No abstract

A consideration of carbamic acid, urea, and guanidine as mono-, di-, and tri-ammonocarbonic acids indicated that the cyclizations of their /3-chloroethyl derivatives might have some properties in common, so compounds of types (I), (II), and (III) were re-examined.In 1885, Nemirowsky (6) prepared N-phenyl /3-chloroethyl carbamate (I, R = phenyl) from /3-chloroethyl chlorocarbonate and aniline. He observed this carbamate to cyclize in alkaline solution to 3-phenyI-2-oxazolidone. This reaction was employed later by Otto (8) to prepare 3-a-napthyl-and 3-,8-napthyl-2oxazolidones. The new carbamates (I) and 2-oxazolidones prepared by these procedures are given in Tables I and II respectively. The cyclization of carbamates to 2-oxazolidones is explained by the participation of a carbonium ion intermediate.

A practical route for the total synthesis of 8,9-secokaurene diterpenes is described. The central step is the [3.3]sigmatropic rearrangement of spirocyclic intermediates such as 35, 40, and 41. All three compounds must necessarily respond identically to properly install the absolute configuration of the bridgehead methine carbon. The total synthesis of (−)-O-methylshikoccin (2b) was realized in 8% overall yield from the Wieland−Miescher ketone 9. Its naturally occurring epoxide 47 was prepared with comparable efficiency. The preparative route developed herein should provide a general entry into this important class of diterpenoids.

A new synthesis of 2,3,5,6-tetrahydro-1-imidaz(1,2-a)imidazole is described. This bicyclic compound is hydrolyzed to l-(β-aminoethyl)-2-imidazolidone. The preparation of l-(β-hydroxyelhyl)- and 1-vinyl-2,3,5,6-tetrahydro-1-imidaz(1,2-a)imidazole also is described.

Radical‐Type Cyclization of Dienes, VIII[1]. — On the Regioselective Synthesis of Unsaturated Cyclopenta[b]furan‐2‐ones from (S)‐(+)‐Carvone — Building Blocks for the Synthesis of Enantiomerically Pure Triquinanes The diastereomeric products 2a and 2b, which were synthesized from (S)‐(+)‐carvone (1), are converted to (3aS,5S,6aS)‐(‐)‐5‐acetoxyhexahydro‐6a‐methyl‐2H‐cyclopenta[b]furan‐2‐one (4) in ca. 50% yield. 4 is regioselectively converted to (3aR,6aS)‐(‐)‐3,3a,6,6a‐tetrahydro‐6a‐methyl‐ (8a) and (3aS,6aR)‐(+)‐3,3a,4,6a‐tetrahydro‐6a‐methyl‐2H‐cyclopenta[b]‐furan‐2‐one (8b). These are used at the 10‐g scale for the synthesis of enantiomerically pure triquinanes.