Cis- and trans-1,7,9-trioxadispiro[5.1.5.3]hexadecane: Synthetic studies

Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products

Fostriecin and related natural products present a significant challenge for synthetic chemists due to their structural complexity and chemical sensitivity. This review will chronicle the successful efforts of synthetic chemists in the construction of these biologically active molecules. Key carbon-carbon bond forming reactions will be highlighted, as well as the methods used to install the numerous stereocenters present in this class of compounds.

Synthesis of the C10-C24-bis-spiroacetal core of 13-desmethyl spirolide C based on a sila-Stetter-acetalization process

Synthesis of the bis-spiroacetal core of 13-desmethyl spirolide C has been completed based on a sila-Stetter-acetalization process. The acylsilane and enone partners in the Stetter reaction were prepared in 7 and 11 steps, respectively, from (S) and (R)-aspartic acid. The quaternary center at C19 in the enone moiety was controlled by relying on the Seebach's chiral self-reproduction method using an enantiopure (S)-lactic acid based dioxolanone. The final acid-catalyzed spiroacetalization provided the desired spiroacetal as a mixture of diastereoisomers in 13 linear steps. Whatever the conditions used, the non-natural transoid isomer was formed preferentially. However, both cisoid and transoid isomers were isolated pure and their structure assigned unambiguously through NMR spectroscopic studies.

A stereoselective synthesis of the C10-C31 (BCDEF ring) portion of pinnatoxin A

[reaction: see text] An efficient synthesis of the C10-C31 (BCDEF ring) portion of pinnatoxin A has been achieved. The key step is a highly stereoselective construction of the dispiroketal (BCD ring) system employing an intramolecular hetero-Michael reaction of a reversibly formed hemiketal alkoxide through the use of LiOMe.

Preorganized ligand arrays based on spirotetrahydrofuranyl motifs. Synthesis of the stereoisomeric 1,8,14-trioxatrispiro[4.1.4.1.4. 1]octadecanes and the contrasting conformational features and ionic binding capacities of these belted ionophores

The cis,trans trispiro ether 4 is accessible from several synthetic directions as a consequence of a crossover in reaction selectivity when proceeding from nucleophilic attack on the cis dispiro ketone to oxygenation of the alpha,beta-unsaturated ester 17. Its cis,cis isomer 3 was obtained in 17 steps and 14.6% overall yield from 3, 5-dimethoxybenzoic acid by making use of the alicyclic side chain in N as a "conformational lock". Although 4 shows no measurable tendency to complex with alkali metal ions, 3 binds strongly to Li(+) and Na(+) ions, as well as to CH(3)NH(3)(+). Whereas the 3eq conformation is populated in the solid state and in solution, complex formation occurs readily. (13)C NMR studies have defined slow exchange limits as the 2:1 sandwich complex with lithium ion is initially formed and transformed progressively into a 1:1 species upon the addition of more LiClO(4). Only the 2:1 complex with sodium ion is formed during comparable titration with NaClO(4). Association constants, molecular mechanics calculations, and X-ray crystallographic studies provide insight into the binding capacity of this belted tridentate ionophore.

Design and synthesis of a new class of nonmacrocyclic alkali metal host compounds

A new class of nonmacrocyclic metal ion hosts has been examined that features a polyspirocyclic framework that offers a 1,3,5-triaxial presentation of ligating centers. These compounds are easily synthesized and exploit stereoelectronic influences to preorganize the metal ion binding site. While compounds bearing oxygen substituents (X = OH, OMe) failed to show appreciable binding of alkali metals, the aminated host (X = NHBn) exhibitied strong binding with association constants (K(a)) greater than 10(7)-10(8) as measured by picrate extraction analysis.