Perchloric acid, cyclization using

Cyclization of hydrazone 38 with mercuric oxide and EDTA gave dihy-drotriazine 39 (87AP198). On the other hand, methyl hydrazone 38, under 4-electron withdrawal and neighboring group participation reacts with the same reagent to give lactam 40, a useful precursor for the synthesis of the pyrrolo[2,l-c]triazinium salt 41 by cyclization with perchloric acid (87AP258) (Scheme 12). 

The oxidative cyclization of thioacylamidines 83 is one of the best methods for the synthesis of unsymmetrical 3,5-diaryl- or dialkyl-1,2,4-thiadiazoles 84 (Equation 23) <2004HOU277>. Typical oxidants used in the cyclization step include bromine, iodine, or nitric acid, and, more recently, hydrogen peroxide in the presence of perchloric acid has been used. N-Substituted thioacylamidines give rise to 1,2,4-thiadiazolium salts <1997JOC3480>. 

An interesting variant uses acyl- or aroyl-methylpyridinium salts with ammonia or an amine to produce dipyridiniumdihydropyridine salts (655). With aromatic aldehydes the dihydropyridines are produced directly or the intermediate (654) can be isolated and subsequently treated with perchloric acid (75LA849). With formaldehyde (giving dihydropyridines without a substituent in position 4), the 1,5-diketone can be isolated and cyclized by ammonia (75LA864). The dipyridinium salts (655) undergo some useful transformations they can be oxidized to pyridines (656), and these dipyridinium pyridine salts give... 

The 1-benzothiopyrylium salts are best prepared from the easily accessible thiochroman-4-one [Eq. (35)].6,51,52,302-304 The common dehydrating agents include potassium hydrogen sulfate8 and phosphorus pentoxide51,302-304while sulfuryl chloride/perchloric acid51,302-304 and trityl perchlorate 6 have been useful as hydride extractors. Another method involves the cyclization of /1-ketosulfides (e.g., 81) or j8-arylthio-acroleins (82) with perchloric acid [Eq. (36)].305,306... 

Pyran decomposes in contact with moist air or adds but its tetrasubstituted derivatives, e.g. (155), are stable and are usefully converted into naphthalenes (156) by treatment with perchloric acid. A 4-benzyl or 4-naphthylmethyl substituent is essential. Pyran ring opening occurs and is followed by cyclization and elimination of a simple ketone which is formed from the 2- or 6-substituent. Alkyl groups at C-2 or C-6 are converted into the ketone more easily than a phenyl group (64LA(678)202). 

Hydroxybenzyl alcohols are cyclized by nitriles in either perchloric acid or sulphuric acid-acetic acid. The usefulness of nitriles in synthesis has been reviewed [3332a]. 

Chromans. - 2,4-Diethoxychromans (78) (mainly cis) represent a new type of chroman and have been prepared in high yield from a phenol (but not a deactivated phenol) and malonaldehyde bis(diethyl acetal) in the presence of SnCU they are converted into the benzopyrylium salts (79), in excellent yield, by treatment with perchloric acid. A Wittig-Horner reaction of the chiral phosphoryl ( S)-sulphoxide (81) and the ketone (80) gave the chiral sulphoxide (82), which was cyclized by aqueous alkali to a mixture which contained 22% diastereoisomeric excess of (25 )-chroman (i )-sulphoxide. This was converted, in three steps, into the aldehyde (83), which is a useful synthon for a-tocopherol. ... 

Benzopyrylium Salts. - Dichloromethoxybutane has been used in the cycliz-ation of the benzyl phenyl ketone (134) to give (after addition of perchloric acid) the pyrylium salt (135). The latter reacted normally with methylamine to form the isoquinolinium salt (136). A complex series of transformations has been suggested to account for the formation of the pyrylium salt (138) by the reaction of the chromone (137) with malononitrile-acetic anhydride. ... 

The cyclization of olefin has been modified using mercuric acetate and perchloric acid or mercuric perchlorate in acetic acid. ... 

Cationic cyclization of 2-(but-3-enyl)-3-methylcyclohex-2-enone to 9-methyl-decalin-2-ol-5-one (34) has been accomplished using perchloric acid and acetic acid in acetic anhydride, followed by a basic work-up. "... 

Another way to control the unpredictability of the hydrolysis is to use milder acidic conditions. The so-called modified Wichterle reaction was developed by Kobayashi and Matsumoto in the 1970s that changed the hydrolysis conditions from sulfuric acid to a 1 1 mix of formic acid and perchloric acid. This system is compatible with saturated ketones and efficiently combines the hydrolysis and cyclization steps. The use of... 

In their total synthesis of 6-methyl-5-androstene-3,l 1,17-trione 3-ethylene acetal, which is a building block for 6a-methylprednisolone, Daniewski and co-workers used a modified Wichterle reaction to install the A-ring of the natural product/ A racemic mixture of 56 was used in a reductive alkylation with Wichterle s reagent to afford the product 57 as a mixture that could be separated by chromatography. Upon treatment of each diastereomer of 57 with formic acid and perchloric acid to induce cyclization, it was observed that both diastereomers resulted in the same enantiopure product 58. This preferred conformation of the steroidal skeleton was confirmed by comparison to an authentic sample of the product that can be obtained directly from natural cortisone. 

The cyclization of amic acid does not always give the imide. Acetyl chloride was reported to cyclize p-methoxymaleanilic acid to the isomaleimide derivative 130. " Product 130 can also be obtained using perchloric acid... 

Silica-supported perchloric acid (HC104-Si02) has also been used as an effective, heferogeneous, recyclable catalyst for the S5mthesis of quinoxalines and dihydropyrazines using a-bromokefones and 1,2-diamines [48] (Scheme 28). The reaction takes place at room temperature presumably via cyclization-oxidation. 

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