Rita Annunziata

Through-space interactions between face-to-face, center-to-edge oriented arenes: importance of polar-π effects

Two series of conformationally restricted polycyclic compounds (1-3 and 4-7) have been synthesized as model systems for studying the through-space interactions between face-to-face, center-to-edge (parallel-offset) oriented arenes. These compounds feature different X substituents on one of the interacting rings. By monitoring the variation of the delta Gz for the rotation around the aryl-aryl bond in 1-7 as a function of X by 2D [1H,1H] EXSY NMR spectroscopy, it was found that the barriers increase on passing from electron-donating to electron-withdrawing substituted derivatives. Quantum mechanical calculations [MP2/DVZ (2d,p)//B3LYP/DVZ(2d,p)] gave barrier values and variations in agreement with the experimental data. The results are consistent with a repulsive arene-arene interaction dominated by electrostatic effects.

A convenient procedure for the synthesis of tetrathia-[7]-helicene and the selective α-functionalisation of terminal thiophene ring

This paper describes a convenient preparation of tetrathia-[7]-helicene (TH[7]), the generation of the α-anion on the terminal thiophene ring, and the synthesis of the 2-formyl-tetrathia-[7]-helicene (2-CHO-TH[7]). The key intermediate trans-1,2-dibenzodithiophene-ethene, prepared in 97% yield by McMurry coupling of the 2-formyl-benzo[1,2-b;4,3-b′]dithiophene, was transformed into TH[7] using a known procedure. The described method affords TH[7] in 46% overall yield, which is more than four times the yield previously reported in the literature. The α-anion of TH[7], which is easily generated on the α-position of one of the terminal thiophene rings, reacts with electrophilic reagents such as D2O and DMF. The latter reaction proved to be the best way to prepare 2-CHO-TH[7], a key intermediate for the preparation of new substituted heterohelicenes.

Oxindole alkaloids. A novel non-biomimetic entry to (−)-Horsfiline.

Abstract A novel non-biomimetic synthesis of horsfiline has been developed. The key pyrrolidine forming reaction is the 1,3-dipolar cycloaddition of the thermally generated N -methylazomethine ylide to a suitable 3-alkylidene-indolin-2(3 H )one. The same strategy was also applied to the synthesis of pure ( R )-(−)-enantiomer.

Immobilization of catalysts derived from Cinchona alkaloids on modified poly(ethylene glycol)

Abstract The straightforward immobilization of some derivatives of Cinchona alkaloids on modified poly(ethylene glycol)s is reported. The compounds, obtained by simple reactions exploiting different sites for the attachment of the alkaloids to the polymer, were tested as catalysts in the enantioselective benzylation of the benzophenone imine of glycine t -butyl ester (ee up to 64%) and in the conjugate addition of thiophenol to cyclohexenone (ee 22%). The observed stereoselectivities were compared to those obtained either with the unsupported catalysts or with the catalysts immobilized on different polymeric matrixes. The influence of the poly(ethylene glycol) moieties on the catalytic activity is discussed.

Synthesis of enantiomerically pure C2-symmetric acyclic and cyclic 1,2-diamines via pinacol coupling of imines

Abstract The inter- and intramolecular coupling of imines promoted by samarium diiodide and Lewis acids or by Zn/MsOH was extensively studied. The intramolecular reaction of chiral, enantiomerically pure bis-imines was also considered, and allowed the efficient, stereoselective synthesis of 1,2-diamines with C2-symmetry.

Soluble-polymer-supported synthesis of beta-lactams on a modified poly(ethylene glycol)

A modified poly(ethylene glycol) (PEG) has been developed for the soluble-polymer-supported synthesis of beta-lactams. The monomethylether of PEG (MeOPEG) with an average M(W) of 5000 was used as the support, a 4-(3-propyl)phenyl residue as the spacer, and a 4-oxyphenylamino group as the moiety with the reactive functionality. From this modified PEG representative aromatic, heteroaromatic, unsaturated, and aliphatic imines were obtained in high yields by different procedures. The polymer-supported imines were then employed to prepare several beta-lactams by enolate/imine condensation and ketene/imine cycloaddition. Examples of the control of the absolute stereochemistry during the azetidinone ring formation are also reported. The reactions carried out on the polymer-bound imines showed a remarkable similarity to those performed on nonimmobilized imines, both in terms of yields and stereoselectivities. Removal of the beta-lactams from the polymer has also been accomplished to directly deliver the N-unsubstituted azetidinones.

Highly stereoselective synthesis of optically active masked syn-α,β-dihydroxy aldehydes.

Dihydroquinidine acetate promoted osmylation of acetals of E-α,β- unsaturated aldehydes occurs with up to 40:1 stereoselection to afford the corresponding protected syn-α,β -dihydroxy aldehydes.

1,3-dipolar cycloaddition reactions of azomethine ylides on enantiomerically pure (E)-γ-alkoxy-α,β-unsaturated esters

Abstract Enantiomerically pure (E)-γ-alkoxy-α,β-unsaturated esters were reacted with azomethine ylides obtained from glycine imines in the presence of LiBr and diazabicycloundecene (DBU), to afford tetrasubstituted pyrrolidines with complete regiocontrol and fair to excellent diastereoselectivity (only two diastereoisomers formed in up to 96: 4 diastereoisomeric ratio). The results are compared with those of other 1,3-dipolar cycloadditions, and the origin of stereocontrol is discussed.

Enantiomerically pure phenanthroline or bipyridine containing macrocycles: a new class of ligands for asymmetric catalysis

Abstract New enantiomerically pure phenanthroline- and bipyridine-containing macrocycles have been synthesized by combination of the coordinating unit to inexpensive and readily available chiral templates. The catalytic properties of their Cu(I) complexes have been studied in the cyclopropanation of alkenes as test reaction. A simple structural modification of the chiral cavity allowed us to successfully control the trans or cis diastereoselectivity of the reaction.

Diastereoselective synthesis of 1,2-diphenyl-1,2-diaminoethanes by Yb(OTf)3 accelerated reductive coupling of imines

Abstract New reaction protocols have been established to perform the reductive coupling of N -benzyl benzaldimines to 1,2-diphenyl-1,2-diaminoethanes in mild, stereoselective, and catalytic conditions by the use of SmI 2 and Yb(OTf) 3 .