Pentyl enol ethers

Nitradon of the potassium enolates of cycloalkanones with pentyl n silyl enol ethers with nitroniiim tetraflnoroborate " provides a method for the preparadon of cydic ct-nitro ketones. Tnflnoroacetyl nitrate generated from tnflnoroacedc anhydnde and ammonium nitrate is a mild and effecdve nitradug reagent for enol acetates fEq. 2.411. ... 

Oxidation of chroman-4-one and its thio analogue with Mn(OAc)3 gives the 3-acetates and subsequent basic hydrolysis yields the 3-hydroxychroman-4-one. Enzymatic hydrolysis of the 0-heterocycle using Amano PS lipase in a phosphate buffer selectively cleaved the (+)-isomer <03TA1489>. Enol ethers derived from chroman-4-one are converted into the 3-hydroxy-chromanone with high enantioselectivity, optimal with the pentyl ether, using a modified Sharpless asymmetric dihydroxylation reaction <03JOC8088>. 

This sequence transforms acyclic ketones and aldehydes into a-methylene ketones and a-methyl-a,)5-unsaturated ketones and aldehydes It has been illustrated by the synthesis of eucarvone, ( )-nuciferal and ( )-manicone This ring-opening of chlorosiloxycyc-lopropanes with ClSiMea elimination appears to be a practical route to Z or a,)5-ethylenic aldehydes and ketones depending on the stereochemistry of the reactants. For example, conversion in MeOH-NEta at 20°C of the 2-chloro-2-methyl-3-pentyl-l-trimethylsiloxycyclopropanes (derived from the addition of the chloromethylcarbene to the E and Z silyl enol ethers of n-heptanal) leads either to or Z 2-methyl-oct-2-enal (equation 65). ... 

Cyclic enol ethers are also good substrates for the Sharpless AD reaction with ee being dependant on the chain length of the enol ether side chain. Thus, while the pentyl enol ether (5.32) is converted into a-hydroxyketone (5.36) with 94% ee, the corresponding methyl enol ether is oxidised with only 83% ee. 

Titanium(IV) is a powerful but selective Lewis acid which can promote the coupling of allylsilanes with carbonyl compounds and derivatives In the presence of titanium tetrachlonde, benzalacetone reacts with allyltnmethylsilane by 1,4-addition to give 4-PHENYL-6-HEPTEN-2-ONE. Similarly, the enol silyl ether of cyclopentanone is coupled with f-pentyl chloride using titanium tetrachlonde to give 2-(tert-PENTYL)CYCLOPENTANONE, an example of a-tert-alkylation of ketones. 

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