Ethers cyclic propargylic

Due to this compact transition state, the introduction of substituents on the carbon skeleton leads to only one diastereomer after intramolecular carbometallation [60]. For example, metallation of the propargyl ether 64 with ec-BuLi followed by transmetallation with ZnBr, resulted in the formation of a cyclic propargyl ether as a single diastereomer in which the remaining metal had been trapped by different electrophiles [60] (Scheme 7-55). According to this approach, a quaternary and a tertiary stereogenic center are linked to form a cyclopentane ring with total diastereoselection. 

Thus, for example, the cyclic propargylic allylic ether (242 equation 49) rearranges completely within 30 min upon treatment with Bu"Li in THF-hexane at -20 C, whereas the acyclic analog (244 equation SO) requires 12 h under comparable conditions. Alcohol (243) serves as a useful intermediate for the synthesis of cembranoid diterpenes. ... 

The extention of radical cyclization of (bromomethyl)dimethylsilyl allyl ethers to propargyl analogs 17 has been studied by Malacria and coworkers. The intermediate exocyclic vinyl radical 18 can be either trapped by the hydrogen atom to give, after simple chemical transformations, the trisubstituted alkene 19 (equation 25) or can be added intramolecularly to give cyclic products when suitably located double bonds are present (equations 26 and 27). An attempt to apply this methodology to the stereoselective synthesis of angular and linear triquinane has also been performed. When = t rt-butyl,... 

Ni(cyclam)]2+ was shown to be an efficient electrocatalyst for the intramolecular cyclization-carboxylation of allyl or propargyl-2-haloaryl ethers,200 and for the synthesis of cyclic carbonates from epoxides and carbon dioxide.201... 

Further, a large number of examples with simple alkyl substituents [168, 171, 176-184], cyclic alkanes [185], aryl substituents [177, 186-192], olefmic substituents [78, 177, 193-196], deuterated compounds [172], thioether groups [171], ester groups [197], orthoesters [198, 199], acetals [168, 182, 200-204], silyl-protected alcohols [198, 205-211], aldehydes [212], different heterocycles [213-217], alkyl halides [218, 219] and aryl halides [192, 220-223] have been reported. A representative example is the reaction of 92, possessing a free hydroxyl group, an acetal and a propargylic ether, to 93 [224] (Scheme 1.40). 

Ether was a better solvent in cases where the propargylic position was unsubstituted (methylene group) since, in THF, a competitive [1,2] -Wittig rearrangement took place and led to diminished yields. When applied to the secondary homoallylic propargyl ethers 388, the zinc-ene-allene cyclization afforded the c -2,3,5-trisubstituted tetrahydrofurans 389 with moderate to satisfactory levels of diastereoselectivity, which could be rationalized by the preferential pseudo-equatorial positioning of the homoallylic substituent in the cyclic... 

Intramolecular Lewis acid-promoted reaction of coordinated propargylic ether with the silyl enol ether in 158 has been applied successfully to the construction of the highly strained 10-membered cyclic enediyne system 159, present in esperamycin and calicheamycin [39,40]. The enediyne system 157 was prepared by the Pd-catalysed Sonogashira coupling of (Z)-l,2-dichloroethylene (154) with two different terminal alkynes 155 and 156. 

Last and Hoffmann described 5-exo radical annulation reactions of cyclic p-bromo propargyl ethers or acetals 259 (X=CH2) and their benzo analogs catalyzed by 5 mol% 255 as an approach towards the total synthesis of aflatoxins and analogs (entry 3) [311]. The cyclizations gave the products 260 in 35-95% yield. 

Intramolecular hydrosilylation. The hydrodimethylsilyl ether (2) of a homo-propargyl alcohol (1) undergoes intramolecular hydrosilylation catalyzed by H2PtCl6 to give a cyclic (E)-vinylsilane (3). This can be converted to a 0-hydroxy ketone (4) by hydrogen peroxide oxidation or into (Z)-3-bromo-3-decene-l-ol (5) by brom-ination/bromodesilylation with inversion of geometry. 

Cobalt-complexed propargylic aldehydes can be alkylated using both cyclic and acyclic trimethylsilyl enol-ethers and -esters in the presence of a Lewis acid. Boron trifluoride etherate is the most frequently employed Lewis acid. 

Cyclic ethers (5-10 membered) can be prepared by reaction of the formed carbocation with pendant alcohols. The reaction is usually stereospecific at the propargylic carbon (Scheme 228). Intramolecular formation of ethers has been... 

AUyl propargyl ethers (oxygen-tethered 1,6-enynes) also react intermolecularly with alcohols or water to give six-membered ring acetals or hemiacetals (equation 52) AUyl silyl alkynes react somewhat similarly with Au(I) catalysts in the presence of alcohols to give cyclic or open alkenylsilanes (equation 59). Both types of products are the result of... 

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