Polyether frameworks

Both the Tebbe and Petasis reagents, Cp2CH2ClAlMe2 and Cp2TiMe2, effect the direct conversion of alkenic esters to dihydropyrans. This olefin metathesis has been successfully applied to the synthesis of complex polyether frameworks <96JA1565,96JA10335>. 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

The construction of the highly functionalized cyclic polyether framework 339 in maitotoxin has been achieved by efficient coupling of the alkenylstannane 338 with the enol triflate 337 [159]. The smooth coupling occurs in the presence of CuCl (2 equivalents) and K2CO3 in THF. 

Compound 20 is a third-generation dendrimer based on a [Fe(tpy)2] " core (tpy = 2,2 6, 2"-terpyridine) surrounded by a polyether framework [93]. Electrochemical characterization in CH2CI2 or THF of dendrimers of generation 0-3 indicated that the reversibility of both the metal-centered oxidation and of the tpy-... 

Hong, F.-T., Paquette, L. A. Olefin metathesis in cyclic ether formation. Direct conversion of olefinic esters to cyclic enol ethers with Tebbe-type reagents. Copper(l)-promoted Stille cross-coupling of stannyl enol ethers with enol triflates construction of complex polyether frameworks. Chemtracts t997, 10,14-19. 

Sasaki, M. et al.. New strategy for convergent synthesis of trans-fused polyether frameworks based on palladium-catalyzed Suzuki cross-couphng reaction. Tetrahedron Lett., 39, 9027, 1998. 

Scheme 15. Synthesis of complex polyether frameworks by tandem methylenation/olefin metathesis. (Nicolaou et al., 1996p...

The impetus for developing this technology was provided by the continuing need for methodology amenable to the preparation of polycyclic arrays that constitute the frameworks of numerous polyether natural products such as the bre-vetoxins, ciguatoxin and maitotoxin 124 (Fig. 3). 

Recently, the effect of two proximate cations on anion-binding was investigated [53], Toward this end, the basic calix[4]arene framework was functionalized on the lower rim with two methyl ester units and in two separate positions with two urea-linked bipyridine-functionalized polyether units to give 13. This receptor was designed to allow for the coordination of hard metal cations via the ester groups, as... 

It is important to appreciate that these ditopic macrocycles contain a polyether moiety on one side and present a diamido motif on the other. Addition of a charged pyridinium cation to the substrate-free form of these macrocycles then generates a highly efficient receptor for chloride, presumably as the result of both template-induced organization and electrostatic effects. The further incorporation of hydro-quinone groups and polyether functionalities into the cyclic framework presumably contributes to the stabilization of the cationic pyridinium component within the final interpenetrated structure. Interestingly, while titration of the pyridinium cation building block alone revealed a preference for oxoanions, the final receptor-anion... 

The macrocyclization reaction described above has been used to generate a great number of catenane (12) and rotaxane (13) architectures (Scheme 10.4) using both crown ethers [preformed macrocyclic components 14 (strategy A)] and hydroqui-none-based dumbbell-shaped polyethers [preformed acyclic components 15 ( clipping )] as templates [14b, 15]. These templates are also relatively robust with regard to the substitution of different groups into both the tetracationic cyclophane and the neutral frameworks. 

Opening of epoxides has also been employed in the preparation of tetrahydrofuran frameworks <05OL2945 05OL4867 05SL1630>. The reaction illustrated below is a relevant example <05JA5806>. Moreover, electrophiles generated in an oxidative manner have been shown to serve as initiators in epoxide cascade cyclization routes to form polyethers <05TA3570>. 

Because of their complex chemical structures and significant biological properties, polyether ionophore antibiotics have stimulated intensive studies of their total synthesis, which primarily require new synthetic methodologies for the control of stereochemistry in acyclic systems two excellent reviews covered the results until 1981 [1]. However, since then, only a few total syntheses have been achieved, because the extremely complex structures of the polyethers have required formidable efforts for synthesis, and much attention has focussed on efficient and stereoselective synthesis of the molecular framework of substituted tetrahydrofurans and tetrahydropyrans and spiroketal systems [2]. This review deals with the total synthesis of three representative complex polyethers, okadaic acid (1), antibiotic X-206 (2), and salinomycin (3) mainly focussing on the latter half of their synthesis. 

Hayashita. T. Sawano, H. Higuchi, T. Indo. M. Hiratani, K. Zhang, Z.-Y. Bartsch, R.A. Molecular design of acyclic polyether dicarboxylic acids possessing pseudo-18-crown-6 frameworks for selective lead(II) extraction. Anal. Chem. 1999. 71 (4), 791-795. 

Wendland and Zimmerman [31] had remarked in their 1999 paper that what they referred to as cored dendrimers represented a system closely analogous to polymer imprinting, but within the framework of adiscrete single molecule. In their approach, a dendrimer is prepared by the assembly of branched polyether dendrons , around a central core by esterification. The periphery of the dendrimer is fully substituted... 

Miscibility of segmented rigid-rod polyimide (PI), viz., biphenyl dianhydride perfluoromethylbenzidine (BPDA-PFMB), and flexible polyether imide (PEI) molecular composites was established by differential scanning calorimetry. The composite films of BPDA-PFMB/PEI were drawn at elevated temperatures above their glass transitions. Tensile moduli of the films were evaluated as a function of composition and draw ratio. Molecular orientations of polyimide were determined by birefringence and wide-angle X-ray diffraction. The crystal orientation behavior of the 80/20 BPDA-PFMB/PEI was analyzed in the framework of the affine deformation model. 

---