Studies toward the synthesis of halichlorine and pinnaic acid

Three approaches toward the core of halichlorine and pinnaic acid are described. The first approach entails a racemic transannular nitrone-olefin [3+2] cycloaddition from nitrone 238. Construction of the nitrone 238 began with aldehyde 241. Another key feature in this route involved a ring-closing metathesis for the formation of a 14-membered ring. The route ended upon formation of a key diol 237.;The second approach incorporated the C14 methyl group at an early stage to probe its influence upon an intramolecular nitrone-olefin [3+2] cycloaddition. The approach began with dithiane 258. A key feature in the second plan involved asymmetric induction at the C5 carbon through a conjugate addition of an azide to an alpha,beta-unsaturated imide to form azide 290 with both excellent yield and stereocontrol for this addition. The route was advanced to oxaziridine 299. Conversion of 299 to the nitrone 300 and ultimately the intramolecular nitrone-olefin [3+2] cycloaddition was unsuccessful.;The third and final approach involved using the basic plan of our first successful approach and employing the asymmetric induction of the second approach. This blending of strategies led to an asymmetric intermediate that links up with the first approach. The route began with dithiane 258. Again, the use of an azide addition to a alpha,beta-unsaturated imide led to azide 309. Upon formation of the 14-membered lactone, the azide function was first reduced and protected as carbamate 320 which in turn, increased the yield substantially over this same transformation from the first route. Amine 318 now stands ready to complete a formal synthesis of halichlorine and pinnaic acid.