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Advances in Heterocylic Chemestry. Vol 10 - Boulton A.J.

Boulton A.J. Advances in Heterocylic Chemestry. Vol 10 - Academic Press, 1969. - 347 p.
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Wright and Collins47 reduced o-nitrophenylacetic acids (37) with zinc and sulfuric acid and obtained a mixture of the hydroxamic acid (38) and lactam (39)
48 R. N. Warrener and E. N. Cain, Angew. Chem. Intern. Ed. English 5, 511 (1966).
47 W. B. Wright and . H. Collins, J. Am. Chem. Soc. 78, 221 (1956).
Although the yields of 38 were high, the oxindole (39) was also formed as a by-product. This exemplifies the problem of reducing the nitro group specifically to a hydroxylamine.
Other zinc reductions have been used extensively. Zinc dust in aqueous ammonium chloride is a standard reagent for the reductive cyclization of nitro esters to hydroxamic acids. These reactions are usually carried out at low temperatures (0-10) to avoid further reduction. Despite the fact that good yields can often be obtained, these reductions are highly capricious, depending on the quality of the zinc (impurities seem to improve the reaction) and other unknown factors.
By this method, Chauveau and Mathis48 have prepared cyclic hydroxamic acids (41) containing a sulfur atom in the ring. The acyclic precursors (40) were formed by alkylation of a thiol anion.
R4/C1 Na+ -S. -"CH CH2
CH , CH2 -----------> I I
I I CH2 co2c2h5
ch2no2 co2c2h5 >2
Earlier, the reduction of y-nitro esters with zinc and ammonium chloride had been shown49 to provide a suitable route to A7-hydroxy 2-pyrrolidones, e.g., 42. Various catalytic hydrogenation procedures can also effect the same reductive cyclization.49,60
ch3n ch3
CH3 225 * CH,
CH3 N2 CHa^N
48 A. Chauveau and F. Mathis, Compt. Rend. 261, 481 (1965).
48 R. Bonnett, V. M. Clark, and Sir A. Todd, J. Chem. Soc. p. 2102 (1959).
50 B. Reichert and E. Wegner, Ber. Deut. Chem. Ges. 71, 1254 (1938).
In the catalytic hydrogenation of ethyl 2-nitrophenyl acetate (43), Di Carlo51 observed a small amount of the cyclic hydroxamic acid (44) in addition to the expected lactam (45). Following this result, a
N02 N ''o N
(43) (44) (45)
thorough study of the platinum oxide-catalyzed hydrogenation of 2-nitro-2'-carboxybiphenyl (46, R = OH) and some of its derivatives (46, R = OMe, NH2, ONa) has been carried out.52 In general, both
products (47 and 48) corresponding to reduction of the nitro group to a hydroxylamine or amine are formed.
In all cases, a higher percentage of hydroxylamino trapping occurs in the presence of mineral acid, and the hydroxylamino trapping abilities of the carboxy derivatives increase as R = OH < OMe < NH2. This order is reversed in the absence of mineral acid. Presumably the carboxyl derivative is protonated in the presence of acid and thus becomes a better electrophile.
An interesting comparison between a catalytic hydrogenation and a zinc and acetic acid reduction is observed53 in the case of the oximino acid (49). The cis-fused hydroxamic acid (50) was the sole
51 F. J. Di Carlo, J. Am. Chem. Soc. 66, 1420 (1944).
62 C. W. Muth, J. R. Elkins, M. L. DeMatte, and S. T. Chiang, J. Org. Chem.
32, 1106 (1967).
53 J. T. Edward and P. F. Morand, Can. J. Chem. 38, 1317 (1960).
product of catalytic reduction, whereas zinc and acetic acid gave the (raws-fused structure (51).
The same oximino acid (49) underwent54 a cyclodehydration on treatment with dicyclohexyl carbodiimide to form the unsaturated hydroxamic acid (52) in almost quantitative yield.
Perhaps the most reliable method for the reductive cyclization of a nitro ester to a hydroxamic acid is that which involves treatment with sodium boro hydride in the presence of palladium on charcoal.65 Although under these conditions aromatic nitro compounds are reduced to amines, o-nitro esters such as 53, in which the ester group is suitably oriented with respect to the nitro group, give good yields6867 of cyclic hydroxamic acids (54). Coutts and his co-
co2c2h5 no3
54 N. J. Doorenbos and . T. Wu, Chem. Ind. (London) p. 648 (1965).
55 T. Neilson, H. C. S. Wood, and A. G. Wylie, J. Chem. Soc. p. 371 (1962).
66 R. T. Coutts and I). G. Wibberley, J. Chem. Soc. p. 4610 (1963).
67 Noble and D. G. Wibberley, J. Med. Chem. 9, 974 (1966).
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