Compounds with a Potential Hydroxy Group

Although iV-hydroxypyrroles possess in principle several tautomeric forms, e.g. (76), (77) and (78), only the A-hydroxy form (76) has been observed for l-hydroxy-2-cyanopyrrole (73JOC173). In the case of 1-hydroxyindoles, where the potential loss of aromatic resonance energy will be much less, both tautomers (79) and (80) coexist in solution with the relative proportions being dependent on the solvent 

The 3-hydroxy form is considerably more favored in the more aromatic thiophene system. Thus, 2-methyl-3-hydroxythiophene exists at equilibrium as a mixture of the keto form (92 R1 = Me, R2 = H) (20%) and enol form (93 R1 = Me, R2 = H) (80%) (86HC(44/3)l). For the r-butyl derivative (R1 = Bu1, R2 = H) it was 45% of (92) and 55% of (93). For the 2,5-dimethyl derivative (R1 = R2 = Me), the keto-enol equilibrium constant in cyclohexane was 2.61, while in methanol it was 1.21. 3-Hydroxy- 

Structure of Five-membered Rings with One Heteroatom 

A closely similar pattern of behavior is also found for the corresponding benzo-annulated derivatives. Thus, 3-hydroxybenzofuran (66CB3076) and 3-hydroxyindoles (58JCS1217) adopt the oxo form (91), but enolize to (90) when an acetyl group is present at position 2 (65T3331,60JA1187). 

Equilibrium and rate constants for the keto-enol tautomerization of 3-hydroxy-indoles and -pyrroles are collected in Table 32 (86TL3275). The pyrroles ketonize substantially (103-104 times) faster than their sulfur or oxygen analogues, and faster still than the benzo-fused systems, indole, benzofuran, and benzothiophene. The rate of ketonization of the hydroxy-thiophenes and -benzothiophenes in acetonitrile-water (9 1) is as follows 2-hydroxybenzo[b]thiophene 2,5-dihydroxythiophene 2-hydroxythiophene 3-hydroxybenzo[/ Jthiophene 3-hydroxythiophene. 3-Hydroxythiophene does not ketonize readily in the above solvent system, but in 1 1 acetonitrile-water, it ketonizes 6.5 times slower than 2-hydroxythiophene (87PAC1577). 

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VI. Compounds with a Potential Hydroxy and a Potential Amino Group. 132... 

Selective de-O-benzylation at primary positions by acetolysis has been described, and the procedure (c/. M, M. Ponpipom, Carbohydrate Res., 1977, 59, 311) would appear to be a convenient means of obtaining benzylated carbohydrates with the primary hydroxy-groups free. Formerly, it was necessary to employ sequential tritylation, benzylation, and detritylation in order to achieve these blocked compounds. De-O-benzylation has also been described using EtSH-Bp3, under mild conditions, and although it has not yet been applied to carbohydrates, it would appear to be of potential value. ... 

Thus, metabolites of 24 were synthesized and evaluated as a potential new lead series [11]. The syntheses of these compounds were carried out in a fashion similar to the original synthesis with a preinstalled ketal group at the carbon that would eventually bear the hydroxyl groups (Scheme 19.2). The 7-a-hydroxy derivative (26) and the 7-p-hydroxy derivative (27) showed comparable IC50 values of 17 nM and 23 nM, respectively. The 8-hydroxy analogs (25 and 37) were slightly less active. A weta-CFs-substituted C5 aryl group... 

A great deal of work has appeared on these compounds however, much of it was mutually contradictory and a clear pattern is only now appearing for the tautomcrisra of compounds of this type. Compounds with potential 5-hydroxyl groups, w hich could exist as 44a, 44b, or 44c (X = O or NR), never appear in the hydroxy form (unless this is stabilized by chelation) the tw o oxo forms, 44b and 44c, exist in equilibrium. However, compounds wdth potential 3-hydroxyl groups do exist as such, i.c, as 44d and not as 44e. This... 

From a-trichloromethyl carbinols with the hydroxy group at a tertiary carbon, either dichlorovinyl compounds or, after rearrangement, carbonyl compounds are formed predominantly. The pathway of the reaction can be influenced by the choice of the reaction conditions. As the reduction potentials of some of the substrates are... 

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