Nucleophilic additions to tetrahydropyridinium salts. Applications to alkaloid syntheses

Abstract

tert -BUTANOL (bulk), M 0.2 0.4 0.6 0.8 1.0

A 3,4‐trans‐Fused Cyclic Protecting Group Facilitates α‐Selective Catalytic Synthesis of 2‐Deoxyglycosides

“…[8,9] Nucleophilic addition to each conformer gives different diastereomeric products. [8,10] Protecting groups can influence the reactivity of a glycosyl donor [11] and the conformer equilibrium by influencing thermodynamic factors. [12] For example, bulky silyl protecting groups trans-vicinal to each other in a carbohydrate moiety have been used to achieve higher reactivity, and in some cases selectivity, by way of inducing conformational constraints which favor molecules in axial-rich conformations.…”

An Expeditious Synthesis of the Dendrobatid Indolizidine Alkaloid 167B

“…A simple steric preference (equatorial predisposition of the propyl group, with hydride delivery on to the more remote face of the CϭC bond) provides a possible but glib rationalisation. The attractive stereoelectronic prin-ciples of Stevens [29] do not seem applicable here since it is hard to envisage the formation of an intermediate iminium ion as the reactive form of the substrate.…”

Synthesis and Biological Properties of the Cytotoxic 14‐Membered Macrolides Aspergillide A and B

“…According to the common view, Mukaiyama-type Cglycosidations and reductions of lactols with Et 3 SiH, both Lewis-acid mediated reactions, take place via a kinetically controlled, nucleophilic attack of the enol silane and the silicon hydride, respectively, on an intermediate cyclic oxocarbenium cation. [37,38] The reactions are markedly exergonic and are thus believed to occur through reactant-like transition states. [39] Thus, lactol 28 and lactol acetate 14, both derived from d-lactone 6 (Scheme 9), are transformed into the corresponding oxocarbenium cations where the main halfchair conformations, A and C, respectively, display the propenyl and benzyloxy groups in the energetically more favorable pseudoequatorial positions.…”