Cyclizing reagents, acetals ammonium acetate

Permanganese is a common oxidative reagent, the application of which to the asymmetric oxidative cyclization of 1,5-dienes has been reported by Brown (Scheme 3.14). The addition of acetic acid is quite important for the reaction to proceed, and highly functionalized tetrahydrofurans are obtained in a range of 58 to 75% ee, in diastereoselective manner [35]. Another oxidative transformation using KMn04 with a chiral ammonium salt has been investigated. Scheme 3.15 illustrates the asymmetric dihydroxylation of electron-deficient olefins to chiral diols in the... 

Ceric (IV) ammonium nitrate (CAN) works as the same oxidative radical reagent as Mn(OAc)3. Diethyl 3-furylphosphonates was prepared in good yield under mild conditions by a simple two-step procedure using CAN-promoted oxidative addition of p-ketophosphonate (139) to vinyl acetate (140), followed by acid-catalyzed cyclization as shown in eq. 4.49. 

In a novel transformation the j8-oximes of isatins gave o-N, N -dimeth-ylaminomethyleneaminobenzonitriles (13) with the Vilsmeier reagent (Me2NCHO—POCI3), The nitriles were cyclized to 4-aminoquinazolines with ammonium acetate in 70-80% yields. ... 

Angelini described the preparation of dihydro- 1,4-benzothiazine and related compounds from analogous esters by a two-stage reduction using tin and HC1 or lithium aluminum hydride.129 It is interesting to compare the reaction of 2-nitrophenol with similar reagents to produce benzooxazine derivatives. 2-Nitrophenol reacts with a-bromoethyl acetate, and the adduct is cyclized with zinc and ammonium chloride to the hydroxamic derivative 85.130 The adduct with phenacyl chloride has been similarly cyclized under a variety of conditions.131,132... 

Sodium cyanate, a reagent with a short shelf life, requiring frequent titration, has been used in cold, dilute acetic acid to convert o-aminoamides to 2-ureidobenzamides (155), which can be cyclized to quinazoline-2,4-diones (see 1) by heating at 100°C with 7 N hydrochloric acid, 8 N sodium hydroxide,257 or simply in water.258 No yields were published. Ammonium thiocyanate, similarly acidified, was used to convert 2-aminothiophene-3-carboxamide to 2-thioxothieno[2,3-d]pyrimidin-4-ones (see 12).255... 

Initially, sulfuric acid was used to initate the Borsche—Drechsel cyclization. However, Perkin found that acetic acid provided cleaner products. Recently, other reagents—such as cerium ammonium nitrate (CAN) and the more environmentally friendly acidic ionic liquids —have been used to catalyze the ring formation. It should be noted that people have found it difficult to promote the cyclization when there are substituents at the ortho-position of the phenylhydrazine. This difficulty becomes more pronounced when both the hydrazine and the cyclohexanone have a-substitution This difficulty usually leads to low yields compared with other methods of forming the carbazole. 

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