A. tert-Butylcarbonic diethylphosphoric anhydride. A 500-ml., three-necked flask fitted with a mechanical stirrer, a pressure-equalizing dropping funnel, and a gas-inlet tube is dried in an oven, flushed with nitrogen, and allowed to cool while an atmosphere of nitrogen is maintained in the reaction vessel. To the flask are added successively 250 ml. of anhydrous tetrahydrofuran (Note 1) and 13.9 g. (0.124 mole) of alcohol-free potassium tert-butoxide (Note 2). After the mixture has been stirred for 10 min. under a nitrogen atmosphere to complete dissolution of the salt, the solution is cooled in an ice bath, and a slow stream of anhydrous carbon dioxide (Note 3) is bubbled through the cold solution for 1.5 hours with continuous stirring. While cooling, stirring, and the flow of carbon dioxide are maintained, a solution of 20.6 g. (0.120 mole) of pure diethyl phosphorochloridate (Note 4) in 50 ml. of anhydrous tetrahydrofuran (Note 1) is added dropwise to the reaction mixture, and the cold reaction mixture is stirred for an additional 30 minutes under a carbon dioxide atmosphere. After the solvent has been removed from the reaction mixture by concentration with a rotary evaporator at room temperature, the residual mixture of anhydride and potassium chloride is diluted with 300 ml. of anhydrous diethyl ether and centrifuged, separating the insoluble salt. The salt is successively suspended in two 150-ml. portions of anhydrous ether and centrifuged to remove any remaining ether soluble product. The combined ether solutions are concentrated at room temperature with a rotary evaporator, leaving 25–28 g. (84–91%) of the crude anhydride, as a colorless to pale yellow liquid (Note 5), sufficiently pure for use in the next step.

B. tert-Butyl azidoformate. A 300-ml., three-necked flask fitted with a mechanical stirrer, a pressure-equalizing dropping funnel, and a gas-inlet tube is dried in an oven, flushed with nitrogen, and allowed to cool while a nitrogen atmosphere is maintained in the reaction vessel. To the flask are added 120 ml. of anhydrous dimethyl sulfoxide (Note 6) and 8.1 g (0.10 mole) of powdered potassium azide (Note 7). While a nitrogen atmosphere is maintained in the reaction vessel, the mixture is stirred for 30 minutes to dissolve the bulk of the potassium azide before 25.4 g. (0.100 mole) of tert-butylcarbonic diethylphosphoric anhydride is added dropwise with stirring. During this addition the temperature of the reaction mixture is maintained at approximately 25° with a water bath. The resulting solution is stirred at 20–25° for 1 hour; 120 ml. of water is then added dropwise while stirring and external cooling are maintained. The resulting mixture is extracted with three 120-ml. portions of pentane, and the combined pentane extracts are washed successively with 20 ml. of water and 20 ml. of saturated aqueous sodium chloride. After the pentane solution has been dried over magnesium sulfate, the pentane is removed at room temperature with a rotary evaporator. The residual crude product, 11–14 g. of a pale yellow liquid, is distilled under reduced pressure from a distilling flask heated in a water bath. Caution! This distillation should be conducted in a hood behind a safety shield. (Note 8). The product is collected as 7.2–9.3 g. (50–65%) of colorless liquid, b.p. 57–61° (40 mm.), n_D24 1.4224–1.4230 (Note 9).