A Convergent Route to Enantiomerically Pure Higher [n?2]Triangulanedimethanol Derivatives and [n]Triangulanes (n?7)

Stereoselective Synthesis of Polysubstituted Spiropentanes

A new approach to polysubstituted spiropentanes is developed through a regio- and diastereoselective carbometalation of sp²-disubstituted cyclopropenes. The control of selectivity originates from a combined syn-facial diastereoselective carbometalation with a regio-directed addition. The regio-controlling element subsequently serves as a leaving group in an intramolecular nucleophilic substitution. This method allows the preparation of various polysubstituted spiropentanes with up to five contiguous stereocenters.

Recent advances in the catalytic cyclopropanation of unsaturated compounds with diazomethane

The main achievements and development trends of the past 10–15 years related to the catalytic cyclopropanation of unsaturated compounds with diazomethane are integrated and analyzed. The attention is focused on the most efficient catalysts based on palladium compounds. Data on the effects of substrate structure and nature of catalyst components on the regio- and stereoselectivity of these reactions are systematized. Characteristic features of safe methods for diazomethane generation are considered, including the use of membrane technologies and continuous-flow and in situ preparation methods, which have prospects for industrial application.

The bibliography includes 281 references.

Polytwistane

Twistane, C10H16, is a classic D2-symmetric chiral hydrocarbon that has been studied for decades due to its fascinating stereochemical and thermodynamic properties. Here we propose and analyze in detail the contiguous linear extension of twistane with ethano (ethane-1,2-diyl) bridges to create a new chiral, C2-symmetric hydrocarbon nanotube called polytwistane. Polytwistane, (CH)n, has the same molecular formula as polyacetylene but is composed purely of C(sp(3))-H units, all of which are chemically equivalent. The polytwistane nanotube has the smallest inner diameter (2.6 Å) of hydrocarbons considered to date. A rigorous topological analysis of idealized polytwistane and a C236H242 prototype optimized by B3LYP density functional theory reveals that the polymer has a nonrepeating, alternating σ-helix, with an irrational periodicity parameter and an instantaneous rise (or lead) angle near 15 °. A theoretical analysis utilizing homodesmotic equations and explicit computations as high as CCSD(T)/cc-pVQZ yields the enthalpies of formation Delta(f)H(0)°(twistane) = -1.7 kcal mol(-1) and Delta(f)H(0)°(polytwistane) = +1.28 kcal (mol CH)(-1), demonstrating that the hypothetical formation of polytwistane from acetylene is highly exothermic. Hence, polytwistane is synthetically viable both on thermodynamic grounds and also because no obvious pathways exist for its rearrangement to lower-lying isomers. The present analysis should facilitate the preparation and characterization of this new chiral hydrocarbon nanotube.

From furan to molecular stairs: syntheses, structural properties, and theoretical investigations of oligocyclic oligoacetals

The synthesis of oligocyclic oligoacetals using five-membered rings as repetitive unit is described. Furan was used as the starting material, which is converted by a three-step procedure consisting of twofold cyclopropanation, reduction, and oxidative ring enlargement into a tricyclic bis(enol ether). A repetition of this synthetic procedure leads to the formation of extended oligoacetal systems. Insights into the structures were gained by X-ray crystallographic investigations and revealed helical arrangements of the subunits in the solid-state. DFT (B3LYP) calculations have been carried out to elucidate the transition state of the ring enlargement and the flexibility of the annelated oligocyclic systems. Strain energies and topologies of potential cyclically condensed oligoacetals are predicted.

Bullvalene trisepoxide and its stereospecific rearrangement to 2,8,12-trioxahexacyclo[8.3.0.0(3,9)0(4,6)0(5,13)0(7,11)]tridecane: two new C3-symmetrical oligocycles with propeller chirality

Epoxidation of bullvalene (1) with a neutralized solution of Oxone gave racemic trisepoxide rac-6 in 93 % isolated yield. Its structure was examined by X-ray crystallography. The two enantiomers of 6 were separated by preparative HPLC and exhibited specific rotations of [alpha](25)(D)= +160, [alpha](25)(365)= +567 (c=0.946, CHCl(3)) for the firstly eluted and [alpha](25)(D)= -157, [alpha](25)(365)= -554 (c=0.986, CHCl3) for the secondly eluted enantiomer of 6. The geometry of (+)-6 and the absolute configuration of (-)-6 were determined by X-ray crystal structure analysis and anomalous diffraction, respectively. According to this, (-)-6 possesses (3R,5S,7S,9R,11R,13S)- and (+)-6 has (3S,5R,7R,9S,11S,13R)-configuration. Upon treatment with BF(3)Et(2)O at -78 degrees C, trisepoxide rac-6 rearranges with retention of the skeletal three-membered carbocycle to give the cage trisether rac-8, as proved by X-ray crystal structure analysis, in virtually quantitative yield. Enantiomers of rac-8 were separated by preparative HPLC and exhibited specific rotations of [alpha](25)(D)= +49, [alpha](25)(365)= +170 (c=1.01, CHCl3) (firstly eluting) and [alpha](25)(D)= -46, [alpha](25)(365)= -160 (c=1.02, CHCl(3)) (secondly eluting enantiomer). The absolute configuration of (-)-8 was determined by anomalous diffraction to be (1R,3R,7R,9R,11R,13R). DFT computations at the TD-B3 LYP/6-31+G(d,p)//B3 LYP/6-31+G(d) level of theory for (3R,5S,7S,9R,11R,13S)-6 and (1R,3R,7R,9R,11R,13R)-8 predicted specific rotations of -206.7 and -83.4, respectively. Acid-catalyzed isomerization of the enantiomerically pure (+)-6 proceeded without racemization to give exclusively (-)-8, and (-)-6 provided only (+)-8. Thus, this isomerization occurs with ring opening of the three C--O bonds in the epoxide moieties in the alpha-position relative to the three-membered carbocycle rather than in the beta-position.

Linear and Branched Phospha[n]triangulanes

Novel, highly stable, linear and branched mono- and diphospha[n]triangulanes were synthesized in high yields by the CuCl-catalyzed phosphinidene addition to spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes. The effect of spirofusion on the electronic properties of these esthetically attractive phosphacycles is apparent from X-ray single crystal structure analyses, which reveals a tightening of the phosphirane ring on additional spirocyclopropanation, and from the NMR features that show deshielded chemical shifts for the ring-phosphorus and -carbon atoms. Steric factors play a role in the addition reaction when the substrate alkene carries a second sphere of spirocyclopropane rings and causes the formation of 2-phosphabicyclo[3.2.0]heptenes in small amounts. These by-products most probably result from addition of the [PhP(Cl)W(CO)(5)]-Cu-L (L=alkene or solvent) reagent to the spirocyclopropanated bicyclopropylidene to give an intermediate sigma-complex, which subsequently, facilitated by steric factors, undergoes a cyclopropylcarbinyl to cyclobutyl ring expansion followed by a [1,3]-sigmatropic shift.