manno- versus gluco-Selectivity in reductions of 2-keto-β-d-arabino-hexopyranosides

Self Cite

The susceptibility of acylated 2‐ketohexosyl halides to C‐homologation is demonstrated with the easily accessible tri‐O‐benzoyl‐α‐D‐arabino‐hexos‐ulosyl bromide 1 as the model compound. C‐Glycosidation with an electrophilic anomeric carbon requires prior carbonyl protection, to avoid carbonyl addition by the C‐nucleophile, for example, as the cyanohydrin. Silver triflate‐promoted reaction with the silylenol ether of acetophenone then efficiently yields the β‐phenacyl product. With thermal (AIBN) or photochemical induction, 1 smoothly generates an anomeric radical − comparatively electrophilic, due to its capto‐dative substitution − which exclusively traps hydrogen in the presence of tributyltin and electron‐deficient alkenes. With allyltributylstannanes, however, it reacts with high stereoselectivity to afford α‐C‐allyl glycosiduloses. The α‐bromoketone functionality in ulosyl bromide 1 is susceptible to Reformatsky conditions: treatment with zinc‐copper couple readily generates the 1,2‐enolate, a most simple anomeric nucleophile, which effectively adds to aldehydes to give α‐C‐hydroxyalkyl glycosiduloses or α‐C‐disaccharides (with sugar aldehydes) with a high degree of double stereoselection. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Section: Radical C-glycosidationmentioning

confidence: 99%

C‐Glycosidations of a 2‐Ketohexosyl Bromide with Electrophilic, Radical, and Nucleophilic Anomeric Carbons

Lichtenthaler¹,

Lergenmüller²,

Schwidetzky³

2003

Self Cite

“…When diborane was used as the reducing agent, however, the reduction became gluco-selective, as also previously observed in the -hexosidulose series. [8] Uloside 25, for example, on treatment with the borane/pyridine complex in THF at Ϫ78°C for 30 min, underwent a high-yield conversion (80%) to the 6-deoxy-β--glucoside 35. The ulosyl donor approach thus not only shows promise for the generation of β--rhamnosidic linkages but, through changing the reducing agent, for the acquisition of 6-deoxy-β--glucosides as well.…”

Section: Model Glycosidations Of Ulosyl Donors 1 ؊5 and Uloside Reducmentioning

confidence: 99%

“…When selectrides are used rather than borohydride only, the carbonyl reduction is also essentially manno-specific. [8] Besides its preparative simplicity, the procedure has the further advantage of providing the β--mannosides with free 2-OH groups (i.e., mannosyl acceptors ready for further glycosidation towards the oligosaccharides with β--Man-(1Ǟ2)--Man linkages).…”

Section: Introductionmentioning

confidence: 99%

Efficient Generation of β‐L‐Rhamnosidic Linkages by the 2‐Ulosyl Donor Approach: Synthesis of a Trisaccharide with a Central β‐L‐Rhamnose Unit

Lichtenthaler

Metz

2003

Practical procedures for the production of variously blocked 6‐deoxy‐α‐L‐arabino‐2‐ketohexosyl bromides from L‐rhamnose have been developed. These compounds are highly useful as indirect β‐L‐rhamnosyl donors: they undergo β‐specific glycosidations under Koenigs‐Knorr conditions, and the 2‐keto group of the resulting 6‐deoxy‐β‐L‐hexosiduloses is reduced with high β‐L‐rhamno selectivity. The straightforward application of this 2‐ulosyl donor approach for the synthesis of β‐L‐rhamnose‐containing di‐ and trisaccharides is demonstrated. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

“…2 Ǟ 5. Since reduction of the β-d-glycosiduloses formed is highly manno-selective, [3,6] the two-step-sequence, 2 Ǟ 5 Ǟ 6, has evolved into an expedient procedure for the generation of β-d-mannosides, the ''ulosyl donor approach'', which has successfully been applied to the synthesis of β-d-mannose-oligosaccharides up to the hexasaccharide level, [3,7] as well as to the generation of β-lrhamnosidic linkages. [8] A gluco-selective reduction may also be accomplished in a straightforward manner by using borane as the reductant, e.g.…”

Section: Introductionmentioning

confidence: 99%

“…5 Ǟ 7, providing 2-OH-free β-d-glucosides. [6] As α-bromoketones, ulosyl bromides are also amenable to Reformatsky reactions, leading to C-glycosides on exposure to zinc and aldehydes (2 Ǟ 8). [9] Modest though is the α-selectivity attainable in glycosidations, which result in α,β-mixtures that require chromatographic separation, e.g.…”

Section: Introductionmentioning

confidence: 99%

Pyrano[2,3‐b]dioxanes through Bisacetalic Annulation of 2‐Ketosugars to Glycol

Cuny¹,

Lichtenthaler²,

Lindner³

2004

The utility of 2‐ketohexosyl (“ulosyl”) bromides has been extended to the straightforward generation of cis‐fused pyrano[2,3‐b]dioxanes (1,4,5‐trioxadecalins) of type 11 and 13 by β‐specific and α‐selective glycosidation with glycol, and subsequent intramolecular hemiketalization. Mild basic conditions elicit highly stereoselective rearrangements to trioxadecalinones 22−25, representing the structurally and configurationally correct framework of steroidal natural products in which the sugar portion is doubly glycosidically linked to the aglycon. Exposure of 11 or 13 to mineral acid results in an essentially stereospecific contraction of the pyranoid ring to yield trioxaspiro[4,5]decanes. Mechanistic considerations of the unusual rearrangements observed are discussed in light of available experimental facts and the scarce existing analogies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)