3
44
(entries 11 and 13), proceeded easily even by a similar system using water instead of KOH under mild
conditions (entries 10 and 15–19). In addition, aldehydes were mostly unaffected by this system under
the tested conditions.
In this method, though the actual reductant and the detailed mechanism of these selective reductions
have not been clarified at the present stage, it has been surmised that the rapid acetalization of aldehyde
proceeded with Sm(OTf) –MeOH at first. Subsequently, the reducing ability of SmI can greatly be
3
2
2+
enhanced by the addition of an additive (KOH, MeOH and H O) to Sm and the reduction of carboxylic
2
acids has been facilitated by this system leading to alcohol.
Little work has been reported so far on the selective one-pot reduction of carboxylic acid in the
presence of an aldehyde or that bearing a formyl group. As mentioned above, it can be presumed that the
new SmI –Sm(OTf) –MeOH–base/H O system provides a useful and convenient method in synthetic
2
3
2
chemistry for the selective one-pot reduction of carboxylic acid bearing a formyl group under mild
conditions.
References
1
2
3
. See for review and references cited therein: Kamochi, Y.; Kudo, T. J. Syn. Org. Chem. Jpn. 1994, 52, 45. Idem Rev.
Heteroatom Chem. 1994, 11, 165. Idem Chem. Lett. 1991, 893. Idem Tetrahedron Lett. 1991, 32, 3511. Idem Tetrahedron
1
992, 48, 4301.
. It has been reported that lanthanide triflates are found to be stable in aqueous solution and can act as strong Lewis acid
catalysts. Kobayashi, S. Chem. Lett. 1991, 2187. Kobayashi, S.; Hachiya, I. Tetrahedron Lett. 1992, 33, 1625. Idem J. Org.
Chem. 1994, 59, 3590.
. Luche et al. reported that lanthanide chlorides are efficient catalysts for the acetalization of aldehydes in the presence of
4
trimethyl orthoformate as a water scavenger and also the lanthanide chloride–methanol–sodium borohydride system is used
5
for selective reduction of ketones in the presence of aldehyde. Furthermore, Fukuzawa et al. achieved the chiral acetalization
3
of carbonyl compounds catalyzed by Ln(OTf) . However, these acetalizations necessitated long reaction time for 13 hours–2
6
days until completion of the reactions. We tested the rapid acetalization of aldehydes in MeOH solution with lanthanide
triflates having the strong ability as a Lewis acid in the absence of a water scavenger as shown in Table 3.
Table 3
Acetalization and esterification with lanthanide triflate
4
5
. Luche, J. L.; Gemal, A. L. J. Chem. Soc., Chem. Commun. 1978, 976.
. Gemal, A. L.; Luche, J. L. Tetrahedron Lett. 1981, 22, 4077. Idem J. Am. Chem. Soc. 1979, 101, 5848. Idem J. Org. Chem.
1
979, 23, 4187.
6. Fukuzawa, S.; Tsuchimoto, T.; Hotaka, T.; Hiyama, T. Synlett 1995, 1077.