David A. Ben-Efraim

The use of mid-points or average NMR chemical shifts in stereochemical assignments

Abstract A comparison of the mid-points or average chemical shifts of mirror-symmetrical spin patterns in the NMR spectra of structural isomers can be used in a straightforward manner to obtain stereochemical information. This result is anticipated from analysis of substituent contributions to chemical shifts and has been observed in a variety of chemical systems, especially cyclobutane derivatives, which comprise a group of compounds for which appreciable data is available and whose structure assignments have often entailed difficulty and even controversy. The method of mid-point comparison may also be useful for conformational analysis.

The prototropic rearrangement of secondary propargylic amines

Abstract Secondary and tertiary propargylic amines of types 3 and 4 were synthesized. On prototropic isomerization amines 3a-f isomerized to α,β-unsaturated aldimines of type 6 while 3g and 3h isomerized to conjugated aldimines 18 and 19 respectively. The diyne-diamine 27 isomerized to a fully conjugated diene-diimine 28 . Amine 3i on treatment with base yielded aniline. Alkyl-substituted dipropynylamines 29b-d were prepared and on prototropic isomerization aromatized to alkyl-substituted pyridines. The unsubstituted dipropynylamine 29a however underwent under the above conditions two types of skeletal rearrangement to yield nitriles 44a and b in high yield and oximes 45a and b in low yield.

The synthesis and some reactions of a series of “skipped” polyacetylenes containing terminal acetylene groups

Abstract The synthesis and some reactions of a series of linear 1,4-polyynes containing terminal acetylene groups (Ia–e) are described. Reaction of propargyl bromide (II) with ethynylmagnesium bromide (III) in the presence of CuCl gave 1,4-pentadiyne (Ia). Rearrangement of Ia with KOBut led to 1,3-pentadiyne (IV), while treatment with Br2 furnished 1,2,4,5-tetrabromo-1,4-pentadiene (V). Oxidation of Ia with O2, CuCl and NH4Cl yielded the linear dimer (VIa) [rearranged to 2,4,6,8-decatetrayne (VIII) with KOH] and linear trimer (VIb). Hydrogenation of the crude oxidation product gave saturated hydrocarbons derived from VIa, VIb, the linear tetramer (VIc) and the cyclic tetramer (VII), but none derived from the cyclic dimer or cyclic trimer of Ia. Similarly, oxidation of Ia with Cu(OAc)2 in pyridine and subsequent hydrogenation led to saturated hydrocarbons derived from VIa and VIb, but none derived from the cyclic dimer or cyclic trimer. In order to prepare cyclodecane as a comparison compound, the cycloethylenedithioketal (X) derived from cyclodecanone was treated with Raney Ni in boiling EtOH, but the reaction led mainly to cis-cyclodecene. Treatment of 1,4-dibromo-2-butyne (XIII) with an excess of III in the presence of CuCl yielded 1,4,7-octatriyne (Ib), 1,4,7,10,13-tetradecapentayne (Id) and 1,4,7,10,13,16,19-eicosaheptayne (Ie). Rearrangement of Ib with alkaline Al2O3, or KOBut at room temp gave 2,4,6-octatriyne (XVII), while treatment with boiling ethanolic KOH or KOBut in hot ButOH led to 2-ethoxy-2-octene-4,6-diyne (XVIIIa) or 2-t-butoxy-2-octene-4,6-diyne (XVIIIc), respectively. 1,4,7,10-Undecatetrayne (Ic) was prepared from crude Ia by conversion to the bis-Grignard derivative, followed by reaction with propargyl bromide (II) in the presence of CuCl.

The rearrangement of 1,5-hexadiyne and 1,6-heptadiyne☆

Abstract The rearrangement of 1,5-hexadiyne (I) with potassium t-butoxide in t-butyl alcohol led mainly to a ca. 2:1 mixture of cis-1,3-hexadien-5-yne (IVa) and trans-1,3-hexadien-5-yne (IVb), and a minor amount of 2,4-hexadiyne (II). Both IVa and IVb were isolated and characterized. Contrary to previous reports, a similar result was obtained when the rearrangement of I was carried out with ethanolic potassium hydroxide, and in this case 2-ethoxy-2-hexen-4-yne (VIa) (derived from II) was also formed. The rearrangement of 1,6-heptadiyne (VIII) with potassium t-butoxide in t-butyl alcohol gave toluene and trans-1,3-heptadien-5-yne (Xb) in about equal amounts, but no cis-1,3-heptadien-5-yne (Xa) was detected. Authentic samples of Xa and Xb were prepared by methylation of IVa and IVb.

Light-sensitive amides. Photocleavage of N-acyl-1,2,3,4-tetrahydro-8-nitroquinolines to give free carboxylic acids

The structural assignments of the 5- and 8-nitrotetrahydroquinolines have been reversed on the basis of their n.m.r. and mass spectra. U.v. irradiation cleaved the amide bonds of N-acyl-8-nitrotetrahydroquinolines to furnish free carboxylic acids in high yields, thus indicating the potential of 8-nitrotetrahydroquinolyl as a photosensitive protecting group for carboxylic acids. It has been shown that the release of the carboxylic acids involves a photocleavage and not a solvolysis. It has been experimentally demonstrated for the first time that oxygen transfer takes place from an excited nitro-group. This process precedes hydrogen abstraction and both processes are intramolecular.

Remarkable dynamic NMR spectra and properties of a sterically congested cis-1,2-diarylcyclobutane

Dimethyl 2.2′-dichloro-6,6′-dimethoxy-β-truxinate 2 displays in solution, by 1H NMR spectroscopy, a mixture of two conformers: a meso conformer and a racemic conformer. The other possible meso conformer may be present in a small amount in solution. The NMR parameters of these conformers were extracted from their low-temperature static 1H and 13C spectra. The conformers interconvert mainly by a mechanism involving rotation of one aryl group at a time. Activation parameters of the same dynamic process were obtained by band-shape simulations from four different regions of the variable-temperature 1H spectra. In one spectral region, rate constants in excess of 106 s–1 were recorded.