A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene.

As eries of oxahelicenes composed of ortho/metaannulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)-or nickel(0)-mediated) double,t riple,o rq uadruple [2+ +2+ +2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes,thus allowing the construction of 6, 9, or 12 rings in asingle operation. The use of aflow reactor was found to be beneficial for the multicyclization reactions.T he stereogenic centers present in some of the oligoyness teered the helical folding in such aw ay that the final oxa[9]-, [13]-, [17]-and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically,t he oxa-[19]helicenes beat the current recordinthe length of ahelicene backbone.S ingle-molecule conductivity was studied by the mechanically controllable break-junction method with ap yridooxa[9]helicene.

A series of diverse aromatic azadienetriyne and azatriynes was synthesised. These compounds were subjected to transition metal-mediated [2+2+2] cycloisomerisation to form pentacyclic or hexacyclic helically chiral azahelicene or azahelicene-like structures mostly in moderate yields. Introducing stereogenic centre(s) into selected azatriynes, cyclisation proceeded in a stereoselective fashion providing aza[5]helicenes or aza[6]helicene-like compounds in up to a 100:0 diastereomeric ratio. Gibbs energy differences between corresponding pairs of diastereomers (calculated at the DFT(B3LYP)/TZV+P level) were in good agreement with the experimental data and allowed for the prediction of the stereochemical outcome of the reaction. This study presents for the first time asymmetric synthesis of azahelicene derivatives in high optical purities.

Dedicated to Professor Pavel Kočovský on the occasion of his 60th birthday.C 8 -Alkenylpurines were synthesized starting from purin-8-yl(allyl) acetates using Pdcatalyzed allylic substitution. The described protocol allows, by reaction of purin-8-yl(allyl) acetates with stabilized nucleophiles, an access to novel (E)-8-alkenylpurine derivatives under Pd 2 dba 3 ·CHCl 3 catalysis in dry THF in yields ranging from 31 to 76%. A wide range of nucleophiles showed exclusive E-alkene formation, however, ethyl nitroacetate gave mixture of E/Z-alkenes. On contrary, purin-2-yl(allyl) acetates reacted smoothly only with dimethyl malonate.

Pd-Catalyzed Allylic Substitution of Purin-8-yl(allyl) Acetate: Route to (E)-Alkenylpurines. -(TOBRMANOVA, M.; TOBRMAN*, T.; DVORAK, D.; Collect. Czech. Chem. Commun. 76 (2011) 4, 311-326, http://dx.doi.org/10.1135/cccc2011030 ; Dep. Org. Chem., Prague Inst. Chem. Technol., CZ-166 28 Prague 6, Czech Republic; Eng.) -C. Cyrus 29-160