Chemistry Reference
In-Depth Information
CO
2
Me
BocHN
P(OMe)
2
[Rh(I)(-COD)(
S,S
)-
Et-DuPHOS]
+
OTf
-
BocHN
Me
Me
BocHN
Me
O
87
Me
Me
Me
MeO
2
C
OHC
MeO
2
C
H
2
89%
TMG (
3
), CH
2
Cl
2
86
89
88
88%
NMe
2
Me
Boc
NH
Me
N
HO
2
C
Me
N
EtO
2
C
Me
O
O
N
H
H
martefragin A (
91
)
potential precursor
90
Scheme 7.18
Synthetic studies on martefragin A (91)
7.3 Deprotection
Guanidine/guanidinium nitrate is a selective O-deacetylation reagent in the presence of
base labile protective groups [30]. Endo et al. reported the use of selective O-deacetylation
in the synthesis of ecteinascidin 743 (99) [24] (Scheme 7.20). The twoO-acetates in 97were
cleanly deprotected in the presence of the N-Troc group by using guanidinium nitrate in
methanol.
In the synthesis of the C1-25 fragment of amphidinol 102, selective O-deacetylation was
performed in the presence of carbonate under the same conditions [31] (Scheme 7.21).
7.4 Elimination
The
-position
is promoted by superbases via an E1cB process. Allin et al. described a synthesis of
deplacheine (107) [32], in which the
b
-elimination reaction of carbonyl compounds having a leaving group at the
b
-methanesulfonyloxycarbonyl compound 105
obtained from the aldol reaction of 104 and acetaldehyde was selectively converted into
the desired E-isomer 106 by the use of DBN in THF. This enone 106 was successfully led to
the natural product 107 (Scheme 7.22).
Tanino and Kuwajima used a guanidine base promoted elimination reaction in the
synthesis of ingenol (110) [33]. Reaction of ketone 108 with MTBD resulted in an
elimination reaction and subsequent isomerization to give conjugated diene 109, which
was efficiently led to ingenol (110) (Scheme 7.23).
The DBU promoted epoxide opening reaction through an E1cB process has been applied
to natural product synthesis. Trudeau andMorken reported a synthesis of fraxinellone (113)
[34] (Scheme 7.24). Treatment of epoxide 111 with DBU in benzene gave allylic alcohol
112, which was led to the natural product by oxidation of the resulting alcohol with TPAP.
b
