Acetal Carbon Atom Research Articles

Synthesis and hydrogenolysis of the methylene, ethylidene, isopropylidene, and diastereoisomeric 1-phenylethylidene acetals of β- l-arabino- and α- l-rhamnopyranoside derivatives

Both diastereoisomers of 1-phenylethylidene acetals (acetophenone acetals) of methyl and benzyl β- l-arabinopyranoside and α- l-rhamnopyranoside were prepared. Acetal-exchange reactions gave only the endo-phenyl isomers; their 2- O- and 4- O-acetyl derivatives were isomerised into the exo-phenyl compounds. 1H-N.m.r. data were used to determine the absolute configuration at the acetal carbon atom in these compounds. The protons of the methyl group of the exo-phenyl isomers resonate at lower field than those of the endo-phenyl isomers. Hydrogenolysis of various methylene, ethylidene, and isopropylidene derivatives gave axial ethers. The endo-phenyl isomers of the acetophenone derivatives also gave axial 1-phenylethyl ethers in two diastereoisomeric forms. The exo-phenyl isomers of the arabinosides were stable towards the reagent (LiAlH 4AlCl 3), whereas the corresponding rhamnopyranosides gave the 2-(1-phenylethyl) ethers, but cleavage required prolonged reaction time and higher temperature.

A conformational study of dioxolane-type acetals of some carbohydrate derivatives by N.M.R. spectroscopy

The conformation of the methylene, isopropylidene, benzylidene, and 1-phenylethylidene acetals of some rhamnopyranoside and arabinopyranoside derivatives has been investigated by n.m.r. spectroscopy. In each series, the pyranoside ring adopts a distorted chair conformation. For the benzylidene acetals, the pyranoside ring is more distorted in the endo-phenyl isomers, and these isomers are compared to the methylene and isopropylidene acetals. For the 1,3-dioxolane ring, only the coupling constant of the acetal carbon atom with the axial bridgehead proton is measurable; for the benzylidene acetals, it is larger in the exo-phenyl isomer. For the other types of acetals, the coupling constant depends on the nature of the endo substituent at the acetal carbon atom. If it is bulky, the coupling constant and the conformation of the dioxolane ring are similar to those of the corresponding endo-phenyl benzylidene derivative.

Configuration of the acetal carbon atom of pyruvic acid acetals in some bacterial polysaccharides

The configuration at the acetal carbon atom of pyruvic acid acetals present in some extracellular bacterial polysaccharides has been investigated. Assignment of the absolute configuration was made by comparing signals in the 13C- and 1H-n.m.r. spectra of the polysaccharides with those of model substances. The S-configuration was demonstrated in eight polysaccharides in which pyruvic acid is linked to O-4 and O-6 of D-glucopyranosyl or D-mannopyranosyl residues. The R-configuration was demonstrated in four polysaccharides in which pyruvic acid is linked to O-4 and O-6 of D-galactopyranosyl residues. Consequently, in each of these acetals, which form 1,3-dioxane rings, the methyl group is equatorial and the carboxyl group axial. The S-form was further demonstrated in four polysaccharides in which the pyruvic acid is linked to O-3 and O-4 of D-galactopyranosyl groups.

Benzylidene acetal structural elucidation by N.M.R. Spectroscopy: Application of carbon-13. N.M.R.-Spectral parameters

The 13C-n.m.r. spectra of 19 2-phenyl-1,3-dioxolane, -1,3-dioxane and -1,3-dioxopane derivatives were examined and it was found that both the 13C-n.m.r. chemical shift for the acetal carbon atom and the one-bond coupling constant between the acetal carbon atom and the acetal proton had values that could be used to distinguish between acetals having different ring sizes. In addition, the presence of axial substituents at positions 4 or 6 in substituted 2-phenyl-1,3-dioxane rings and 4 or 7 in substituted 2-phenyl-1,3-dioxepane rings could be readily detected. The structures of a number of carbohydrate examples were determined by using these two parameters and also the chemical shift of the acetal proton from 1H-n.m.r. spectra. The use of all three parameters made assignment of benzylidene acetal ring-size unambiguous.

Acetoxonium ions from acetoxyoxiranes and orthoesters: their conversion into ethylidene acetals, rearrangement, and solvolysis

The acid-catalysed ethylidenation of some methyl pentopyranosides has been studied and the configuration at the acetal carbon atom assigned by p.m.r. spectroscopy. There is a strong preference for the isomer in which the methyl group has the endo configuration. Several cyclic alkyl orthoacetates derived from methyl pentopyranosides have been prepared by orthoester exchange and the endo C-methyl isomer shown to preponderate. Treatment of vicinal acetoxyoxiranes and orthoacetates with boron trifluoride followed by lithium borohydride, or with diborane, yields ethylidene acetals in which the C-methyl group is endo. Rearrangements of the hexachloroantimonate salts of acetoxonium ions derived from methyl lyxo- and arabino-pyranosides, possessing trans-vicinal acetoxyl groups, have been studied. The ions having the arabino configuration are preferred in both the α and β series. The reaction of cyclic orthoacetates of methyl β- L-arabinopyranoside and some derivatives with dry acetic acid proceeds via an acyclic acetoxonium ion to yield only products having the L- arabino configuration.

Polymerization of Bicyclic Acetals. II. Steric Course in the Cationic Polymerization of 6,8-Dioxabicyclo[3.2.1]octane

The polymerization of 6,8-dioxabicyclo[3.2.1]octane (DBO) was carried out in toluene, methylene chloride, and 1-nitropropane over the wide range of temperatures from −78 to 30°C. Boron trifluoride etherate was used as an initiator. The stereoregularity of the polymer obtained was evaluated from the relative intensities of the two NMR peaks appearing at τ 5.15 and 5.60, which were due to the equatorial and axial acetal protons of the tetrahydropyran ring, respectively. The EQ—H% of the polymer, defined as the percent of the equatorial acetal protons to the total acetal protons, decreases from nearly 100 to 70–80 depending on the solvents used with the rise in the polymerization temperature. On the basis of these results, it is concluded that the cationic polymerization of DBO proceeds at lower temperatures through an SN2 mechanism involving the reverse side attachment of the monomer to the acetal carbon atom of a propagating cyclic trialkyloxonium ion to afford a stereoregular polymer. At higher temperatures and in polar solvents, the participation of a car-boxonium ion derived from the cyclic trialkyloxonium ion in the propagation reaction and/or the competitive oxonium ion exchange reaction at the propagating chain end becomes significant, thus causing the loss of stereospecificity of the polymerization.

Mass-spectrometric study of completely substituted 1,3-dioxolanes

1. A study of the mass spectra of the completely substituted 1,3-dioxolanes disclosed that the principal directions of the decomposition of their molecular ions are associated with a rupture of the bonds of the dioxolane ring. The substituents are most easily eliminated from the acetal carbon atom. 2. The isomers with different positions of the substituents can easily be distinguished by means of the mass spectra. 3. The previously not observed elimination of the OH group from nitro compounds, not due to the “ortho-effect,” was described.

1,2( R):5,6( R)-, 1,2( R):5,6( S)-, and 1,2( S):5,6( S)-bis- O-(trifluoromethyl)-ethylidene- D-mannitol

The three possible 1,2:5,6-bis- O-(trifluoromethyl)ethylidene- D-mannitols have been isolated from the reaction between D-mannitol and 1,1,1-trifluoroacetone, and converted into 2,3- O-(trifluoromethyl)ethylidene derivatives of D-glycerol and D-glyceric acid. The assignment of configuration to the acetal carbon atom in the various products was based on n.m.r. studies. Chemical shift non-equivalence is exhibited by the acetal methyl group in 2,3- O-( S)-(trifluoromethyl)ethylidene- D-glycerol ( RS)-atrolactate ( i.e., where the atrolactoyloxymethyl substituent is cis to the acetal methyl group), whereas the corresponding 2,3- O-( R)-trifluoromethyl)-ethylidene- D-glycerol derivative shows only a single methyl signal.

Stereochemistry of carbohydrate derivatives. Part II. Addition of iodine azide to unsaturated sugars: the crystal structure of 6-azido-5,6-dideoxy-5-iodo-1,2-O-isopropylidene-β-L-idofuranose

X-Ray crystallographic analysis has established that the product of the addition of iodine azide to 5,6-dideoxy-1,2-O-isopropylidene-α-D-xylo-hex-5-enofuranose (III) is 6-azido-5,6-dideoxy-5-iodo-1,2-O-isopropylidene-β-L-idofuranose (IV), by anti-Markovnikov addition to the sugar double bond. The trioxabicyclo[3,3,0]octane ring-system is in a ‘twin-envelope’ conformation, with C(4) and the acetal carbon atom, C(7), displaced by 0·7 and 0·45 Å from the planes of the other four atoms of their respective five-membered rings. The crystals of the adduct are monoclinic, space group P21, Z= 2, in a cell of dimensions a= 5·86, b= 10·20, c= 10·80 Å, β= 91·2°. The structure was established by Fourier and least-squares methods and the final value of R is 19·7% for 830 observed reflections.