4 -propargyl ether

The flask was charged with 2.0 mol of dry, freshly distilled (note 1) methyl propargyl ether (VIII-6, Exp. 7), potassium iert.-butoxide (note 2) (5 g) was added with stirring and the mixture was heated under reflux. The reaction was... 

To a solution of 0.40 mol of butyllithium in about 280 ml of hexane were added 300 ml of dry THF at -20 to -40 0. Subsequently 0.40 mol of freshly distilled tert.-butyl propargyl ether was added, keeping the temperature below -30°C. Freshly distilled acetaldehyde (0.40 mol) was then added at the same temperature during about 15 min. The cooling bath was removed and, after an additional 15 min, 200 ml of an aqueous solution of 30 g of ammonium chloride were introduced. After separation of the layers the aqueous layer was extracted twice with diethyl ether and the combined solutions were dried over magnesium sulfate and concentrated in... 

Note 2. A very vigorous explosion has occurred in our laboratory in the course of a distillation of a large amount of the propargyl ether, which had been stored for 3 weeks at room temperature. Traces of peroxide (the product had not been kept under nitrogen) might have been responsible for the explosion. In any case it is not advisable to distil large quantities of propargyl ethers at normal pressure. 

Propargyl ethers are cleaved with TiCl3-Mg in THF, 54-92% yield. Allyl and benzyl ethers were not cleaved phenolic propargyl ethers are also cleaved. ... 

The reaction of propargylic ethers proceeds through an addition-elimination pathway, which docs not involve the coppcr(III) intermediate. The stereochemical outcome varies with the nature of the halogen component of the Grignard reagent of RMgX6Sc. If the halogen is chloride, overall syn selectivity is obtained however, anti substitution results in the case of iodide. 

Scheme 21 Diastereoselective benzannulation of chiral propargylic ethers...

Another effective radical cascade strategy started from bromomethyldi-methylsilyl propargyl ethers. " The synthesis of functionalized cyclopenta-none 108 was achieved as a single diastereomer, starting from the reduction of bromoderivative 107 in the presence of (TMSlaSiH (Reaction 83). When different substituents are used in the skeleton, as in compound 109, a completely different reaction pattern resulted (Reaction 84). 

Cleavage of propargyl ethers by Grignard reagents 12-2 Rearrangement of alkynes... 

The cyclisation of naphthyl propargyl ethers occurs efficiently under microwave irradiation leading to naphthopyrans, but naphthofurans are formed in the presence of base <96JCR(S)338>. The thermal rearrangement of naphthyl 3-trimethylsilylprop-2-ynyl ethers yields the 4-trimethylsilyl derivatives of naphthopyrans <96H(43)751>. 

Whereas the Rh2(OAc)4-catalyzed addition of diazoalkanes to propargyl alcohols readily gives the insertion of the carbene into the 0-H bond, with only a small amoimt of cyclopropenation of the resulting propargylic ether [54] the 2-diazopropane 59 reacts at 0 °C with l,l-diphenyl-2-propyn-l-ol 62a in dichloromethane and exclusively gives, after 10 h of reaction, only the adduct 63a isolated in 75% yield and corresponding to the regioselective 1,3-dipolar cycloaddition of the 2-diazopropane to the alkyne C - C bond (Scheme 15). 

Instead of propargyl alcohol, propargyl ether has been proposed to be used as a corrosion inhibitor. Propargyl alcohol is added to olefins to form the corresponding ether [936]. 

However, the classical version of the ABAC, when triallyl-, trimethallyl-, or tricrotylborane are involved in the reaction with RR1CHC=CH (e.g., propargylic ethers), can be applied only for the synthesis of 1-boraadamantane derivatives 35, 3,5-dimethyl- 36 and 4,6-dimethyl-l-boraadamantanes 37 (Scheme 7). Hence, the stmctures of the final cage compounds are rigidly restricted by the stmctures of the starting allylic boranes. 

Under the catalytic action of Rh2(OAc)4, formation of a propargylic ether from a terminal alkyne (229, R1=H) is preferred as long as no steric hindrance by the adjacent group is felt162,218>. Otherwise, cyclopropenation may become the dominant reaction path [e.g. 229 (R1 = H, R2 = R3 = Me) and methyl diazoacetate 56% of cyclopropene, 36% of propargylic ether162)], in contrast to the situation with allylic alcohols, where O/H insertion is rather insensitive to steric influences. 

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