A versatile stereocontrolled synthesis of 2-deoxyiminosugar C-glycosides and their evaluation as glycosidase inhibitors.

A highly enantioselective synthesis of (R,S) or (S,S)-2,6-disubstituted dehydropiperidines has been previously achieved through Sn/Li transmetalation of the corresponding stannylated dehydropiperidines or of their precursors. Herein, we successively consider their Upjohn's syn dihydroxylation and their anti-dihydroxylation via an epoxidation reaction followed by epoxide opening reaction. The stereochemical course of these reactions was first reported including the use of appropriate protecting groups before considering the conversion of the obtained compounds into NH or NMe iminosugar hydrochlorides. A primary evaluation of the designed iminosugar C-glycosides as glycosidase inhibitors suggests candidates for the selective inhibition of α-galactosidase, amyloglycosidase and naringinase. Beyond the reported results, the method constitutes a highly modulable route for the synthesis of well stereodefined iminosugar C-glycosides, an advantage which might be used for the design of iminosugars to enhance their biological properties.

Preparation of enantiomerically enriched α-aminoorganostannanes and their applications in stereoselective synthesis.

This review deals with the preparation of chiral, nonracemic α-aminoorganostannanes and their applications in asymmetric synthesis. The pioneering works in this field date back almost 20 years ago and since then extensive research has been carried out to develop efficient and selective routes to highly enantioenriched α-aminoorganostannanes. The facile Sn/Li transmetalation of these compounds by n-BuLi has led to various applications in stereoselective synthesis. Selected examples using chiral α-aminoorganostannanes as starting materials will be reported.

Synthesis of highly enantioenriched chiral alpha-aminoorganotins via diastereoselective ring opening of chiral N-(arenesulfonyl) 2-tributylstannyloxazolidines.

trans-N-(Arenesulfonyl)-2-tributylstannyloxazolidines derived from (R)-phenylglycinol were diastereoselectively ring-opened by soft organometallic reagents in the presence of BF(3).OEt(2). Both higher order organocuprates and allyltributyltin afforded the desired products in good-to-excellent yields and high diastereoselectivities (dr up to 99/1). The stereochemical assignments of tributylstannyl-beta-aminoalcohols were firmly established from NMR data and after determination of several radiocrystallographic structures. Mechanisms were proposed in order to rationalize the observed selectivities.

Syntheses of Theaspirone and Vitispirane via Palladium(II)-Catalyzed Oxaspirocyclization.

Total syntheses of theaspirone (A and B) and vitispirane (A and B) are described. The key step in the syntheses is the palladium(II)-catalyzed intramolecular oxaspirocyclization of diene alcohol 4 to either vitispirane or the allylic alcohol 9. The outcome of the oxaspirocyclization is very much dependent on the solvent employed. In water-acetic acid (4:1) a 1:1 mixture of the diastereomeric alcohols 9A and 9B was exclusively formed. In water with 8 equiv of a strong non-nucleophilic acid, vitispiranes A and B (1:1) were obtained. An alternative procedure to obtain vitispirane with the use of LiCl and K(2)CO(3) is described. In the latter reaction vitispirane B is formed preferentially. This result is explained by an equilibrium between the two possible pi-allyl complexes 5A and 5B, the kinetically favored 5B being transformed into vitispirane 3B before isomerization to 5A occurs.

Stereodivergent synthesis of iminosugars from stannylated derivatives of (S)-vinylglycinol.

An original access to iminosugars from a cis/trans mixture of stannylated oxazolidinones 5 is reported. The dehydropiperidines 7-trans and 7-cis were obtained stereoselectively with an RS and SS configuration depending on the order of the Sn-Li transmetalation (followed by electrophilic trapping) and of the ring closing metathesis reactions due to the stereoselective epimerization of the α-aminoanion intermediate. The dehydropiperidines 7-trans and 7-cis were subsequently used for the synthesis of enantiopure homonojirimycin analogs.

Electrochemical cleavage of sulfonamides: an efficient and tunable strategy to prevent ß-fragmentation and epimerization.

The electrochemical reduction of sensitive sulfonamides is described. The addition of a benzoyl group on the nitrogen atom facilitates the reductive cleavage of sulfonamides preventing ß-fragmentation and epimerization. This strategy was successfully applied to the cyclopropylamine and to α-amino stannanes.

Preparation of allyltin reagents grafted on solid support: clean and easily recyclable reagents for allylation of aldehydes.

The preparation of polymer-supported allyltin reagents was shown to be possible for both unfunctionalized and functionalized allyl units. These reagents were treated with aldehydes in the presence of cerium(III) or indium(III) salts to afford high yields of homoallylic alcohols, practically uncontaminated with organotin residues (less than 5 ppm). Some mechanism aspects are briefly discussed and the potential for regeneration and reuse of these supported reagents is pointed out.

Polymer-supported organotin reagents for regioselective halogenation of aromatic amines.

[reaction: see text] Polymer-supported triorganotin halides were used in the halogenation reaction of aromatic amines. Treatment of aromatic amines with n-butyllithium and polymer-supported organotin halides gave the corresponding polymer-bound N-triorganostannylamines, which by treatment with bromine or iodine monochloride gave the para-halogenated aromatic amines with high yields and high selectivities. The polymer-supported organotin halides reagents regenerated during the course of the halogenation reaction can be reused without loss of efficiency. The presence of tin residues in halogenated aromatic amines was also investigated and evaluated at under 20 ppm after three runs.

Preparation of gamma-siloxyallyltributylstannanes and their use in the synthesis of (+/-)-1-deoxy-6,8a-di-epi-castanospermine.

gamma-siloxyallyltributylstannanes were selectively obtained as E or Z isomers from beta-tributylstannylacrolein upon reaction with lithium or magnesium alkylcyanocuprates. The ability of the reagents to give a high syn selectivity when added to iminium salts has been used for the efficient synthesis of (+/-)-1-deoxy-6,8a-di-epi-castanospermine from succinimide. The key step of the synthesis was the allylstannation of the N-allyliminium intermediate followed by ring closing metathesis.

Synthesis of oligothiophene-bridged bisporphyrins and study of the linkage dependence of the electronic coupling.

A set of twelve porphyrin dimers has been prepared to give information on how the type of connectivity between a porphyrin core and a bridge can influence the interporphyrin electronic interaction. The new porphyrin systems are substituted directly at the meso position with an oligothiophene chain tethered either with a single C-C sigma bond, a trans ethylenyl group, or a acetylenyl group. The compounds are easily obtained by palladium-catalyzed cross-coupling reactions (Stille, Heck, and Sonogashira) between 5-iodo-10,15,20-(3,5-ditert-butylphenyl)porphyrin and the appropriate oligothiophene derivative. This synthetic approach is straightforward and very effective for preparing oligothiophene-based prophyrin systems. The absorption spectra and the fluorescence properties of the dimers demonstrated the crucial importance of the characteristics of the chemical bond used to connect the bridge to the porphyrin unit. The magnitude of the electronic communication can thus be significantly modulated by altering the type of bond connectivity used to link the chromophore to the bridge. The present work shows that an oligothiophene spacer is a viable class of linker for connecting porphyrins, and that a quaterthiophene appended with ethynyl linkages affords a high electronic interaction over a distance as large as 28 A. A detailed computational study of these dimers has clarified the conditions needed for a conjugated system to behave as a molecular wire. These conditions are full planarity of the molecule and proper energy matching between the frontier orbitals of the bridge and the porphyrin. Intermolecular energy transfer in asymmetrical dyads composed of a zinc porphyrin and a freebase porphyrin has been studied by fluorescence spectroscopy. In all systems, this process is more than 98% efficient, and its rate constant decreases steadily in the order 4ZH > 1ZH > 3ZH approximately 2ZH. Thus, the largest rate (kEnT = 1.2 x 10(11) s-1) was found in the dyad linked with bisethynyl quaterthiophene, which represents the longest bridge within the series. These results clearly demonstrate that strong communication and also efficient photoinduced processes can be promoted over a large distance if the electronic structure of the molecular connector is appropriately chosen.