Darzen reaction

Darzens Condensation (Darzens-Claisen Reaction, Darzens Glycidic Ester Condensation)... 

Darzens-type reactionThe Darzens reaction2 of a-halocarbonyl compounds with an aldehyde requires a base, which can promote side reactions. Darzens reactions can proceed under neutral conditions when effected with this neutral reagent (I cquiv.). Presumably, the reaction proceeds via an organotin(lV) enolate. When the reaction is conducted in THF, the rron.v-epoxidc is generally favored, but addition of HMPA can favor cis-sclectivity. ci.v-Selectivity is also favored in reactions of aliphatic aldehydes. The Darzens-type reaction can also be extended to or-halo esters, particularly in the presence of Bu4NF or LiBr. 

Since this pioneering study, a breathtaking number of naturally occurring alkaloid derivatives have emerged, finding application in a wide range of processes such as alkylations, Michael additions, aldol reactions, Darzens... 

Other strong base reactions such as Michael additions, aldol condensations, Wittig reactions, Darzens condensations, carbene reactions Oxidations using hypochlorite, hydrogen peroxide, oxygen, permanganate Epoxidations... 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

This, the Darzens reaction, is useful in other circumstances (frames 280-1) but a nuisance here. We must use some means to make the ketone act as the nucleophile in the initial condensation. One effectiye way is to conyert it into an enamine. Draw a mechanism for this reaction. 

This is the Darzens reaction (frames 172-3) (see Norman, p.231 if you want more details). How would you make... 

It is also possible to convert carbonyl groups into oxirane rings with cenain carbenoid synthons. The classical Darzens reaction, which involves addition of anions of a-chloroacetic esters, has been replaced by the addition of sulfonium ylides (R. Sowada, 1971 C.R. Johnson, 1979). 

Ba.se Catalyzed. Depending on the nature of the hydrocarbon groups attached to the carbonyl, ketones can either undergo self-condensation, or condense with other activated reagents, in the presence of base. Name reactions which describe these conditions include the aldol reaction, the Darzens-Claisen condensation, the Claisen-Schmidt condensation, and the Michael reaction. 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Class (2) reactions are performed in the presence of dilute to concentrated aqueous sodium hydroxide, powdered potassium hydroxide, or, at elevated temperatures, soHd potassium carbonate, depending on the acidity of the substrate. Alkylations are possible in the presence of concentrated NaOH and a PT catalyst for substrates with conventional pX values up to - 23. This includes many C—H acidic compounds such as fiuorene, phenylacetylene, simple ketones, phenylacetonittile. Furthermore, alkylations of N—H, O—H, S—H, and P—H bonds, and ambident anions are weU known. Other basic phase-transfer reactions are hydrolyses, saponifications, isomerizations, H/D exchange, Michael-type additions, aldol, Darzens, and similar... 

Darzens glycidic ester condensation generally involves the condensation of an aldehyde or ketone 2 with the enolate of an a-halo ester 1 which leads to an a,P-epoxy ester (a glycidic ester) (3). Thus the reaction adds two carbons to the electrophile however, the reaction has been primarily developed as a one-carbon homologation method. That is, subsequent to the condensation, the ester is saponified and decarboxylation ensues to give the corresponding aldehyde or ketone 5.2... 

Several years ago, there was much debate concerning the mechanism of the Darzens condensation.2.3 The debate concerned whether the reaction employed an enolate or a carbene intermediate. In recent years, significant evidence that supports the enolate mechanism has been obtained, wherein the stabilized carbanion (11) of the halide (10) is condensed with the electrophile (12) to give diastereomeric aldolate products (13,14), which subsequently cyclize via an internal Sn2 reaction to give the corresponding oxirane (15 or 16). The intermediate aldolates have been isolated for both a-fluoro- and a-chloroesters 10. 

In recent years, several modifications of the Darzens condensation have been reported. Similar to the aldol reaction, the majority of the work reported has been directed toward diastereo- and enantioselective processes. In fact, when the aldol reaction is highly stereoselective, or when the aldol product can be isolated, useful quantities of the required glycidic ester can be obtained. Recent reports have demonstrated that diastereomeric enolate components can provide stereoselectivity in the reaction examples include the camphor-derived substrate 26, in situ generated a-bromo-A -... 

The Darzens condensation reaction has been used with a wide variety of enolate equivalents that have been covered elsewhere. A recent application of this important reaction was appljed toward the asymmetric synthesis of aziridine phosphonates by Davis and coworkers.In this application, a THF solution of sulfinimine 34 (0.37 mmol, >98% ee) and iodophosphonate 35 (0.74 mmol) was treated with LiHMDS (0.74 mmol) at -78 °C to give aziridine 36 in 75% yield. Treatment of 36 with MeMgBr removed the sulfinyl group to provide aziridine 37 in 72% yield. 

Darzens reaction can be used to efficiently complete the stereoselective synthesis of a"-substituted epoxy ketones. As an example, Enders and Hett reported a technique for the asymmetric synthesis of a"-silylated a,P-epoxy ketones. Thus, optically active a -silyl a-bromoketone 38 was treated with LDA followed by the addition of benzaldehyde to give a"-silyl epoxyketone 40 in 66% yield with good... 

In a separate report, the Darzens reaction was recently used by Barluenga, Concellon, and coworkers for the preparation of enantiopure a"-amino a,P-epoxy ketones. Accordingly, the Z enolate of a"-amino a-bromo ketone 41 was generated with KHMDS at -100°C. Benzaldehyde was added, and trans epoxyketone 42 was isolated in 87% yield and >95% de. ... 

Recendy, Darzens reaction was investigated for its synthetic applicability to the condensation of substituted cyclohexanes and optically active a-chloroesters (derived from (-)-phenylmenthol). In this report, it was found that reaction between chloroester 44 and cyclohexanone 43 provided an 84% yield with 78 22 selectivity for the axial glycidic ester 45 over equatorial glycidic ester 46 both having the R configuration at the epoxide stereocenter. 

Of interest is a recent report of a rapid synthesis of efaroxin (51), a potent, selective O2 adrenoceptor antagonist, using Darzens Reaction. Accordingly, a-bromoester 48 was condensed with aldehyde 47. The glycidic ester (49) was then hydrogenated to reduce the more labile epoxide bond to give alcohol 50. Subsequent standard transformations subsequently lead to a completed 4-step synthesis of efaroxin. o... 

The Darzens reaction involves a two-step, base-catalyzed condensation of ethyl chloroacetate with a ketone to yield an epoxy ester. The first step is a carbonyl condensation reaction, and the second step is an SK2 reaction. Write both steps, and show their mechanisms. 

Fischer projections of, 980 Dacron, structure of, 819 Danishefsky, Samuel, 1002 Darzens reaction. 913 DCC (dicyclohexylcarbodiimide),... 

There have been two general approaches to the direct asymmetric epoxidation of carbonyl-containing compounds (Scheme 1.2) ylide-mediated epoxidation for the construction of aryl and vinyl epoxides, and a-halo enolate epoxidation (Darzens reaction) for the construction of epoxy esters, acids, amides, and sulfones. 

Epoxides bearing electron-withdrawing groups have been most commonly synthesized by the Darzens reaction. The Darzens reaction involves the initial addition of an ct-halo enolate 40 to the carbonyl compound 41, followed by ring-closure of the alkoxide 42 (Scheme 1.17). Several approaches for inducing asymmetry into this reaction - the use of chiral auxiliaries, reagents, or catalysts - have emerged. 

Table 1.7 2-Oxazolidinones as chiral auxiliaries in Darzens reactions.

Table 1.8 Use of 8-phenylmenthyl esters to induce asymmetry in the Darzens reaction.

Ohkata [30, 31] and co-workers have employed an 8-phenylmenthyl ester to induce asymmetry in the Darzens reaction (Table 1.8). Moderate to high diaster-... 

Yan s group has used the camphor-based chiral thioamide 62 in asymmetric Darzens reactions (Scheme 1.21) [32]. The addition of the titanium enolate of 62 to... 

Table 1.10 Chiral reagent 75 in asymmetric Darzens reactions. Ph Ph...

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