Enone Conversions

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

The epiperoxides-to-hydroxy enones conversion is best accounted for in terms of a competing Pd(0)/Pd(II) redox mechanism (Scheme 22). On the other hand, the formation of (hols, diepoxides, and hydroxy enones from monocyclic substrates involves radical... 

Dehydrogenation of steroidal ketones to give the a,/8-unsaturated species can be accomplished efficiently using benzeneseleninic anhydride.Ketone to enone conversion can also be attained by hydride-ion abstraction from the corresponding trialkylsilyl enol ether [equation (26)]. ... 

Ryu, I., Murai, S., Hatayama, Y., and Sonoda, N., A ketone-enone conversion via the reaction of silyl enol ethers with DDQ, Tetrahedron Lett., 37, 3455, 1978. 

Conversion of PGA2 to the highly sensitive PGC2 was accomplished by deconjugation of the enone system by formation of the y-extended enolate using rm-alkoxide as base and a-protonation by pH 4 buffer. 

There are ample precedents for reductions of double bonds in conjugated enones with lithium in deuterioammonia (see section V-C). Examples of the reduction of saturated ketones in deuterated media appear only as side reactions (over reductions) during the above mentioned conversions. For experimental details, therefore, one should consult the literature for the analogous reductions in protic medium (see also chapter 1). The use of deuterioammonia is essential for labeling purposes since by using liquid ammonia and methanol-OD the resulting alcohol contains no deuterium. For the preparation of deuterioammonia see section IX-D. 

When the enone chromophore of the diketone (148) is excited selectively using 2537 A-light, a smooth conversion to the two stereoisomeric cyclopropyl diketones (149) and (150) takes place exclusively. Experiments... 

Analogous to DPNH (144-146), 1,4-dihydropyridines (147) act as reducing agents. For instance, the conversion of aromatic nitro compounds to amines (148) and reduction of enones to ketones (749) has been achieved. 

It was anticipated that two of the three stereochemical relationships required for intermediate 12 could be created through reaction of the boron enolate derived from imide 21 with a-(benzyloxy)ace-taldehyde 24. After conversion of the syn aldol adduct into enone 23, a substrate-stereocontrolled 1,2-reduction of the C-5 ketone car-... 

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

The direct conversion of 3-methylcyclohex-2-enone into 2-allyl-3-methylcyclohexanone provides an interesting example of the utility of the reduction-alkylation procedure. Synthesis of this compound from 3-methy I cyclohexanone would be difficult because the latter is converted mainly into 2-alkyl-5-methylcyelohexanones either by direct base-catalyzed alkylation11 or by indirect methods such as alkylation of its enamine (see Note 13) or alkylation of the magnesium salt derived from its cyclohexylimine.12... 

The reaction of the aldehyde 174, prepared from D-glucose diethyl dithio-acetal by way of compounds 172 and 173, with lithium dimethyl methyl-phosphonate gave the adduct 175. Conversion of 175 into compound 176, followed by oxidation with dimethyl sulfoxide-oxalyl chloride, provided diketone 177. Cyclization of 177 with ethyldiisopropylamine gave the enone 178, which furnished compounds 179 and 180 on sodium borohydride reduction. 0-Desilylation, catalytic hydrogenation, 0-debenzyIation, and acetylation converted 179 into the pentaacetate 93 and 5a-carba-a-L-ido-pyranose pentaacetate (181). 

Enones and enoates undergo 1,2-reduction [115, 191]. Lipshutz et al. reported the effective protection of carbonyl functions by the triisopropylsilyl acyl silane group (TIPS), which allowed the selective conversion of alkenes or alkynes to the corresponding zirconocene complexes [24]. The aldehyde could subsequently be regenerated by desilylation with TBAF [186]. 

A milder protocol for the conversion to enones involves use of a catalytic amount of TMSOTf and a pyridine base.47... 

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