NIR-FT Raman and infrared spectra and ab initio computations of glycinium oxalate

The single crystals of glycinium oxalate are grown by slow evaporation technique and vibrational spectral analysis is carried out using NIR-FT Raman and FT-IR spectra. The ab initio quantum computations are also performed at HF/6-31 G(d) level to derive the optimized geometry, atomic charges and vibrational frequencies of the glycinium oxalate molecule. Vibrational analysis indicates the presence of peculiar intermolecular C-H...O hydrogen bonding interaction producing "blue shift" of C-H stretching frequency. The vibrational spectra confirm the existence of NH3(+) in glycinium oxalate. Hydroxyl vibrations with different inter and intra molecular H-bonding are analysed, supported by computed results.     

Diode laser spectrum and analysis of the ν15 band of s-tetrazine

The analysis of the diode laser spectrum of the 1103-cm^?1 band of s-tetrazine has shown it to be A type, confirming its assignment to the ν₁₅(b_2u) mode. The results of the analysis of this band have been merged with the results of the earlier analysis of the ν₁₄ band and a set of parameters consistent with all available data has been obtained for the ground, ν₁₄, and ν₁₅ states of s-tetrazine.     

Indium(III) Triflate: A Highly Efficient Catalyst for Reactions of Sugars[1]

Indium(III) trifluoromethanesulfonate has been found to be extremely efficient in catalyzing acyl transfer reactions of various carbohydrates and their derivatives. Selective acetolyses of certain benzyl ethers/isopropylidene acetals of sugars have been possible using In(OTf)₃ in Ac₂O (neat). Reaction of the per-O-acetate of 2-deoxy-2-phthalimido-D-glucose with benzyl mercaptan in the presence of In(OTf)₃ led to the formation of the corresponding thioglycoside in high yield. Facile formation and hydrolysis of the isopropylidene and benzylidene acetals of various carbohydrates have also been achieved very efficiently in the presence of In(OTf)₃. The results show great promise for In(OTf)₃ in synthetic carbohydrate chemistry.     

Synthetic Studies on Sialoglycoconjugates 9: An Efficient Method for the Selective Acetolysis of 2-(Trimethylsilyl)Ethyl Glycosides Using Ferric Chloride in Acetic Anhydride

Abstract

After the first report of the synthesis and deblocking of some 2-(trimethylsilyl)ethyl (SE) glycosides by Lipshutz and coworkers,¹ a few more papers appeared describing facile deprotection of the SE group in such glycosides or some of their derivatives.² We have employed the SE glycosides of D-galactose,^3a,b D-glucose, D-galacto-samine, D-glucosamine,^3c D-lactose,^3d and neuraminic acid^3e very extensively in our work on oligosaccharide synthesis. We have found that the SE group possesses a useful combination of properties, namely, stability over a wide range of pH which makes many experimental manipulations possible, and easy removability under certain specific but mild conditions.     

Novel Selectivity in Carbohydrate Reactions III. Selective Deprotection of p-Methoxybenzyl (PMBn) Ethers of Carbohydrates by Tin(IV)Chloride 1

In multi-step syntheses involving polyhydroxylated natural products such as carbohydrates that are variously derivatized at different positions, orthogonal removal of one or another type of protecting group is of vital importance. Discrimination of different classes of protecting groups, such as ethers, esters, etc., is often possible with a great degree of success, as for example, selective removal of an 0-acetyl by catalytic transesterification in the presence of an ether protecting group, or hydrogenolytic removal of a benzyl ether protection in the presence of ester groups such as acetates.³ Differentiation of different types of protecting groups within a given class of protecting groups has also been similarly achieved with great success, as for example, hydrogenolytic removal of a benzyl ether group in the presence of a methyl ether.³ However, the situation becomes more challenging when the same protecting group is used to mask more than one position in a polyfunctional molecule and their preferential partial deprotection is required. Selective unmaslung of one or more of such protecting groups has been achieved in some cases.4 Of particular interest to us was the regioselective deprotection of the 2-0-benzyl group of per- 0-benzylated 1,6-anhydromannopyranose mediated by SnC14 (1) and Tic14 (2). Considering the greater susceptibility of p-methoxybenzyl (PMBn) ethers to Lewis acid catalysts5 and the complexation of benzyl ethers with 14b and 24b16 we decided to investigate the action of 1 on PMBn ethers of some carbohydrates, We expected the methoxy substituent on the phenyl group in the PMBn moiety to enhance complexation with 1, possibly resulting in a facile reaction under mild conditions. Since 1 is a strong Lewis acid, the need to use chlorotrimethylsilane and anisole, as in the tin(I1)chloride- chlorotrimethylsilane-anisole system for deprotection of PMBn ethers, can be eliminated. Moreover, the complex formation in the case of 1 presents possibilities for unusual regioselectivity in partial de-0-p-methoxybenzylation reactions, a problem that has not been addressed in reports on the oxidative cleavage of PMBn ethers by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)⁷ ceric ammonium nitrate (CAN),⁸ N-bromosuccinimide (NBS)⁸ or bromine⁸.

     

Interplay between H‐Bonding and Preorganization in the Evolution of Self‐Assembled Systems

Cooperative π–π interactions and H‐bonding are frequently exploited in supramolecular polymerization; however, close scrutiny of their mutual interplay has been largely unexplored. Herein, we compare the self‐assembly behavior of a series of C₂‐ and C₃‐symmetrical oligophenyleneethynylenes differing in their amide topology (N‐ or C‐centered). This subtle structural modification brings about drastic changes in their photophysical and supramolecular properties, highlighting the reciprocal impact of H‐bonding vs. preorganization on the evolution and final outcome of supramolecular systems.     

Novel Selectivity in Carbohydrate Reactions Ii. Selective 6-O-Glycosylation of a Partially Protected Lactoside 1 2

Abstract

A novel regioselectivity was observed in the silver salt promoted glycosylation of 2-(trimethylsilyl)ethyl 3′-O-benzyl-β-D-lactoside using acetobromogalactose as the glycosyl donor. The resulting trisaccharide, obtained in 67% yield, was shown to have the newly formed β-glycosidic linkage at the O-6 position of the lactoside. This was confirmed by synthesis of the authentic product by an alternate route. The novel regioselectivity observed is attributed to the presence of the axially disposed 4′-OH group in the lactoside acceptor.

     

Recording multiple holographic gratings in silver-doped photopolymer using peristrophic multiplexing

Plane-wave transmission gratings were stored in the same location of silverdoped photopolymer film using peristrophic multiplexing techniques. Constant and variable exposure scheduling methods were adopted for storing gratings in the film using He-Ne laser (632.8 nm). The role of recording geometry on the dynamic range of the material was studied by comparing the results obtained from both techniques. Peristrophic multiplexing with rotation of the film in a plane normal to the bisector of the incident beams resulted in better homogenization of diffraction efficiencies and larger M/# value.