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

28 NITRILE OXIDES
bonding effect. The chelation of Mg metal instead of H bonding, in the same
reaction, results in excellent regioselectivity in addition to a good diastereoselectivity
(221).
Cycloaddition of 2-alkoxy-1,3-butadienes, H2C=C(OAlk)CH=CH2, and
nitrile oxides to give isoxazolines 51 proceeds with the participation of only one
of the conjugated C=C bonds. With benzonitrile oxide, only the vinyl group in
alkoxydienes participates in cycloaddition reactions while in the case of phenylglyoxylonitrile
oxide both double bonds react (222). Nitrile oxides RC≡NO
react with iron complexed trienes 52. The reaction proceeds with good yield
and diastereoselectivity (∼90/10) to give isoxazolines 53 (223).
R 2
O N
51
R1 = Ph, R2 = C(OAlk)=CH2, R3 R
= H
3
R1 = PhCO; R2 = C(OAlk)=CH2, R3 = H + R2 = CH=CH2, R3 = OAlk
Alk = Me, Et, i-Pr, t-Bu
R 1
R 1
R 1
Fe (CO) 3
Fe (CO) 3
52 53
R = Me, Et, CMe3, Ph; R 1 = CO 2Me, Me, Si(CMe 3), Ph 2OCH 2
Allenes add nitrile oxides either to one or two double bonds. For mono- and
1,1-disubstituted allenes, relative activity of the two bonds depends on the nature
of substituents. The reaction (Scheme 1.18) of N-propadienylanilines 54 with
3,5-dichloro-2,4,6-trimethylbenzonitrile oxide proceeds site- and regioselectively
to give 5-substituted 4-methylene-4,5-dihydroisoxazoles 55, which add a second
molecule of nitrile oxide to afford 4,5 ′ -spirobi-(4,5-dihydroisoxazoles) 56. Dihydroisoxazoles
55 isomerize to 4-(2-aminobenzyl)isoxazoles 57 via a Claisen-type
rearrangement (224).
N,N-diarylaminoallenes 54 (R 1 = 2-R 3 -3-R 4 -C6H3,R 2 = R 3 = R 4 = H; R 2 +R 3
= CH=CH, CH2CH2, R 4 = H), undergo similar transformations with 3,5dichloro-2,4,6-trimethylbenzonitrile
oxide affording methyleneisoxazolines 55
and 4,5 ′ -spirobi-(4,5-dihydroisoxazoles) 56, respectively (225). The difference
is that only 55 (R 1 = Ph, R 2 = H) undergoes a Claisen rearrangement to 57 on
treatment with Lewis acids while under similar conditions complicated rearrangements
and degradation reactions are observed with other compounds 55.
Allenyl sulÞdes RSCH=C=CH2 and the same nitrile oxide undergo cycloadditions
which occur exclusively or predominantly at the external double bond to
give 4-alkylidenedihydroisoxazoles 58 and 5-(methylthio)isoxazoles 59 (226).
O
N
R