Unsaturated groups allylic series

If the alkyl groups in any R SnX4 series are replaced by phenyl, the U9Sn signal moves to lower frequencies (Table XI), which is inconsistent with the greater electron withdrawing capability of a phenyl substituent on tin. Similar effects are found with benzyl, vinyl, and allyl groups (Tables XXX and XXXI) and may be due to the increased polarizability of these unsaturated substituents. (23) This topic is discussed further in Section III.B.3. 

Vinyl allyl ethers and allyl aryl ethers are converted by heating to y,6-unsaturated carbonyl compounds and allyl phenols, respectively. The two groups of reactions are mechanistically related, and are referred to as Claisen rearrangements. Claisen rearrangements of allyl aryl ethers have been reviewed and their kinetics are discussed in Chapter 3 Volume 13 of this series. The present discussion is limited to thermal isomerizations of vinyl allyl ethers. 

Foods of animal origin are suspected to contain some amount of COP formed by autoxidation. Cholesterol autoxidation is a well-established free radical process that involves the same chemistry that occurs for the oxidation of unsaturated lipids. Cholesterol contains one double bond at the carbon-5 position therefore, the weakest points in its structure are at the carbon-7 and carbon-4 positions. However, due to the possible influence of the hydroxyl group at carbon-3 and the tertiary carbon atom at carbon-5, the carbon-1 position is rarely attacked by molecular oxygen, and therefore the abstraction of an allylic hydrogen predominantly occurs at carbon-7 and gives rise to a series of A and B ring oxidation products. In the chain reaction,... 

The 15-deoxy-16-hydroxyprostaglandins proved to be potent bronchodilators, whereas the other derivatives were relatively uninteresting. Therefore, further studies were concentrated in this series and followed three approaches 1) homologation of the p-chain, 2) introduction of a 16 or 17-methyl group to steri-cally crowd the 16-hydroxy function, and 3) restoration of allylic character by introducing unsaturation at C17-C18 (55, see 24,35, 40,42 and 47 for related studies). The requisite p-chain intermediates were prepared in a similar fashion to that of 178 or 1T9 (Scheme 31) by condensation of propargyl magnesiunPbromide with the appropriate aldehyde or ketone (55). 

Terpene-derived bis-N-oxide 21.22 represent the most recent addition to the successful catalyst series. The catalyst was shown to be particularly efficient in the allylation and crotylation of aromatic and a,p-unsaturated aldehydes (<99% enantiomeric excess at —60°C), however, with aliphatic aldehydes the selectivity dropped to 50% ee. It is noteworthy that 21.22 was synthesised in four easy steps from inexpensive (I )-myrtenal and the protocol is amendable to scaling up. In contrast, synthesis of enantiopure catalysts 21.19-21.21 requires either resolution of enantiomers or separation of diastereoisomeric mixtures, which hampers their larger-scale application. In this group of polydentate IV-oxides it is also worth mentioning terpyridine N,IV IV"-trioxide, the related bis-imidazole Af,M -dioxides and chiral dinitrones,but their efficiency was inferior to the best pyridine-type dioxides, such as 21.20-21.22. 

Investigation of allylic and acetyleneallenic rearrangements in a series of 3-substituted quinuclidines lead to an elucidation of the effect of these processes on the position of divisible bonds in unsaturated quinuclidine compounds [114]. In the course of these studies it was shown that oxidation and ozonolysis of substances containing functional groups in the allylic position to the double bond resulted in the breaking, not only of the divisible, but also of adjacent C-C-bonds. 

The same group also reported the selective allylic oxidation of unsaturated steroids and valencene by a series of metal complexes based on Co(II), Cu(II), Mn(II) and V(II) immobilized on mesoporous silica using a synthetic strategy similar to that reported in Scheme 23.71. The catalysts could be recycled with only minor loss of activity. 

The most useful synthetic methods are those based on the transfer of unsaturated residues such as allyl, vinyl, propargyl, acetylene and allene groups. It has been found, however, that saturated alkyl groups are also capable of being transfered from a silicon atom to a carbon atom. It was found at first [572] that trimethylsilylacety-lenes reacted with acyl halides in the presence of aluminium chloride to give ketones of the acetylene series (Eq. 4.33) ... 

On the basis of this finding, Shibasaki s group developed a series of asymmetric reactions using soft copper(I) Brpnsted base catalysts and a wide variety of pre-nucleophiles (thioamides [34 3], isocyanide [44, 45], unsaturated butyrolactones [46, 47], nitroalkanes [48], allyl cyanide [49, 50], and a-trifluoromethylacetamide [51]) via proton transfer strategy (Fig. 5). 

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