Carbonyl, masked

Carbonyl masking. Dithioacetalization and formation of a-silylamines-aldehydes exploit the Lewis acidity of LiC104—Et20. 

Carbonyl masking and regeneration. Tin(II) chloride is effective as catalyst for dithioacetalization as well as hydrolysis of acetals. ... 

The reaction of the 1- and (2-ethoxyvinyl)tributylstannanes (721) and (723) as masked carbonyls with aryl halides proceeds smoothly and the products 722 and 724 are used for further reactions[592,593]. 

Azomethine ylides are also frequently obtained by ring opening of aziridines, and the analogous carbonyl ylides from oxiranes. These aspects are dealt with in Section 3.03.5.1. A variety of five-membered heterocycles can also function as masked 1,3-dipoles and this aspect is considered in Section 3.03.5.2. 

Alkylation of ethyl tnfluoroacetoacetate enolate occurs at the carbon atom provided that the carbonyl function is masked as a dioxolane or an N,N dimeth-ylhydrazone [IIS] (equation 102)... 

Let us consider the separation of polymethylmethacrylate (PMMA) on a nonmodified silica column as an example. In THE (medium polar eluent) the PMMA eludes in size exclusion mode because the dipoles of the methylmethacrylate (MMA) are masked by the dipoles of the THE. Using the nonpolar toluene as the eluent on the same column, the separation is governed by adsorption because the dipoles of the carbonyl group in the PMMA will interact with the dipoles on the surface of the stationary phase. The separation of PMMA in the critical mode of adsorption can be achieved by selecting an appropriate THF/toluene mixture as the eluent. In this case all PMMA samples... 

All of these ehimnddon reacdons contain fi-carbonyl groups in the nltro compounds Of course, masked carbonyl groups are also frequently employed for such fi-elimination of HNO, as shown in Eq 7131, Eq 7 133, and Eq 7 133In these cases, the sulfinylmethyl or hydroxymethyl group is converted into the carbonyl group by the Pummerer rearrangement or by simple oxidation... 

Reductive alkylations have been carried out successfully with compounds that are not carbonyls or amines, but which are transformed during the hydrogenation to suitable functions. Azides, azo, hydrazo, nitro and nitroso compounds, oximes, pyridines, and hydroxylamines serve as amines phenols, acetals, ketals, or hydrazones serve as carbonyls 6,7,8,9,12,17,24,41,42,58). Alkylations using masked functions have been successful at times when use of unmasked functions have failed (2). In a synthesis leading to methoxatin, a key... 

Formation of C —C Bonds by Addition to Chiral Dicarbonyl Compounds Where One Carbonyl Group is Modified or Masked with a Chiral Auxiliary... 

Since double bonds may be considered as masked carbonyl, carboxyl or hydroxymethylene groups, depending on whether oxidative or reductive methods are applied after cleavage of the double bond, the addition products from (E)-2 and carbonyl compounds can be further transformed into a variety of chiral compounds. Thus, performing a second bromine/lithium exchange on compound 4, and subsequent protonation, afforded the olefin 5. Ozonolysis followed by reduction with lithium aluminum hydride gave (S)-l-phenyl-l,2-ethanediol in >98% ee. 

P 55] Before synthesis, a micro-mixing tee chip micro reactor (Figure 4.85) (with two mixing tees and four reservoirs) was primed with anhydrous tetrahydrofuran (THF). A 40 gl volume of a 0.1 M solution of tetrabutylammonium fluoride trihydrate in anhydrous THF is filled into one reservoir of a micro-mixing tee chip reactor [15], 40 gl of a 0.1 M solution of 4-bromobenzaldehyde in anhydrous THF is added to a second reservoir, 40 gl of a 0.1 M solution of the silyl enol ether (masking the enolate of a carbonyl compound such as cyclohexanone) in anhydrous THF is added to a third reservoir and anhydrous TH F is filled into the fourth collection reservoir. Electrical fields of417,455,476 and 0 V cm are applied to transport the reaction species from the respective reservoirs. The reaction is carried out at room temperature. 

If only the O-H, as opposed to the carbonyl, of a carboxyl group has to be masked, it can be readily accomplished by esterification. Alkaline hydrolysis is the usual way for regenerating the acid. t-Butyl esters, which are readily cleaved by acid, can be used if alkaline conditions must be avoided. 2,2,2-Trichloroethyl esters, which can be reductively cleaved with zinc, are another possibility.267 Some esters can be cleaved by treatment with anhydrous TBAF. These reactions proceed best for esters of relatively acidic alcohols, such as 4-nitrobenzyl, 2,2,2-trichloroethyl, and cyanoethyl.268... 

The synthetic utility of the D-A reaction can be expanded by the use of dienophiles that contain masked functionality and are the synthetic equivalents of unreactive or inaccessible compounds. (See Section 13.1.2 for a more complete discussion of the concept of synthetic equivalents.) For example, a-chloroacrylonitrile shows satisfactory reactivity as a dienophile. The a-chloronitrile functionality in the adduct can be hydrolyzed to a carbonyl group. Thus, a-chloroacrylonitrile can function as the equivalent of ketene, CH2=C=0,63 which is not a suitable dienophile because it has a tendency to react with dienes by [2 + 2] cycloaddition, rather than the desired [4 + 2] fashion. 

Figure I indicates the approach used to synthesize poly(oxyethylene)-b-poly(pivalolactone) telechelomers. An acetal capped anionic initiator, X (13) polymerizes ethylene oxide (EO) to give 2> a potassium alkoxide of a masked polyether, and this "new" initiator is to be used to polymerize pivalolactone (PVL). Since potassium alkoxides are strong nucleophiles, they can randomly attack at both the carbonyl carbon and the 3-methylene carbon in lactones, (Figure 2) such a random attack would result in a pivalolactone segment containing irregularities. Lenz (15), and Hall (16), and Beaman (17) have investigated PVL polymerization and have shown that the less nucleophilic carboxylate anion is preferable in polymerizing PVL smoothly. The weaker carboxylate anion will attack only at the methylene...

For internal olefins, the Wacker oxidation is sometimes surprisingly regioselective. By using aqueous dioxane or THF, oxidation of P,y-unsaturated esters can be achieved selectively to generate y-keto-esters (Eq. 3.18).86 Under appropriate conditions, Wacker oxidation can be used very efficiently in transforming an olefin to a carbonyl compound. Thus, olefins become masked ketones. An example is its application in the synthesis of (+)-19-nortestosterone (3.11) carried out by Tsuji (Scheme 3.5).87... 

Figure 1 Left illustration of the application of a mask to produce different samples . Right FT-IR images of the carbonyl band of poly(vinyl cinnamate), showing different levels of crosslink density as revealed from a decrease in conjugated carbonyls. Reproduced from Rafferty et al. [9], Copyright 2002, with permission from the Society for Applied Spectroscopy.

Later, Patterson and Hibbert (S3) stated that ultraviolet absorption of lignin corresponded with the absorptions of hydroxy derivatives of 4-(4,hydroxy-3,methoxyphenyl)-l-propanone and l-(4,hydroxy-3, 5,dimethoxyphenyl)-4-propanone. These authors, therefore, assumed the presence in lignin of a carbonyl group, or of an ethylenic double bond conjugated with an aromatic nucleus. They ascribed the 280 mp band to m-position freedom in the benzene ring and the 310 mp baud to a masked carbonyl group in conjugation with a benzene nucleus. 

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