Diastereoselectivity of the cyclization of hexos-5-uloses by Sm2-mediated pinacol coupling

Useful access to enantiomerically pure protected inositols from carbohydrates: the aldohexos-5-uloses route

The intramolecular aldol condensation of aldohexos-5-ulose derivatives of the D-xylo and L-ribo stereoseries has been studied. Only one of the four possible inososes was isolated from both stereoseries in reasonable yields (30–38%). The results obtained, together with the previous findings for the L-arabino and L-lyxo stereoseries, allowed for the rationalisation of a mechanism of the reaction based on open-transition-state models and electron-withdrawing inductive effects. Complementary reductions of the intermediate inososes were possible by changing the reaction conditions, and two isomeric inositol derivatives were obtained with complete stereoselection from each inosose. The presented approach permits us to control the configuration of three out of the six stereocentres of the inositol frame and gives access to seven of the nine inositols. Noteworthy, for the D-xylo derivative, the two-step sequence (condensation followed by reduction with NaBH(OAc)₃) represents the biomimetic synthesis of myo-inositol. Furthermore, the sugar-based pathway leads directly to enantiomerically pure selectively protected inositols and does not require any desymmetrisation procedure which is needed when myo-inositol and other achiral precursors are employed as starting materials. As an example of application of the method, the indirect selective protection of secondary inositols’ hydroxy functions, by placing specific protecting groups on the aldohexos-5-ulose precursor has been presented.

Synthetic Approaches to L-Iduronic Acid and L-Idose: Key Building Blocks for the Preparation of Glycosaminoglycan Oligosaccharides

L-Iduronic acid (IdoA) is an important monosaccharide component of glycosaminoglycans (GAGs) such as heparin, heparan sulfate and dermatan sulfate. GAGs are complex, highly sulfated polysaccharides that mediate a multitude of physiological and pathological processes via their interactions with a range of diverse proteins. The main challenge in the synthesis of GAG oligosaccharides is the efficient gram-scale preparation of IdoA building blocks since neither IdoA nor L-idose is commercially available or readily accessible from natural sources. In this review, the different synthetic approaches for the preparation of IdoA and its derivatives, including L-idose, are presented and discussed. Derivatives of the latter are often used in GAG synthesis and are elaborated to IdoA via selective oxidation at C-6 after incorporation into a GAG chain. Particular focus will be given to the preparation of IdoA synthons most commonly used for GAG oligosaccharide synthesis, and on the progress made since the last systematic review in this area.

Towards the synthesis of new dideoxy delta-dicarbonyl heptoses

The preparation of a delta-dicarbonyl sugar thorough ring-opening, by a methoxymercuration-demercuration procedure, of a 5-spirocyclopropanated d-galactose derivative, is reported. This method constitutes a new route for the transformation of a hexose into new and interesting delta-dicarbonyl sugars, synthetic precursors of cyclitols, carba- and azasugars. Moreover this is, to our best knowledge, the first reported example of an elongation to a higher sugar starting from a spirocyclopropanated saccharide.

Synthesis of D-Galactopyranosylphosphonic and (D-Galactopyranosylmethyl)phosphonic Acids as Intermediates of Inhibitors of Galactosyltransferases

Employing the Michaelis-Arbuzov reaction of 1-O-acetyl-2,3,4,6-tetra-O-benzyl-D-galactopyranose with triethyl phosphite and trimethylsilyl trifluoromethanesulfonate, α- and β-D-galactopyranosylphosphonic acids were prepared in 33 and 28% yields, respectively. (α-D-Galactopyranosylmethyl)phosphonic acid was synthesized by a five-step route from 2,3,4,6-tetra-O-benzyl-D-galactopyranose in 52% overall yield. When tested against bovine α-1,3- and β-1,4-galactosyltransferases, all three compounds showed, at best, very poor inhibitions. Both enzymes were inhibited more effectively by β-D-galactopyranosylphosphonic acid (IC50 = 17 mmol l-1 at 13.5 μmol l-1 of UDP-Gal for β4GalT).

Intramolecular aldol cyclization of L-lyxo-hexos-5-ulose derivatives: a new diastereoselective synthesis of D-chiro-inositol

The DBU-promoted intramolecular aldol condensation of two partially protected L-lyxo-hexos-5-ulose derivatives (8 and 9), in turn obtained starting from methyl beta-D-galactopyranoside, takes place with fairly good yield and complete diastereoselectivity to give 2L-(2,3,6/4,5)-pentahydroxycyclohexanone derivatives, 10 and 11. The stereoselective reduction of inosose 10 with sodium triacetoxyborohydride leads, after catalytic debenzylation, to D-chiro-inositol (1), while the sodium borohydride reduction furnishes, with opposite stereoselectivity, a derivative of allo-inositol.

Total synthesis of calditol: structural clarification of this typical component of Archaea order Sulfolobales

The original structure of calditol--that is, an open-chain branched nonitol--has recently been questioned by various research groups and cyclopentane-based structures have been proposed. To unambiguously clear up this confusion, four isomeric cyclopentane candidates 26-29 have been synthesized. Of these, compound 27 was found to be fully identical to the natural product present in Sulfolobus solfataricus (A.T.C.C. 49155). The synthesis of 27 uses a samarium-diiodide-induced pinacolization reaction of the ketoaldehyde 15 as the critical step.

Studies on the SmI2-promoted pinacol-type cyclization: synthesis of the hexahydroazepine ring of balanol

The efficiency of the samarium(II) iodide induced pinacol-type coupling for the construction of seven-membered cyclic amino alcohols has been investigated. With the acyclic carbonylhydrazones 6 and 16, good yields of the hexahydroazepines 22 and 23 were obtained (56-57%) with high trans-selectivity (= 10:1), which compares well with similar reactions generating the corresponding five- and six-membered carbocycles (Fallis, A. G.; Sturino, C. F. J. Am. Chem. Soc. 1994, 116, 7447). It is essential for ring formation that the strongly electron-donating ligand, hexamethylphosphoramide, be present, as in its absense intermolecular pinacol coupling forming the diols 27-30 is the dominant reaction. Hence, the role for HMPA appears not only to increase the rate of electron transfer but also to modulate rate constants for the subsequent reactions (cyclization and pinacol coupling) of the intermediate ketyl. This ring forming reaction has been applied to the construction of the fully functionalized hexahydroazepine ring of the PKC inhibitor, balanol. Initial attempts to develop an asymmetric version of this reaction indicate the use of chiral ligands based on the structure of HMPA.

Synthesis of Trehazolin from D-Glucose

Trehazolin (2) is a specific inhibitor of trehalase, an enzyme that cleaves the reserve carbohydrates of many insects. We describe a short and efficient synthesis of trehazolin (2) and trehazolamine (5) that mimics its hypothetical biosynthesis. Starting molecule for the synthesis of trehazolamine (5) is glucose from which three chiral centers are conserved during the reaction sequence. The remaining two chiral centers of trehazolamine (5) are formed stereoselectively in a reductive cyclization of ketooxime ether 16 and the reduction of oxime ether 18. The overall yield of trehazolamine (5) is 22% over 8 steps from 15. The synthesis of trehazolin (2) from trehazolamine (5) follows a known procedure and is achieved in 63% over 3 steps.