Ring-opening aldol-type reaction

Ring-Opening Aldol-Type Reaction of 2,2-Dialkoxycyclopropane-carboxylic Ester [7]... 

An acid-catalyzed lactonization of 8-hydroxy esters has been used in the diastereoselective synthesis of cA-4,5-substituted 8-lactones by Saigo et al. [48] (Scheme 11). The ring-opening aldol-type reaction of 2-methoxy-2-(trimethylsiloxy)cyclobutanecarboxylic ester 58 with aldehydes 59 in the presence of Lewis acid afforded the corresponding adducts 60 and 61. Subsequent treatment of these products with a catalytic amount of p-TsOH gave cis- and (rans-4,5-substituted lactones in favor of the c/r-isomer 62. 

Darzens reaction of (-)-8-phenylmethyl a-chloroacetate (and a-bromoacetate) with various ketones (Scheme 2) yields ctT-glycidic esters (28) with high geometric and diastereofacial selectivity which can be explained in terms of both open-chain or non-chelated antiperiplanar transition state models for the initial aldol-type reaction the ketone approaches the Si-f ce of the Z-enolate such that the phenyl ring of the chiral auxiliary and the enolate portion are face-to-face. Aza-Darzens condensation reaction of iV-benzylideneaniline has also been studied. Kinetically controlled base-promoted lithiation of 3,3-diphenylpropiomesitylene results in Z enolate ratios in the range 94 6 (lithium diisopropylamide) to 50 50 (BuLi), depending on the choice of solvent and temperature. ... 

A possible origin of this ring-opened alkaloid is from the methyl chanofruticosinate (238) which on oxidation provides the iminium ion 360. Hydrolysis of this iminium ion gives the presumably unstable carbinol amine 361 which could then undergo a retro-aldol-type reaction... 

To explain all the results it was necessary to assume that the aromaticity of the arylamine had to be broken by adding an ammonia molecule to the arylamine (Scheme 6). This explains why a high NH3 pressure is required and why the reaction runs better with naphthylamine than with aniline. Then ring opening occurs through a reverse aldol-type reaction (Scheme 7) and ring closure might occur... 

Some of the important click reactions are (Kolb and Sharpless 2003 Adzima and Bowman 2012) azide-alkyne azide-sulfonyl cyanide benzyne-azide cycloaddition and hetero Diels-Alder reactions nucleophilic ring opening of strained molecules such as epoxides, aziridines, etc. non-aldol-type reactions and additions to carbon-carbon multiple bonds through oxidation-type reactions such as epoxidation, hydrox-ylation, etc. For more details, the reader is referred to a recent review by Adzima and Bowman (2012). 

Base-induced eliminative ring fission, in which both the double bond and the sulfone function take part, has been observed in thiete dioxides253. The reaction can be rationalized in terms of initial Michael-type addition to the double bond of the ring vinyl sulfone, followed by a reverse aldol condensation with ring opening. The isolation of the ether 270c in the treatment of 6c with potassium ethoxide (since the transformation 267 -> 268 is not possible in this case) is in agreement with the reaction mechanism outlined in equation 101253. 

Treatment of cycloprop [c]pyrans 520 with acid leads to chromans 521 with a 5,8-substitution pattern, which is difficult to obtain by other synthetic methods (Scheme 114). This novel reaction is believed to proceed via a retro-hDA opening of the dihydropyran ring to afford 522 followed by an acid-catalyzed aldol-type cyclization and dehydration (Scheme 114) <20040L3191>. 

The reaction with a, -epoxy carbonyl compounds 12 leads to the corresponding reductive ring-opened products, -hydroxy carbonyl compounds 13, in good yields (Scheme 22). Electrochemically generated benzeneselenolate [21,22] and sodium phenylseleno(triethoxy)borate (1) [35, 36], have been applied for this type of reaction as a nucleophilic selenolate. In the latter case, the reaction mechanism was suggested as shown in Scheme 23 [36]. Reaction of 1 with 12 first produces the ring-opened adduct 14, which is then reacted with an excess amount of 1 to produce the final product 13. This method is important as a simple synthetic procedure to aldols and -hydroxy esters that are rather difficult to obtain by other methods. The reactions have been extended to the reduction of more functionalized a, -epoxy carbonyl compounds [37] and have been successfully applied for the synthesis of several natural products [38]. 

Click chemistry [174] involves the use of reactions that fulfill certain conditions they are high-yielding reactions that use readily available reagents. Moreover, these reactions are performed in green solvents like water, and allow easy isolation of products. In this context, several groups of reaction fit in this profile, such as (i) cycloaddition reactions (ii) nucleophilic ring-opening reactions (iii) carbonyl chemistry of the non-aldol type and (iv) addition to carbon-carbon multiple bonds... 

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