Cyclodehydration chloride

Benzisothiazoles are best prepared by oxidative cyclization of o-aminothiobenz-amides (see Section 4.17.9.1.1), reaction of o-toluidines with thionyl chloride (see Section 4.17.9.2.1) or by sulfuration of 2,1-benzisoxazoles (see Section 4.17.10.2). 1,2-Benzisothiazoles can also be prepared from o-disubstituted benzene compounds, cyclodehydration of o-mercaptobenzaldoximes or oxidative cyclization of p-mercaptobenzylamines (see Section 4.17.9.1.1) being the most convenient. Both series of benzo compounds are readily substituted at the 5- and 7-positions by electrophilic reagents. 

Reduction of the imine with sodium borohydride leads to an intermediate amino-ester that cyclizes spontaneously to the <5-lactam function. Solvolysis of the acetyl group with methoxide followed by acylation of the hydroxyl group thus liberated with trimethoxybenzoyl chloride leads to 38. Bischler-Napieralski cyclodehydration (phosphorus oxychloride) effects closure of the remaining ring. Reduction of the imine thus formed with sodium borohydride gives 39. This, it should be noted, leads to the... 

Mendelson et al. [78] also investigated a number of cyclization reactions. One of these involved the cyclodehydration of N-benzylethanolamine chloride in a molten salt derived from AICI3 and NFL4CI (X(A1C13) = 0.73). This gave rise to the corresponding tetrahydroisoquinoline in 41-80 % yield, as shown in Scheme 5.1-48. 

The scheme used to prepare the direct 8-aza-analogue 21 of estrone bears at least formal similarity to the Torgov-Smith steroid total synthesis sequence. Acylation of the phenethylamine 9 with acryloyl chloride gives amide 16. Michael addition of dimethylamine followed by Bischler-Napieralski cyclodehydration gives the dihydroisoquinoline, 17. 

Anthraquinone itself is traditionally available from the anthracene of coal tar by oxidation, often with chromic acid or nitric acid a more modern alternative method is that of air oxidation using vanadium(V) oxide as catalyst. Anthraquinone is also produced in the reaction of benzene with benzene-1,2-dicarboxylic anhydride (6.4 phthalic anhydride) using a Lewis acid catalyst, typically aluminium chloride. This Friedel-Crafts acylation gives o-benzoylbenzoic acid (6.5) which undergoes cyclodehydration when heated in concentrated sulphuric acid (Scheme 6.2). Phthalic anhydride is readily available from naphthalene or from 1,2-dimethylbenzene (o-xylene) by catalytic air oxidation. 

Cyclodehydration of 2-phenylthiocyclohexanone with a variety of reagents yielding 1,2,3,4-tetrahydrodibenzothiophene (64) as an oil has been reported,and represents the simplest way of obtaining this material (88%). Alternatively, reduction of 4-keto-1,2,3,4-tetrahydrodibenzothiophene under Huang-Minlon conditions affords 64 in high yield.Trace amounts of 64 were detected in the reduction of dibenzothiophene with calcium hexamine and during electrolysis in ethylenediamine-lithium chloride solution (Section III, C,4). Peracetic acid oxidizes 64 to its sulfone (65%), which... 

Ph R = H). Other cyclodehydrating agents used to prepare meso-ionic l,3-oxazol-5-ones (66) include oxalyl chloride or dicyclohexyl-carbodiimide. ... 

In addition to the typical cyclization procedures described above, methods involving the use of other mild, cyclodehydrating agents have been published. For example, cyanuric chloride in the presence of triethylamine, 2-ethoxy-A-ethoxycarbonyl-l,2-dihydroquinoline (EEDQ) or 2-isobutoxy-A-isobutoxy-carbonyl-1,2-dihydroquinoline (IIDQ), ° A,A-dimethylchlorosulfitemethanimi-... 

Wuts and co-workers recently reported that the Vilsmeier reagent is superior to thionyl chloride for the cyclodehydration of primary and secondary p-hydroxy amides to prepare oxazolines, in particular, for oxazoline 18b, which is used in Taxol synthesis (Scheme 8.10). Some other examples are shown in Table 8.5 (Fig. 8.3). As expected, inversion of configuration at the alcohol bearing carbon atom is observed. Of the examples examined, serine afforded low yields due to the formation of dehydroalanine. The reaction is conveniently carried out in pyridine at room temperature. p-Chloro amides are also formed, which can be converted to the oxazoline with DBU, generally using the same mixture without isolation. The... 

The standard route to sydnones is the cyclodehydration of iV-nitroso-P-amino acids. 2-Chloro-4,5-dihydro-13-dimethylimidazolium chloride 97 has been shown to be an excellent reagent for this conversion, as, for example, in the synthesis of 3-phenylsydnone 98 <99JOC6989>. Functional group manipulations of fused sydnones have led to the isolation of several new compounds including 99 and 100 <99H(51)1433>. 4-Acetylsydnones react with hydrazine at room temperature to give 2,4-dihydropyrazol-3-ones 101 <99H(51)95>. ... 

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