Carbonyl compounds masked

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. 

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... 

The condensation of a,/3-unsaturated carbonyl compounds with phenols has received much attention as a route to chromenes. Problems associated with the instability of alkenals have been largely overcome by the use of masked forms of these reagents. The derived acetals have proved particularly useful in this respect. It is also advantageous to use the acetals of hydroxyalkanals and to introduce the double bond during the course of the reaction. Thus, 4,4-dimethoxy-2-methylbutan-2-ol serves as the equivalent of the unstable 3-methylbut-2-enal. 

If a C6-unsubstituted DHPM derivative needs to be synthesized, the corresponding 3-oxopropanoic ester derivative in which the aldehyde function is masked as an acetal can be employed [116]. Apart from ester-derived CH-acidic carbonyl compounds, nitroacetone also serves as a good building block, leading to 5-nitro-substituted DHPM derivatives in generally high yields [117]. 

These masked enolate complexes also react smoothly with carbonyl compounds, in reactions surprisingly reminiscent of the reaction of dimedone with formaldehyde (Fig. 5-13). The dinuclear products are of some interest as models for the dinuclear sites of some metalloproteins. 

C=0 —> C=Se.2 The reagent reacts with enones and several masked carbonyl compounds to form selenocarbonyl compounds. 

A sequence in which a carbonyl group has been masked as a sulfur derivative, alkylated with an electrophile, and then revealed again is a nucleophilic acylation. These nucleophilic equivalents of carbonyl compounds are known as acyl anion equivalents. In the retrosynthetic terms of Chapter 50 they are d1 reagents corresponding to the acyl anion synthon. 

Having served their purpose as masked carbonyl functionalities in the preceding 4 steps of the synthesis, the benzylidene groups in fused terpyridyl 11 are cleaved by ozonolysis in Protocol 7. As shown in Scheme 6.10, the reaction solvent is a mixture of methanol and dichloromethane. Reactant 11 is insoluble in methanol, so dichloromethane is used as a cosolvent to enhance its concentration in the cold (-78°C) reaction mixture. Methanol is a convenient cosolvent for conducting ozonolysis reactions safely because it cleaves ozonides in situ to carbonyl compounds and peroxyhemiacetals or... 

Even the very efficient enantioselective catalysts used in organozinc addition reactions to carbonyl compounds failed to catalyze the corresponding addition reactions to nonactivated imines such as A-silyl-, A-phenyl-, or iV-benzyl-imines. However, enantioselective additions of diaUcylzinc compounds to more activated imines, like iV-acyl- or iV-phosphinoyl-imines, in the presence of catalytic or stoichiometric amounts of chiral (see Chiral) aminoalcohols, have been recently reported. For example, in presence of 1 equiv of (A,A-dibutylnorephedrine) (DBNE) diethylzinc reacts with masked A-acyl imines like A-(amidobenzyl)benzotriazoles, to give chiral A-(l-phenylpropyl)amides with up to 76% e.e. (equation 68). 

Pedro and coworkers developed a strategy for asymmetric Michael additions of a masked benzoyl anion equivalent (199) to a,/3-unsaturated carbonyl compounds (equation 54)9°. (5)-mandelic acid (198) as a source for the benzoyl anion and chiral information, the corresponding Michael adducts were obtained in good yields and with high... 

Oxycyclopropanes can be regarded as masked carbonyl compounds and for this reason display a particularly rich and very practicable chemistry. 

Furthermore, several syntheses of a,p-unsaturated carbonyl compounds have involved the alkylation of suitably functionalized sulfones, followed by the base-promoted elimination of the sulfenate moiety on related alkyl sulfones bearing an oxygen atom at the -y-position (Scheme 106, entries b-d Scheme 110, entry c Scheme 105, entry b). - 46i Particularly interesting is the synthesis of nuciferal (Scheme 106, entry d) which involves sequential alkylation of y,-y-dioxy sulfones, deblocking of the acetal moiety and sulfenate elimination. In this and related reactions y, y-dioxy-a-metalloalkyl sulfone has played the role of a masked p-metallo-a,p-unsaturated carbonyl compound (M(>aC—C—C=0), which is not a directly available synthon. 

In the Reformatsky reaction, an ester with an a-halogeno substituent is treated with zinc, and so forms a masked carbanion, which in turn can react with a carbonyl compound to form a 3-hydroxyester. In the Darzens condensation, an a-halogenoester, on treatment with a base, reacts with a carbonyl compound to form an a,P-epoxyester. 

Dioxolans 146 can be considered as masked carbonyl functionalities, and are cleaved under acidic conditions. In solution, the olefination of a dioxolan-protected ketone would therefore be a two-step transformation consisting of deprotection and olefination. Using polymer-supported adds, e.g., strongly addic Amberlyst resin 147 and polymer-supported phosphonates 135 (Scheme 25), the two-step transformation involving the carbonyl compound 137 as intermediate could be performed simultaneously in one pot (Scheme 27) [113]. The product 136 was isolated by filtration. This procedure would not work as a one-pot sequence in solution because the acidic catalyst would immediately quench the basic phospho-nate resin. 

Circular dichroism measurements have been considered in order to observe the carbonyl tautomers which are particularly quickly fading. Carbonyl compounds absorb near 280 nm due to the utt transition of the C=O double bond. The beginning of this band is visible in aqueous solutions of sugars, but because of its weak intensity, it is largely masked by the shoulder of a more intense one. For this reason, extinction cannot be directly determined. But what is characteristic is the presence of a circular dichroism in this region, that is to say an extinction difference of fit - = Aff between left- and right-circulatory polarized light. This is... 

As can be seen from equations 2-5, the formation of A -acylenamines proceeds from derivatives of carboxylic acids (having an oxidation level equal to three ), namely anhydrides or add halides RCOX or amides RCONH2, on the one hand, and from carbonyl compounds or their derivatives (oxidation level equal to two ), namely enamines 3, acetylenes 5, or vinyl ethers 6, on the other. Indeed, the acetylenes 5, being the dehydration products of enols, may be regarded as having a masked carbonyl functionwhile the enamines 3 rank with vinyl ethers, vinyl esters and enols, as the heteroanalogs of these carbonyl derivatives (see Section IV). 

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