Chemistry Reference
In-Depth Information
Yamamoto et al. devised a new chiral catechol-derived LBA
3
as an artifi cial cyclase
for hydroxypolyprenoids [42]. The synthetic utility of LBA
3
was demonstrated by
enantioselective cyclizations of various 2-(polyprenyl)phenol derivatives with good to
excellent enantioselectivities (88-90% ee), leading to very short and effi cient total syn-
theses of (
−
) - chromazonarol, (+) - epi - puupehedione, and (
−
) - 11
′
- deoxytaondiol methyl
ether (Scheme 3.24 ).
F
O
O
O
O
LBA
3,
toluene
-78°C, 2 days
O
then, CF
3
CO
2
H
SnCl
4
,
i
-PrNO
2
-78°C, 1 day
SnCl
4
O
OH
O
H
H
40% yield
69% dr, 88% ee
OMe
LBA
3
OMe
O
O
O
O
H
40% yield
69% dr, 88% ee
ent
-LBA
3
was used
H
62% yield
73% dr, 89% ee
HO
LBA
3,
toluene
-78°C, 2 days
OMe
OMe
H
H
22% yield, 48% dr, 90% ee
Scheme 3.24.
Yamamoto and his coworkers extended the utility of chiral LBA
3
as an artifi cial
cyclase to the asymmetric syntheses of the (
−
) - caparrapi oxide and (+) - e - epicarparrapi
oxide [43,44] . (
) - Caparrapi oxide and (+) - e - epicarparrapi oxide can be diastereoselec-
tively synthesized from optically active hydroxypolyprenoid by the reagent control of
LBA
3
and
ent
- LBA
3
, respectively, regardless of the chirality of starting material
(Scheme 3.25 ).
In 2005, the Hall group found that strong Brønsted acids, such as trifl ic acid, could
catalyze the addition of allylboronates to aldehydes [45]. Because, in general, chiral
Lewis acid-catalyzed allylation to aldehydes led only to low levels of enantioselection,
−
