Showing papers by "J. Cristobal Lopez published in 2004"

Further Insight into “Matching” of Donors and Acceptors via Reciprocal Donor Acceptor Selectivity (RDAS) Studies

Abstract: Experiments with n-pentenyl glycosides and thioglycosides show that protecting groups have a dominant effect in controlling regioselective glycosidations whereby the best 'match' of donor with acceptor emerges. The experiments also show that for this outcome to be observed, the armed and disarmed donors must be allowed to compete on an equal level. If this condition is not met, coupling may still occur, but it will not be the best 'match'.

Reciprocal Donor Acceptor Selectivity (RDAS) and Paulsen′s Concept of “Match” in Saccharide Coupling

Abstract: Three modes of selectivity — enantio, stereo, and regio — are critically important for efficient organic synthesis. In the sub-domain of oligosaccharide synthesis, the first is (usually!) irrelevant in view of the chiralities of the reacting partners. The challenge posed by the other two is compounded by the reality that “protecting groups” are the major, if not only, instruments by which control can be exercised. The role that O2 protecting groups play in stereocontrol of coupling reactions was formulated 60 years ago. In this microreview, evidence is presented which shows that O2 protecting groups also exercise regiocontrol. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Stereodivergent Synthesis of 5a-Carba-hexopyranoses from Carbohydrates via 6-exo-dig Radical Cyclization: Preparation of 5a-Carba-β-D-manno-, α-D-allo-, β-L-talo- and α-L-Gulopyranose Pentaacetates from D-Mannose.

Abstract: Four carbasugars, 5a-carba-β- d -manno-, α- d -allo-, β- l -talo- and α- l -gulopyranose pentaacetates, have been prepared in a stereodivergent manner from d -mannose. Alkynyl derivatives of 2,3:4,6-di- O -isopropylidene- d -mannopyranose, which are prepared by homologation at C-1, by reaction with phenyl acetylide, undergo a 6- exo - dig radical cyclization, from a radical located at C-5, to yield a mixture of highly functionalized cyclohexanes. Some of these compounds, after transformation of their exocyclic double bond in a hydroxy function, were correlated with polyhydroxylated cyclohexanes, which were then selectively deoxygenated either at position C-4 or C-5a (carbohydrate numbering) to afford carbasugars of the d - or l - series.

A Novel Entry to C-Glycals via Diethylzinc-Mediated Umpolung of π-Allyl Palladium Derived from 1-exo-Methylene 2,3-Anhydrofuranoses.

Abstract: The reaction of aldehydes with allylzinc reagents resulting from the umpolung of π-allyl palladium complexes from 1-exo-methylene 2,3-anhydrofuranoses provides a novel entry to C-glycals.

An Expeditious Entry to Carbohydrate Derived Enynes and Ene-Diynes via Sonogashira Coupling of Halo-exo-Glycals.

Abstract: Sonogashira coupling of bromo or iodo-exo-glycals, readily prepared from 1-exo-methylene furanoses and pyranoses, provides an efficient entry to furanose- and pyranose-derived enyne and ene-diyne moieties found in biologically relevant structures.

Stereoselective Synthesis of C- and N-Ketosides by Lewis Acid Catalyzed C- and N-Glycosidation of Alkynyl, Phenyl, and Methyl Ketoses.

Abstract: C-Ketosides can be prepared conveniently, in a stereoselective manner, from alkynyl, phenyl and methyl glucopyranose hemiketals by reaction with carbon nucleophiles in the presence of Lewis acids. The reaction of the hemiketals with trimethylsilyl azide provides an efficient route to the corresponding N-ketopyranosides. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)