WITTIG AND RELATED APPROACHES

E+ = H20, RX, RCHO, R2CO, RCOQ, CO2, aCOzMe, aCONR2, R0C02R, R3SiCl, I2 

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On the other hand, spontaneous [2,3]-Wittig rearrangement of the anion 69 affords the epimers 70 (Scheme 22) [102], illustrating that useful 2,5-dihydro-furans with C-substituents on the carbon atoms on either side of the ring oxygen atom can be obtained by this and related approaches. 

As discussed below, the strategies for asymmetric induction through olefination by Wittig-type reactions can be broadly classified into four groups. According to the type of these approaches to optically active olefinic compounds, asymmetric olefi-nations based on Wittig and related reactions will be reviewed in this chapter. 

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. 

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