Biomedical Engineering Reference
In-Depth Information
H
Nu
R
2
OR
2
O
O
O
- [Au]
[Au]
O
O
R
2
Nu
[76]
R
1
R
1
-
R
1
[Au]
NuH = ROH, ArH, RSO
2
NR'H
O
n
(PO)
O
[Au]
O
O
Glycosylation
n
(PO)
NuH
Nu
n
-Bu
[75]
NuH = ROH, RCO
2
H, RCONH
2
SCHEME 4.70
actually the formation of an amide via an intermolecular hydro-oxycarbonylation of
an alkyne. Hence, treatment of alkyne
with 1mol% of (Ph
3
P)AuNTf
2
resulted in
the formation of the enol ester intermediate
244
by a 6-
exo
-
dig
nucleophilic addition
of the carboxylic acid on the alkyne moiety. Compound
245
represents an activated
form of the acid precursor. Its subsequent coupling with primary amine
245
246
furnished
the keto amide
247
, a precursor in the construction of the A-E ring system of
ecteinascidin 743
.
Finally, an efficient and versatile gold-catalyzed procedure for the highly
a
-selective glycosylations of alcohols, sugars, acids, and amides was developed by
Yu and coworkers [74]. This work is based on the results previously reported by Asao
and coworkers regarding the use of
ortho
-alkynyl benzoic acid esters as efficient
alkylating agents (Scheme 4.70) [75].
The glycosylation procedure was applied to the synthesis of the complex cyclic
triterpene glycoside
248
(Scheme 4.71a), whose structure is closely related to that of
the natural product lobatoside E
249
250
(Scheme 4.71b) [74a].
4.6. CONCLUSION
Although the recent efforts carried out in the field of gold catalysis have been
mainly focused on the development of new methods, the interest in this tool in
synthesis is already incontestable, as reflected in the above presented collection of
natural product total syntheses. Notably, gold catalysis has found applications in the
synthesis of a great diversity of molecules belonging to the major families of natural
products (terpenoids, alkaloids, macrolides, or prostaglandins). The generation of
cyclic structures of various sizes and functionalizations, by far themost frequently used
transformation, can reach an extreme level of complexity (see the synthesis of englerins
(Section 4.4.2.1)). Gold-catalyzed reactions also exhibit a high degree of functional
groups tolerance: the transformations can be carried out on polyfunctionalized
molecules since a complete compatibility is generally observed with functionalities,
such as silyl ethers, alcohols, esters and ketals (see the syntheses of bryostatin 16 and
cephalostatin 1 (Section 4.2.1.2) or azadirachtin (Section 4.5)), or even iodoalkynes
and iodoalkenes (see the syntheses of lycopladine A and fawcettimine (Sec-
tion 4.4.1.1)).Moreover, it is noticeable that the electrophilic behavior of gold catalysts
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