GOLDEN OPPORTUNITIES IN THE SYNTHESIS OF NATURAL PRODUCTS AND BIOLOGICALLY ACTIVE COMPOUNDS - Modern Tools for the Synthesis of Complex Bioactive Molecules - page 118

MeO OMe

MeO MeO

HO O

HO

AuCl (10 mol%)
K 2 CO 3 (10 mol% )
CH 3 CN, rt

OMe

OMe

OH

MnO 2
CH 2 Cl 2 , rt, 1 h
64% (2 steps)

O O

83%

OMe OMe

OMe

MeO MeO

OMe

Aurone 46

45

44

SCHEME 4.16 Synthesis of aurone by Pale and coworkers.

OBn (Ph 3 P)AuCl (2 mol%)

AgOTf (2 mol%)
THF, rt, 1 h

OBn

OH O

91%

47 48

O

Ph

OBn

O

H

O

steps

O

O

H

O

49

OH

Englerin A 50

SCHEME 4.17 Synthesis of englerin A by Nicolaou and coworkers.

Another 5- exo gold-catalyzed cyclization of a g -hydroxyalkyne was used by
Nicolaou and coworkers in their total synthesis of englerin A 50 (Scheme 4.17) [14].

The treatment of enynol 47 with 2mol% of (Ph 3 P)AuOTf in THF for 1 h furnished

furan in an excellent 91% yield. The reaction involved the formation of the exo -

methylidene dihydrofuran intermediate

49

48 that underwent aromatization in the

presence of the gold catalyst to finally afford .

Finally, Shair and coworkers used a gold-catalyzed 5- endo cyclization of a
b -hydroxyalkyne

49

53
(Scheme 4.18) [15]. Notably, the reaction conditions were compatible with several
other functional groups, such as silyl ethers or esters. The sensitive tertiary alcohol at
carbon C17 was not even affected by the gold catalyst and the desired dihydrofuran

51 in the synthesis of the eastern half of ( þ )-cephalostatin 1

52

was obtained in an excellent 88% yield.

4.2.2. Spiroketalizations
The gold-catalyzed addition of an oxygen nucleophile onto an alkyne generally leads
to the formation of an enol ether of type 54 (Scheme 4.19).