Regio- and stereoselective intermolecular carbolithiation reactions.

The inter-molecular carbolithiation of unfunctionalized and functionalized alkenes, followed by electrophile trapping, provides a versatile and useful tool for the construction of complex molecular systems, allowing the simultaneous creation of up to two novel carbon-carbon or carbon-heteroatom bonds. In this review a full overview on inter-molecular carbolithiation processes is reported, with a main focus on the chemo-, regio- and stereo-control of the reaction and on the strategies for asymmetric processes.

Central-to-axial chirality induction in biphenyl chiroptical probes for the stereochemical characterization of chiral primary amines.

Flexible biphenyls have been applied as efficient and practical chiroptical probes for absolute configuration assignment to chiral primary amines. The mechanism of the central-to-axial chirality transfer from the amine moiety to the conformationally flexible biphenyl system has been determined by NMR and computational studies. This allowed proposing a general non-empirical rule in order to establish, simply by looking at the sign of the 250 nm A band in the ECD spectrum of the biphenyl derivative, the torsion of the biphenyl and thus the absolute configuration of the amine. The method proved to be very reliable and sensitive, allowing treatment of samples on the µmol scale and permitting the simultaneous determination of the amine sample's absolute configuration and enantiopurity.

Determination of absolute configuration using vibrational circular dichroism spectroscopy: the chiral sulfoxide 1-thiochromanone S-oxide.

We reexamined the absolute configuration (AC) of the chiral sulfoxide 1-thiochromanone S-oxide (1) using vibrational circular dichroism (VCD) spectroscopy. The VCD spectrum of 1 was analyzed using density functional theory (DFT). DFT predicts two stable conformations of 1, separated by <1 kcal/mole. Their VCD spectra were calculated using the DFT/GIAO methodology. The VCD spectrum predicted for the equilibrium mixture of the two conformations of (S)-1 is in excellent agreement with the experimental spectrum of (+)-1. The AC of 1 is therefore definitively R(-)/S(+).

Synthesis and stereochemical characterization of optically active 1,2-diarylethane-1,2-diols: useful chiral controllers in the Ti-mediated enantioselective sulfoxidation.

The series of phenylsubstituted 1,2-diphenylethane-1,2-diols 2a-h was prepared in high chemical (70--80%) and optical yields (approximately 90%) by Sharpless syn-dihydroxylation of the corresponding (E)-1,2-diarylethenes, in turn obtained by McMurry or Wittig reactions. The enantiomeric excesses of the samples were determined by HPLC analysis using Chiralcel OD chiral stationary phase (CSP). This CSP was able to resolve all the diols, except for 2g, with alpha values ranging between 1.10--1.64. In all cases the (R,R) antipode was eluted first. (R,R) absolute configuration was assigned to the dextrorotatory (CHCl(3)) diols 2a--h by analyzing the CD spectra of their 2,2-dimethyl-1,3-dioxolanes 3a--h. In fact, the CD spectra of all these dioxolanes present a positive couplet (210--180 nm range) which can be nonempirically related to an (R,R) absolute configuration of the two stereocenters.