2,2 Difluoro unsaturated acids compounds

The double bond in chlorotrifluoroethylene is polarized by back-donation of electrons of fluorine in such a way that the negative charge is on the carbon linked to chlorine and fluorine. Consequently, the difluorome-thylene end of the double bond is more electrophilic and is attacked by the ethoxide anion. Subsequent ejection of fluoride anion gives an unsaturated intermediate, l-chloro-l,2-difluoro-2-ethoxyethylene, compound V. This compound reacts with another ethoxide anion in a similar way and yields l-chloro-2,2-diethoxy-1-fluoroethylene. Nucleophilic addition of a third molecule of ethanol gives the final product, the orthoester of chlorofluoroacetic acid, compound W [75]. 

HNF2 reacts with saturated and unsaturated mono- and diacetals at ambient temperature to 100°C (250) and also smoothly in the presence of a sulfonic acid ion-exchange resin (Amberlyst-15) (140, 276) to give a-difluoroamino ethers (140). These compounds react with a second molecule of HNF2 in the presence of sulfuric acid to give bis(difluoro-amino) derivatives (140). 

The transformation of 4,4-difluoro-3-trifluoromethylbut-3-ene-l-ones 53, easily prepared from hexafluoroacetone (HFA), into 2-fluoro-3-trifluoromethylthio-phenes 54 proceeded on heating with phosphorus pentasulflde [32]. Yields of 2-fluoro-3-trifluoromethylthiophenes 54 depend on the reaction conditions and the progress of the transformation should be monitored by T NMR spectroscopy. The starting compounds were formed by elimination of water from hexafluoroacetone aldols 51 obtained by reaction of HFA with enol silyl ethers in the presence of Lewis acid such as SnCU. The unsaturated ketones 52 were reduced with SnCla and 53 were cyclized to the desired thiophenes 54 with phosphorus pentasulflde [33]. 

Diethyl trimethylsilyloxycarbonylmethanephosphonate" and diethyl car-boxymethanephosphonate " are the preferred reagents for the two-carbon homologation of aldehydes and ketones to give a,/8-unsaturated carboxylic acids, while 2,2-difluoro-l-tosylvinyl-lithium is a novel effective reagent for the preparation of a-keto-acids from carbonyl compounds (Scheme 21). a-Methoxy-aliphatic acids have been prepared from aliphatic aldehydes and chloroform using... 

---