Effect of Remote Trigonal Carbons on the Kinetics of Bergman Cyclization: Synthesis and Chemical Reactivity of Pyridazinedione-Based Enediynes

Synthesis and biological properties of maleimide-based macrocyclic lactone enediynes

Natural enediyne antibiotics are powerful DNA-cleavage agents due to the presence of the highly reactive hex-3-ene-1,5-diyne units. However, the complicated chemical structure and thermal instability make their synthesis, derivatization, and storage challenging. Heterocycle-fused enediynes, which exhibit strong antineoplastic activity, are promising analogues of natural enediynes for medicinal applications. To this end, a series of maleimide-based enediynes with macrocyclic lactone moieties were synthesized through the Sonagashira coupling reaction. Differential scanning calorimetry and electron paramagnetic resonance results showed that these macrocyclic enediynes exhibited a rather low onset temperature and the ability to generate radicals at physiological temperature. In addition, the structure-activity relationship of enediynes was analyzed by changing the ring size and the substituents on the propargyl group. Cellular experiments indicated that the diradicals produced by these enediynes efficiently cleaved DNA and disrupted the cell cycle distribution, and consequently induced tumor cell death via an apoptosis pathway at low half inhibitory concentrations. Computational studies suggested that the maleimide moiety promoted the propargyl-allenyl rearrangement of the cyclic enediyne, enabling the generation of diradical species through the Myers-Saito cyclization, and then abstracted hydrogen atoms from the H-donors.

Bergman cyclization of main-chain enediyne polymers for enhanced DNA cleavage

The design of novel polymers bearing multiple ene-diynes in the main chain are reported, allowing to tune activity of DNA cleavage via the Bergman cyclization.

Strain-Induced Nucleophilic Ring Opening of Donor-Acceptor Cyclopropenes for Synthesis of Monosubstituted Succinic Acid Derivatives

1,2,3-Trisubstituted donor-acceptor cyclopropenes (DACPs) generated in situ from enoldiazo compounds react with nucleophiles to form α-substituted succinic acid derivatives in high yields. Initial dirhodium(II) carboxylate catalysis rapidly converts enoldiazo-acetates or -acetamides to DACPs that undergo catalyst-free Favorskii ring opening with amines, and also with anilines, alcohols, and thiols, when facilitated by catalytic amounts of 4-dimethylaminopyridine (DMAP). This methodology provides easy access to mixed esters and amides of monosubstituted succinic acids, including derivatives of naturally occurring compounds. It also affords dihydrazide, dihydroxamic acid, and diamide derivatives, as well as α-substituted tetrahydropyridazine-3,6-diones in high yields. Attempts to generate optically enriched DACPs were not successful because their populations having the R and S configurations formed with a chiral dirhodium catalyst are quite similar, and the loss of enantiocontrol likely originates from the DACP ring forming step which is reversible with its intermediate metal carbene.

Efficient and accurate characterization of the Bergman cyclization for several enediynes including an expanded substructure of esperamicin A1

Incorporation of enediynes into anticancer drugs remains an intriguing yet elusive strategy for the design of therapeutically active agents. Density functional theory was used to locate reactants, products, and transition states along the Bergman cyclization pathways connecting enediynes to reactive para-biradicals. Sum method correction to low-level calculations confirmed B3LYP/6-31G(d,p) as the method of choice in investigating enediynes. Herein described as MI:Sum, calculated reaction enthalpies differed from experiment by an average of 2.1 kcal x mol(-1) (mean unsigned error). A combination of strain energy released across the reaction coordinate and the critical intramolecular distance between reacting diynes explains reactivity differences. Where experimental and calculated barrier heights are in disagreement, higher level multireference treatment of the enediynes confirms lower level estimates. Previous work concerning the chemically reactive fragment of esperamcin, MTC, is expanded to our model system MTC2.

Photoisomerization as a trigger for Bergman cyclization: Synthesis and reactivity of azoenediynes

Cyclic enediynes 1a and 2a containing stable E-azo moiety (azoenediynes) have been synthesized. These compounds upon irradiation with long wavelength UV isomerize to the Z-compounds 1b and 2b, which can be thermally reisomerized to the Z compounds. Reactivity studies toward BC using DSC predictably indicate higher reactivity for the Z-isomers. Our studies may provide a novel way to modulate the reactivity of enediynes under thermal or photochemical conditions.

The synthesis and evaluation of 10- and 12-membered ring benzofused enediyne amino acids

The enediyne moiety is a versatile functional group found in natural anticancer and anti-infective agents, undergoing the Bergman cyclization reaction to afford a diradical which cleaves double-stranded DNA. We have incorporated the enediyne group into 10- (4-10) and 12-membered ring (11) cyclic amino acids and dipeptides, respectively, and explored their relative reactivity toward cyclization, varying N-substitution in the case of the 10-membered ring substrate, which gave the expected cyclization products in good yields when using either thermal conditions in the presence or absence of microwave irradiation. The N-tosyl substituted derivative (4) was shown to nick double-stranded supercoiled DNA. N-Arylsulfonyl substitution on the ring promoted the cyclization, when compared to N-mesyl or acyl substitution, possibly because of a pi-pi stacking effect as an endo-relationship of the aryl group with the enediyne was demonstrated in both the solid state and in solution. The 12-membered ring enediyne dipeptide (11) was inert to the Bergman cyclization under a variety of conditions. When this substrate was irradiated with ultraviolet light, regio- and stereospecific reduction was observed in which one of the alkynes was reduced to a Z-olefin (47).