O-Heterocyclics s. a. Oxa

Osmic acid s. Osmium tetroxide Osmium tetroxide OC CC Oxa... s. a. O-Heterocyclics 3-Oxabicyclo[3.1.0]bexanes... 

A soln. of startg. ester in dichloromethane treated with 2 eqs. SnC at —70° for 15 min, and the mixture allowed to warm slowly to room temp, over 2 h - product. Y 58%. Reaction is thought to proceed via a cationic oxa-Cope rearrangement. F.e, inch oxepin analogs, also satd. O-heterocyclics from ethylenic derivs., and 3-vinyl-tetrahydrofuran-2-carboxylic acid esters with an allylsilane group as internal nucleophile, s. L.D.M. Lolkema et al.. Tetrahedron Letters 29, 6365-8 (1988). 

O-Heterocyclics (s. a. Oxa...) addition to carbon-carbon double bonds 23, 709... 

The heterocyclic Mannich bases of oxa- and thia-cyclohexanone (290, X = O, S) obey the general rule of predominant B attack in hydride reductions. The synthesis of A diastereomer requires the use of aluminum isopropoxide (Meerwein-Pondoif), which always favors the opposite direction of attack with respect to metal hydride. 

CHj) and several oxa- and ihia-derivatives (X - O, S). In this case, the preferred direction of attack is usually A, regardless of the substituent s position, type of amino group, and homo- or heterocyclic ring, the only exceptions being those of the lithium-acetylene reagent with dimethylaminomethyl cyclohexanone derivatives. 

---