Pyridine diphosgene

Homopolycarbonates based on 1 and 2 have been prepared by several groups. The interfacial polycondensation typical for the synthesis of aromatic polycarbonates is not useful with alditols, including 1, because they are water-soluble and less acidic than diphenols. The 1-based homopolycarbonate was prepared by phosgena-tion of the sugar diol, with phosgene or diphosgene in pyridine-containing solvent mixtures at low temperatures. The polycondensation of the isosorbide bischloro-formate in pyridine is an alternative approach. 

The decomposition of diphosgene is catalysed in the presence of various nitrogen-containing derivatives, such as pyridine, quinoline, or tertiary amines, to give complete... 

Interestingly, the chloroformate (diphosgene) and the carbonate (triphosgene) react with pyridine to give the same product (see also Chapter 12). The material described as (py)j. jCOClj, prepared from the reaction of phosgene with pyridine in benzene solution, was noted to lose half a molar equivalent of pyridine upon standing over PjOj in vacuo [1809a]. 

Esters and thiol esters. Treatment of acids with diphosgene and pyridine at —40°C and then with alcohols or thiols produces esters and thiol esters. 

Recently, DAS and equimolar amounts of various diols were polycondensed with diphosgene in pyridine. Different bisphenols, l,3-bis(4-hydroxybenzyloxy) propane, and 1,4-cyclohexanediol were used as comonomers [42]. In some cases, large amounts of cyclic oligo- and polycarbonates were formed. 

Protection of the 7-hydroxyl group in Paditaxd (taxol ) 10 as the carbonate 13 of the maleimido-peptide-benzyl alcohol 12 was carried out through the corresponding chloroformate using diphosgene in the presence of pyridine/DIE [19]. 

Typical procedure. (+)-(R)-(Cyclopent-2-enYl)methYl chloroformate 50 [34] To a magnetically stirred solution of diphosgene (2.42 mL, 20 mmol) in dichloromethane (18 mL), dry pyridine (0.82 mL, 10 mmol) was added at 0 °C, and the resulting suspension was stirred at room temperature for 30 min. Thereafter, a solution of (+)-(R)-(cydopent-2-enyl)methanol (980 mg, 10 mmol) in dichloromethane (8 mL) was added dropwise at 0 °C, and the mixture was stirred at room temperature for 5 h. It was diluted with diethyl ether, resulting in a dark-brown liquid (+)-(R)-fcyclopent-2-enylJ methyl chloroformate (1.30 g, 81%). 

Chloroformates 65 of 4-substituted ben2yl alcohols 64, intermediates for berrzyl carbamate disulfide drug derivatives, have been prepared with diphosgene in dioxane/pyridine [42]. 

Chloroformylation of unsaturated alcohols has been carried out by treatment with either diphosgene or triphosgene [46]. A two-step procedure for the preparation of the unsaturated chloroformate 82 was developed. The first step is treatment of alcohol 81 with 2 equiv. of diphosgene in boiling dichloromethane to give a 1 1 mixture of chloroformate 82 and trichloromethyl 4-penten-2-yl carbonate 83. The second step is conversion of the latter into chloroformate 82 by treatment of the mixture with a catalytic amount of pyridine (0.05 equiv.) in dichloromethane. After careful evaporation of the solvent, the residue was soaked in dry pentane and then pyridinium hydrochloride was removed by filtration. Chloroformate 82 was obtained in 78% yield after distillation (bp 75 °C/66 mmHg). 

Isopropyl benzyloxy-(2-oxo-5-phenyl-oxazolidin-4-yl)acetate 595 has been prepared in high yield with diphosgene and pyridine in dichloromethane at 0 °C [420, 421]. 

Typical procedure. (4R,5R)-5-[(R)-(1-BenzYloxy-1-isopropoxYcarbonYl)ntethYl]-4-phenYl-2-oxazolidone, 595, and its enantiomer [421] At 0 °C, pyridine (200 pL, 2.5 mmol) and diphosgene (72 pL, 0.60 mmol) were successively added to a solution of 594 (207 mg, 0.60 mmol) in dichloromethane (2 mL). After stirring at the same temperature for 10 min, the mixture was diluted with water and ethyl acetate. The upper ethyl acetate layer was separated, washed successively with saturated NaHC03 solution and saturated brine, dried over anhydrous MgS04, and then concentrated in vacuo. The residue was purified by column chromatography (hexanes/EtOAc, 4 1—> 3 1) to afford (4R,5R)-5-[(R)-(l-ben2cyloxY-l-isopropoxYcar-bonyl) methyl]-4-phenyl-2-oxazolidone 595 as a colorless oil (199 mg, 90%). 

Furthermore, polycondensations of bisphenol-A and diphosgene were performed in a homogeneous phase with pyridine as catalyst and HCL acceptor. The excess of diphosgene was varied to compensate for the loss due to side reactions. After optimization of the reaction conditions the MALDl-TOF mass spectra displays again an almost exclusive formation of cyclic polycarbonates, but the highest Mn amounted only to 15 kDa. In other words, the interfacial polycondensations... 

---