2- Hydroxypyridine esters

The migration reaction has been reported for both heteroaromatic acid phenyl esters154 184 and hydroxypyridine esters.185 The former report describes the process for both 3- and 4-isomers of (150) and for the 5-ring heteroaromatic compounds... 

Displacement reactions with oxygen nucleophiles are of potential commercial interest. Alkaline hydrolysis provides 2-fluoro-6-hydroxypyridine [55758-32-2], a precursor to 6-fluoropyridyl phosphoms ester insecticides (410—412). Other oxygen nucleophiles such as bisphenol A and hydroquinone have been used to form aryl—pyridine copolymers (413). 

An interesting class ot covalent Inflates are vin l and ar>/ or heteroaryl Inflates Vinyl inflates are used for the direct solvolytic generation of vinyl cations and for the generation of unsaturated carbenes via the a-elimination process [66] A triflate ester of 2-hydroxypyridine can be used as a catalyst for the acylation of aromatic compounds with carboxylic acids [109] (equation 55)... 

The Guareschi-Thorpe pyridine synthesis is closely related to the Hantzsch protocol. The primary point of difference lies in the use of cyanoacetic esters. This modification assembles pyridine 23 by the condensation of acetoacetic esters 21 with cyanoacetic esters 22 in the presence of ammonia. A second variation of this method involves reaction of cyanoacetic ester 22 with P-diketone 24 in the presence of ammonia to generate the 2-hydroxypyridine 25. 

Reductive decarbonylation and decarboxylation can be carried out by (TMSlsSiH using acyl chlorides, phenylseleno esters, or N-hydroxypyridine-2-thione esters. Examples are shown in Reactions (17)-(19). Hydrolysis of the methyl ester followed by decarbonylation at the C2 position of hexahydropyrro-loindole (+)-17 afforded the desired tricycle (+)-18 in 84% yield and >99% ee. ... 

The acyl selenide 19 affords the decarbonylated )S-lactam in good yield. A N-hydroxypyridine-2-thione ester 20 is used in the key step to construct the chiral cis-cyclopropane structure in compounds designed as antidopaminergic agents. The observed high cis selectivity is due to the hydrogen abstraction from the sterically demanding (TMSlsSiH, which occurs from the less-hindered side of the intermediate cyclopropyl radical. 

Condensation of aminopyrazole 116 with ethoxy-methylene malonic ester gives the product of addition-elimination (117), which is then cyclized to the piperidone by heating in diphenyl ether. The product tautomerizes spontaneously to the hydroxypyridine 118. The hydroxyl group is then converted to the chloro derivative by means of phosphorus oxychloride (119). Displacement of halogen by n-butylamine gives... 

The 2D NOESY spectrum of the Schiff base being a derivative of histidine ethyl ester and 3-hydroxypyridin-4-carboxaldehyde has shown that in THF-d8 solution the E conformation of the molecule is preferred.22... 

Photochemical rearrangement has been reported for heterocyclic compounds, as shown in the esters of hydroxypyridine [79-81]. An example is shown... 

The esters of A-hydroxypyridine-2-thione are another versatile source of radicals.195 The radical is formed by decarboxylation of an adduct formed by attack at sulfur by the... 

A very efficient methodology for generating carbon radicals utilizes esters of A-hydroxypyridine-2-thione as radical precursors, and these have been used by Blood-... 

SCHEME 10. Preparation of hydroperoxides via oxygenation of carbon radicals generated from iV-hydroxypyridine-2-thione esters as radical precursors... 

Hydroxypyridine, available from pyridylpyridinium chloride hydrochloride, is the substrate for diodone (95) and its n-propyl ester, propyliodone. Both compounds are used as... 

Adams and Reifschneider 22 have reported ring opening and formation of 6-hydroxypyridine-2-crotonic acid by strong alkaline or acid hydrolysis of 6-hydroxy-4-quinolizone with ester groups in the 1-and 3-positions. However, hydrolysis with barium oxide affords the corresponding l-carbethoxy-3-carboxy-6-hydroxy-4-quinolizone. 

The construction of the furan ring in a Feist-Benary type reaction starting from 4-hydroxy-6-methylpyran-2-one (66) has also proved to be useful for the synthesis of furo[3,2-c]pyridines (Scheme 14) (75JHC461, cf. 71BSF4041). Quite recently the synthesis of the hitherto unknown furo[3,2-c]pyridin-3-ols has been described (79LA371). 4-Hydroxypyridine-3-carboxylates (e.g. 69), which are available from diketene and /3-aminocrotonic esters, react with a-halogenoketones in the presence of potassium carbonate to give compounds of type 70. 

The cycloaddition of acetylenic esters (e.g., DMAD) to betaines (e.g., 58) derived from 3-hydroxypyridine gives 1 1-molar adducts (e.g., 59), and has been reviewed.266... 

Newcomb and coworkers have very elegantly demonstrated that N-hydroxypyridine-2-thione carbamates lc (PTOC carbamates, X = NR2) are directly analogous to the A-hydroxy-pyridine-2-thione esters la. They can be prepared readily from primary and secondary amines. Reaction of these carbamates in a radical chain process gives carbamoyl radicals that decarboxylate to give aminyl radicals, Scheme 40.66 71 In the... 

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