Alcohols, alkynic asymmetric epoxidation

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 two stereogenic centers of alcohol 560 were generated applying Sharpless asymmetric epoxidation following by carbamate-assisted epoxide opening (Scheme 9.12). Hydrolysis of the carbonate, which involved the deprotection of the terminal alkyne, led to triol 561. After specific protection of the primary alcohol, acetonide formation resulted in... 

The use of inorganic supramolecular compounds in catalysis has also been successful in recent years. Hupp etal. incorporated a Mn(IIl)-porphyrm (see Porphyrin) epoxidation catalyst inside a molecular square, a system that shows enhanced catalyst stabihty and substrate selectivity as compared to the free catalyst. In another example, chiral metaUocyclophanes were constructed from Pt(PEt3)2 units and enantiopme atropoisomeric t,t -binapthyl-6,6 -bis-(acetylenes) and used in enantioselective diethyl zinc addition to aldehydes to afford chiral secondary alcohols. The first organometaUic triangle based on Pt(II) and alkyne-di-substituted-binaphfhyl system was reported and found to effect asymmetric catalysis reactions of aldehydes to alcohols with excellent conversion rates and enantiomeric excess/ ... 

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