| Abstract: | Ureas of type 3 with sterically protected carbonyl groups such as 4a-7a are prepared by successively adding 2 equivalents of a highly substituted sec. amine and an excess of dimethylamine to benzene or toluene solutions of phosgene. The piperidine derivatives 6a and 7a are metalated in high yields to 6b and 7b , respectively, with sec.-butyllithium/TMEDA in THF at 0° see derivatives 6, 7 , ( c-i ) in Table 1]. Methods of cleaving urethanes 2 (see 8 → 9 ) and ureas 3 are described, among which the retro-Mannich reaction, removing the piperidine ring from 7 under acidic hydrolysis conditions, appears to be promising also for other applications of sterically blocked carbonyl compounds in organic synthesis. |
