Propargylic alcohols, sensitivity

Silver acetylides also reacted with epoxides yielding propargylic alcohols.27 As for related alkylations with silver acetylides (see Section 10.6.1), this reaction required zirconocene dichloride and catalytic amount of silver triflate. This method proved useful for both electron-rich and electron-deficient alkynes and compatible with various acid- and base-sensitive functional groups (Table 10.2). 

The dehydration of alcohols under mild conditions is affected by copper(II) Lewis acids. Copper sulfate has long been utilized as a dehydrating agent. An example of its effectiveness for alcohol dehydration is demonstrated in the conversion of the sensitive propargylic alcohol 1 to enyne 2 (Sch. 1) [4]. A carbocation mechanism is suggested by the formation of bis ethers in these reactions [5]. The addition of pyridine... 

In additions to alkynols, both possible orientations are noted. The balance between them is sensitive to steric effects bulky substituents at or near one carbon of the triple bond tend to favor carbon—carbon bond formation at the other carbon. In reactions with propargylic alcohols anti-addition products are seen that result from carbon—carbon bond formation at the triple-bond carbon nearer the OH group. These are often accompanied by allenes, the result of addition of the opposite orientation followed by elimination ... 

Notably, this methodology tolerates acetyl, ketal, free hydroxy and acid sensitive glycosidic groups. In the case of propargyl alcohol, ( )-dihydrokawain was produced in good yields [100]. In addition, this methodology was applied in the total synthesis of annularin G and annularin H [101]. 

The base-sensitive propargyl alcohol is separated by precipitating 9-BBN as its ethanolamine adduct. In many cases the product propargyl alcohol is entrapped in the precipitate. This problem is overcome by oxidative workup, and propargylic alcohol is separated from c/s-1,5-cyclooctanediol by distillation or precipitation of the diol from hexane or ether. 

Palladium-catalysed carbonylation of an iodoallylic alcohol, stereoselectively formed from the propargylic alcohol 4.57 through organoaluminium chemistry, was employed to form the base sensitive butenolide moiety of parviflorin 4.62 in a two-directional synthesis (Scheme After oxidative removal of the protecting... 

Other Unsaturated Alcohols. An improved procedure that has been reported recently for the preparation and utilization of alkali-metal acetylides in liquid ammonia allows the ethynation of sensitive carbonyl compounds to be carried out, in good yield, to give propargylic alcohols. The reduction of ap-acetylenic ketones to (S propargyl alcohols of high enantiomeric purity can be performed with NB-Enantrane (27) [cf. (6) earlier], which is derived from nopol benzyl ether as a low-cost alternative to (—)-a-pinene. a-Allenic alcohols have been prepared by the new routes shown in Schemes... 

Oxidation of sensitive divinyl alcohol 20 to dienone 4 is achieved by treatment with activated manganese dioxide. Commercially available active Mn02 21 is a synthetic nonstoichiometric hydrated material. This reagent provides mild conditions for oxidation of allylic, propargylic, and benzylic alcohols.10... 

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