Hydroxy aldehydes cleavage

Cleavage of the oxathiane moiety can be carried out with iV-chlorosuccinimide/silver nitrate and leads to the a-hydroxy aldehyde 21 along with a diastereomeric mixture of sultines 20. The sultines can be reduced to the hydroxy thiol 22 which can be reconverted to the chiral auxiliary 16 in ail overall recovery of about 70%39. 

Cleavage of the addition product 5 can be easily performed by treatment with dilute ( 1 %) aqueous alcoholic hydrochloric acid which liberates the chiral amino alcohol 3 and an a-hydroxy aldehyde. The latter can be further oxidized to the oc-hydroxy carboxylic acid 6 with sodium chlorite33. 

Originally, the term aldol condensation referred specifically to the reaction of an aldehyde (having an a-hydrogen) with an aldehyde/ketone to form a j8-hydroxy aldehyde (the aldol). The reverse reaction is often referred to as a retrograde aldol reaction, a retro-aldol condensation (or reaction), or an aldol cleavage. March categorizes aldol condensations into five classes. The first is condensation between two identical aldehydes... 

Transfer of hydrogen from C-4" of uridine 5 -(a-D-glucopyranosylur-onic acid pyrophosphate) to C-3 of the apiosyl group in the ester 117 has been demonstrated.4458 The conversion of 116a into 118 was hypothesized446 to involve aldol cleavage, isomerization of the resulting a-hydroxy aldehyde, and intramolecular, aldol reaction as shown. 

Conversion of the cryrtro-aldols to eryrAro-a-inethyl-/J-hydroxy aldehydes is effected by reduction with LiAlH4 to a mixture of isomeric diols followed by glycol cleavage by sodium periodate in aqueous methanol.3... 

Reversible Modifications. Reversibility of a modification is frequently a desired characteristic in order to regain the protein in its original native form. A variety of methods (I) is available for this purpose. Sometimes a further intermediate modification is necessary as used recently in the reductive alkylation of amino groups with an a-hydroxy aldehyde or ketone and subsequent removal of the hydroxyalkyl group by periodate cleavage (85) (see Figure 18). 

Chiral secondary a-hydroxy aldehydes, Rf HOHCHO. The chiral acyl derivatives 3, obtained from 1 by lithiation and reaction with an aldehyde followed by Swem oxidation, can be reduced stereoselectively before cleavage to the secondary a-hydroxy aldehydes. 

When the vinyl ether 6 was the substrate, AD-mix (3 led to significant amounts of cleavage of the diol product. A change to the pyrimidine-based ligands, in particular the trimethoxy derivative 7 gave high selectivity for the formation of the desired a-hydroxy aldehyde 8 (Scheme 3.7) [255]. 

On a related front, the reactions of carbonyl compounds with metaliated derivatives of 2-methylthia-zoline furnish adducts (85). Although the initial nucleophilic addition occurs smoothly with a wide variety of aldehydes and ketones, the intermediate P-hydroxythiazolines (85) suffer thermal reversion upon attempted purification by distillation. Moreover, attempted cleavage of the corresponding P-hydroxythia-zolidines, which are readily produced from (85) upon dissolving metal reduction (Al-Hg), leads to the formation of p-hydroxy aldehydes only in simple systems numerous complications arising from dimerization, dehydration and retroaldol processes of the products usually intervene. Consequently it is necessary to protect the initial 1,2-adducts (85 = H) as the corresponding 0-methoxymethyl ether... 

Oxidative Cleavage Reactions. Among the numerous methods for 1,2-diol cleavage there exist only a few that involve catalytic ruthenium reagents, for example Ruthenium(III) Chloride with Sodium Periodate Attempted selective monooxidation of a 1,2-diol to the hydroxy aldehyde with catalytic TPAP and NMO resulted in carbon-carbon bond cleavage to provide the aldehyde (eq 11). Furthermore, attempted oxidation of an anomeric a-hydroxy ester failed instead, in this case decarboxy-lation/decarbonylation and formation of the lactone was observed (eq 12). However, Dimethyl Sulfoxide-Acetic Anhydride provided the required a-dicarbonyl unit. Retro-aldol fragmentations can also be a problem. ... 

The C21-C37 segment (183) The construction of the polyene chain from aldehyde 185 involved the use of lithiated phosphonate 127 twice as follows Horner-Emmons condensation generated a , , -triene ester which was reduced and oxidized to aldehyde 208 (Scheme 28). Iteration of this process, including a THP cleavage, generated the desired hydroxy aldehyde 183. 

C-Glycosidation of enol silane 279 to lactol acetate 278, prepared from 277 in two steps, furiushed ynone 280 as a single isomer. Reduction of the ketone with L-selectride furnished alcohol 270 with poor selectivity, but the minor isomer can be converted into the desired isomer via the Mitsunobu protocol Dihydroxylation of the terminal alkene, reduction of alkyne, and oxidative cleavage of the resulting triol gave the intermediate hydroxy aldehyde, which was spontaneously transformed into macrolactol 281 as a single diastereomer. 

In a pioneering investigation, the lithium enolate derived from 3-methyl-2-pentanone was added to aldehydes. Only moderate diastereoselectivity was obtained, however [119]. Exceptionally high induced stereoselectivity was observed when camphor-derived ketone 69 was converted into the lithium enolathe and subsequently added to aldehydes. a-Cleavage at the carbonyl group enabled the formation of y -hydroxy aldehydes and acids in high enantiomeric excess (Eq. (31)) [60]. The work on the aldol addition of methyl ketones led to a variety of stereoselective variants which rely mainly on boron enolates and will be discussed in Chapter 3 of Part I of this book. 

In additional studies, this work also demonstrated the oxidative cleavage of these silacycles using the Tamao and Fleming oxidation (Equation 8.3). Interestingly, the allylsilane 88 (n = 1) provided the (Z)-l,5-diol 89, whereas the vinylsilane 88 (n = 0) furnished the y-hydroxy aldehyde 90 [39]. 

Cleavage reactions of carbohydrates also occur on treatment with aqueous base for prolonged periods as a consequence of base catalyzed retro aldol reactions As pointed out m Section 18 9 aldol addition is a reversible process and (3 hydroxy carbonyl com pounds can be cleaved to an enolate and either an aldehyde or a ketone... 

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