Achievement of ring inversion of myo-inositol derivatives due to silyloxy/silyloxy repulsion enhanced by the trans-substituents on both sides

Allylic Strain as a Stereocontrol Element in the Hydrogenation of 3-Hydroxymethyl-cyclohex-3-en-1,2,5-triol Derivatives. Synthesis of the Carbasugar Pseudo-2-deoxy-α-D-glucopyranose

By adaptation of a literature method developed in the glucose series and standard protecting group manipulations a 2-deoxy-D-glucose derivative is converted to a series of 1R,2R,5R-3-hydroxymethyl-cyclohexen-1,2,5-triol derivatives whose reduction to the corresponding diastereomeric D-gluco and L-ido-2-deoxy carbasugars is studied. When the hydroxymethyl group is tied up in a cyclic isopropylidene acetal with the adjacent 6-hydroxy group the cis-fused products with the ido-configuration are formed preferentially whereas protection of the hydroxymethyl group as a methoxymethyl ether results in the formation of the D-gluco isomer on hydrogenation over Raney nickel. This result is rationalized in terms of the minimization of allylic strain in the course of the reduction, enables the preparation of the carbasugar pseudo-2-deoxy-α-D-glucopyranose, and suggests that the conformation of the side chain is an important control element in the chemistry of the pseudosugars as it is in the sugars themselves.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Turning the Nitrogen Atoms of an Ar2 P-CH2 -N-N-CH2 -PAr2 Motif into Uniquely Configured Stereocenters: A Novel Diphosphane Design for Asymmetric Catalysis

Hexahydropyridazines with CH₂ PAr₂ groups at both N atoms are newly designed 1,4-diphosphanes and were synthesized for the first time. Their N atoms assume a single configuration under the influence of stereocenters at C-5 and C-6. In the solid state, these N-atoms bind the CH₂ PAr₂ substituents axially. Combined with Pd⁰ , N,N'-chiral diphosphanes of this kind catalyzed Tsuji-Trost type allylations of dialkyl malonates with racemic 1,3-diphenylallyl acetate efficiently and with up to 91 % ee.

Synthesis of 3,6-O-(o-xylylene)glucopyranosyl fluoride, an axial-rich glycosyl donor of β-glycosylation

Despite the reported complete β-selectivity in glycosylation with 2,4-di-O-benzyl-3,6-O-(o-xylylene)glucopyranosyl fluoride, its preparation has been inefficient. This paper describes an improved route for the donor, including the formation of the 3,6-bridge on 1,2,4-orthoacetylglucose, the preparation of which was also refined, along with a discovered feature that the 3,6-bridged glucose prefers the furanose form. Although this feature made the synthesis of the desired glucopyranosyl donor difficult, application of thermal glycosylation solved the problem. With a modifiable intermediate, the improved availability of the donor would expand the applications.

The 4-(tert-butyldiphenylsiloxy)-3-fluorobenzyl group: a new alcohol protecting group, fully orthogonal with the p-methoxybenzyl group and removable under desilylation conditions

A new benzyl ether-type protecting group for alcohols, the 4-(tert-butyldiphenylsiloxy)-3-fluorobenzyl group, is introduced. The protecting group is introduced by means of the readily prepared benzyl bromide and is cleaved with tetrabutylammonium fluoride in dimethylformamide under microwave irradiation. The fluoride substituent provides stability to oxidizing conditions, such that the new protecting group is fully compatible with the removal of p-methoxybenzyl ethers with DDQ. Applications of the new protecting group in the direct stereocontrolled synthesis of beta-mannopyranosides are presented.

Synthetic and structural studies on macrocyclic amino cyclitols--conformational chameleons

Starting from quinic acid the synthesis of 1,4-butanediol-linked macrocyclic aminocyclitols 30, 32, 34, 36 and 38 is described. Assembly was achieved by olefin cross-metathesis of appropriate cyclohexyl allyl ethers followed by ring-closing metathesis of bis-O-allyl homodimers. In all five cases studied, the only products that were formed were those resulting from direct ring-closing metathesis; the formation of larger rings was not detected. These macrocycles exhibited diverse conformational behaviour which included formation of stable separable conformers 31a and 31b as well as conformationally dynamic macrocycles 35 in which a ring flip in one cyclohexane chair conformer induces a ring flip of the other cyclohexane ring through the linking chains of the macrocycles. The activation energy for the inversion of the chair conformation in this process was determined to be about 38 kJ mol(-1), which is about 7 kJ mol(-1) lower than the activation energy for the ring flip of the unsubstituted cyclohexane ring. In all cases, the conformational studies strongly suggest that intramolecular H-bonding between 1,3-diaxially oriented amido and alcohol or ether groups exerts a decisive contribution to the overall stabilisation of the preferred cyclohexane chair conformation.

1,4-Diferrocenylbutane-1,4-dione

In the crystal structure of the title compound, [Fe(2)(C(5)H(5))(2)(C(14)H(12)O(2))], each carbonyl group is coplanar with the adjacent cyclo-penta-dienyl ring, thus maximizing the π-orbital overlap and electronic inter-actions between the groups. In the crystal structure, there are inter- and intra-molecular C-H⋯O contacts.

Highly β-Selective and Direct Formation of 2-O-Glycosylated Glucosides by Ring Restriction into Twist-Boat

Three disaccharide donors, ethyl 2-O-(2,3,4-tris-O-tert-butyldimethylsilyl-beta-D-xylopyranosyl)-3,4,6-tris-O-tert-butyldimethylsilyl-1-thio-beta-D-glucopyranoside, ethyl 2-O-(2,3,4-tris-O-tert-butyldimethylsilyl-alpha-L-rhamnopyranosyl)-3,4,6-tris-O-tert-butyldimethylsilyl-1-thio-beta-D-glucopyranoside, and ethyl 2-O-(2,3,4,6-tetrakis-O-tert-butyldimethylsilyl-beta-D-glucopyranosyl)-3,4,6-tris-O-tert-butyldimethylsilyl-1-thio-beta-D-glucopyranoside, produced a highly beta-selective glycosidation up to alpha/beta = 2/98 using MeOTf as the activator and 2,6-lutidine as an additive. The ring conformations of the glucose part in these disaccharide donors were all restricted to 3S1, and the conformation would lead to the stereoselectivity.

Inositols: The Effect of Bulky Substituents on Conformations

Inositols have been converted into their triphenylsilyl derivatives. In all cases only five substituents could be introduced, the remaining free hydroxy group being hindered. The derivative of myo-inositol and one of two of neo-inositol were found to be in the inverted chair conformation, but those of other inositols are in the normal chair form. The structures of two penta-substituted derivatives have been determined by X-ray crystallography.

Facile and regioselective preparation of partly O-benzylated d-glucopyranose acetates via acid-mediated simultaneous debenzylation–acetolysis

Fully O-benzylated methyl alpha-D-glucopyranoside shows a steady order in stepwise debenzylation when it is treated with sulfuric acid in acetic anhydride. Based on the order of debenzylation, regioselective preparations of 2,3,4-tri-, 2,3-, 2,4-, 3,4-di-, and 2-O-benzyl-D-glucopyranose acetates were facilitated in greater than 80% yields. The key points of the preparative reactions were the control of the acid strength and choice of suitable substrates.