Hydroxy aldehydes rearrangement

Lithiated allylic carbamates (35) (prepared as shown) react with aldehydes or ketones (R C0R ), in a reaction accompanied by an allylic rearrangement, to give (after hydrolysis) y-hydroxy aldehydes or ketones. The reaction is called the homoaldol reaction, since the product is a homolog of the product... 

If the reaction is carried out with ketone labeled in the C=0 group with the first pathway predicts that the product will contain all the " C in the C=0 carbon, while in the second pathway the label will be in the a carbon (demonstrating migration of oxygen). The results of such experiments have shown that in some cases only the C=0 carbon was labeled, in other cases only the a carbon, while in still others both carbons bore the label, indicating that in these cases both pathways were in operation. With a-hydroxy aldehydes and ketones, the process may stop after only one migration (this is called the a-ketol rearrangement). 

Oxidation of silyl enol ethers. Oxidation of silyl enol ethers to a-hydroxy aldehydes or ketones is usually effected with w-chloroperbenzoic acid (6, 112). This oxidation can also be effected by epoxidation with 2-(phenylsulfonyl)-3-( p-nitrophenyl) oxaziridine in CHC1, at 25-60° followed by rearrangement to a-silyloxy carbonyl compounds, which are hydrolyzed to the a-hydroxy carbonyl compound (BujNF or H,0 + ). Yields are moderate to high. Oxidation with a chiral 2-arene-sulfonyloxaziridine shows only modest enantioselectivity. 

Ring enlargement.1 A new route to seven-membered ring systems from a cyclohexenone (1) involves a photocycloaddition of ethylene to provide the bicy-clooctanone 2. Addition of lithio-1,3-dithiane to 2 provides the adduct 3, which on reaction with HgO and HBF4 forms an unstable rearranged hydroxy aldehyde... 

Rearrangement of aryl propargyl ethers.1 Claisen rearrangement of the naphthyl propargyl ether 1 at 215° results in the benzopyran 2 as the only isolable product (40% yield). Addition of 1 equiv. of CsF results in the benzofuran 3 as the major product, presumably formed via an a-allenyl ketone (a). Related salts such as KF, RbF, or BaF2 are completely ineffective. This modified Claisen rearrangement provides a route to o-hydroxy aldehydes such as 4 from a phenol. 

Few methods for the a-hydroxylation of aldehydes are available. This is a reflection of their notorious instability towards polymerization and rearrangement to a-hydroxy ketones. All the useful procedures generate protected a-hydroxy aldehydes. 

A recent application of the furan-carbonyl photocycloaddition involved the synthesis of the mycotoxin asteltoxin (147)." Scheme 16 shows the synthetic procedure that began with the photoaddition of 3,4-dimethylfuran and p-benzyloxypropanal to furnish photoaldol (148), which was epoxidized with MCPBA to afford the functionalized product (149) in 50% overall yield. Hydrolysis (THF, 3N HCl) provided the monocyclic hemiacetal which was protected as its hydrazone (150). Chelation-controlled addition of ethylmagnesium bromide to the latent a-hydroxy aldehyde (150) and acetonide formation produced compound (151), which was transformed through routine operations to aldehyde (152). Chelation-controlled addition of the lithium salt of pentadienyl sulfoxide (153) followed by double 2,3-sigma-tropic rearrangement provided (154) as a 3 1 mixture of isomers (Scheme 17). Acid-catalyzed cyclization of (154) (CSA/CH2CI2) gave the bicyclic acetal (155), which was transformed in several steps to ( )-asteltoxin (147). ... 

Lewis Acid-Catalyzed Rearrangement of 2,3-Epoxy-1-ols. Epoxides with adjacent hydroxyl groups, as in 2,3-epoxy-1-ols, undergo rearrangement on treatment with Bp3 Et20 to afford aldol-like products (e.g., P-hydroxy-aldehydes). ... 

The defined geometry of these steps has permitted the synthesis of some cycloheptane acids related to terpenoid natural products. The key reaction was the rearrangement of a suitably substituted bicy-clo[4.2.0]octane. Thus the photoaddition of ethylene to 3-methylcyclohexenone gave the ketone (42), which was converted to the alcohol (43). On treatment with HgO and HBF4 this gave the unstable hydroxy aldehyde (44), which was readily oxidized to the dicarboxylic acid (45 Scheme 17).- Ring expansion methodology was also used in an approach to the synthesis of the trichothecenes (see Scheme 18).32... 

Lipides, 238, 239, 241-243, 261 Lipidosis, 239, 242, 243 Lobry de Bruyn-Alberda van Ekenstein transformation, 63, 291 acid catalysis of, 79 aldolization in, 77 base catalysis of, 79-81 catalysis of, by metal ions, 81 dealdolization in, 77 dehydration reactions in, 73 enzyme-catalyzed, 66, 70 formation of reductones in, 79 of or-hydroxy aldehydes, 71 mechanism of, 84 of noncarbohydrate a-ketols, 71 non-enzymic, 66, 67, 83 in paper chromatography, 81 rearrangement of carbon chain, 79 scope of, 65 of steroids, 72 use of, for synthesis, 82 Lyxonic acid, 3-deoxy-D-, 300 Lyxose, D-, condensation of, with urea, 218... 

Homoallylic amines containing an allylic hydroxy group rearrange in the presence of an aldehyde and an acid catalyst to yield 3-acylpyrrolidines. If the starting amino alcohol is cyclic, this transformation provides a pyrrolidine-annulated product, in which the initial ring is expanded by one member. The mechanism has been proposed to proceed via a tandem cationic aza-Cope rearrangement/Mannich cyclization1134. 

Pummerer rearrangement (5,3-4). The Pummerer rearrangement of P-hydroxy sulfoxides to derivatives of a-hydroxy aldehydes has been extended to the rearrangement of 3-keto sulfoxides. Thus rearrangement of (1) with acetic anhydride-sodium acetate in toluene under reflux gives the S-aryl thioester (2) in 74% yield. The ester is hydrolyzed by base to mandelic acid (3). In the absence of sodium acetate the normal product of the Pummerer rearrangement... 

The Amadori rearrangement is a reaction between a-hydroxy aldehydes and suitable amines leading to a-amino ketones. When applied to aldoses, for example, D-glucose (1), it allows the introduction of an amino group at position C-1 with concomitant isomerization leading to 1-amino-1-deoxyketoses such as 2 (Scheme 1). 

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