L. Monti

A highly diastereoselective synthesis of dl-oleandrose

Abstract Racemic oleandrose (2,6-dideoxy-3-O-methyl-arabino-hexone) has been obtained starting from the Diels-Alder adduct between 4-methoxy-3-buten-2-one and isobutyl vinyl ether, which was converted into a mixture of the isobutyl β- and α-oleandrosides through hydroboration-oxidation. The latter reaction is highly diastereoselective since attack by borane takes place exclusively anti to the OMe group: none of the diasteroisomeric cymarosides are formed. The only side-products are small amounts of the isobutyl β- and α-amicetodies formed by a demthoxylation occuring during the hydroboration step.

Synthesis of d-amicetose and l-rhodinose from l-glutamic acid

Abstract l -Glutamic acid has been converted into a separable mixture of d -amicetono- and l -rhodinono-γ-lactones by a sequence involving transformation into (S)-γ-carboxy-γ-butyrolactone (2), conversion of 2, conversion of 2 into the corresponding methyl ketone by the diazoketone route, and selective reduction with zinc borohydride or borane-methyl sulfide. Reduction of the two lactones with di-isobutylaluminium hydride gave the corresponding deoxy sugars. In spite of some improvements in the preparation of 2, the optical yield of this step was only ∼80%, but one crystallisation from chloroform raised the optical purity to 96%. The subsequent steps produced a loss in optical purity of only 4%.

Regio- and stereoselectivity of the 3-membered ring opening of some 3,4-epoxytetrahydropyrans and of the corresponding epibromonium ions.

Abstract The reactions of 3,4-epoxytetrahydropyran and of its cis - and trans -2-methyl derivatives with hydrogen halides and with lithium aluminum hydride have been investigated in order to assess the influence of an O atom in the β position on the regioselectivity of the epoxide ring opening. All these reactions exhibit a high preference for nucleophilic attack at position 4, which decreases moderately only when the inductive effect of the O atom and the stereoelectronic requirements of the attack act in opposite directions. Similar trends are observed in the reactions of the 5,6-dihydro-2 H -pyrans with NBA, which occur with preferential nucleophilic attack by water at position 4 of the intermediate epibromonium ions. A remarkably high preference (96%) for electrophilic attack syn to the 2-Me group is observed in the latter type of reaction, in accordance with a previous proposal of a mechanism in which the nucleophilic step is rate determining.

Conformational analysis of 3,4-epoxytetrahydropyran by n.m.r. spectroscopy

Abstract The conformational equilibrium of the title compound has been determined by correlating its n.m.r. parameters with those of its 2,2,6,6-tetradeuterio derivative and trans - and cis -2- tert -butyl-4,5-epoxytetrahydropyran. The high preference (Δ G ∼ −0.8 kcal/mol) for the half-chair conformation in which the pyranoid oxygen is furthest from the oxirane oxygen atom can be interpreted in terms of electrostatic interactions between the two oxygen atoms.

The mechanism of N-haloamide promoted electrophilic additions. High regio- and stereoselectivity in the conversion of 2-tert-butyl-3,6-dihydro-2H-pyran into bromohydrins and in the opening of the corresponding epoxides

Abstract The reactions of cis- and trans-2-tert-butyl-4,5-epoxytetrahydropyran with HBr and with LAH have been examined as a model for the nucleophilic step of the reaction of the corresponding olefin with NBA in aqueous dioxane. A remarkable 90:10 preference for electrophilic attack syn to the tert -butyl group in the NBA reaction is found and shows that the two epoxides, as well as the intermediate epibromonium ions, undergo nucleophilic attack with high preference for diaxial opening, even when this requires reaction at carbon 5, which is more subject than carbon 4 to the unfavourable inductive effect of the pyran ring oxygen. These results constitute a further proof in favour of a mechanism of N-haloamide promoted electrophilic additions in which the electrophilic step is rapidly reversible and product composition is determined during the nucleophilic step.

The steric course of the reactions of 2,3-dihalotetrahydropyrans with grignard reagents

Abstract The steric course of some reactions of 2,3-dihalotetrahydropyrans with sodium methoxide and with methyl- and phenyl-magnesium bromides has been reinvestigated. Whereas the methanolysis of the cis - and trans -dichlorides occurs with practically complete inversion, it has been confirmed that their reactions, as well as those of the dibromides, with methyl-magnesium bromide are non-stereospecific, yielding mixtures of trans - and cis -3-halo-2-methyl-tetrahydropyrans in ratios that are independent of the configurations of the starting dihalides. It has been further established that Grignard reagents cause equilibration of cis - and trans -dihalide mixtures in a reaction that is much faster than the Grignard coupling step.