General Route to Purin-2-ylmagnesium Halides by Metal–Halogen Exchange in Dichloromethane

Treatment of a solution of a 9-alkyl- or 9-aryl-2-iodopurine in dichloromethane with an ethereal solution of ethylmagnesium bromide at –5 °C generates the corresponding purin-2-ylmagnesium bromide, which reacts with aldehydes to give the corresponding 2-(hydroxyalkyl)purines in yields of 53–84%. The purin-2-yl Grignard reagents show good functional-group tolerance to ester and nitro groups, and the method permits the synthesis of the previously unknown 6-unsubstituted 2-magnesiopurines for the first time. Performing the same procedure in THF as solvent resulted either in extensive decomposition or rapid isomerization to give purin-8-ylmagnesium halides.

An Improved Route to Purin-6-yl Magnesium Halides by Metal–Halogen Exchange in Dichloromethane

Treatment of a solution of 9-benzyl or 9-phenyl 6-iodopurine in dichloromethane with an ethereal solution of ethylmagnesium bromide at ambient temperature generates the corresponding purin-6-yl magnesium halides which react with aldehydes to give carbinols in 55–80% yield. Performing the same procedure with THF as solvent gave carbinols in much lower yields (≤15%).

Das Anion [R2Al-AlR2Br]- (R = CH(SiMe3)2) mit Aluminium-Aluminium- und terminaler Aluminium-Brom-Bindung / The Anion [R2Al-AlR2Br]- (R = CH(SiMe3)2) with an Aluminium -Aluminium and a Terminal Aluminium -Bromine Bond

Tetrakis[bis(trimethylsilyl)methyl]dialane(4) (1) with an Al-Al bond reacts with LiBr in n-pentane in the presence of chelating N.N.N'.N'-tetramethylethylenediamine (TMEDA) to yield the monobromo adduct [Li(TMEDA)2]+[R2Al-AlR2Br]- 4. Compound 4 is unstable in ethereal solution and decomposes with quantitative formation of LiBr and the starting compound 1. Crystal structure determination of 4 C5H10 shows a terminal Al-Br bond in an anion with a four-coordinated aluminium atom neighbouring a three-coordinated, coordinatively unsaturated aluminium atom. While the Al-Al bond length is not significantly different (264.3 pm) as compared to the data for 1, the Al-Br bond is rather long (247.6 pm) indicating a weak coordinative bonding.

Mixed aromatic-aliphatic acetals; Their preparation, 1H and 13C-NMR spectra and hydrogenolysis by ethereal solution of chloroalane

The corresponding p-X-phenylisobutyl acetals to acetaldehyde (I), 2-(p-X-phenoxy)tetrahydropyrans (II), 2-(p-X-phenoxy)tetrahydrofurans (III), and cyclohexyl(p-X-phenyl)acetals of acetaldehyde (IV) were prepared by addition of p-X-substituted phenols to isobutyl vinyl ether, 2,3-dihydro-4H-pyran, 2,3-dihydrofuran, and cyclohexyl vinyl ether, respectively. The structures of acetals I-IV were confirmed by their 1H and 13C NMR spectra. Diastereotopic protons of the methylene and geminal methyl groups are anisochronous in the 1H NMR spectra of acetals I. 13C NMR spectra of acetals IV demonstrate anisochronous behaviour of cyclohexane ring diastereotopic carbon atoms. The constants σp+ for several groups calculated using 13C NMR spectra are: 1-isobutoxy-1-ethoxy- (-0.56), 2-tetrahydropyranyloxy- (-0.58), 2-tetrahydrofuryloxy- (-0.59), and 1-cyclohexyloxy-(1-ethoxy) (-0.57). Experimental relative rate constants of hydrogenolysis of acetals I and II by ethereal solution of chloroalane yield best correlations with σp+ constants of the substituent X, providing the values ρ = 2.61 and ρ = -1.09 for compounds of the series I and II, respectively. The importance of these results for finding the rate determining step of the studied reaction is discussed.