Ethers, cyclic, conversion from diols

The scope and synthetic utility of dioxyphosphoranes, and particularly, diethoxytriphenylphosphorane (DTPP), as useful cy-clodehydrating "reagents" for the conversion of diols to cyclic ethers have received only superficial attention (1). The DTPP-diol - ether route has several unique advantages over existing methods (a) the reaction conditions are effectively neutral and mild, (b) the stereoselectivity in the closure of both unsymmet-rical and symmetrical diols to cyclic ethers is high, and (c) the isolation of the product(s) from triphenylphosphine oxide (TPPO) is convenient. 

Koreeda and Hamann have reported the use of silyl tethers in stereocontrolled syntheses of branched-chain 1,4-diols and 1,5-diols [61]. Exposure of (bromomethyl)silyl ethers prepared from the corresponding homoallylic alcohols with Bu SnH in the presence of AIBN allowed smooth conversion to the corresponding cyclic siloxanes, from which diol products were obtained using standard, oxidative cleavage protocols. While monosubstituted olefin 149 selectively underwent 1-endo cyclization, di- and trisubsti-tuted olefins 150 and 151 preferentially reacted through the 6-exo mode with complete stereocontrol, affording the diol products 152 and 153, respectively (Scheme 10-50). 

This procedure has been adapted to transformation reactions however, most of the reported transformations were achieved from AM polymerization of cyclic ethers to conventional radical polymerization by using thermal or photochemical activation. For instance, AM polymerization of epichlorohydrin (ECH) was performed in the presence of 4,4 -azobis(4-cyanopentanol) yielding polymers with azo linkages in the main chain. Polymerization was conducted under typical conditions, that is, by slow addition of ECH to the solution of initiator containing catalyst. The reaction was considerably slower than in the presence of simple diols (e.g., EO) and only 28% conversion was achieved under conditions sufficient to reach complete conversion in the polymerization initiated by EO. Poly(epichlorohydrin) (PECH) prepared this way was consequently used in the polymerization of St to produce block polymer (Scheme 59). This polymerization yielded PSt with PECH segments at each end since termination occurs through radical-radical combinations. 

The relative amounts of the two types of polymers are determined by reaction conditions. Hydrolysis with water alone yields 50—80% linear polydimethyl-siloxane-a,co-diols and 50—20% polydimethylcyclosiloxanes. Hydrolysis with 50—85% sulphuric acid gives mostly high molecular weight linear polymers with only small amounts of cyclosiloxanes. Conversely, the hydrolysis of dimethyl-dichlorosilane with water in the presence of immiscible solvents (e.g., toluene, xylene and diethyl ether) results in the preferential formation of lower polycyclosiloxanes. Such solvents, in which the organochlorosilane is readily soluble, lead to a reduction in concentration of dimethyldichlorosilane in the aqueous phase and thus intramolecular condensation is favoured over inter-molecular condensation. Further, the hydrolysis products are also soluble in the organic solvent and the cyclic compounds are protected from the action of the aqueous acid. (See later.)... 

Our first climbing route, or synthetic path, to aureothin, relied on the reduction of lactone 17, prepared by the condensation of the anion of 18 with aldehyde 15, an approach entailing nucleophilic addition and subsequent lactonization (Scheme 11). We envisioned the conversion of 17 into aureothin (1), either by the direct reduction of the lactone moiety into the cyclic ether or by the cyclization of the diol resulting from the exhaustive reduction of the lactone. 

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