Alkynes allylic dihydroxylation

Allyl alcohols can be produced catalytically by oxidation of alkenes with TBHP in the presence of small amounts of Se02 (equation 128). In contrast to the stoichiometric reaction, this catalytic oxidation can be performed under mild conditions (r.t., CH2C12 solvent).356 Alkynic compounds undergo a predominant allylic dihydroxylation upon reaction with TBHP/CH2C12 (equation 129) 357... 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

The masked propargylic anfz-l,3-diols obtained in these reactions are useful precursors to more functionalized systems. Lindlar reduction of alkyne 171 generated the (Z)-allylic diol 172, which underwent diastereoselective osmium tetraoxide-catalyzed dihydroxylation to provide the partially protected tetraol 173 (Scheme 28). The propargylic anfz-l,3-dioxane 175,obtained in 88% yield from... 

This reaction was employed to synthesize the perfumery compound, rosefuran 8.180 (Scheme 8.48) by the ruthenium-catalysed combination of alkyne 8.177 with an allylic alcohol. Dihydroxylation of the product 8.178 was followed by acid-catalysed dehydration. Hydrolysis of the ester and thermal elimination of water gave the natural product 8.180. Another synthesis of rosefuran can be found in Scheme 2.15. 

Walsh developed an efficient protocol for the asymmetric catalytic vinylation of a variety of aldehydes and ketones, using the vinylzinc species generated in situ by treatment of terminal alkynes with Schwartz s reagent for hydrozirconation, followed by transmetallation to zinc via reaction with Me2Zn. Under the catalysis of titanium complex of the chiral dihydroxyl bis (sulfonamide) ligand, the reaction provides access to the corresponding secondary or tertiary allylic alcohols in high yields and excellent enantiomeric excesses [65] (Scheme 14.13). 

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