Isocyanates from diazoketones

Silyl-substituted diazoketones 29 cycloadd with aryl isocyanates to form 1,2,3-triazoles 194 (252) (Scheme 8.44). This reaction, which resembles the formation of 5-hydroxy-l,2,3-triazoles 190 in Scheme 8.43, has no analogy with other diazocarbonyl compounds. The beneficial effect of the silyl group in 29 can be seen from the fact that related diazomethyl-ketones do not react with phenyl isocyanate at 70 °C (252). Although the exact mechanistic details are unknown, one can speculate that the 2-siloxy-1-diazo-1-alkene isomer 30 [rather than 29 (see Section 8.1)] is involved in the cycloaddition step. With acyl isocyanates, diazoketones 29 cycloadd to give 5-acylamino-l,2,3-thiadiazoles 195 by addition across the C=S bond (252), in analogy with the behavior of diazomethyl-ketones and diazoacetates (5). 

Benzo[6]thiophene-2-carboxamides have been prepared by treatment of 2-benzo[6]thienyllithium with isocyanates,564 but are more conveniently obtained from the appropriate acid chloride by the conventional procedure.334,481,548,556,564,686-688 The acid chlorides react with diazomethane to give the diazoketone,218,689 and with dimethylcadmium 76,90 or diethyl ethoxymagnesium malonate218,336... 

The total syntheses of clivonine (114) and clividine (138) have been achieved from the anhydride 75 already used in the syntheses of dihydrolycorine (85), y-lycorane (87), and S-lycorane (92). Treatment of 75 with MeOH mainly gave the half ester 127 which was converted into the urethane 131 via the acid chloride 128, the azide 129, and the isocyanate 130. The desired methyl ester 132 was obtained from 131 through the corresponding chloride and diazoketone. The... 

Like diazoketones, acyl azides lose nitrogen when heated or irradiated. The end result is an isocyanate, formed either directly from the acyl aade, in a reaction called the Curtius rearrangement after Theodore Curtius (1857-1928), or by intramolecular rearrangement of a nitrene, which is the nitrogen analogue of a carbene (Fig. 18.70). For most acyl azides, it is known that the isocyanate is formed by direct rearrangement of the acyl compound and that the nitrene, though also formed from the azide, is not involved in isocyanate production. 

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