Isoxazoles reductive ring fission

Isoxazoles are readily reduced, usually with concomitant ring fission (e.g. 262 -> 263). They behave as masked 1,3-diketones <79AHC(25)147>. 1,2-Benzisoxazoles are easily reduced to various products (Scheme 28) (67AHC(8)277). Chemical or catalytic reduction of oxazoles invariably cleaves the heterocyclic ring (Scheme 29) (74AHC(17)99). For similar reactions of thiazoles, see Section 4.02.1.5.1. 

The 4-(chloromethyl)isoxazole 13, which is readily accessible from 3,5-dimethyloxazole, serves as a C4-building-block in annulations to cycloalkanones (isoxazole annelation according to Stork). The primary step is alkylation leading to product 14, a masked triketone. On hydrogenation, the isoxazole ring is reductively opened and cyclization via the enaminone 15 leads to the enamine 16. On treatment with sodium hydroxide, this is converted into the bicycloenone 17 by hydrolysis, acid fission of the y dicarbonyl system and an intramolecular aldol condensation (analogous to a Robinson annu-lation) ... 

---