Applications of NMR Spectroscopy in Organic Chemistry.

Methodology for the construction of quaternary carbon centers

The Balz-Schiemann synthesis of aryl fluorides via the thermal decomposition of aryl diazonium tetrafluoroborate salts represents both the earliest and the most proverbial example of the BF4 ion as a nucleophilic fluoride source. Mechanistic studies of the Balz-Schiemann reaction have accrued strong experimental support for an SN1 mechanism via the intermediacy of an aryl cation, which is quenched by fluoride transfer directly from the BF4 ion. A notable advance in this area has been the development by Ohmori and co-workers of a fluorinative variant of the Mitsunobu reaction, involving the electrooxidative generation and subsequent thermal decomposition of alkoxytriphenylphosphonium tetrafluoroborates, allowing for the conversion of alcohols to the corresponding alkyl fluorides with inversion of configuration. Although stereoselectivity during C-F bond formation for reactions proceeding via (putative) carbocation intermediates can be highly variable, the ring-opening of strained rings or ?iranium intermediates via fluoride transfer from fluoroborate ions typically proceeds with high stereoselectivity.

Beyond the Balz-Schiemann reaction: the utility of tetrafluoroborates and boron trifluoride as nucleophilic fluoride sources.

Construction of cyclopentyl units by ring contraction reactions

Enaminones in heterocyclic synthesis

The skeletal rearrangement of α-halogeno-ketones, which is known as the Favorskii rearrangement, is met most frequently in aliphatic monocyclic, and polycyclic halogenated ketones. This molecular rearrangement is used in the synthesis of branched carboxylic acids and cis-αβ-unsaturated acids and for obtaining smaller rings in alicyclic and to a less extent heterocyclic compounds. The sterochemistry and the mechanism of the Favorskii rearrangement are also considered, these being of great interest for theoretical organic chemistry. A list of 261 references is included.

https://iopscience.iop.org/article/10.1070/RC1970v039n09ABEH002019/pdf

The Favorskii Rearrangement

The mass spectra of a number of the epimeric 1,2-dimethyl-4-alkyl-4-hydroxydecahydroquinolines (alkyl: CCH, CHCH2, C2H5 and COCH3) have been studied. The configuration at C-2 and C-4 in these molecules is proposed on the basis of the data obtained. Some aspects of the fragmentation pathways under electron-impact are discussed.

Mass spectrometric determination of the configuration of the 4-substituted 1,2-dimethyl-4-hydroxydecahydroquinolines

CONTENTS I. Introduction 311 II. Five-membered heterocycles with one heteroatom 312 III. Five-membered heterocycles with two heteroatoms 314 IV. Five-membered heterocycles with three heteroatoms 319 V. Six-membered heterocycles with one heteroatom 320 VI. Six-membered heterocycles with two heteroatoms 320 VII. Condensed heterocyclic systems with several heteroatoms 322

http://iopscience.iop.org/article/10.1070/RC1967v036n05ABEH001616/pdf

Derivatives of steroids with condensed heterocycles

The configuration at C-2 and C-4 in the molecules of 2-methyl- and 1,2-dimethyl-4-vinylethinyl(n-butyl)-4-hydroxyperhydroquinolines was determined by mass spectrometry. The principal conclusions concerning the stereochemistry were made on the basis of differences in the values of the I[M−15]+/I[M]+·, I[M−17]+/I[M]+·, I[M−43]+/I[M]+· and I[M−57]+/I[M]+· ratios in the mass spectra of the epimeric vinylethinylic alcohols, and of the I[M−15]+/I[M]+· and I[M−15]+/I[M]+· ratios in the case of the n-butylic alcohols.

Mass spectrometric determination of the configuration of 2-methyl-and 1,2-dimethyl-4-vinyl-ethinyl(n-butyl)-4-hydroxyperhydroquinolines

Information on the opening of an epoxide ring with retention of the initial configuration under conditions of catalytic hydrogenation and under the action of various nucleophilic reagents is examined. It is noted that when an electron-donating p- or π-grouping is present in the α position to the epoxide ring the retention of the initial configuration is more probable than its inversion. Bibliography of 182 references.

http://iopscience.iop.org/article/10.1070/RC1968v037n06ABEH001652/pdf

A. A. Akhrem

Papers

The Favorskii Rearrangement

Mass spectrometric determination of the configuration of the 4-substituted 1,2-dimethyl-4-hydroxydecahydroquinolines

Derivatives of steroids with condensed heterocycles

Mass spectrometric determination of the configuration of 2-methyl-and 1,2-dimethyl-4-vinyl-ethinyl(n-butyl)-4-hydroxyperhydroquinolines

Retention of Configuration in Reactions with Opening of an Epoxide Ring