A Ravi Sankar

Synthesis and structural studies of oligomers of 6-amino-2,5-anhydro-6-deoxy-d-mannonic acid

Abstract A novel cycloetherification process involving a facile 5- exo S N 2-type ring closure by intramolecular opening of a terminal aziridine ring by a γ-hydroxyl group, led to the stereoselective synthesis of 6-amino-2,5-anhydro-6-deoxy- d -mannonic acid ( 1 ). Oligomerization of 1 by solution phase peptide coupling methods gave oligomers 2 – 5 . While most of the oligomers, in either protected or deprotected form, did not show any significant secondary structure, octamer 5 (P=H) exhibited a very strong positive band at 216 nm in its CD spectrum in MeOH and TFE, indicating the possibility of the presence of an ordered structure in solution. Its 1 H NMR spectra in various polar solvents, however, failed to produce any distinct dispersion of the amide proton chemical shifts. Compounds 1 – 5 were found to be inactive in hypoglyceamic tests in rats.

Towards the synthesis of (+)-discodermolide ☆

Abstract An approach to the asymmetric synthesis of fragments corresponding to C1C7 and C15C24 of (+)-discodermolide is reported. Key elements of the successful strategy include elaboration of two advanced fragments from a common precursor.

Stereoselective synthesis of octahydro-3bH-[1,3]dioxolo[4",5":4',5']furo[2',3':5,6]pyrano[4,3-b]quinolines via intramolecular hetero-Diels-Alder reactions catalyzed by bismuth(III) chloride

A new, efficient and stereoselective synthesis of furo[2′,3′:5,6]pyrano[4,3-b]quinoline derivatives 3 and 3′ has been achieved by intramolecular hetero–Diels–Alder reactions of aldimines generated in situ from aromatic amines and the O-allyl derivative of the chiral sugar derived aldehyde 2 in acetonitrile in the presence of a catalytic amount of BiCl3. The products are formed with extremely high trans selectivity in good to excellent yields.

Furanoid sugar amino acid based peptidomimetics: well-defined solution conformations to gel-like structures

Abstract Furanoid sugar amino acid based peptide and a dimer fold into pseudo β-turn-like structures in DMSO-d6 with intramolecular H-bonds between LeuNH and sugarOH. In CDCl3, they display a repeating β-turn-type secondary structure at lower concentrations and start to form aggregates that gradually turn into excellent organogels as the concentrations are increased, a phenomenon observed for the first time in sugar amino acid containing peptides.

‘Off-template site’ intramolecular nitrone cycloaddition (INC) reactions on sugar-derived allylic ethers—a study on the substituent effect and synthesis of furano-pyrans☆

Abstract Sugar-derived allylic ethers having one or two substituents on the terminal olefinic carbon centre were subjected to intramolecular nitrone cycloaddition (INC) reactions resulting in bicyclo[4.3.0] systems. The exclusiveness of product formation may be attributed to the effect of substituents.

Highly efficient synthesis of 3-alkyl/aryl-4-aryl-1,2,3,4-tetrahydroisoquinolines from N,N-dibenzylaminols

Abstract Substituted tetrahydroisoquinolines are synthesized in optically pure form using a Grignard reaction of N , N -dibenzyl aminol and Friedel–Crafts cyclization as the key steps.

Synthesis and conformational studies of peptidomimetics containing a carbocyclic 1,3-diacid

Abstract A rigid carbocyclic scaffold comprising of an all- cis 4,5-dihydroxy-1,3-cyclopentanedicarboxylic acid is developed. Attachment of peptide strands to the carboxylic groups of this novel template led to the peptidomimetics 2 and 3 . Conformational analysis by circular dichroism and NMR studies revealed that these molecules adopt a unique folded structure in nonpolar solvent involving intramolecular hydrogen bonding between PheNH of one strand and LeuCO (in 2 ) or GlyCO (in 3 ) of the other strand. This structure is very different from the structures observed earlier in their sugar counterparts ( 1 ). The paper describes in detail the synthesis and structural studies of compounds 2 and 3 .

Synthesis of spiro carbon linked disaccharides from d-glucose, d- and l-arabinose ☆

Abstract The synthesis of new spiro carbon linked disaccharides from d -glucose, d - and l -arabinose is described. In the present study furan is used as a masked sugar synthon, while chirality is transferred from the sugar derived chiral templates.

Development of 2,3-diazabicyclo[2.2.1]heptane as a constrained azapeptide template and its uses in peptidomimetic studies

2,3-Diazabicyclo[2.2.1]heptane 1 is used for the first time as a peptidomimetic scaffold in the synthesis of a conformationally constrained analogue 6 of azaproline (or azapipecolic acid) containing peptides.

Unusual cyclo-tetra and hexa peptidation of bis-boc-cystine with cystine-di-OMe: one step preparation of the novel 32- and 48-membered cyclotetracystine and cyclohexacystine

The unprecedented formation of 32- and 48-membered macrocycles that inscribe 4 and 6 cystine units, in the peptidation of bis-Boc-cystine with cystine di-OMe is reported.