O-Binucleophiles. Formation of Cyclic Carbonates

The formation of several cyclic carbonates by methods employing phosgene equivalents has been discussed in part in Section 4.3.3 Carbonates . In this Section, additional reported cyclization procedures with triphosgene are illustrated. 

Typical procedure. (S)-( )-5-EthYl-5-methYl-h3-dioxolane-Z4-dione 1148 [810] To a solution of (S)-(+)-2-hydroxy-2-methylbutyric acid 1147 (1.06 g, 8.97 mmol) in THF 

Typical procedure. l,3-Dithia-[3]ferrocenophan-2-one Fe(CsH4S)2CO 1150 [811] To a solution of Fe(C6H4SLi)2-2THF 1149 (406 mg, 1 mmol) in THF (20 mL) at -78 °C, a solution of triphosgene (0.33 mmol, 98 mg) in THF (5 mL) was added dropwise. After warming to ambient temperature, the solution was decanted off from the precipitated LiCl, filtered through Celite, and concentrated in vacuo. Recrystalhza-tion of the residue from CH2CI2 afforded colored crystals, which decomposed at 194 °C yield 224 mg (82%). 

The regioselective preparation of five-membered cycUc carbonates by treatment of 1,2,3-, 1,2,4-, or 1,2,5-triols with triphosgene, dimethyl carbonate, or 1,1 -carbonyldiimidazole has been reported [812]. 

The substituted 1,2,3-triol 1151 reacts with triphosgene in the presence of pyridine (Method A) to afford five-membered 1,2-cycUc carbonate 1152 as the sole product. However, reaction of 1,2,3-triol 1151 with dimethyl carbonate (Method B) affords the more stable internal cyclic carbonate 1153. 

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