Phosphoranes, cleavage ethers

The chlorination of (72) at —100 °C gives the trichlorophosphorane (73), which rearranges above — 40 °C to the dichlorophosphate (74) with no evidence for the formation of the phosphonium salt (75). Likewise, the halogenation of (76a,b) at low temperature gives the phosphoranes (77a,b) which, with the exception of (77b, X = Br), are stable at room temperature. The latter compound, which was shown to be in equilibrium with the phosphonium salt (78), slowly gave a spirophosphorane (79) by cleavage of the ether linkage. The... 

The other way to generate thioesters at the penultimate step of penem synthesis is acylation of silver thiolates 226. These compounds are conveniently obtained by cleavage of the corresponding trityl sulfides 225 with silver nitrate and methanol [51a, 144] trityl methyl ether and nitric acid (trapped by pyridine or, better, imidazole) are liberated in the process. Tetrahydropyranylthioethers 227 [145] and acetyl thioesters 228 [51a, 52] were also reported to be cleavable to the corresponding argentiothio-phosphoranes of general formula 226. 

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