Recognition of cyclic, acyclic, exocyclic, and spiro acetals via structurally diagnostic ion/molecule reactions with the (CH3)2N-C(+)=O acylium ion

Reactions of the model acylium ion (CH3)2N-C(+)=O with acyclic, exocyclic, and spiro acetals of the general formula R(1)O-CR(3)R(4)-OR(2) were systematically evaluated via pentaquadrupole mass spectrometry. Characteristic intrinsic reactivities were observed for each of these classes of acetals. The two most common reactions observed were hydride and alkoxy anion [R(1)O(-) and R(2)O(-)] abstraction. Other specific reactions were also observed: (a) a secondary polar [4(+) + 2] cycloaddition for acetals bearing alpha,beta-unsaturated R(3) or R(4) substituents and (b) OH(-) abstraction for exocyclic and spiro acetals. These structurally diagnostic reactions, in conjunction with others observed previously for cyclic acetals, are shown to reveal the class of the acetal molecule and its ring type and substituents and to permit their recognition and distinction from other classes of isomeric molecules.     

Reactions of cyclopropanone acetals with alkyl azides: carbonyl addition versus ring-opening pathways

The Lewis acid-mediated reactions of substituted cyclopropanone acetals with alkyl azides were found to strongly depend on the structure of the ketone component. When cyclopropanone acetal was treated with alkyl azides, N-substituted 2-azetidinones and ethyl carbamate products were obtained, arising from azide addition to the carbonyl, followed by ring expansion or rearrangement, respectively. When 2,2-dimethylcyclopropanone acetals were reacted with azides in the presence of BF3.OEt2, the products obtained were alpha-amino-alpha'-diazomethyl ketones, which arose from C2-C3 bond cleavage of the corresponding cyclopropanone, giving oxyallyl cations that were captured by azides. Aryl-substituted cyclopropanone acetals, when subjected to these conditions, afforded [1,2,3]oxaborazoles exclusively, which were also the result of C2-C3 bond rupture, azide capture, and then loss of nitrogen. In the reactions of n-hexyl-substituted cyclopropanone acetals with alkyl azides, a mixture of 2-azetidinones and regioisomeric [1,2,3]oxaborazoles was obtained. The reasons for the different behavior of the various systems are discussed.     

Communication: Use of an α-Haloether for the Acetonation of Carbohydrates.

Cyclic acetals are important and commonly used protective groups for polyhydroxy compounds, e.g., carbohydrates. The formation of cyclic acetals has been thoroughly investigated and much is now known about the factors that influence the outcome of these reactions. Several reviews have been written on this subject.^1,2,3     

Anodic coupling reactions: the use of N,O-ketene acetal coupling partners

Intramolecular anodic olefin coupling reactions utilizing N,O-ketene acetals have been studied. Coupling reactions with both enol ether and allylsilane terminating groups were examined. The reactions involving the coupling of the N,O-ketene acetals with allylsilane groups were found to be much more efficient than corresponding reactions utilizing dithioketene acetal groups and allylsilanes. The reactions were also more efficient than the intramolecular coupling reactions between enol ethers and allylsilanes studied earlier.     

Tetranuclear BINOL-titanium complex in selective direct aldol additions

The extremely robust and water-stable tetranuclear complex Ti(4)(micro-BINOLato)(6)(micro(3)-OH)(4) (1) catalyzes the direct aldol addition with high degrees of regioselectivity at the sterically more encumbered alpha-side of unsymmetrical ketones. The formation of quaternary stereocenters is described. Oxygen-containing ene components can also be used as starting material in these reactions. When used with aliphatic aldehydes, acetals 18 or acetals of aldol adducts 20 were observed. As few as 0.2 mol % loadings with this catalyst 1 were enough to complete the reactions. Mechanistical aspects of the reactions are discussed.     

Electrostatic effects on the reactions of cyclohexanone oxocarbenium ions

Nucleophilic substitution reactions of 4-substituted cyclohexanone acetals display different selectivities depending upon the electronic nature of the substituent. Alkyl groups favor equatorial positions in the oxocarbenium ions, but alkoxy groups prefer axial conformers. The reactions of acetals with alkoxy groups constrained to either equatorial or axial positions suggest that the presence of an axial alkoxy group distorts the oxocarbenium ion, changing its inherent preferences for facial attack.     

An Examination of the Scope and Stereochemistry of the Ireland–Claisen Rearrangement of Boron Ketene Acetals

The Ireland–Claisen rearrangement of boron ketene acetals is described. The boron ketene acetal intermediates are formed through a soft enolization that obviates the use of strong bases and the intermediacy of alkali metal enolates. Yields and diastereoselectivities of these rearrangements are very sensitive to the choice of boron reagent, even among those that have been shown to effect quantitative formation of boron ketene acetals from esters. The rearrangement occurs at room temperature for all substrates with generally high levels of stereoselectivity. In contrast to previous reports using boron triflates, the use of a commercially available boron iodide reagent allows for a wider substrate scope that extends to propionates and arylacetates, as well as the previously described α‐oxygenated esters. This work also provides insight into the dynamic nature of boron ketene acetals and the ramifications of this behavior for reactions in which they are intermediates.     

Synthesis of macrocyclic acetals containing lipophilic substituents

A series of new macrocyclic acetals all containing lipophilic substituents were prepared by reacting the appropriate diols and lipophilic acetal-containing dichlorides or ditosylates. The reactions using the ditosylates gave the best yields. Several of the macrocycles contained pyridine subcyclic units. The lipophilic acetals were obtained by reacting a long-chain aldehyde with 2-hydroxyethyl chloride or tosylate and 3-hydroxypropyl chloride or tosylate. At least two of the new pyridino ligands complexed with metal ions as shown by the use of these materials as carriers for silver nitrate through a water-methylene chloride-water bulk liquid membrane system.     

Structural determination of mannopeptimycin cyclic acetals

In structure-activity relationship (SAR) studies on mannopeptimycin antibiotics, mannopeptimycin α(1) was acetalized by reacting with certain dialkyl acetals under acidic conditions. The major products of these reactions were determined to be cyclic acetals at the 4,6-positions of the terminal mannose (Man-B), by exemplary spectroscopic analyses of two typical acetalization products 2 and 3.     

Mechanism of Mukaiyama-Michael Reaction of Ketene Silyl Acetal: Electron Transfer or Nucleophilic Addition?

Mechanism of Mukaiyama-Michael reaction of ketene silyl acetal has been discussed. The competition reaction employing various types of ketene silyl acetals reveals that those bearing more substituents at the beta-position react preferentially over less substituted ones. However, when ketene silyl acetals involve bulky siloxy and/or alkoxy group(s), less substituted compounds react preferentially. The Lewis acids play an important role in these reactions. Enhanced preference for the more sterically demanding Michael adducts is obtained with Bu(2)Sn(OTf)(2), SnCl(4), and Et(3)SiClO(4) in the former reaction while TiCl(4) gives the highest selectivity for the less sterically demanding products in the latter case. These results are interpreted in terms of alternative reaction mechanisms. The reaction of less bulky ketene silyl acetals are initiated by electron transfer from these compounds to a Lewis acid. On the other hand, bulkier ketene silyl acetals undergo a ubiquitous nucleophilic reaction. Such a mechanistic change is discussed based on a variety of experimental results as well as the semiempirical PM3 MO calculations.     

Structural evidence that alkoxy substituents adopt electronically preferred pseudoaxial orientations in six-membered ring dioxocarbenium ions

Nucleophilic substitution reactions of monosubstituted tetrahydropyran acetals give opposite selectivities when a remote alkyl or alkoxy substituent is present at C-4. Coupled with earlier computational reports on the intermediates of glycosylation reactions, these results are consistent with the preference for the oxocarbenium ion intermediate derived from alkoxy-substituted tetrahydropyran acetals to occupy a pseudoaxial conformation. Theoretical, spectroscopic, and X-ray crystallographic evidence of this conformational preference in an analogous and more stable dioxocarbenium ion are provided. This contrasteric preference for an axial orientation of an alkoxy substituent likely derives from the electronically favored pseudoaxial orientation of remote electronegative substituents in neutral six-membered rings possessing electron-deficient carbon atoms.     

The chemistry of formamide acetals

The formamide acetals enter into two main categories of reactions, namely alkylation and formylation. As alkylating agents, they have been used in the synthesis of esters from acids, ethers and thioethers from phenols and aromatic and heterocyclic thiols, and the alkylation of active methines. As formylating agents, formamide acetals are useful in the synthesis of enamines from active methylenes and amidines from amines and amides. Also formamide acetals are efficient reagents for the conversion of α-hydroxycarboxylic acids to alkenes while trans-vic-diols yield epoxides. The above conversions as well as othere listed in the review proceed in almost all cases under mild conditions in high yields making these reagents valuable synthetic tools.     

Unexpected highly chemoselective deprotection of the acetals from aldehydes and not ketones: TESOTf-2,6-lutidine combination

Acetal functions are recognized as good protecting groups of carbonyl groups. Although many deprotecting methods of acetals to carbonyl functions have already been developed, there is no methodology which can deprotect acetals in the presence of ketals because the usual acidic or radical reactions occur more easily via the more stable cationic or radical intermediates from the ketals. On the other hand, this new method can proceed in a reverse manner to that described in previous reports. That is, the method can deprotect aliphatic acetals in the presence of ketals. The reaction condition is common for silylation, i.e., the TESOTf-2,6-lutidine combinations. Although the TMSOTf-2,6-lutidine combination can also deprotect acetals, it lacks chemoselectivity in deprotection of the acetals from aldehydes and ketones. The treatment of acetals with TESOTf and 2,6-lutidine in CH2Cl2 followed by a H2O workup gave the corresponding aldehydes. Of course, the compounds, which have both acetal and hydroxyl functions afforded the compounds obtained by the usual silylation of an alcohol and deprotection of an acetal without any problem. However, deprotection of the ketals from ketones was not observed during the conversion reaction of acetals from aldehydes. This chemoselectivity was confirmed in the reactions of the compounds that have the acetal and ketal in the same molecule. In both cases, the acetal functions were deprotected to give aldehydes with intact ketals. Furthermore, under the conditions described here, many functional groups such as methoxy, acetoxy, allyl alcohol, and silyloxy ether are intact. This method is very mild and available for many compounds.     

Fast-atom bombardment of the cyclic acetals: Evidence indicating the predominant involvement of condensed-phase processes in ionization

A series of cyclic acetals, the 2-phenyl-l,3 dioxolanes, and their deuterated analogues were studied by electron ionization (EI), chemical ionization (CI), and fast-atom bombardment (FAB) mass spectrometry to gain insight into the primary ionization processes for these compounds in FAB/liquid secondary ion mass spectrometry. Comparison of EI and CI data with that of FAB led to the conclusion that the predominant [M - H]+ ion observed in FAB for the nondeuterated cyclic acetals cannot to a large extent be rationalized in thermodynamic terms by known gas-phase ion-molecule reactions. Instead, a condensed-phase model in which the multicharged transition state for hydride abstraction is better solvated than the transition state for proton transfer appears to be a plausible explanation for the FAB data obtained for the nonlabeled cyclic acetals; however, this explanation is not entirely sufficient to rationalize the FAB data for the deuterated cyclic acetals. For these compounds, a dramatic time dependence of protonation versus hydride abstraction is observed that suggests that beam-induced reactive species are responsible for hydride abstraction in the condensed phase. This time dependence can be interpreted in terms of a buildup of highly reactive beam-induced species in the bulk of solution. Comparison of the results obtained for deuterated acetals with different surface activities support this hypothesis. (J Am Sot Mass     

Chemical ionization mass spectrometry of crown ether acetals using isobutane as reagent gas

Isobutane chemical ionization mass spectrometry of crown ether acetals (M), derived from ethanal, give [MH]⁺ ions from which species corresponding to 44 u are successively eliminated. Mechanisms are presented in which these units correspond to (i) ethanal and (ii) oxirane. An accompanying process is the elimination of ethyne. Similar reactions occur in the chemical ionization mass spectrometry of benzo crown ether acetals.     

Lewis base activation of Lewis acids: catalytic, enantioselective addition of silyl ketene acetals to aldehydes

The concept of Lewis base activation of Lewis acids has been reduced to practice for catalysis of the aldol reaction of silyl ketene acetals and silyl dienol ethers with aldehydes. The weakly acidic species, silicon tetrachloride (SiCl4), can be activated by binding of a strongly Lewis basic chiral phosphoramide, leading to in situ formation of a chiral Lewis acid. This species has proven to be a competent catalyst for the aldol addition of acetate-, propanoate-, and isobutyrate-derived silyl ketene acetals to conjugated and nonconjugated aldehydes. Furthermore, vinylogous aldol reactions of silyl dienol ethers are also demonstrated. The high levels of regio-, anti diastereo-, and enantioselectivity observed in these reactions can be rationalized through consideration of an open transition structure where steric interactions between the silyl cation complex and the approaching nucleophile are dominant.     

Cationic ring-opening polymerization of bicyclic amide acetals

Various bicyclic amide acetals were synthesized from the cycloaddition reactions of 2-substituted-2-oxazolines with styrene oxide. Ring-opening polymerization of the bicyclic amide acetals occurred upon heating in the presence of methyl tosylate. Characterization of the bicyclic amide acetals and their polymers was accomplished by NMR and elemental analysis. Vapor pressure osmometry showed the highest polymer molecular weight was only 2,400. The mechanisms for cycloaddition and polymerization are discussed. © 1994 John Wiley & Sons, Inc.     

Semisynthesis of long-chain alkyl ether derivatives of sulfated oligosaccharides via dibutylstannylene acetal intermediates

Long-chain alkyl ether derivatives of sulfated oligosaccharides were semisynthesized as follows: two naturally occurring red seaweed galactans (neutral agarose and kappa-carrageenan) were submitted to partial reductive hydrolysis to give neutral and sulfated oligosaccharide alditols. The neutral disaccharide alditol (1) and its trityl ether (5) were sulfated and/or alkylated through formation of their dibutylstannylene or (bis)dibutylstannylene acetals. In these reactions, the dibutylstannylene acetals of the terminal 1,2-diols in the alditol units were more reactive than those formed on the cis-diols of the galactopyranosidic units. This property allowed the regioselective monoalkylation of a neutral tetrasaccharide alditol (2), which contained eleven free hydroxyl groups, the highest selectivity ever observed with dibutylstannylene acetals. An alkylated/sulfated derivative (11) was also obtained through the regioselective alkylation of a naturally sulfated disaccharide alditol (10, a kappa-carrageenan derivative).     

Reactivity of pyridines bearing EWG with bis-(TMS)ketene acetals. Substituent-induced lactonization reaction

The nucleophilic addition of bis-(TMS)ketene acetals to pyridines substituted by electron-withdrawing groups, activated with triflic anhydride, leads to their corresponding dihydropyridine carboxylic acids in a first instance. These acids can be efficiently turned into new bicyclic lactones by either of three different methods: first, bicyclic γ-lactones were successfully obtained by an unprecedented Michael-type ring closing procedure assisted by silica gel; second, bicyclic δ-bromolactones were achieved by a NBS-promoted process; and third, the unexpected spontaneous formation of new hydroxy γ-spirolactones was observed. Along the work presented herein, the scope, limitations, and regioselectivity of these lactonization reactions were studied, confirming the utility of bis-(TMS)ketene acetals as 1,3-dinucleophiles in this type of reactions, extending thus the range of synthesis of a wide variety of new aza-compounds.     

Reactions of polyhydric phenols with nitrogen-containing acetals in the synthesis of polyphenols and heterocyclic compounds

The review systematizes and summarizes the data on the reactions of nitrogen-containing acetals with phenols. Both intra- and intermolecular reactions leading to acyclic, macrocyclic, and heterocyclic compounds are considered.