Carboxylic acids exocyclic enolates

The A -acyl derivatives of 4-substituted-3,4,5,6-tetrahydro-27/-l,3-oxazin-2-ones proved to behave as effective chiral auxiliaries in asymmetric enolate alkylations and aldol reactions, the stereoselectivities of which were found to be higher for 4-isopropyl than for 4-phenyl derivatives <2006OBC2753>. The transformations of 4-isopropyl-6,6-dimethyl-3-propa-noyl-3,4,5,6-tetrahydro-2/7-l,3-oxazin-2-one 251 to 252 or 253 proceeded with excellent diastereoselectivities (Scheme 47). 6,6-Dimethyl substitution within the oxazine ring facilitated exclusive exocyclic cleavage upon hydrolysis of the C-alkylated and the aldol products 252 and 253, to furnish a-substituted carboxylic acids 254 or a-methyl-/ -hydroxycarboxylic acids 256. 

Enol lactones of carboxylic acids [X = —COR— (to C2) with exocyclic and X = —COR— (to C3) with endocyclic lactones Section VIII]. 

Enol lactonization. The last step in a synthesis of ( )-cyanobacterin (2), an antibiotic isolated from a cyanobacterium, is the enol lactonization of the y,8-acetylenic carboxylic acid 1. Cyclization catalyzed by AgNO, results in the desired exocyclic y-... 

Diasteroselective Alkylations of Exocyclic and Endocyclic Enolates of Carboxylic Acid Derivatives... 

Enolates derived from cyclic compounds such as cyclohexane carboxylic acid or cyclohexane carboxalde-hyde generate enolates that are unique. These enolates have an exocyclic double bond that can exist as ( ) and (Z) isomers. The facial and orientational bias in alkylation and condensation reactions of such enolates is influenced by the conformation of the ring it is attached to. Alkylidene cyclohexane enolates show a preference for equatorial attack, just as cyclohexanone derivatives do (sec. 4.7.C,D). 

---