Carbonyl conjugated double bonds

5-double bond of a linear dienone is selectively reduced over palladium in the presence of a trace of Presaturation of the 

The product stereochemistry obtained on hydrogenation of a, -unsaturated ketones is generally the same as that observed on saturation of the corresponding desoxy olefin. However, the stereochemistry of hydrogenation of these polarized species can be affected by the nature of the solvent (see section II-C). 

The 5a-isomer is obtained from hydrogenation of A -7-keto steroids over platinum or palladium. In the latter case the addition of pyridine to the reaction mixture greatly increases the amount of 5a- product formedd a Attack also takes place on hydrogenation (deuteration or tritiation ) 

5y9-isomer is formed almost exclusively using hydrobromic acid in acetic acid as the solvent. 

The addition of weak base (e.g. triethyl amine) to the reaction mixture gives a product having somewhat more of the 5j8-isomer than is observed in the neutral medium but in strong base the 5j8-product is formed almost exclusively. This latter result is also obtained with other substituents on the molecule (however, cf. below) and even occurs on hydrogenation of dienones and trienones incorporating the A -3-keto entity. 

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Isolated and conjugated double bonds / 118 Carbonyl conjugated double bonds / 124 Homogeneous catalysis / 131... 

In neutral medium A16-, A17(20)-, and A20-olefins are hydrogenated over palladium in preference to the double bonds of A4-3-ketones.67 The double bonds of A4-3-ketones and A16-20-ketones are reduced in preference to A5-,93 A7-48 155 and A9(I -olefins.2 169 The double bond of a A14-16-ketone is saturated before a 5-ene.70,163 In basic medium the carbonyl conjugated double bond is preferentially saturated in all cases92 presumably because of the strong adsorption of the enolate anion (see section II-C). Debromination (9a-bromo 11-ketone) occurs before saturation of the A4-3-keto double bond14 over Raney nickel but hydrogenation of benzyl ethers takes place concurrently with the hydrogenation of this double bond over palladium in neutral medium.96... 

Kamer and Stuninger 17 provided further support- for this premise -when they ahowod that tx-iouone undergoes epoxidation only at tlu-carbonyl-conjugated double bond (Eq. 60). 

Addition of dihydrosilane to a, /J-unsaturated carbonyl compounds such as citral (49), followed by hydrolysis, affords saturated citroneJlal (50) directly. The reaction is used for the selective reduction of conjugated double bonds[45,46]. In addition to Pd catalyst, the use of a catalytic amount of... 

Step 1 of Figure 29.3 Introduction of a Double Bond The /3-oxidation pathway begins when a fait)7 acid forms a thioester with coenzyme A to give a fatty acyl Co A. Two hydrogen atoms are then removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently jn biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

Dehydration of cortisone (198) affords the diene 199. This is then converted to ketal 200. The selectivity is due to hindrance about both the 11- and 20-carbonyl groups. The shift of the double bond to the 5,6-position is characteristic of that particular enone. Treatment of protected diene 200 with osmium tetroxide results in selective oxidation of the conjugated double bond at C-16,17 to afford the cis-diol (201). Reduction of the ketone at C-ll (202) followed by hydrolysis of the ketal function gives the intermediate 203. Selenium dioxide has been... 

A similar transformation occurs as a critical step in the total synthesis of (+)-estrone by a Diels-Alder cycloaddition-cycloreversion pathway (Eq. 80).227 It is worth noting that in this reaction the conjugated double bond is stereoselectively reduced while both an isolated double bond and a ketone carbonyl are preserved. 

The selective 1,4-reduction of a,p-unsaturated carbonyl compounds is always a challenge, but it has been met successfully by the use of dithionite under phase-transfer conditions. Reduction proceeds in high yield to the total exclusion of saturated or allylic alcohols (Table 11.10) [5, 6], Exocyclic and endocyclic conjugated C=C double bonds are reduced with equal ease, whereas non-conjugated double bonds remain intact. The predominant reduction pathway for conjugated dienoic... 

As shown in Scheme 4.16, the retrosynthetic process proceeds as follows i) FGI (substitution of the unconjugated aldehyde by an acetal group and the conjugated double bond by an OH group) ii) retro-aldol disconnection of the 1,3-C system iii) "reconnection" of the resulting 1,6-D system to a 6-membered ring iv) FGI (substitution of the double bond by an OH group and the acetal by a carbonyl... 

The domain of hydrides and complex hydrides is reduction of carbonyl functions (in aldehydes, ketones, acids and acid derivatives). With the exception of boranes, which add across carbon-carbon multiple bonds and afford, after hydrolysis, hydrogenated products, isolated carbon-carbon double bonds resist reduction with hydrides and complex hydrides. However, a conjugated double bond may be reduced by some hydrides, as well as a triple bond to the double bond (p. 44). Reductions of other functions vary with the hydride reagents. Examples of applications of hydrides are shown in Procedures 14-24 (pp. 207-210). 

If the triple bond is conjugated with an aromatic ring or with carbonyl groups complete reduction can be achieved with reagents which are capable of reducing conjugated double bonds (p. 42). 

Complex hydrides can be used for the selective reduction of the carbonyl group although some of them, especially lithium aluminum hydride, may reduce the a, -conjugated double bond as well. Crotonaldehyde was converted to crotyl alcohol by reduction with lithium aluminum hydride [55], magnesium aluminum hydride [577], lithium borohydride [750], sodium boro-hydride [751], sodium trimethoxyborohydride [99], diphenylstarmane [114] and 9-borabicyclo[3,3,l]nonane [764]. A dependable way to convert a, -un-saturated aldehydes to unsaturated alcohols is the Meerwein-Ponndorf reduction [765]. 

Lithium aluminum hydride reduces preferentially the carbonyl function (p. 98) but alanes prepared by reactions of aluminum hydride with two equivalents of isopropyl or /er/-butyl alcohol or of diisopropylamine reduce the conjugated double bonds with high regioselectivity in quantitative yields [871] (p. 121). 

Reduction of unsaturated ketones to unsaturated alcohols is best carried out Nit v complex hydrides. a,/3-Unsaturated ketones may suifer reduction even at the conjugated double bond [764, 879]. Usually only the carbonyl group is reduced, especially if the inverse technique is applied. Such reductions are accomplished in high yields with lithium aluminum hydride [879, 880, 881, 882], with lithium trimethoxyaluminum hydride [764], with alane [879], with diisobutylalane [883], with lithium butylborohydride [884], with sodium boro-hydride [75/], with sodium cyanoborohydride [780, 885] with 9-borabicyclo [3.3.1]nonane (9-BBN) [764] and with isopropyl alcohol and aluminum isopro-... 

The absorption of the alkene bond in conjugation with the carbonyl group occurs at a lower frequency than that of an isolated C=C bond the intensity of the conjugated double-bond absorption, when in an s-cis system, is greater than that of an isolated double bond. 

The most common method for the synthesis of phosphinopeptides is the addition of a nucleophilic, trivalent phosphorus species to a carbon electrophile, including conjugated double bonds, alkylating agents, imines, and carbonyl compounds. By analogy with the phosphite ester additions described above, the nucleophilic form of a phosphinic acid is the trivalent species, generated from the more stable, pentavalent PH derivative by deprotonation or by silylation (cf. Scheme 2). 

Chiral centres within the nucleophile or the conjugated double bond will control the stereochemistry of the chiral centres that are formed. For example, the Felkin-Ahn model has been applied to nucleophilic 1,4-addition to an ,/3-unsaturatcd carbonyl bearing a... 

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