Phosphorus-oxygen rings

Binary oxides of phosphorus, arsenic and antimony are polycyclic compounds, e.g. E4O6 (E = P, As, Sb), with an adamantane-like structure. Monocyclic 

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CARBON-PHOSPHORUS-OXYGEN (AND SULFUR) -METAL RINGS... 

Phosphorus—phosphorus bonds, addition, to alkynes, 10, 751 Phosphorus rings, chromium carbonyl link arsenic rings, 5, 226 As, Sb, Se, Te, Sn rings, 5, 226 boron rings, 5, 222 carbon rings, 5, 224 characteristics, 5, 221 oxygen rings, 5, 227... 

The oxazoliumcarboxylic acid (147) is easily decarboxylated via the ylide (148) the neutral compound (149) is much more stable due to the low equilibrium concentration of the zwitterionic tautomer (150 Scheme 7). Oxazolium salts lacking substituents at the 2-position react with dialkyl acylphosphonates in the presence of triethylamine to give mixtures of l,4-oxazin-3-ones and 2-azetidinones the reaction (see Scheme 8) proceeds by electrophilic attack of the phosphonate on an oxazolium ylide, e.g. (151), followed by insertion of oxygen into the carbon-phosphorus bond, ring-opening, and formation of the enolate anion (152) which can cyclize in two alternative ways with expulsion of the phosphonate group. 

Ylids can be isolated, but are usually used in reactions immediately they are formed. They are nucleophilic species that will attack the carbonyl groups of aldehydes or ketones, generating the four-membered ring oxaphosphetane intermediates. Oxaphosphetanes are unstable they undergo elimination to give an alkene (65% yield for this particular example) with a phosphine oxide as a byproduct. The phosphorus-oxygen double bond is extremely strong and it is this that drives the whole reaction forward. 

The structural types observed for phosphorus-sulfur rings are rather more diverse than for the oxygen congeners. Figme 50 shows representative examples. 

Oxaphosphetane (Section 21.10B) An intermediate in the Wittig reaction consisting of a four-membered ring containing a phosphorus-oxygen bond. 

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