Synthesis of Enantiopure C-Glycosides and Pseudo C-Glycosides. Lewis Acid Mediated Cleavage of [3.3.1] Oxabicyclic Lactones

Discrete chains of metal atoms are of current interest from both theoretical and practical points of view. One synthetic approach to this type of compounds is based on ligandassisted reactions using polydentate ligands, which is exemplified by several families of metal string complexes supported by oligo-a-pyridylamido ligands. [1] A second approximation involves the formation of metal ± metal bonds between partially oxidized d 8 square-planar complexes, for which the platinum blues are the most representative examples. [2] Most of the reported platinum blues have been prepared from disymmetric square-planar dinuclear complexes with two amidate N-C-O-type ligands as bridges, and it has been claimed [2a±c] that the head-to-head (HH) configurations are essential to achieve the tetrametallic platinum chains. Moreover, it has been argued that the head-to-tail (HT) structures do not dimerize for steric reasons. In fact, all the reported structures of platinum blues are based on HH dimers. Other metals, for which square-planar complexes are common, such as rhodium, iridium, and gold, are suitable for forming such metallic chains, but few examples for these metals are known. [3] Herein we describe two new tetrametallic mixedvalence iridium compounds, which substantiate that dinuclear complexes with HT configurations are actually able to participate in metallic chains, while those with HH configurations generate the thermodynamically stable compounds.An appropriate precursor for the formation of iridium pyridonate blues is the binuclear complex [{Ir(m-OPy)-(CO) 2 } 2 ] (Opy 2-pyridonate (1)). It was prepared, as purple microcrystals with metallic luster, by bubbling carbon monoxide through a solution of the complex [{Ir(m-OPy)(cod)} 2 ] (cod 1,5-cyclooctadiene) in toluene. [4] While the HT configuration of the a-pyridonate ligands was found in the solid state and in solution for the diolefinic complex, a bridging ligand rearrangement occurs on carbonylation. Thus, complex 1 was found to be a 1:1 mixture of the HH and the HT isomers in solution (Scheme 1). Noteworthy, these configurational isomers were found to be in a chemical equilibrium, as shown by the 1 H two-dimensional exchange (EXSY) spectrum.Oxidation of a solution of 1 in toluene with diiodine (in a 2:1 molar ratio) at 50 8C gives immediately an EPR-silent purple solution from which a crystalline blue solid (2) was isolated in excellent yield (95 %) on addition of hexane. [5] This acetal 2, [a] 20 D À 21.7 (c 0.67 in CHCl 3 ), was spectroscopically identical to that prepared previously [3] ([a] 20 D À 22.2 (c 1.15 in CHCl 3 ) [3] ). The sense of asymmetric induction was the same as those observed in kinetic resolutions of terminal epoxides under similar reaction conditions. The enantiomeric excess of the desymmetrized product 14 was determined by chiral HPLC analysis of the corresponding benzoate 16; comparison of 16 with a sample of low enantiomeric excess (prepared by mixing 16 with its enantiomer ent-16, also synthesized by desymmetrization of 3) showed t...

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“…Selected bond lengths [] and angles[8]: Ir(1)-Ir( Primed atoms are related to the unprimed ones by the symmetry transformation À x, y, À z 1/2.…”

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confidence: 99%

Discrete Mixed-Valence Metal Chains: Iridium Pyridonate Blues

et al. 2001

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“…The following bicyclic ketones were synthesized according to literature procedures: 8-oxabicyclo[3.2.1]oct-6-en-3-one (1), [21] 1,5-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (3), [22] 2-benzyloxy-8-oxabicyclo[3.2.1]oct-6-en-3-one (5), [23] 2-benzyloxy-1,5-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (7), [24] 2-methoxy-1,5-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (9), [25] 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (11), [26] 2,4-dimethyl-8-oxabicyclo[3.2.1]octan-3-one (13), [14] 8-thiabicyclo[3.2.1]octan-3-one (15). [16] General Procedure for the Nonaflation of 8-Oxabicyclo[3.2.1]oct-6-en-3-ones (GP 1)…”

Section: Resultsmentioning

confidence: 99%

Nonaflates from 8‐Oxabicyclo[3.2.1]oct‐6‐en‐3‐ones as Building Blocks for Diversity‐Orientated Synthesis: Preparation, Heck‐Couplings and Subsequent Diels–Alder Reactions

et al. 2004

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Dedicated to Prof. H. M. R. Hoffmann on the occasion of his 70 th birthday.Abstract: A series of bicyclic alkenyl nonaflates was prepared in good yields by deprotonation of the corresponding 8-oxabicyclo[3.2.1]oct-6-en-3-ones and a sulfur relative with LDA followed by trapping with nonafluorobutanesulfonyl fluoride (NfF). The resulting compounds undergo Heck couplings to methyl acrylate under standard conditions, generally providing bicyclic dienes in satisfactory yields. For nonaflates 2 and 12 a novel base-promoted fragmentation reaction to substituted furan derivatives was observed. Several Diels-Alder reactions of the bicyclic dienes were conducted leading to polycyclic compounds. Whereas the diastereofacial selectivity with respect to the oxygen or sulfur bridge of the dienes is excellent, the exo/endo selectivity strongly depends on the substitution patterns of the bicyclic diene and the dienophile. The results presented demonstrate the potential of bicyclic nonaflates to serve as versatile building blocks in diversity-orientated synthesis.

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“…[13] Application to marine metabolite synthesis: To exemplify our approach, we have targeted three segments of prominent polyoxygenated marine natural products i) phorboxazole A and B, ii) ()-discodermolide and iii) spongistatin 1. [14] Again, the polar solvent acetonitrile and equimolar trimethylsilyl triflate are important for Cglycosidation. After reduction of the carboxylic acid to the alcohol (94 %) and quantitative protection of the alcohol as TIPS-ether ()-31 the nitrile group was converted into methyl ketone ()-32 using MeMgBr in toluene, ultrasound and then 0.5 n HCl (76 %, 93 % based on recovered starting material).…”

Section: Introductionmentioning

confidence: 99%

“…Furthermore, various monocyclic methyl acetals, here exemplified by eight stereochemical tetrades 14 3 21, serve as precursors of Cglycosidic tetrahydropyrans. [14,29] Chem. Eur.…”

Section: Introductionmentioning

confidence: 99%

High Stereochemical Diversity and Applications for the Synthesis of Marine Natural Products: A Library of Carbohydrate Mimics and Polyketide Segments

Misske

Hoffmann

2000

Chem. Eur. J.

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We have developed a powerful concept for the rapid assembly of a series of twenty-four homochiral building blocks from simple racemic trans-2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one. The series comprises eight stereochemical pentades of anomeric [3.3.1]lactone acetals, eight stereochemical tetrades of anomeric carbohydrate mimics, and eight stereotetrades of acyclic polypropionate units. The utility of these enantiopure materials (average 94% ee) in natural product synthesis is demonstrated and shown to complement the popular aldol method.

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Synthesis of Enantiopure C-Glycosides and Pseudo C-Glycosides. Lewis Acid Mediated Cleavage of [3.3.1] Oxabicyclic Lactones

Abstract: Lewis acid mediated cleavage of substituted 2,9-dioxabicyclo[3.3.1]nonan-3-ones affords enantiopure 2,6-trans-C-glycosides in excellent yield. Variation of nucleophile, solvent and Lewis acid were investigated.

Cited by 30 publications

References 3 publications

Discrete Mixed-Valence Metal Chains: Iridium Pyridonate Blues

Discrete Mixed-Valence Metal Chains: Iridium Pyridonate Blues

Nonaflates from 8‐Oxabicyclo[3.2.1]oct‐6‐en‐3‐ones as Building Blocks for Diversity‐Orientated Synthesis: Preparation, Heck‐Couplings and Subsequent Diels–Alder Reactions

High Stereochemical Diversity and Applications for the Synthesis of Marine Natural Products: A Library of Carbohydrate Mimics and Polyketide Segments

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