Table 1.

Rate constants for the cycloaddition of dibenzocyclooctynol 2 with nitrones 4 a–f.^[a]

4	R1	R2	R3	k[b] [m−1 s−1]	k′	Yield [%]
a	H	Ph	Me	1.3×10−2	1	95
b	H	CH2CH2Ph	Me	3.2×10−2	3	80
c	H	Ph	Ph	>0.2[c]	>17	89
d	Me	CH2CH2CO2Et	Me	<1×10−3	<0.1	33
e	H	CO2Et	Me	3.9	330	92
f	H	C(O)NHBn	Me	2.2	180	93

^[a]The nitrone substrates (except 4 d) were formed as pure Z isomers. Isoxazoline 5 was formed as a mixture of regio- and diastereoisomers. See the Supporting Information for reaction conditions.

^[b]Method A: The rate constant was determined by ¹H NMR spectroscopy in CD₃CN/D₂O (3:1); [2 a]=18 mm, [4 a–f]=16.4 mm. Method B: The rate constant was determined¹⁸ by UV spectroscopy in CH₃CN/H₂O (3:1); [2 a]=0.33 mm, [4 a–f]=0.30 mm. These reactions were too fast for monitoring by NMR spectroscopy.

^[c]The reaction was too fast for accurate determination of the rate constant by NMR spectroscopy. Determination by UV spectroscopy was not possible owing to overlapping absorptions. Bn=benzyl.