Ring closure acetals

An interesting application is the preparation of 1 2 3 4-tetrahydrocarb azole (VI), which is formed when phenylhydrazine is added to a boiling aolutiai of cyclohexanone in acetic acid the plienylhydrazone (V) Intermediately produced undergoes ring closure directly ... 

The thiazolyl-2-thioglycollic acid (119) undergoes intramolecular ring closure to give mesoionic compound 120 under treatment with acetic anhydride and triethylamine (Scheme 60) (192). The parent acid (119) can be recovered from 120 by hydration with hot 50% aqueous sulfuric add. Compound 120 affords monohydrate of bis(-cyclopentenothiazolyi-2-thio)acetone (121) (192). 

Oxidation of cycHc sulfites with permanganate in acetic acid solution gives cycHc sulfates (102). Heating monohydroxyalkyl hydrogen sulfates with thionyl chlohde causes ring closure (103). 

A second synthesis of cobyric acid (14) involves photochemical ring closure of an A—D secocorrinoid. Thus, the Diels-Alder reaction between butadiene and /n j -3-methyl-4-oxopentenoic acid was used as starting point for all four ring A—D synthons (15—18). These were combined in the order B + C — BC + D — BCD + A — ABCD. The resultant cadmium complex (19) was photocyclized in buffered acetic acid to give the metal-free corrinoid (20). A number of steps were involved in converting this material to cobyric acid (14). 

A Methylanthrapyridone and Its Derivatives. 6-Bromo-3-methylanthrapyridone [81-85-6] (75) is an important iatermediate for manufacturiag dyes soluble ia organic solvents. These solvent dyes are prepared by replacing the bromine atom with various kiads of aromatic amines. 6-Bromo-3-methylanthrapyridone is prepared from 1-methyl amino-4-bromoanthra quin one (43) by acetylation with acetic anhydride followed by ring closure ia alkaU. The startiag material of this route is anthraquiaoae-l-sulfonic acid (16). 

Numerous variations of this reaction have been studied, principally those involving a prior inclusion of the nuclear sulfur atom in a thioacylamino compound. Thus, thiobenz-amido acetaldehyde diethyl acetal (8) underwent ring closure to 2-phenylthiazole (9) on gentle heating (57JCS1556). Similarly, iV-thioacyl a-amino acids also undergo ready ring closure to thiazoles. 

Substitution of the nitrogen atom in (289) and subsequent ring closure of (293) under acid cyclodehydration conditions gave the mesoionic system anhydro-5-hydroxythiazoIium hydroxide (294). These reactions are analogous to the cyclodehydration of the A-nitrosogly-cines (295) with acetic anhydride to give the sydnones (296) (see Chapter 4.21). 

Alcoholic potassium hydroxide or sodium hydroxide are normally used to convert the halohydrins to oxiranes. Other bases have also been employed to effect ring closure in the presence of labile functional groups such as a-ketols, e.g., potassium acetate in ethanol, potassium acetate in acetone or potassium carbonate in methanol.However, weaker bases can lead to solvolytic side reactions. Ring closure under neutral conditions employing potassiunT fluoride in dimethyl sulfoxide, dimethylformamide or A-methyl-pyrrolidone has been reported in the patent literature. 

In cases where decarboxylation is anticipated, the quinazolinone could be obtained by heating ammonium o-formamidobenzoate or o-forra ami do benz amide for several hours. This ring closure could be effected more conveniently by boiling the o-formamidobenzamide with 3% aqueous sodium hydroxide for a few minutes, o-Amino-benzamides have also been converted to quinazolinones by refluxing with ethyl orthoformate alone or preferably in the presence of acetic anhydride. ... 

Reaction of tryptamine with simple ketones has not been widely explored. Acetone in the presence of benzoyl chloride has been reported to yield 2-benzoyl-1,1 -dimethyl-1,2,3,4-tetrahydro-j8-carbo-line. That the keto group is much less reactive than the aldehyde group is indicated by the fact that j8-keto aldehydes, in the form of their acetals or sodium salts, react with tryptamine at the aldehyde function to yield the conjugated enamine 24, which undergoes ring closure via an intramolecular Michael addition. The potentialities of this interesting modification of the Pictet-Spengler reaction have not yet been fuUy explored. 

It is worth noting that cyclization reactions of o-trichlorotellurodiphenylamine 40 proceed under milder conditions (refluxing their acetic acid solutions) as compared to those required for the ring closure of o-trichlorotellurodiphenyloxide and the corresponding sulfide (heating at 200°C and 240-250°C, respectively) (26JCS223 60T15). 

The Smiles rearrangement of sulfones 352 led to intermediates 353 the final ring closure was done in glacial acetic acid to provide the corresponding products of denitrocyclization 354 in 66% yield (Scheme 54) (72JHC699). 

The diazo function in compound 4 can be regarded as a latent carbene. Transition metal catalyzed decomposition of a diazo keto ester, such as 4, could conceivably lead to the formation of an electron-deficient carbene (see intermediate 3) which could then insert into the proximal N-H bond. If successful, this attractive transition metal induced ring closure would accomplish the formation of the targeted carbapenem bicyclic nucleus. Support for this idea came from a model study12 in which the Merck group found that rhodi-um(n) acetate is particularly well suited as a catalyst for the carbe-noid-mediated cyclization of a diazo azetidinone closely related to 4. Indeed, when a solution of intermediate 4 in either benzene or toluene is heated to 80 °C in the presence of a catalytic amount of rhodium(n) acetate (substrate catalyst, ca. 1000 1), the processes... 

The piperidide 30 a of o-aminohippuric acid cyclizes to the benzodiazepinedionc 31 a in refluxing acetic acid.209 Similarly, the free acids 30b-e210 and their ethyl esters 30f—I211 undergo ring closure on heating. The reaction has been extended to hydrazine derivatives 30m-q.212... 

A completely different concept13 makes use of a highly reduced bilane 5 which is oxidatively cyclized to an isobacteriochlorin 6 with copper(II) acetate. The ring closure is initiated by ester cleavage with trifluoroacetic acid and decarboxylative formylation with trimethyl orthoformate to yield a dialdehyde. One of the aldehyde functions forms the desired methine bridge whereas the other is lost during cyclization. 

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