Ketone protection

Steric hindrance due to the presence of other substituents a to the carbonyl group may diminish reactivity during the formation of ketone protecting groups. Thus a marked decrease in reactivity has been observed with 2- or... 

Tn general. Nelson and Schut s procedure is recommended for expansion of an unsymmetrically substituted cyclohexanone (for example, the A ring of a steroid 3-ketone). The yield is satisfactory in most instances and the precursors are readily available. However, the relative proportions of the two possible ketones may change as substituents are varied. The procedure is applicable to systems with additional ketones protected as ketals, double bonds, ethers and hydroxyl groups. 

Selective ketone protection The -CHO group is converted in Step 1 to a siloxysulfonium salt [R CH(OTMS)S Me2 OTf] that is reconverted to an aldehyde group in Step 3." ... 

Lactones can be protected as dithiolane derivatives using a method that is analogous to ketone protection. The required reagent is readily prepared from trimethyl-aluminum and ethanedithiol. 

After the known intermediate 79 (contaminated with ca. 6 % < /.v isomer) [39] was prepared from Hajos-Parrish ketone [40] 78, the tert-butyl ether was cleaved (quant.) and the ketone protected as the acetal (96 %). The secondary alcohol was oxidized by pyridinium chlorochromate (PCC) to provide ketone 80 in good yield (71 %) and after fractional crystallization afforded material absent of any m-hydrindane (Scheme 10.6). [NOTE All compounds shown in Schemes 10.6 and 10.7 are shown in the ent-configuration, as published]. The oxidation of protected hydrindane 80 under Saegusa-Ito conditions [41, 42] gave enone 81 (82 %), confirmed by X-ray crystallography. 

The methyl ether was chosen for 14 and it turned out that the enol ether 13 could be converted directly to the acetal 12. It is essential to have one ketone protected before the other is introduced. The diazoketone (with copper catalysis) was used to put in the three-membered ring.2... 

Aldol-type reactions.1 Reaction of lithium enolates with SEM chloride results in a-hydroxymethyl ketones protected as the P-trimethylsilylethyl ethers, which can undergo deprotection with Bu4NF or TFA. Yields are in the range of 55-80%. 

The alkyltelluro group is generally introduced into aldehydes and ketones protected in the form of their acetals. The acetal groups in the protected acetylphenyl alkyl and 2-formylphenyl alkyl tellurium compounds are hydrolyzed by strong sulfuric or hydrochloric acid to carbonyl groups without affecting the C — Te — C moiety. 

Alcohols, see Hydroxyl group Aldehydes and ketones, protection of, 175-223... 

Everything is straightforward except the final reduction where a less reactive ester must be in the presence of a more reactive ketone. Protection is the answer and an acetal is suitable... 

An obvious disadvantage of a-amino ketone dimerizations is their unsuitability for crosscoupling. The alternative of condensing a 1,2-dicarbonyl component with a 1,2-amine is also inappropriate for most cephalostatins and ritterazines, which are unsymmetrical both from right to left and top to bottom. It is possible that unsymmetrical condensation could be carried out in a stepwise manner using two a-amino ketones protected in different ways. However, this would... 

Our final example is a base-labile 4-(phenyIsulfonyl)methyI-l,3-dioxolane protecting group for aldehydes and ketones. Protection is carried out by the reaction of diol 17.1 (obtained by dihydroxylation of allyl phenyl sulfone) with a carbonyl compound in the presence of pyridinium p-toluenesulfonate [Scheme 2.17). Cleavage is accomplished by treatment with DBU rm-ButyIdimethylsilyl ethers, p-toluenesulfonate esters, tetrahydropyranyl ethers, carboxylic esters and benzoates are well tolerated. A disadvantage to the use of 17.1 is the introduc-... 

Full details have been reported on the use of Et2NSp3 for the conversion of hydroxy-ketones into fluoro-ketones. Protection of the hydroxy-ketone by acety-... 

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