Phosphorus ring oxygen replacement

Phosphates, nomenclature, 113-115 Phosphinates, nomenclature, 116 Phosphonates, nomenclature, 115-116 Phosphorus, ring oxygen replacement by, 140-141... 

As with five-membered heterocycles, phosphorus pentasulfide may replace a ring oxygen atom by sulfur, or it may effect replacement in an acyclic precursor and then bring about cyclization. Thus isochroman-l-one (99) is converted into thioisochroman-l-thione (100) under the same conditions a 3-arylisocoumarin( 101) produces a 3-ary 1-1-thioisocoumarin (102) only, with no replacement of the ring oxygen atom.122 A typical example of thiation, followed by cyclization, is the formation of 2-alkyl- or 2-aryl-3,l-benzo-thiazine-4-thiones (103) from /V-acylanthranilic acid esters (104) by treatment with phosphorus pentasulfide in boiling xylene.122... 

A number of hypoxanthine derivatives have resulted from rearrangement of 7-oxoimidazo-[4,5-rfl[l,3]oxazines with an amine hydrochloride in a mixture of Af,A -dimethylcyclohexylamine and phosphorus pentoxide in which the overall effect is the replacement of the ring oxygen by nitrogen, e.g. formation of 9. ... 

A piridazine ring forms the nucleus for a rather unusual nontricyclic antidepre.ssant. Condensation of the keto ester 136 with hydrazine leads to the cyclic hydrazide 137. Oxidation, for example with bromine, gives the corresponding pyridazone 138. The oxygen is then replaced by chlorine by reaction with phosphorus oxychloride. Displacement of the halogen in 139 with N-ethylami-nomorpholine affords minaprine 140 [30]. 

Phosphorus pentasulfide is used to replace oxygen atoms with sulfur atoms the reaction is commonly carried out in a solvent heated under reflux. Solvents employed include carbon disulfide, aromatic hydrocarbons, and pyridine. If an oxygen atom is part of a heterocycle, then the reagent may replace it with sulfur, as in the formation of 2,1-benzisothiazoles from 2,1-benzisoxazoles.119 Such replacements are, however, not general some prior ring opening appears to be necessary before the reagent can act. For example, under normal conditions furan is not attacked. 

The betaine intermediate is not isolated rather, it spontaneously decoiq poses through a four-membered ring to yield alkene and triphenyiphosphi oxide, (Ph)3P=0, The net result is replacement of the carbonyl oxygen ata by the R2C= group originally bonded to phosphorus (Figure 19.13). 

As in the case of monocyclic 1,3-azaphospholes/arsoles (Section 3.16.4.1.4) tris(trimethyl-silyl)phosphine/arsine provides the phosphorus/arsenic ring member by replacing the oxygen atom of an oxazolium ring. 

When an ester reacts with a ring-carbon atom in the presence of a Friedel-Crafts catalyst, a cyclic ketone (or its tautomeric enol) is usually formed (p. 378), but in boiling phosphorus oxychloride, the oxygen function may be replaced by a chlorine atom [2634]. A carboxylic acid may react with a suitably placed ring-carbon by heating with either PPA or phosphorus oxychloride [2127] phosphorus pentoxide in xylene is also effective [2347]. When an electron-rich ring such as that of pyrrole is available, heating with acetic anhydride for I h suffices [3913]. 

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