amide 4aaa was obtained in 40% yield after 24 h; however, the
hydrolysis product 5aa6c was also obtained in 20% yield. When
1.0 equiv of isocyanide and 2.0 equiv of sulfinic acid were used,
4aaa was obtained in 50% yield, but the yield of 5aa increased
to 49% (entry 2). The hydrolysis side reaction was suppressed
using 1.5 equiv of isocyanide and 2.0 equiv of sulfinic acid,
giving the desired 4aaa in 79% yield as well as 5aa in 11% yield
(entry 3). Finally, we were pleased to observe that 4aaa was
obtained in 87% yield, while the yield of 5aa was decreased to
8%, when 2 equiv of 2a were used (entry 4). This reaction
proceeded efficiently in both toluene and diethyl ether to afford
4aaa in high yields (entries 5 and 6). Use of the cyclic ether
THF as a solvent was less effective, resulting in the formation of
α-hydroxyamide 5aa in 27% yield, probably due to hydrolysis of
4aaa (entry 7). Methanol, which has been used in the Ugi
reaction as a protic solvent, produced a very sluggish reaction
from which only 5aa was obtained in 20% yield after 24 h
(entry 8).
Having established a method for the O-sulfinative Passerini
reaction, we then set out to evaluate the effectiveness of sulfinic
acids bearing other substituents (entries 9−12). When (4-
trifluoromethylphenyl)sulfinic acid (3b), (4-chlorophenyl)-
sulfinic acid (3c), or phenylsulfinic acid (3d) was used, the