RESUMO
Acylsilanes are able to react as nucleophilic carbene precursors, electrophiles, and directing groups in C-H functionalization. To date, some of the products reportedly formed during transition-metal-catalyzed and photochemical reactions involving acylsilanes have been incorrectly assigned. To provide clarity, we herein address these structural misassignments and detail the revised structures. New insights into the reactivity of acylsilanes were also afforded via the discovery that light-induced siloxy carbenes participate in intramolecular 1,2-carbonyl addition to proximal esters.
RESUMO
Investigations into C-H amidation reactions catalysed by cationic half-sandwich d6 metal complexes revealed that the indenyl-derived catalyst [Ind*RhCl2 ]2 significantly accelerated the directed ortho C-H amidation of benzoyl silanes using 1,4,2-dioxazol-5-ones. Ring slippage involving a haptotropic η5 to η3 rearrangement of the indenyl complex proposedly enables ligand substitution at the metal centre to proceed via associative, rather than dissociative pathways, leading to significant rate and yield enhancements. Intriguingly, this phenomenon appears specific for C-H amidation reactions involving weakly coordinating carbonyl-based directing groups with no acceleration observed for the corresponding reactions involving strongly coordinating nitrogen-based directing groups.
RESUMO
Despite their synthetic utility, practical methods to prepare diversely functionalized aromatic acyl silanes (benzoyl silanes) remain scarce. We herein report that cobalt complexes can successfully engage acyl silanes as weakly coordinating directing groups to catalyse the ortho C-H functionalisation of benzoyl silanes. Under Cp*Co(III) catalysis, installation of allyl or amido functionality at the 2-position of benzoyl silanes was achieved, while reaction with internal alkynes led to a desilylative annulation to afford indenone scaffolds. A Co(II)/dppp catalytic system was also investigated to achieve the acyl silane directed hydroarylative cyclisation of 1,6-enynes to access unique benzoyl silane derivatives.
Assuntos
Cobalto , Silanos , Catálise , Alcinos , CiclizaçãoRESUMO
The total synthesis of viridiofungins A (1) and B (2) via ß-lactone 3 in 13 steps is reported. Key steps included an HF-mediated rearrangement of cyclobutene diester 9 to form a bicyclic lactone 6, an olefin cross metathesis between disubstituted alkene 3 and alkene 4 in which isomerization was suppressed, and a novel ß-lactone ring opening to form the amide. Deprotection then gave either viridiofungin A (1) or B (2) in high yield.
RESUMO
The nargenicin family of antibiotics are macrolides containing a rare ether-bridged cis-decalin motif. Several of these compounds are highly active against multi-drug resistant organisms. Despite the identification of the first members of this family almost 40 years ago, the genetic basis for the production of these molecules and the enzyme responsible for formation of the oxa bridge, remain unknown. Here, the 85 kb nargenicin biosynthetic gene cluster was identified from a human pathogenic Nocardia arthritidis isolate and this locus is solely responsible for nargenicin production. Further investigation of this locus revealed a putative iron-α-ketoglutarate-dependent dioxygenase, which was found to be responsible for the formation of the ether bridge from the newly identified deoxygenated precursor, 8,13-deoxynargenicin. Uncovering the nargenicin biosynthetic locus provides a molecular basis for the rational bioengineering of these interesting antibiotic macrolides.
Assuntos
Antibacterianos/biossíntese , Éteres/química , Macrolídeos/metabolismo , Antibacterianos/química , Antibacterianos/farmacologia , Proteínas de Bactérias/metabolismo , Dioxigenases/metabolismo , Escherichia coli/efeitos dos fármacos , Lactonas/química , Lactonas/metabolismo , Lactonas/farmacologia , Macrolídeos/química , Macrolídeos/farmacologia , Testes de Sensibilidade Microbiana , Família Multigênica , Nocardia/genética , Staphylococcus aureus/efeitos dos fármacosRESUMO
An efficient and step-economic new approach to alkyl citrate natural products from a cyclobutene diester is presented. The key sequence involves a formal [2 + 2]-cycloaddition of a silylketene acetal with dimethylacetylene dicarboxylate to provide the cyclobutene diester 14 with 4.5:1 stereoselectivity. Exposure of diester 14 in acidic methanol effected a hydrolysis, intramolecular oxy-Michael reaction, and cyclobutanone methanolysis cascade to give the triester 15. Iodination and elimination then afforded a key alkyl citrate alkene intermediate, which was converted into the natural products (-)-CJ-13,982 (1), (-)-CJ-13,981 (2), and (-)-L-731,120 (3) via a cross-metathesis and subsequent reduction.