Alexander F. Khlebnikov

Bicyclopropylidene — A Unique Tetrasubstituted Alkene and a Versatile C6-Building Block

Strain in an organic molecule often correlates with increased reactivity, at least for certain types of reactions. Thus, the elucidation of the chemistry of highly strained alkenes like the unusual tetrasubstituted alkene, bicyclopropylidene (1), which is a particularly strained derivative of methylenecyclopropane (2), has proved to be fruitful both with respect to synthetic applications as a C6 building block, as well as understanding certain reaction principles. The different, steadily improved methods which have been developed for the preparation of this unusual alkene in the last thirty years as well as rather recent methods for the synthesis of functionally substituted, and spirocyclopropanated derivatives as well as bis(bicyclopropylidenyls) are presented. The special aspects of molecular geometry in bicyclopropylidenes as well as strain and its influence on chemical properties are discussed.The rich chemistry of bicyclopropylidene beginning with its well-known thermal rearrangement and dimerization, its dihalocarbene additions all the way to its recently developed organometallic chemistry, especially its reactions under the catalysis of palladium and other transition metals, is covered. Finally, some synthetically useful chemical transformations of bicyclopropylidene derivatives, e.g., synthetic approaches to the cyclopropanated analogs of natural products, are presented.

The First Enantiomerically Pure Triangulane (M)‐Trispiro[2.0.0.2.1.1]nonane Is a σ‐[4]Helicene

A remarkably high specific rotation, even at 589 nm, is shown by (M)-1, the first enantiomerically pure unbranched [4]triangulane, although it has no chromophore that would lead to any significant absorption above 200 nm. This outstanding rotatory power is in line with a helical arrangement of its σ bonds, as confirmed by high-level computations. Thus, it is appropriate to call (M)-1 a “σ-[4]helicene”, the first σ-bond analogue of the aromatic [n]helicenes.

The first example of intramolecular cycloaddition of carbene-derived azomethine ylides in a domino reaction of difluorocarbene with Schiff bases

Abstract Domino reactions of difluorocarbene with Schiff bases containing a tethered olefinic or acetylenic dipolarophile moiety involve intramolecular 1,3-dipolar cycloaddition of iminiodifluoromethanides and yield chromeno[4,3- b ]pyrrole derivatives.

Recent advances in isoxazole chemistry

The preparation methods and reactions of isoxazoles are described and systematized on the basis of analysis of the literature published from 2005 to present. In the discussion of synthesis, major attention is focused on the most efficient approaches: condensation of hydroxylamine with 1,3-dielectrophiles and reactions of nitrile oxides with alkenes and alkynes. Five-membered ring opening reactions leading to acyclic functionalized or other heterocyclic compounds are considered. The transformations of isoxazole derivatives that occur without ring cleavage to form fused heterocyclic systems, as well as reactions that lead to the introduction of C-substituents into isoxazoles, are considered. Data on the biological activity of some isoxazole derivatives are reported. The bibliography includes 439 references.

Electrophilic Cyclization of Aryldiacetylenes in the Synthesis of Functionalized Enediynes Fused to a Heterocyclic Core

An efficient strategy for the synthesis of asymmetrically substituted enediynes fused to benzothiophene, benzofuran, and indole was developed. The proposed approach is based on the electrophilic cyclization of diacetylenes and Sonogashira coupling. Thus, iodocyclization of readily available ortho-functionalized (buta-1,3-diynyl)arenes was used as a direct way for the synthesis of 2-ethynyl-3-iodoheteroindenes. These substrates and their modified derivatives were easily converted by Sonogashira coupling with acetylenes to a variety of asymmetrically substituted acyclic enediynes fused to heterocycles. The tolerance of the developed methodology to a variety of functional groups is a great advantage in the synthesis of macrocyclic enediyne systems fused to a heterocyclic core. Synthesis of indole-fused 12-membered macrocyclic dienediyne was achieved using ring-closing metathesis as a key step.

Dibenzoxazepinium Ylides: Facile Access and 1,3-Dipolar Cycloaddition Reactions

An effective approach to azirino-fused heterocycles is disclosed. The approach, involving formation of heterocycle/C-(arylchloromethyl)-subsituted CN double bond via domino isomerization of a gem-dichloroaziridine-intramolecular Friedel-Crafts acylation of the O-tethered benzene ring and subsequent intramolecular cyclization induced by hydride, was realized for 1-aryl-1,11b-dihydroazirino[1,2-d]dibenz[b,f][1,4]oxazepines. The latter are excellent precursors of azomethine ylides, especially in solvent-free conditions, which can undergo a completely stereoselective 1,3-dipolar cycloaddition to CC dipolarophiles giving derivatives of dibenzo[b,f]pyrrolo[1,2-d][1,4]oxazepine.

Das erste enantiomerenreine Triangulan: (M)-Trispiro[2.0.0.2.1.1]nonan ist ein σ-[4]Helicen

Eine auserordentlich hohe spezifische optische Drehung selbst bei 589 nm kennzeichnet (M)-1, das erste enantiomerenreine, unverzweigte [4]Triangulan, obwohl es keinen Chromophor enthalt, der zu einer nennenswerten Absorption oberhalb von 200 nm fuhren wurde. Diese hohe spezifische Drehung hat mit der helicalen Anordnung der σ-Bindungen zu tun, was durch Rechnungen auf hohem Niveau bestatigt wird. Damit erscheint es gerechtfertigt, das [4]Triangulan in Analogie zu den aromatischen π-[n]Helicenen als σ-[4]Helicen zu bezeichnen.

Pseudopericyclic 1,5- versus Pericyclic 1,4- and 1,6-Electrocyclization in Electron-Poor 4-Aryl-2-azabuta-1,3-dienes: Indole Synthesis from 2H-Azirines and Diazo Compounds

Transformations of 2-azabuta-1,3-dienes, formed in Rh2(OAc)4-catalyzed reactions of diazo carbonyl compounds with 2H-azirines, dramatically depend on the nature of substituents. 4,4-Diphenyl-2-azabuta-1,3-dienes with two electron-acceptor substituents at C(1) undergo thermal 1,5-cyclization to give indoles in good yields. The increase in electron-withdrawing ability of C(1)-substituents facilitates the reaction that proceeds via pseudopericyclic 1,5-electrocyclization of 2-azabutadiene into 7aH-indolium ylide followed by prototropic shift. 3,4-Diphenyl-2-azabuta-1,3-dienes, resulting from reaction of 2,3-diphenyl-2H-azirine and diazo compounds, do not produce indoles via 1,5-cyclization due to the trans-configuration of the 4-Ph-group and the nitrogen, but undergo 1,4-cyclization to 2,3-dihydroazetes. 1,6-Cyclization into 2H-1,4-oxazines with participation of the oxygen of ester or amide group at C(1) of corresponding 2-azabuta-1,3-dienes does not take place due to kinetic and thermodynamic reasons. Instead of this, 1,6-electrocyclization with participation of phenyl substituent at C(4) of the 2-azabuta-1,3-dienes, providing isoquinoline derivatives, can occur at elevated temperatures. The DFT-calculations (mPWB1K/6-31+G(d,p)) confirm the dependence of 2-azabuta-1,3-diene transformation type on the nature of substituents.

Raman Optical Activity of a Purelyσ-Bonded Helical Chromophore: (−)-(M)-σ-[4]Helicene

The recent synthesis of enantiomerically pure (−)-(M)-σ-[4]helicene has provided an archetype helical model system for vibrational optical activity, comparable to what π-helicenes represent in the field of electronic optical activity. We present the first measurements and the first calculations of the Raman optical activity (ROA) of this interesting molecule. Observed and calculated ROA is large throughout the vibrational spectrum, in agreement with expectations, and spectacular effects, with Δ values close to 0.5%, occur in the 900-cm−1 region. Agreement between the experimental spectrum and the theoretical one, calculated with density-functional theory for the vibrational part and Hartree-Fock linear response theory for the molecular electronic tensors, is excellent, clearly the best that has been achieved to date in the field. This allows us to place confidence in the results of an analysis of Raman and ROA scattering generation in the molecule, obtained by a newly developed graphical procedure for extracting this kind of information from ab initio calculations. One finds that relative contributions made by carbon and hydrogen atoms can be comparable in size, but can also vary considerably, even between closely lying vibrations, and that, for most vibrations, the generation of ROA difference intensity is distributed rather differently than that of Raman intensity over the shape of the molecule. The sign of the ROA is, for the set of vibrations in the 900-cm−1 region, which we analyze in detail, determined by coupling terms between the two halves of the molecule, while Raman intensity is primarily generated within the two fragments, with coupling terms between them only adding to or substracting from it.

Azirinium ylides from alkoxycarbonylcarbenoids and 2H-azirines: Generation and transformations

Dirhodium tetraacetate-catalyzed decomposition of diazo esters in the presence of 3-aryl-2H-azirines having no substituent in the 2-position gives rise to azirinium ylides which then undergo isomerization into 2-azabuta-1,3-diene derivatives or (in the presence of excess diazo ester) react with the corresponding rhodium carbenoid to form substituted 3,4-dihydro-2H-pyrroles. 2-Mono-and 2,2-disubstituted 3-phenyl-2H-azirines react with rhodium carbenoids generated from diazo esters to give azirinium ylides which are converted into the corresponding 2-azabuta-1,3-dienes.