An extremely facile aza-Bergman rearrangement of sterically unencumbered acyclic 3-aza-3-ene-1,5-diynes

[4+1] Heteroannulation to Form 1-Pyrrolines through Alder-Ene Reaction of in situ Generated Ynimines: Development and Application in Total Synthesis of Borrecapine

We report in this paper a novel Cu-catalyzed synthesis of polysubstituted 1-pyrrolines. The reaction of β,γ-unsaturated oxime esters 4 with terminal alkynes 5 in the presence of a catalytic amount of Cu(OAc)2 and 2,2'-biquinoline affords the corresponding 1,6-enynimines, which undergo a highly stereoselective Alder-ene reaction to afford 1-pyrrolines with concomitant generation of a quaternary carbon and a 2-azadiene motif. It represents an unusual [4+1] heteroannulation reaction wherein terminal alkynes act as a one carbon donor and are 1,1-difunctionalized. Mechanistic studies suggest that the allylic hydrogen trans to the oxime ester main chain is selectively transferred to the electron-rich alkyne during the pericyclic reaction. The resulting 1-pyrrolines undergo facile acid-catalyzed regioselective Wagner-Meerwein rearrangement to provide 2H-pyrroles in excellent yields. The 2H-pyrroles are also accessible from 4 and 5 in a one-pot manner without isolation of the 1-pyrrolines. Leveraging this heteroannulation reaction as a key step, the first total synthesis of borrecapine, a 2,3,3,5-tetrasubstituted pyrrolidine, is accomplished.

Spirocyclic Products via Carbene Intermediates from Thermolysis of 1,2-Dialkynylpyrrole and 1,2-Diethynylimidazole

The thermal rearrangements of 1,2-dialkynylimidazoles have been shown to lead to trapping products of cyclopenta[b]pyrazine carbene intermediates. Here we show that a similar rearrangement also occurs in the case of 1,2-diethynyl-1H-pyrrole, and that trapping the intermediate cyclopenta[b]pyridine carbene with solvent THF affords an ylide that undergoes a Stevens rearrangement to a spirocyclic product. An analogous rearrangement and trapping is observed for thermolysis of 1,2-dialkynylimidazoles in THF or 1,4-dioxane.

Photochemistry of 3,6-Didehydropyridazine Biradical─An Untraceable Para Benzyne Analogue

We report matrix isolation infrared spectroscopic studies to characterize 3,6-didehydropyridazine 6, a heterocyclic analogue of para benzyne, combined with computations. In this regard, we have utilized 3,6-diiodopyridazine 11 as a photolytic precursor. The experiments toward the generation of the biradical are carried out in argon and nitrogen matrices at 4 K. Instead of the elusive biradical, we have observed a ring-opening product maleonitrile (Z)-7 upon irradiation at 254 nm. In contrast, prolonged irradiation at 254 nm leads only to Z-E isomerization, forming fumaronitrile (E)-7. The mechanistic aspects of ring-opening, product selectivity, and Z-E photoisomerization steps have been investigated in detail using high-level ab initio computations. These studies have found that 3,6-didehydropyridazine 6 is an untraceable intermediate, and the ring-opening step leading to maleonitrile is barrierless. In addition, we have proposed the involvement of the S₁ (π-π*) state via conical intersection in the Z-E photoisomerization of maleonitrile.

Transition-Metal-Free Synthesis of Ynones via Decarboxylative Alkynylation of α-Keto Acids under Mild Conditions

A transition-metal-free synthetic method of various ynones via decarboxylative alkynylation of α-keto acids is described. The reaction is carried out under mild conditions and exhibits remarkable tolerance of functional groups. The mechanism of a radical process is proposed in the reaction.

Cytotoxic 1,2-dialkynylimidazole-based aza-enediynes: aza-Bergman rearrangement rates do not predict cytotoxicity

A new class of potential antitumor agents inspired by the enediyne antitumor antibiotics has been synthesized: the 1,2-dialkynylimidazoles. The aza-Bergman rearrangement of these 1,2-dialkynylimidazoles has been investigated theoretically at the B3LYP/6-31G(d,p) level and experimentally by measuring the kinetics of rearrangement in 1,4-cyclohexadiene. There is a good correlation between the theoretical and experimental results; subtle substituent effects on the initial aza-Bergman cyclization barrier predicted by theory are confirmed by experiment. Yet, despite the ability of these 1,2-dialkynylimidazoles to undergo Bergman rearrangement to diradical/carbene intermediates under relatively mild conditions, there is no correlation between the rate of Bergman cyclization and cytotoxicity to A459 cells. In addition, cytotoxic 1,2-dialkynylimidazoles do not cause nicking of supercoiled plasmid DNA or cleavage of bovine serum albumin. An alternative mechanism for cytotoxicity involving the unexpected selective thiol addition to the N-ethynyl group of certain 1,2-dialkynylimidazoles is proposed.

Highly diastereoselective synthesis of α-difluoromethyl amines from N-tert-butylsulfinyl ketimines and difluoromethyl phenyl sulfone

The first highly efficient and stereoselective difluoromethylation of structurally diverse N-tert-butylsulfinyl ketimines has been achieved with an in situ generated PhSO(2)CF(2)(-) anion, which provides a powerful synthetic method for the preparation of a variety of structurally diverse homochiral α-difluoromethyl tertiary carbinamines, including α-difluoromethyl allylic amines and α-difluoromethyl propargylamines. The stereocontrol mode of the present diastereoselective difluoromethylation of ketimines was found to be different from that of other known fluoroalkylations of N-tert-butylsulfinyl aldimines, which suggests that a cyclic six-membered transition state may be involved in the reaction.

Fast oxy-cope rearrangements of bis-alkynes: competition with central C-C bond fragmentation and incorporation in tunable cascades diverging from a common bis-allenic intermediate

Fast anionic oxy-Cope rearrangements of 1,5-hexadiyn-3,4-olates can be incorporated into cascade transformations which rapidly assemble densely functionalized cyclobutenes or cyclopentenones via a common bis-allenic intermediate. The competition between fragmentation, 4π-electrocyclic closure, and aldol condensation can be efficiently controlled by the nature of the acetylenic substituents. The rearrangement of bis-alkynes with two hydroxyl substituents opens a conceptually interesting entry in the chemistry of ε-dicarbonyl compounds and suggests a new approach to analogues of rocaglamide/aglafolin.

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.

Pyridyne radical cations produced by photodissociation of Mg˙+(multifluoro-pyridine) complexes: A combined experimental and theoretical study

Gas phase complexes Mg*+ (2,6-difluoropyridine) (1) and Mg*+ (pentafluoropyridine) (2) have been subjected to photodissociation in the spectral range of approximately 230-440 nm. Except for the evaporative photofragment Mg*+ , the primary photoproduct for is C(5)H(3)N*(+), which is associated with the rupture of two C-F bonds by the photoexcited Mg*+ , forming very stable MgF(2). In contrast, the direct loss of MgF(+) is more favorable for due to fluorine substitution. Given enough energy, C(5)H(3)N*(+) can undergo decomposition to form C(4)H(2)*(+) and HCN. These results are very different from those for Mg*+ (2-fluoropyridine), highlighting the significance of the additional F at C6 of and . Density functional theory (DFT) calculations have been employed to examine the geometries and energetics of the complexes as well as relevant reaction mechanisms. All of the complexes feature the direct attachment of Mg*+ to the N atom. The key intermediate is found to be FMg(+) (C(5)H(x)F(4-x)N) (x = 3 or 0), which can lead to the formation of MgF(+) directly or MgF(2) through activation of another C-F bond adjacent to N, producing the pyridyne radical cations. However, hydrogen-transfer prior to the rupture of the second C-F bond followed by ring-opening of C(5)H(3)N*(+) may result in the formation of chain forms of C(5)H(3)N*(+). The influence of the fluorine substitution on the competition of the two routes have been demonstrated.

3,5-PyridyneA Heterocyclic meta-Benzyne Derivative

3,5-Pyridyne (3) has been generated by flash vacuum pyrolysis of 3,5-diiodopyridine (20) and 3,5-dinitropyridine (21) and characterized by IR spectroscopy in cryogenic argon matrices. The aryne can clearly be distinguished from other side products by its photolability at 254 nm, inducing a rapid ring-opening presumably to (Z)-1-aza-hex-3-ene-1,5-diyne. As byproducts of the pyrolysis, HCN and butadiyne were identified, together with traces of acetylene, cyanoacetylene, (E)-1-aza-hex-3-ene-1,5-diyne, and the 3-iodo-5-pyridyl radical (from 20). Several pathways for rearrangements and fragmentations of 3 and of the parent meta-benzyne (1) have been explored computationally by density functional theory and ab initio quantum chemical methods. The lowest energy decomposition pathway of biradicals 1 and 3 is a ring-opening process accompanied by hydrogen migration, leading to (Z)-hex-3-ene-1,5-diyne [(Z)-10] and (Z)-3-aza-hex-3-ene-1,5-diyne [(Z)-24], respectively. Both reactions require activation energies of 45-50 kcal mol(-1). Mechanisms leading from (Z)-24 or directly from 3 to the experimentally observed byproducts are discussed. Upon replacement of the C(5)H moiety by N in meta-benzyne, high-level calculations predict a modest shortening of the interradical distance by 5-7 pm and a reduction of the singlet-triplet energy splitting by 3 kcal mol(-1), in good agreement with isodesmic equations, according to which the singlet ground state of 3 is destabilized relative to 1 by 3-4 kcal mol(-1). In contrast to 3,5-borabenzyne (2), which is found to be doubly aromatic, nucleus-independent chemical shifts of 3 are almost identical to that of pyridine, indicating the absence of paramagnetic ring current effects that may be associated with "in-plane antiaromaticity". As compared with 1, the overall perturbation caused by the nitrogen atom in 3 is weak, and four electron, three center interaction is of minor importance in this molecule.