure-biological activity relations in detail and to find ways of improving their biological activity, four new modified belactosin congeners have been synthesized and tested. One of them (compound 6) turned out to be a more potent inhibitor against HeLa cells than the known proteasome inhibitor MG132.

Rh-Catalyzed rearrangement of vinylcyclopropane to 1,3-diene units attached to N-heterocycles.

Dienes embedded in quinolizidine and indolizidine structures can be prepared in four steps from cyclic nitrones and bicyclopropylidene. The key intermediates α-spirocyclopropanated N-heterocyclic ketones, generated via a domino 1,3-dipolar cycloaddition/thermal rearrangement sequence, were converted by Wittig methylenation to the corresponding vinylcyclopropanes (VCPs), which underwent rearrangement to 1,3-dienes in the presence of the Wilkinson Rh(I) complex under microwave heating. The previously unexplored Rh(I)-catalyzed opening of the VCP moiety embedded in an azapolycyclic system occurs at high temperature (110-130 °C) to afford the corresponding 1,3-dienes in moderate yield (34-53%).

Scalable synthesis of (1-cyclopropyl)cyclopropylamine hydrochloride.

1-Cyclopropylcyclopropanecarboxylic acid (2), which is accessible on a large scale (900 mmol) from 1-bromo-1-cyclopropylcyclopropane (1) in 64% yield (89% on a 12.4 mmol scale), has been subjected to a Curtius degradation employing the Weinstock protocol to furnish the N-Boc-protected (1-cyclopropyl)cyclopropylamine 3 (76%). Deprotection of 3 with hydrogen chloride in diethyl ether gave the (1-cyclopropyl)cyclopropylamine hydrochloride (4·HCl) in 87% yield.

Cyclopropylamines from N,N-dialkylcarboxamides and Grignard reagents in the presence of titanium tetraisopropoxide or methyltitanium triisopropoxide.

Thirty-three different N,N-dialkyl- and N-alkyl-N-phosphorylalkyl-substituted carboxamides 9-17 were treated with unsubstituted as well as with 2-alkyl-, 2,2-dialkyl-, and 3-alkenyl-substituted ethylmagnesium bromides 6 in the presence of stoichiometric amounts of titanium tetraisopropoxide or methyltitanium triisopropoxide to furnish substituted cyclopropylamines 20-25 in 20-98% yield, depending on the substituents with no (1:1) to excellent (>25:1) diastereoselectivities. Generally higher yields (up to 98%) of the cyclopropylamines 20-28 without loss of the diastereoselectivity were obtained with methyltitanium triisopropoxide as the titanium mediator. Under these conditions, even dioxolane-protected ketones and halogen-substituted and chiral as well as achiral alkyloxyalkyl-substituted carboxamides could be converted to the correspondingly substituted cyclopropylamines with unsubstituted as well as phenyl- and a variety of alkyl-substituted ethylmagnesium bromides in addition to numerous heteroatom-containing (e.g., halogen-, trityloxy-, tetrahydropyranyloxy-substituted) Grignard reagents (62 examples altogether). The transformation of N,N-diformylalkylamines 54 with ethylmagnesium bromide in the presence of methyltitanium triisopropoxide to N,N-dicyclopropyl-N-alkylamines 55 can be brought about in up to 82% yield (6 examples). An asymmetric variant of the titanium-mediated cyclopropanation of N,N-dialkylcarboxamides has been developed by applying chiral titanium mediators generated from stoichiometric amounts of titanium tetraisopropoxide and chiral diamino or diol ligands, respectively. The most efficient chiral mediators turned out to be titanium bistaddolates that provided the corresponding cyclopropylamines with enantiomeric excesses (ee) of up to 84%. Evaluation of several silyl-based additives revealed that the reaction can also efficiently be carried out with substoichiometric amounts (down to 25 mol%) of the titanium reagent, as long as 2-aryl- or 2-ethenyl-substituted ethylmagnesium halides are used and a concomitant slight decrease in yields is accepted. The newly developed methodology was successfully applied for the preparation of analogues with cyclopropylamine moieties of known drugs and natural products such as the nicotine metabolite (S)-Cotinine as well as the insecticides Dinotefuran and Imidacloprid.

Isocyanides in the synthesis of nitrogen heterocycles.

Isocyanides have long proved themselves to be irreplaceable building blocks in modern organic chemistry. The unique features of the isocyano group make isocyanides particularly useful for the synthesis of a number of important classes of nitrogen heterocycles, such as pyrroles, indoles, and quinolines. Several cocyclizations of isocyanides via zwitterions and radical intermediates as well as transition-metal-catalyzed syntheses of different types of heterocycles have recently been developed. Methods starting from isocyanides often have distinct advantages over alternative approaches to the same heterocycles because of their enhanced convergence, the great simplicity of most of the operations with them, and the great variety of isocyanides readily available for use. Isocyanides have also been used in some enantioselective syntheses of chiral heterocyclic compounds, including natural products as well as precursors thereof.

Oligosubstituted pyrroles directly from substituted methyl isocyanides and acetylenes.

The formal cycloaddition of alpha-metallated methyl isocyanides 1 onto the triple bond of electron-deficient acetylenes 2 represents a direct and convenient approach to oligosubstituted pyrroles 3. The scope and limitations of this reaction (24 examples, 25-97% yield) are reported along with an optimization of the reaction conditions and a rationalization of the mechanism. In addition, a related newly developed Cu(I)-mediated synthesis of 2,3-disubstituted pyrroles by the reaction of copper acetylides derived from unactivated terminal alkynes with substituted methyl isocyanides is described (11 examples, 5-88% yield).

Reactions of ortho-lithiophenyl (-hetaryl) isocyanides with carbonyl compounds: rearrangements of 2-metalated 4H-3,1-benzoxazines.

ortho-Lithiophenyl (-hetaryl) isocyanides react with aldehydes and ketones providing isocyanoalcohols 8 (36-89%, nine examples), 4H-3,1-benzoxazines 9 (45-78%, six examples) or, after two types of rearrangements, isobenzofuran-1(3H)-imines (iminophthalanes) 18 (52-75%, four examples), or indolin-2-ones 19 (42-79%, two examples), depending on the reaction conditions and substitution patterns. Isocyanoalcohols 8, in turn, were converted to 9 or 18 under Cu(I) catalysis (66-86%, eight examples). 4H-3,1-Benzoxazin-4-ones 39-Nu and isatoic anhydride 40 were obtained by the reaction of 2 with carbon dioxide followed by trapping of the lithiated intermediate with iodine and subsequent reactions with nucleophiles (45-60%, three examples).

Synthesis of alpha-cyclopropyl-beta-homoprolines.

1-(2-Pyrrolidinyl)cyclopropanecarboxylic acids (alpha-cyclopropyl-beta-homoprolines) were prepared by 1,3-dipolar cycloadditions of cyclic nitrones onto bicyclopropylidene followed by trifluoroacetic acid induced thermal fragmentative rearrangement. With the use of enantiopure pyrroline N-oxides derived from easily available chiral pool molecules, beta-homoprolines were formed with high stereocontrol. The incorporation of one of these new cyclic beta-amino acids into a simple tripeptide was also evaluated. In particular, the sterically hindered nitrogen atom of the highly substituted pyrrolidine 30 was smoothly acylated through the intermediate formation of a mixed anhydride.

Facile synthesis of structurally diverse 5-oxopiperazine-2-carboxylates as dipeptide mimics and templates.

A sequence of Michael addition of a primary amine onto methyl 2-chloro-2-cyclopropylidene-acetate (1), acylation of the adduct with alpha-bromo acid chlorides under modified Schotten-Baumann conditions and ring-closing twofold nucleophilic substitution on the thus formed bishalides 3a-e with aliphatic or aromatic amines according to a very simple protocol with final acid/base extraction or filtration over silica gel for purification leads to the 3-spirocyclopropanated 5-oxopiperazine-2-carboxylates 2 or in two cases, after intermolecular transesterification of 2, to bicyclic oxopiperazines 6, with a remarkable variability of the substituents R1-R3 in 39-99% yields (20 examples). Starting with alpha-bromophenylacetic acid chloride, the trans-configured 6-phenyl-5-oxopiperazine-2-carboxylates are formed preferentially.

High yielding selective access to spirocyclopropanated 5-oxopiperazine-2-carboxylates and 1,4-diazepane-2,5-diones from methyl 2-chloro-2-cyclopropylideneacetate.

The 2-spirocyclopropanated methyl 5-oxopiperazine-2-carboxylate and the 3-spirocyclopropanated 6-chloro-1,4-diazepane-2,5-dione could both be prepared at choice in 93 and 88% yield, respectively, from methyl 2-chloro-2-cyclopropylideneacetate () in a sequence of Michael addition of 3-benzyloxypropylamine, peptide coupling with N-Boc-glycine, Boc-group removal and cyclization. Transformation of the benzyloxypropyl side chain, peptide coupling with N-Boc-(S)-asparagine, deprotection and repeated cyclization led to the octahydro[2H]pyrazino[1,2-a]pyrazinetrione scaffold containing a rigidified mimic of a tripeptide with a DGR motif. The overall yield of after deprotection of (a total of 13 steps in 8 distinct operations) was 30%.