Y. Takemoto et al. / Tetrahedron Letters 42 (2001) 1725–1728
1727
Acknowledgements
H
R1
H
This work was supported in part by The Japan Health
Sciences Foundation and Grant-in-Aid for Scientific
Research (C) from the Ministry of Education, Science,
Sports, and Culture, Japan.
R3
O
X2In
NXMts
X = In, H
H
H
Figure 1. Transition state model (A).
References
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5. A new method for synthesizing chiral 4-amino
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Pd(PPh3)4 reagent. Contrary to our results, the reaction
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in Table 2, both trans-1 and cis-1 gave the 1,3-amino
alcohols 2a–c8 and 1,5-amino alcohols 3a in a similar
ratio (entries 1,2). In both cases, syn,syn-1,3-amino
alcohol 2a was predominantly produced. In addition,
reaction of the allylindium reagent derived from cis-1
with several aldehydes also proceeded in good yields to
afford 5a–7a as a major product among four possible
diastereomers, irrespective of aromatic aldehydes hav-
ing an electron-withdrawing or electron-donating group
on the aromatic ring and aliphatic ones (entries 3–5).
These results strongly suggest that diastereoselectivity
of the InI-mediated allylation is independent of the
chirality of the allylic carbon center of the substrates
and the substituent (R3) of the aldehydes. The latter
result is in sharp contrast to that of allyltitanium
reagents, where diastereoselectivity of the reaction with
aryl aldehyde is very low.3a It would be desirable to
employ a Boc group for protection of the amino moiety
in place of the Mts group. Therefore, we next examined
the umpolung of the N-Boc-aziridines 8 and 9 and
N-Boc-allylic acetates 10 and 11 under identical
transmetalation conditions. The reaction of 8 and 9
with benzaldehyde gave the corresponding N-Boc-1,3-
amino alcohols 12a and 13a as a major product with
similar diastereoselectivity to that of the N-Mts-
aziridine 1, but the chemical yields are low (entries 6
and 7). On the other hand, employing the allylic acetate
10 as a substrate, both the chemical yield and
diastereoselectivity of 12a were increased (entry 8).9
From comparison with the diastereoselectivity obtained
from the reactions of 9, 10 and 11 (entries 7–9), it was
revealed that bulkiness of the alkyl group (R1) of the
substrates would be crucial to achieve good diastereose-
lectivity. The good stereoselectivity attained in the ally-
lation reaction can be explained by assuming that the
reaction proceeds via the six-membered chair-like tran-
sition state (Fig. 1, A),2c,10 which is the most favorable
according to the Felkin–Ahn model, because the most
sterically demanding alkyl moiety (R1>NHR2>H) is
located at the anti position.
In conclusion, we have demonstrated a novel utility of
N-activated 2-vinylaziridines as a precursor of chiral
allylmetals by umpolung with an indium(I) salt. The
allylindium reagents possessing a protected amino
group (Mts and Boc) possess different characters from
the reported allyltitanium3a and allenylindium reagents5
in terms of stereochemistry of the major products (2a
versus 2b–c) and the stereodetermined chiral centers
(C-2 versus C-3), respectively.
8. Stereochemistry of 12a–14a was confirmed by NOE anal-
ysis of tetrahydro-1,3-oxazin-2-ones prepared from 12a–
14a by treatment with NaH, and that of the others, 2a
and 5a–7a, was deduced by comparison of their TLC
1
behavior and H NMR spectra.
Takemoto, Yoshiji
