Quinol acetates

Hydroxylation at C-5 or C-l3 has also been successfully achieved by lead tetraacetate oxidation, which was extensively studied in connection with isoquinoline alkaloids by Umezawa s group. (+)-Govanine (96) and (+)-discretine (97) were oxidized with lead tetraacetate in acetic acid to afford 5-acetoxy products 100,101, and 102 via p-quinol acetates (e.g., 99) (Scheme 23)... 

Hydroxyprotoberberine 59a and ( )-corytencine (98) led to 13-acetoxy compounds 104,105, and 107 moreover, the 2,3,9,10,12-pentaoxy-genated protoberberine 108 was also obtained from 98 via the p-quinol acetate 106 through a retro-Mannich reaction followed by recyclization (74,75). Oxidation in dichloromethane instead of acetic acid proceeded differently, namely, 97 and 98 led to pentaoxygenated protoberberines 103 and 109 by introduction of an acetoxyl group at C-4 and C-12, respectively, via o-quinol acetates (76). 

Lead tetraacetate oxidation was applied to construct a benzo[c]-phenanthridine skeleton. The Hofmann degradation product 224 derived from the phenolic protoberberine 59a was oxidized with lead tetraacetate to afford the p-quinol acetate 225, which was cyclized to the benzo[c]-... 

Lead tetraacetate in acetic acid oxidizes phenolic 1-benzylisoquinolines to p-quinol acetates which usually rearrange to aporphines in trifluoroacetic acid (25). However, Blasko et al. (24) recently reported that lead tetraacetate oxidized ( )-A -norlaudanosine (34) to dibenzopyrrocoline 35 in 16% yield. 

A full paper has appeared describing the oxidation of 6-hydroxylated tetrahy-drobenzylisoquinolines of type (38) with lead tetra-acetate, to furnish the corresponding ortho-quinol acetates (39), which can readily undergo cyclization to the corresponding aporphines in acid solution. Predicentrine (40), isodomesticine (41), boldine (42), and 2,10-dihydroxy-1,9-dimethoxyaporphine (43) were prepared by such a route, which is, therefore, a practical pathway for the synthesis of 2-hydroxylated aporphines.27... 

Phenols with a free ortho position can also give rise to ortfeo-quinone diacetates such as 39a and 41b, in addition to or instead of ortfeo-quinol acetates such as 41a [1, 6]. Phenols bearing a 2-methoxy group are particularly prone to regioselective Wessely oxidation to give 6-... 

In a continuing study of the synthesis of aporphines via o-quinol acetates, 1,2-diacetoxyaporphines of the type (32) were obtained in good yields by treatment of solutions of the o-quinol acetates (31), in acetonitrile, with concentrated sulphuric acid in acetic anhydride.45... 

A full paper has appeared describing the synthesis of the aporphines (34) and (35) through treatment of the p-quinol acetate (33) with trifluoroacetic acid.46 The non-identity of (35) with natural lirinine confirmed structure (36) for lirinine. 

Treatment of corypalline (13) with lead tetraacetate leads57 to the quinol acetate (14) which rearranges in acetic anhydride-sulfuric acid to yield the 4-acetoxytetrahydroisoquinoline (15). The reduction of the lactol (16) to 4-hydroxytetrahydroisoquinoline (17) has been reported.70... 

Pattenden and coworkers have recently evaluated the relative merits of LTA and electrochemical oxidation of phenolic compounds with particular reference to synthesis of the antiallergic compounds sodium chromoglycate (lOTAL 53) and proxicromil (54), which are used for the prophylactic treatment of asthma. The 2-carboxychromone moieties in the compounds (53) and (54) are synthesized from the appropriate 2, 6 -dihydroxyacetophenones. Oxidation of the 2 -hydroxyacetophenone (55) by LTA in (U-chloromethane gave almost exclusively the quinol acetate (56), which was subsequently converted to the 2, 6 -dihydroxyacetophenone (57), a precursor to proxicromil (54 Scheme 21). By contrast, electro-... 

In addition to these reactions typical of the (n,nr ) state, 6-acetyloxycy-clohexadienones (o-quinol acetates) form phenols from the state. 

Figure 7.50. Schematic state correlation diagram for o-quinol acetate photoreactions. Wavy arrows designate physical as well as chemical radiationless processes (by permission from Quinkert et al., 1986).

A preference of 1,4-addition by the alkylmagnesium halide species rather than the dialkylmagnesium species, was observed during the reactions of di-/-propylmagnesium and /-propylmagnesium bromide in diethyl ether with the o-quinol acetate 40 [53]. Between concentrations of 0.06 and 0.24 M in diethyl ether, only 2% of 1,4-addition product 41 was observed with the dialkylmagnesium species. However, at 0.24 M concentration of... 

The migration of oxygen from a quaternary center in a cyclohexadienone may be preferred to a carbon shift, when present as an ether or ester function rather than free hydroxy. Thus the p-quinol acetate (117) yields the orcinol monoacetate (118 79%) on treatment at room temperature with trifluoroacetic anhydride, and the p-quinol ether (119) forms the resorcinol diethyl ether (120 71%) in ethanolic sulfuric acid. In the second case, hemiketalization must intervene also some methyl shift (12%) is observed. With the quinol (121), treatment with acetic anhydride-sulfuric acid leads to the lactone (122) acetylation or lactonization probably precedes oxygen shift. A number of related examples can be found in the steroid area. - Thermal 1,3-shifts of p-quinol acetates can also be induced acetate (117) yields catechol acetate (123 50-60%, 45 °C) by way of isomerization of the first-formed acetate (124). In the o-quinol acetate series, 1,2-acetoxy shift is seen in (125) (126 92%) and in (127) (128 90%), both in... 

The conventional dienone-phenol rearrangements of o-quinol acetates are described above. An alternative mode of restructuring to an aromatic system has been observed, exemplified by the quantitative conversion of the 2,4-dienone (155) on heating at 110 C in acetic acid to the benzylic acetate (156). The related 2,4-dienone (157) rearranges at 70-80 °C in dimethyl sulfoxide-sodium bicarbonate to the... 

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