Acetals open-chain

Open-chain carbon skeletons (including cyclic acetals, lactones, lactams, cyclic anhydrides, etc.)... 

The general pattern of alkylation of 2-acylaininothiazoles parallels that of 2-aminothia2ole itself (see Section III.l). In neutral medium attack occurs on the ring nitrogen, and in alkaline medium a mixture of N-ring and N-amino alkylation takes place (40, 43, 161. 163). In acidic medium unusual behavior has been reported (477) 2-acetamido-4-substituted thiazoles react with acetic anhydride in the presence of sulfuric acid to yield 2-acetylimino-3-acetyl-4-phenyl-4-thiazolines (255) when R = Ph. but when R4 = Me or H no acetylation occurs (Scheme 151). The explanation rests perhaps in an acid-catalyzed heterocyclization with an acetylation on the open-chain compound (253), this compound being stabilized... 

Compounds that contain both carbonyl and alcohol functional groups are often more stable as cyclic hemiacetals or cyclic acetals than as open chain compounds Examples of several of these are shown Deduce the structure of the open chain form of each... 

The anomenc carbon m a cyclic acetal is the one attached to two oxy gens It IS the carbon that corresponds to the carbonyl carbon m the open chain form The symbols a and (3 refer to the configuration at the anomenc carbon... 

Analogous open-chain precursors also lead readily to 1,3-dithiolylium salts. S-a-Oxoalkyl thioesters such as (20) on treatment with perchloric acid in glacial acetic acid and HaS undergo ready cyclization to the 1,3-dithiolylium perchlorate (22) (66AH0.7)39). The oxoalkyl dithioesters (21) are probably intermediates in this cyclization as they themselves undergo cyclization with warm 70% perchloric acid or sulfuric acid (80AHC(27)15l). 

In specific applications to phase transfer catalysis, Knbchel and his coworkers compared crown ethers, aminopolyethers, cryptands, octopus molecules ( krakenmole-kiile , see below) and open-chained polyether compounds. They determined yields per unit time for reactions such as that between potassium acetate and benzyl chloride in acetonitrile solution. As expected, the open-chained polyethers were inferior to their cyclic counterparts, although a surprising finding was that certain aminopolyethers were superior to the corresponding crowns. 

However, these authentic cyclic compounds are converted by acetic anhydride or methyl iodide into derivatives of the hypothetical amino-aldehyde form. Methylation gives first the cyclic methiodides (23a-b) and (24) which occur by the addition of a single molecule of methyl iodide in the presence of sodium hydroxide and excess methyl iodide these are converted into the open-chain quaternary methiodides (25a-b) and (26). Similar examples have been given by Garden and by Gensler and co-workers. ... 

Glycosides are named by first citing the alkyl group and then replacing the -ose ending of the sugar with -oside. Like all acetals, glycosides are stable to neutral water. They aren t in equilibrium with an open-chain form, and they don t show mutarotation. They can, however, be converted back to the free monosaccharide by hydrolysis with aqueous acid (Section 19.10). 

Reactions. PE readily forms esters with organic and inorganic acids (Refs 9, p 157 13, pp 54, 212 246) and cyclic as well as open chain acetals and ketals (Refs 9, p 195 13, p 141). Some of these prods are used in expls, while others... 

This section is completed with a brief review of the synthesis and properties of this epimer (20) of the precursor of thiazole in bacteria. This pentulose is conveniently accessible by an unconventional route (Scheme 19). Methyl 2,3 4,6-di-O-isopropylidene-a-D-mannopyranoside, readily available from methyl ot-D-mannopyranoside, is converted to the ketonic glycoside by butyllithium in 91% yield, following a method first published by Klemer and Rodemeyer43 and scaled up by Horton and Weckerle.44 This was converted by means of lithium hydroxide in a water-ether mixture into 3,5-0-benzylidene-l-deoxy-D-eryf/iro-2-pen-tulose in 55% yield. Hydrolysis to the free pentulose (20) proceeded in 73% yield in aqueous acetic acid. This product was obtained as a syrup with a characteristic absorption band at 1705 cm 1 as a film. Thus, there is a fair proportion of the open-chain ketone under these conditions, as with the D-threo epimer.45... 

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

Although a maj ority of research activities were dedicated to cycloketone converting BVMOs, the recently discovered novel MOs also enable stereoselective oxidation of noncyclic ketones to esters. An aliphatic open-chain monooxygenase (AOCMO) from Pseudomonas Jluorescens DSM 50106 displays stereoselective biooxidation of terminal acyl-groups in proximity to hydroxyls (Scheme 9.23). The biooxidation gives acetic... 

The abasic sites (3, Scheme 8.2) resulting from the loss of alkylated bases from DNA are both cytotoxic and mutagenic. " The cyclic acetal (3) exists in equilibrium with small amounts (—1%) of the open chain aldehyde (4). The acidic nature of a-proton in the aldehyde form of the abasic lesion facilitates 3-elimination of the 3 -phosphate residue to yield a strand break. " This reaction occurs with a half-life of about 200 h under physiological conditions (pH 7.4, 37°C), but can be accelerated by heat, basic conditions, or the presence of various amines. " ... 

Theoretical calculations at DFT level agree that the reactions of nitrones with silyl ketene acetal proceeds via 1,3-dipolar cycloaddition followed by the transfer of the silyl group, yielding an open-chain product (641). 

Addition of alcohols to lactones results in the formation of orthoacid or orthoester derivatives. Thus, reaction of lactone 95a with potassium cyanide in ethanol led to displacement of the tosyl group by cyanide and addition of ethanol to the lactone carbonyl group, to give the orthoacid derivative 95b, which was isolated as its acetate 95c. Mild deacylation of 95c led back to 95b, but under more vigorous reaction conditions the open-chain methyl aldon-ate was obtained (90). 

The pyrrolidine derivative 314, a skeletal analog of the antitumor antibiotic anisomycin, was synthesized from the acetal derivative 16b. The 5-OH group of 16b was tosylated and then substituted with sodium azide. Reduction (sodium borohydride) of the lactone group afforded an open-chain derivative, which was selectively protected to give 313. Hydrogenation of the azide function, followed by p-toluenesulfonylation, led to 314 by an intramolecular nucleophilic displacement (284). 

Rattet, L. S., Williams, A. D., Goldstein, J. H. Medium Dependent Geminal Coupling in Open Chain sp3 Hybridized Systems (Acetals). J. Mol. Spectry, 26, 281 (1968). 

The photo-Fries rearrangement of aryl hydrogen (or methyl) succinates 267 leads to 4-(2-hydroxyaryl)-4-oxobutanoic acids (or methyl esters 268), which are readily cyclized to 5-(2-acetoxyaryl)-2(3//)-furanones (269) (Scheme 68). [189-191] Photolysis of 269 [191] or the analogous open-chain enol acetates [192,193] leads to chromones. 

In aqueous solution, both glucose (hemiacetal) and frnctose (hemiketal) exist as equilibrium mixtures of cyclic and open-chain carbonyl forms. Sucrose, however, is a single stable substance (acetal and ketal), and conversion back to glucose and frnctose requires more rigorous hydrolytic conditions, such as heating with aqueous acid. 

Reaction With Diols - With pentan-2,4-d1ol, both 1 and 2 (R =He) are converted with high selectivity to open chain cyclic acetals 11. In excess methanol or water with one equivalent of added diol, acetals H form In 8054 yield at room temperature. 

Addition of a catalyst containing sulfuric acid and acetic anhydride to 3, 5 -di-0-acetylthymidine in acetonitrile results in an equilibrium mixture of a- and ]3-anomers that, after time, gave a substantial quantity of a diastereoisomeric mixture of fully acetylated open-chain nucleosides. These were of type 1.5 with 2 -deoxy and 4 -acetoxy substituents (93TL6779). Open-chain nucleoside was also obtained as well as the expected nucleoside when 5-nitrouracil was condensed with methyl-2,3-dideoxy-3-fluoro-5-0-(4-phenylbenzoyl)-]3-D-cryr/zro-pentofuranoside (94S516). 

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