英语翻译Scheme 1 141IntroductionThe synthesis of nitrogen-contai

英语翻译
Scheme 1 141
Introduction
The synthesis of nitrogen-containing heterocycles such as
indoles has been one of the most active areas in organic
chemistry due to their extensive existence in a great number
of biologically active compounds.1 Among the numerous
methods for the preparation of these azaheterocycles,2
the intramolecular hydroamination reaction represents a
highly atom-economic way and many kinds of catalysts
have been developed for this transformation.3 Recently,as
a continuation of their pioneering work in zinc-mediated
cyclization of 2-ynylphenol or -aniline,Nakamura and coworkers
have developed a novel tandem cyclization/
nucleophilic addition procedure of N-benzyl-protected
alkynylanilines with electrophiles to form indole derivatives
mediated by one equivalent of n-BuLi,ZnCl2,
[Pd2(dba)3] (cat.) or n-BuLi,ZnCl2,CuCN·2LiCl.4
Our group has demonstrated that the use of alkynyl sulfonamides
bearing a more acidic NH permits the use of diethylzinc
instead of the more basic n-butyllithium.5 The
use of this weaker base allows for the generation of a sufficiently
nucleophilic nitrogen anion for cyclization,
while conceding for a broader functional group tolerance.
Moreover,a catalytic amount of diethylzinc was found to
be able to catalyze the cyclization reaction and when an
excess of diethylzinc was used,the resulting indole zinc
salt intermediate could undergo reactions with acid chloride
or halides to form C2,C3-disubstituted indoles.Notably,
the choice of sulfonyl groups as the protecting group
was crucial to effect such a cyclization process and the use
of unprotected primary amine or acyl-protected substrates
all failed in this transformation.5
Scope and Limitations
Firstly,a series of 4-substituted 2-ethynyl-N-tosylanilines
were prepared and studied in the cyclization reaction with
20 mol% of diethylzinc in toluene (Scheme 1,Table 1).In
general,both the substrates with electron-withdrawing or
electron-donating groups could provide the desired indole
derivative products in excellent yields,while a shorter reaction
time was observed for the electron-withdrawing
ones (Table 1,entries 3–7).It was found that both halogen
and nitro group were well tolerated in the reaction.In addition,
compounds with alkyl and functionalized alkyl
substituents on the acetylene terminal (R1) were also tested
under similar reaction conditions and the corresponding
indole products could also be obtained in high yields
(Table 1,entries 8–11).Unfortunately,when the substituent
was TMS,the cyclization was inhibited completely,
affording only the deprotected aniline 1i after 24 hours
(1l/1i = 6:1) (Table 1,entry 12).6