Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

398 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
EtO2C CO2Et
799
Ph
Ph
N
NO 2
I
Pd(PPh3)4CuI
i-Pr2NEt, r.t.
97%
O
EtO2C
803
CO 2Et
NO2
EtO 2C CO2Et
800
I 2, THF, 0 ° C to r.t.
Scheme 2.332
Co(CO)8
CH2Cl2, r.t.
69%
Ph
Ph
O2N
EtO2C CO 2Et
Sc(OTf)3 (10 mol)
CH2Cl 2, r.t.
57%
N
O
EtO2C
802
801
CO 2Et
N O Ph
−
+
Ph
NO 2
Co(CO)3
Co(CO)3
Co(CO)3
Co(CO)3
in the tetrahydro-1,2-oxazine products (797) (entries 1-4). The use of bisoxazoline
ligands (798e) and (798f) with Cu(OTf)2 and MgI2 was also ineffective
(entries 5-7). The chiral Lewis acid system, derived from nickel perchlorate and
ligand (798 g), proved to be very effective (96% yield, > 80% ee, entry 8). Molecular
sieves were important for obtaining good yields (entry 9). THF as a solvent
also gave good results, as long as molecular sieves were included (entry 10)
(Scheme 2.331) (855).
The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when
treated with Co2(CO)8, affords the formation of the dicobalt hexacarbonyl complex
(801). It undergoes a smooth cycloaddition with α,N -diphenylnitrone, in the
presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex
of tetraydro-1,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyltetrahydro-1,2-oxazine
(803) (Scheme 2.332) (856).
2.12. CONCLUSION
The analysis of the literature on nitrone chemistry shows that these organic compounds
are still of special interest for researchers due to their diversity in chemical