Organic Letters
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(14) The conditions of the four-step transformation of ester is as
follows: (1) CSA, HC(OMe)3, MeOH, reflux, 24 h, 70%; (2) KOH,
H2O, THF, 40 °C, 3 h; (3) N,N′-diisopropyl-O-tert-butylisourea,
NH4Cl, CH2Cl2, rt, 15 h; (4) 1 M HCl, THF, rt, 1.5 h, 80% (three
steps). Direct formation of the tert-butyl ester 14 from 12 by a
carbonylation reaction in the presence of t-BuOH did not proceed. In
the carbonylation reaction of 12, a higher temperature was necessary,
probably because of the larger steric hindrance compared with 10.
Thus, n-butanol was used instead of methanol. The carbonylation
reactions of 10 and 12 were promoted by either Pd(OAc)2/DPPF or
Pd(dppf)Cl2.
(15) See Supporting Information for details.
(16) Undesired decomposition might be initiated by t-BuOK-
mediated deprotonation at the C-4 position. In contrast, such
deprotonation did not occur in the case of 27b. Although we do
not know the exact reason, steric repulsion between the tert-butyl ester
and the pyrrolidine ring might prevent the proton at the C-4 position
from taking a perpendicular position with respect to the pyridine ring,
thereby decreasing its acidity.
̀
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(17) Selective removal of the tert-butyl ester of 21 was difficult due to
the labile nature of the Dpm ester under acidic conditions. When the
obtained diacid was subjected to the same epimerization conditions, an
anhydride was formed intramolecularly and no epimerization occurred.
Thus, ester 21 should be converted to the corresponding methyl ester
22.
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̃
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(13) A condensation reaction of 13 with nitromethane provided the
double alkylated product 31. In this nitro-aldol reaction, the generated
hydroxyl group was readily captured by the n-butyl ester to give a five-
membered lactone 29. The subsequent β-elimination and conjugate
addition of nitromethane afforded predominantly 31. However, the
bulky tert-butyl ester 14 did not undergo such a side reaction.
1983
dx.doi.org/10.1021/ol500529w | Org. Lett. 2014, 16, 1980−1983