The chemical ionization mass spectra of 2-tert-butyl-substituted 1,3-cycloalkanediol diacetates and dimethyl ethers

The chemical ionization (CI) mass spectra of the 2-tert-butyl-substituted 1,3-cyclopentane- and 1,3-cyclohexanediol diacetates and dimethyl ethers have been determined using isobutane and methane as reagent gases. From the differences in the spectra of these compounds, it clearly follows that steric and conformational effects are expresssed in the CI mass spectra. The relative impact of these effects, however, is strongly dependent on diol derivatization and 2-alkyl substitution.     

Stereochemical effects in the chemical ionization mass spectra of cyclic diols

The chemical ionization mass spectra of configurational isomers of many cyclic diols give substantial differences which are characteristic of their stereochemistry. For the cis-isomer of 1,3- and 1,4-cyclohexanediols, formation of a stable intramolecular proton bridge involving the OH groups gives rise to dominant MH⁺ peaks, suppressing the (M-H)⁺ peaks found in the spectra of the trans-isomers and monoalcohols. The stability of the proton bridge in cis-1,3-cyclohexanediol structures is decreased by a sterically interfering cis-5-methyl substituent, but increased by a cis-5-OH group due to additional proton solvation. cis-Stereochemistry also gives increased formation of the dimeric M₂H⁺ ions, but decreased formation of trimers, at higher diol concentrations, for the 1,3- and 1,4-diols. The similarity of the CI spectra of cis- and trans-1,2-cyclohexanediol are explicable in terms of the similarities of the most stable proton-bridged conformers; the reduced ring flexibility in cis- and trans-1,2-cyclopentanediols makes such similar structures unfavorable, as shown by the substantial differences between their CI spectra. The substantial, but expected, variations in behavior with temperature and reagent gas are useful for maximizing stereochemical effects on CI spectra; conditions of lowest energy are usually, but not always, the most useful.     

Synthesis of a series of isomers of coumestrol

Three position isomers of the coumestan (2) coumestrol have been synthesized from appropriately substituted flavylium salts. The dimethyl ethers and diacetates of the isomers and several possible monomethyl ethers and their acetates have been prepared.     

A one-pot selective deprotective acetylation of benzyl ethers and OTBDMS ethers using the BF3·Et2O-NaI-Ac2O reagent system

An efficient selective deprotection followed by acetylation of several benzyl ethers, including 6-OBn ethers of monosaccharides, and -OTBDMS ethers has been developed by using the BF₃·Et₂O-NaI-Ac₂O reagent system. In addition, both benzylidene and isopropylidene groups are deprotected to form the corresponding diacetates.     

The addition reaction of α-azido ethers and α-azido thioethers to phenylacetylene

Several v-triazoles were synthesized by 1,3-dipolar cycloaddition of certain α-azido ethers and α-azidothioethers to phenylacetylene. In most of the cases the reaction led to the formation of the two isomeric v-triazoles. Structural assignments for the products obtained were made on the basis of NMR data and chemical reactions. Characteristic differences between the NMR spectra of the isomers have been noted.     

NMR experiments on acetals—XXXIX: Conformational insights through NMR studies at 300 MHz of 2-methyl-4-halogenomethyl-1,3-dioxolanes

NMR spectra of cis- and trans-2-methyl-4-halogeno-methyl-1,3-dioxolanes have been analysed at 300 MHz. Some of the extracted parameters facilitate easy distinction between these 1,3-dioxolanes and the corresponding structurally isomeric 2-Methyl-5-halogeno-1,3-dioxanes. Criteria enabling configurational assignments to be made for the cis-trans isomers of the dioxolane series are tested. The Me-2 group causes an upfield shift (0·2 to 0·3 ppm) of a trans proton at position 5, but the reversed shift for the corresponding cis proton. This competes with, or even overwhelms the effect of the CH₂X-4 substituent, which by virtue of its pronounced preferential rotameric orientation and in comparison with a simple Me-group, has no large upfield effect on the shift of the syn-adjacent proton. Shift criteria and coupling constants J_{H-4, H-5} in cis- and trans derivatives allow further conformational insights into these 1,3-dioxolanes.     

Synthesis and 13C-NMR spectroscopic investigations of rhamnobioses

A convenient method has been developed for the synthesis of all mono- and di-O-benzyl ethers of methyl α-L-rhamnopyranoside applying a new stereoselective method for the hydrogenolytic ring-cleavage of benzylidene acetals. Using the prepared dibenzyl ethers as aglycones, the (1→2)-, (1→3)- and (l→4)-linked rhamnosyl-rhamnose derivatives (13–15) were synthesised. Hydrogenolysis of the latter compounds and subsequent acetylation gave the pentaacetates (16–18) of methyl dirhamnosides, which on saponification furnished the free methyl dirhamnosides (19–21). Acetolysis of 16–18 gave the corresponding dirhamnose-hexaacetates which were transformed into the three disaccharides by saponification. The structure of each product was investigated by ¹³C-NMR spectroscopy, and for the purpose of ¹³C-NMR studies the mono-O-methyl ethers of methyl α-L-rhamnopyranoside, the diacetates and di-O-benzyl ethers of the latter compounds, and, also the diacetates of methyl α-L-rhamnopyranoside were synthesised.It has been established that, for ¹³C-NMR investigations of oligosaccharides, the benzyl ethers of monosaccharides are more suitable model compounds than the currently used monosaccharide methyl ethers.     

17O NMR spectra of equatorial and axial hydroxycyclohexanes and 5-hydroxy-1,3-dioxanes and their methyl ethers

¹⁷O chemical shifts of axial hydroxyl groups in cyclohexanols are upfield of those of corresponding equatorial groups, but in 5-hydroxy-1,3-dioxanes the opposite is observed: the axial OH resonates downfield of the equatorial OH. The situation is the same in the corresponding methyl ethers and is, thus, not a result of intramolecular hydrogen bonding in the axial 5-hydroxy-1,3-dioxane, but appears to parallel the effect on ¹³C and ¹⁹F shifts observed in corresponding equatorial and axial 5-methyl- and 5-fluoro-1,3-dioxanes, which has been attributed to an upfield shifting effect of the antiperiplanar γ-located heteroatoms. Surprisingly, the reciprocal effect is not seen in the ring ¹⁷O shifts of the 5-hydroxy-1,3-dioxanes. A δ compression shift is seen in the ¹⁷O spectrum of trans-3,3,5-trimethylcyclohexanol (syn-axial OH and CH₃), analogous to the effect earlier reported in ¹³C spectra. Conversion of four of the alcohols to methyl ethers produces a large upfield effect on the ¹⁷O shift, larger in the cyclohexanols than in the 1,3-dioxane-5-ols. Similar upfield shifts have been recorded in the literature; their extent depends on whether the alcohols are primary, secondary or tertiary.     

Carbon-13 chemical shift assignments of the nitration products of pyrene

¹³C and ¹H nuclear magnetic resonance (NMR) spectra have been obtained in order to identify the six nitration products of pyrene—1-nitropyrene, 1,3-dinitropyrene, 1,6-dinitropyrene, 1,8-dinitropyrene, 1,3,6-trinitropyrene and 1,3,6,8-tetranitropyrene. ¹³C chemical shifts in DMSO-d₆ were assigned using empirical rules, with particular emphasis on the additivity of the substituent effects. Carbon spectra of separated dinitromixtures enabled identification of the previously unreported 1,3-isomer, and proton spectra differentiated the 1,6- and 1,8- isomers.     

Chiral Zinc(II)‐Catalyzed Enantioselective Tandem α‐Alkenyl Addition/Proton Shift Reaction of Silyl Enol Ethers with Ketimines

A new catalytic asymmetric tandem α‐alkenyl addition/proton shift reaction of silyl enol ethers with ketimines was serendipitously discovered in the presence of chiral N,N′‐dioxide/Zn^II complexes. The proton shift preferentially proceeded instead of a silyl shift after α‐alkenyl addition of silyl enol ether to the ketimine. A wide range of β‐amino silyl enol ethers were synthesized in high yields with good to excellent ee values. Control experiments suggest that the Mukaiyama–Mannich reaction and tandem α‐alkenyl addition/proton shift reaction are competitive reactions in the current catalytic system. The obtained β‐amino silyl enol ethers were easily transformed into β‐fluoroamines containing two vicinal tetrasubstituted carbon centers.     

N,N,N′,N′‐Tetrabromobenzene‐1,3‐Disulfonamide and Poly(N‐Bromo‐N‐Ethyl‐Benzene‐1,3‐Disulfonamide) as Efficient Catalysts for the Methoxymethylation of Alcohols under Solvent‐Free Conditions

Methoxymethylation of a variety of alcohols was performed using formaldehyde dimethyl acetal in the presence of N,N,N′,N′‐tetrabromobenzene‐1,3‐disulfonamide [TBBDA] and poly(N‐bromo‐N‐ethylbenzene‐1,3‐disulfonamide) [PBBS] as catalysts at room temperature and solvent‐free conditions. The methoxymethyl ethers (MOM‐ethers) were obtained with good to excellent yields.     

Synthesis of 7-[4-alkoxy-3-methoxy(hydroxy)phenyl]-10,11-dihydrobenzo[c]acridin-8(7H,9H,12H)-ones and 4-(8-oxo-7,8,9,10,11,12-hexahydrobenzo[c]acridin-7-yl)-2-methoxy(ethoxy)phenyl esters of carboxylic acids

Reactions of azomethines (Schiff bases) prepared from vanillin and vanillal ethers and 1-naphthylamine with cyclohexane-1,3-dione in butanol afforded in 40–64% yields 7-[4-alkoxy-3-methoxy(hydroxy)phenyl]-10,11-dihydrobenzo[c]acridin-8(7H,9H,12H)-ones and 4-(8-oxo-7,8,9,10,11,12-hexahydrobenzo[C]acridin-7-yl)-2-methoxy(ethoxy)phenyl esters of carboxylic acids. The reaction products presumably formed by the rearrangement of the azomethine adduct with the cyclohexane-1,3-dione proceeding by the type of Hofmann-Martius rearrangement. The structure of compounds synthesized was confirmed by the elemental analysis, UV, IR, and ¹H NMR spectra.     

Effects of functional group interactions in the chemical ionization mass spectra of some substituted cis-1,3-cyclopentane dicarboxylic acids and their derivatives

Ammonia, isobutane and methane chemical ionization mass spectra have been measured for some substituted cis-1,3-cyclopentane dicarboxylic acids and their derivatives. The relative proton affinities of the different functional groups determine the protonation site in the molecule and thereby greatly affect the fragmentation. Intramolecular catalysis clearly facilitates the elimination of water in cases where functional groups can interact with each other.     

Conformational equilibria in isomeric methylcyclohexanols studied by 13C NMR spectroscopy

The conformational equilibrium constants for isomeric methylcyclohexanols (cis- and trans-1,2-, 1,3- and 1,4-methylcyclohexanols) have been determined from peak area measurements in the completely proton decoupled low temperature ¹³C NMR spectra of the individual conformers. The ¹³C chemical shifts are discussed in terms of the additive model.     

Characterization of tri-o-methylcellulose by one- and two-dimensional NMR methods

Tri-O-methylcellulose was prepared from partially O-methylated cellulose and its chemical shifts (¹H and ¹³C), and proton coupling constants were assigned using the following NMR methods: (1) One-dimensional ¹H and ¹³C spectra of the title compound were used to assign functional groups and to compare with literature data; (2) double quantum filtered proton–proton correlation spectroscopy (¹H, ¹H DQF-COSY) was used to assign the chemical shifts of the network of 7 protons in the anhydroglucose portion of the repeat unit; (3) the heteronuclear single-quantum coherence (HSQC) spectrum was used to establish connectivities between the bonded protons and carbons; (4) the heteronuclear multiple-bond correlation (HMBC) spectrum was used to connect the hydrogens of the methyl ethers to their respective sugar carbons; (5) the combination of HSQC and HMBC spectra was used to assign the ¹³C shifts of the methyl ethers; (6) all spectra were used in combination to verify the assigned chemical shifts; (7) first-order proton coupling constants data (J_H,H in Hz) were obtained from the resolution-enhanced proton spectra. The NMR spectra of tri-O-methylcellulose and other cellulose ethers do not resemble the spectra of similarly substituted cellobioses. Although the ¹H and ¹³C shifts and coupling constants of 2,3,6-tri-O-methylcellulose closely resemble those of methyl tetra-O-methyl-β-D -glucoside, there are differences with regard to the chemical shifts and the order of appearances of the resonating nuclei of the methyl ether appendages and the proton at position 4 in the pyranose ring. H4 in tri-O-methylcellulose is deshielded by the acetal system comprising the β-1→4 linkage, and it resonates downfield. H4 in the permethylated glucoside is not as deshielded by the equitorial O-methyl group at C4, and it resonates upfield. The order of appearance of the ¹H and ¹³C resonances in the spectra of the tri-O-methylcellulose repeat unit (from upfield to downfield) are H2 < H3 < H5 < H6a < H3a < H2a < pro R H6B < H4 < pro S H6A ≪ H1 and C6a < C3a < C2a < C6 < C5 < C4 < C2 < C3 ≪ C1, respectively. Close examination of the pyranose ring coupling constants of the repeat unit in tri-O-methylcellulose supports the ⁴C₁ arrangement of the glucopyranose ring. Examination of the proton coupling constants about the C5-C6 bond (J_5,6A and J_5,6B) in the nuclear Overhauser effect difference spectra revealed that the C6 O-methyl group is predominantly in the gauche gauche conformation about the C5-C6 bond for the polymer in solution. © 1999 John Wiley & Sons, Inc.* J Polym Sci A: Polym Chem 37: 4019–4032, 1999     

Isoxazolium enolates derived from α,β-unsaturated and 1,3-dicarbonyl compounds. The through-space effect of the enolate moiety on the chemical shift of α-protons

The reaction of 4-phenyl-3,5-dihydroxyisoxazole with α,β-unsaturated cyclic ketones and 1,3-cyclic diketones was studied. β-Substituted α,β-unsaturated ketones give pairs of isomeric isoxazolium enolates. The remarkable influence of the heterocyclic betaine on the proton chemical shifts is discussed. 1,3-Cyclic diketones reacted spontaneously with 4-phenyl-3,5-dihydroxyisoxazole yielding isoxazolium enolate enols and enol ethers.     

Novel transformations of nitraminodiol diacetates

A novel scission reaction of nitraminodiol diacetates of the formula AcO[CH₂N(NO₂) _x CH₂OAc (x = 1–4) in concentrated sulfuric acid to form formaldehyde, nitramide, and 2-nitro-2-azapropane-1,3-diol disulfate was found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 3, pp 761–762, March, 1996.     

Synthesis and Enantiodifferentiating Properties of Chiral Aza Crown Ethers

Alkylation of 2-substituted (4S,5S)-4,5-bis(hydroxymethyl)-1,3-dioxolanes with 9-benzyl-1,17-diiodo-3,6,12,15-tetraoxa-9-azaheptadecane afforded new chiral aza and diaza crown ethers as a result of [1+1] and [2+2] additions. Their catalytic debenzylation gave the corresponding derivatives with a secondary amino group. The reaction of diethyl (+)-tartrate and diethyl (4S,5S)-1,3-dioxolane-4,5-diacetates with 1,8-diamino-3,6-dioxaoctane led to formation of chiral macrocyclic lactams which were reduced with lithium aluminum hydride. The resulting diaza crown ethers were tested for enantioselectivity in complex formation with L- and D-valine methyl ester by the potentiometric method. In most cases, the aza crown ethers showed better enantioselectivity than their oxygen analogs.     

Enantioselektive Verseifung der Diacetate von 2-Nitro-1,3-diolen mit Schweineleber-Esterase und Herstellung enantiomerenreiner Derivate von 2-Nitro-allylalkoholen (chirale Verknüpfungsreagenzien)

Enantioselective Saponification of Diacetates of 2-Nitro-1,3-propanediols by Pig-Liver Esterase and Preparation of Enantiomerically Pure Derivatives of 2-Nitro-allylic Alcohols (Chiral Multiple-Coupling Reagents) The reproducible enantioselective saponification of open-chain and cyclic diacetates of meso-2-nitro-1,3-propanediols (see 4b – 13b ) with pig-liver esterase (PLE) gives monoacetates (see 4c – l3c ) of > 95% enantiomeric excess. The Re enantiotopic acetate group appears to be saponified preferentially, as proved by the X-ray crystal structure analysis of three camphanoates 4d , 6d , and 7d . Elimination of H₂O or AcOH from the hydroxy acetates thus available gives derivatives of nitro-allylic alcohols (see 20 – 24 , 27 , and 29 ) which are subjected to diastereoselective Michael additions or S_N2′ substitutions.     

Nucleophilic addition to acetylenes in superbasic catalytic systems: XVII. Vinyl ethers with furyl and cycloacetal fragments: Synthesis and structure

4-Hydroxymethyl-2-(2-furyl)-1,3-dioxolane and 5-hydroxy-2-(2-furyl)-1,3-dioxane consisting of mixtures of cis- and trans-isomers react with acetylene in the superbasic catalytic system KOHH-DMSO at the atmospheric or higher pressure (80–85°C, 2–3 h) giving the corresponding vinyl ethers in 88–90% yield. The ratio of the structural and configurational isomers in vinyl ethers remains the same as in the initial compounds.