Dehydrations oximes, acetic anhydride

If the hydrazone is unsubstituted, dehydration with acetic anhydride gives a 2-acetyltriazole, which is readily converted into an unsubstituted triazole.Oxidation of a-diketone hydrazone oximes with cupric sulfate in aqueous pyridine leads to the formation of 2.ff-triazole-1-oxides. ... 

Thiazine-5-carboxaldehyde (254) has been converted to esters 255 by the action of AcOH and NaCN, followed by MnC>2 (Scheme 103) (85MI1). Thiazine-5-carbonitrile (256) has also been obtained from compound 254. This is first converted to the oxime, which is then dehydrated by acetic anhydride. Alternatively, 254 can be treated with CF3CONO2CCF3 in benzene and pyridine (Scheme 104), to also give 256. 

Carbonation of the sodium salt of 7-azaindole (105, R = H) gives 7-azaindole-3-carboxylic acid (106, R = OH) in low yield. Robison and Robison also prepared it from 7-azaindole-3-carboxaldehyde (105, R = CHO) either by direct oxidation (28 % yield) or through the oxime, which was dehydrated with acetic anhydride to l-acetyl-3-cyano-7-azaindole. This compound was hydrolyzed in water to give 3-cyano-7-azaindole (105, R = CN) in refluxing hydrochloric acid it... 

D-Galactopyranosyl-D-arabinose was prepared from lactose oxime by dehydration with acetic anhydride and degradation of the resultant nitrile with sodium methoxide in chloroform. This galactosylarabinose has also been prepared by the oxidation of calcium lactobionate with hydrogen peroxide and a ferric salt (the Ruff degradation ). Under... 

The oxime 590 derived from the aldehyde 588 was dehydrated with acetic anhydride, and the resulting nitrile (591) methanolized to ( )-cylindrocarine (117). As expected, acetylation of 117 gave ( )-cylinflro-carpidine (121), and cinnamoylation gave ( )-cylindrocarpine (120 Scheme 34) (249). 

The Wohl deffvdation, an alternative to the Ruff d radation, is nearly the reverse of the Kiliani-Fischer synthesis. The aldose carbonyl group is converted to the oxime, which is dehydrated by acetic anhydride to the nitrile (a cyanohydrin). Cyanohydrin formation is reversible, and a basic hydrolysis allows the cyanohydrin to lose HCN. Using the following sequence of reagents, give equations for the individual reactions in the Wohl degradation of D-arabiiK>se to D-erythrose. Mechanisms are not required. 

Of these compounds, the dichloromethylthiadiazole (134) is the key intermediate in the synthesis of the desired compound (140). It is hydrolysed to the 2-carboxyaldehyde, which is isolated as the oxime (135). This is next simultaneously acetylated and dehydrated by acetic anhydride to yield 5-acetamido-2-cyano-l,3,4-thiadiazole (136). Application of Pinner s synthesis affords the amidine (138), which is dehydrated and cyclized by sulphuric acid to the imidazole (139) methylation and nitration under carefully controlled conditions complete the synthesis. ... 

Oximes derived from aldehydes (aldoximes) can be dehydrated to nitriles by many different dehydrating reagents acetic anhydride is one of the most common reagents used ... 

The creation of the N—N bond as the last step of the ring synthesis is common in indazoles and very rare in pyrazoles. In indazoles this method is well known (type B synthesis (67HC(22)l), for example, the dehydration of oximes (570) with acetic anhydride yields 1-acetylindazoles (571), and in basic medium the indazole 1-oxides (573) are formed from the nitro derivatives (572). 

Quinazoline 3-oxides were first prepared by Awers and von Meyen-burg in 1891 by the dehydration of o-acylamino benzophenone oximes with acetic acid-acetic anhydride and were assigned the in-dazole structure (33). This was then modified by Bischler to the... 

Conversion of the aldehyde into a nitrile is accomplished by treatment of an aldose with hydroxvlamine to give an oxime (Section 19.8), followed by dehydration of the oxJme with acetic anhydride. The Wohl degradation does not give particularly high yields of chain-shortened aldoses, but the reaction is general for all aldopentoses and aldohexoses. For example, D-galactose is converted by Wohl degradation into n-lyxose. 

By Dehydration of a-Diketone Hydrazone Oximes Another general method of synthesis of 2i/-triazoles involves intramolecular elimination of water from adjacent hydrazone and oxime functions (Scheme 32). The usual reagent is acetic anhydride, but others, including phosphorus pentachloride and urea, have been used. 

The indirect conversion of an aromatic aldehyde into the corresponding nitrile by dehydration of an oxime is illustrated by the synthesis of veratronitrile (Expt 6.170). The dehydrating agent is acetic anhydride which probably effects an initial acetylation of the oximino group followed by the elimination of acetic acid. 

Dehydration of the oxime to a nitrile occurs with acetic anhydride and sodium acetate. The nitrile product is a cyanohydrin. 

To synthesize the triazole (71), another approach (63) was used, where dehydration of the mixed hydrazone oxime (70) (61) with acetic anhydride afforded the triazole (71). 

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