V. G. Kul'nevich

Stereostructure of 2-ethynyl-substituted 2,4,4-trimethyl-1,3-dioxanes

It has been shown by PMR spectrometry and x-ray structure analysis that the six-membered 1,3-dioxane ring of 2-ethynyl-substituted 2,4,4-trimethyl-1,3-dioxanes, either in solution or in the solid state, exists in a nearly ideal chair conformation with an axial position of the ethynyl substituent.

4H-3, 1-benzoxazines. 5. PMR and mass spectrometric study of 2,4-substituted 1,2-dihydro-4H-3, 1-benzoxazines

The formation of 2,4-substituted 1,2-dihydro-4H-3, 1-benzoxazines from tertiary o-aminophenylcarbinols and carbonyl derivatives has been demonstrated by means of PMR and mass spectrometry. Interconversion of the six-membered heterocycle of the half-chair ⇄ half-chair type has been established on the basis of the PMR data when identical substituents are present in positions 2 and 4. The cisarrangement of the protons in the H-C(2)-N-H fragment of the heterocycle has been determined. The general laws governing the initial fragmentation of 1,2-dihydro-4H-3, 1-benzoxazines under the effects of electron impact have been revealed.

Synthesis and nonchair conformations of 2,2-disubstituted 4,4-dimethyl-1, 3-dioxanes

The fundamental possibility of the synthesis of 2-substituted 1,3-dioxanes by reaction of 1,3-dioxanium perchlorates with organometallic compounds is demonstrated. A method for the synthesis of acyl derivatives of heterocycles was developed on the basis of these compounds. The existence of 2,2,4,4-substituted 1,3-dioxanes in the twist conformation was shown by 1H NMR spectroscopy; the twist conformation is explained by the effect of nonbonded 1,3-syn-axial interactions.

SYNTHESIS AND NONCHAIRLIKE CONFORMATIONS OF 2,2-DISUBSTITUTED 4,4-DIMETHYL-1,3-DIOXANES

The fundamental possibility of the synthesis of 2-substituted 1,3-dioxanes by reaction of 1,3-dioxanium perchlorates with organometallic compounds is demonstrated. A method for the synthesis of acyl derivatives of heterocycles was developed on the basis of these compounds. The existence of 2,2,4,4-substituted 1,3-dioxanes in the twist conformation was shown by 1H NMR spectroscopy; the twist conformation is explained by the effect of nonbonded 1,3-syn-axial interactions.

Synthesis, structure, and stereochemistry of 2-alkyl-4,4,5-trimethyl- and 2-alkyl-4,4,6,6-tetramethyl-1,3-dioxanium perchlorates

A number of 2-alkyl-4,4,5-trimethyl- and 2-alkyl-4,4,6,6-tetramethyl-1,3-dioxanium perchlorates were obtained by acylation of 1,3-diols in the presence of 70% perchloric acid. Acid-catalytic cleavage of the heteroring with subsequent cyclization to 2-alkyl-1,3-dioxanium perchlorates occur in the reaction of 2-aryl(alkyl)-substituted and 2-unsubstituted 1,3-dioxanes with carboxylic acid anhydrides and 70% HClO4. Hypothetical detachment of a hydride ion by means of the acceptor acylium cation does not occur under the investigated conditions. The structures of the synthesized perchlorates were studied by PMR spectroscopy and it was shown that 1,3-dioxanium cations at room temperature exist in a state of rapid conformational isomerization of the “chair-chair” form. The rate of interconversion of the 2,4,4-trimethyl-1,3-dioxanium cation decreases appreciably as the temperature is reduced to −90°. as evidenced by marked broadening of the lines of the protons of the gemdimethyl groupings.

UNTERSUCHUNGEN UEBER FURANACETALVERBINDUNGEN 7. MITT. NMR-SPEKTREN, KONFIGURATION UND KONFORMATION SUBSTITUIERTER 2-FURYL-1,3-DIOXANE

Fur eine Reihe von stereoisomeren Furandioxanen (I)-(III) wird mit Hilfe der NMR-Spektroskopie die Konfiguration und die bevorzugte Konformation bestimmt.