Tetrahedron Letters
Synthesis of substituted isatins
⇑
Larry L. Klein , Michael D. Tufano
Institute for Tuberculosis Research, College of Pharmacy, University of Illinois at Chicago, 833 S. Wood Street, Chicago, IL 60612, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Isatins are valuable intermediates for heterocyclic chemistry. Most of the common methods for their pro-
duction are less than adequate when the number and lipophilicity of substituents on the targeted isatin
are increased. Our group desired such molecules and identified an alternative method for their
production.
Received 16 November 2012
Revised 10 December 2012
Accepted 12 December 2012
Available online 21 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Isatin
Sandmeyer isatin synthesis
Oximinoacetanilides
Isatins (1H-indole-2,3-diones, 1) are valuable intermediates in
the field of heterocyclic and pharmaceutical chemistry.1 Several
of these derivatives show activities of biological interest2 but most
serve as templates in the construction of pharmaceutically active
agents. Recently, we required multiply substituted isatins as
intermediates and found the standard methods for their prepara-
tion to be less than optimal on a gram scale.3
The most common procedure for the synthesis of isatins is the
Sandmeyer process4 which utilizes a mixture of chloral hydrate,
an aniline (or its hydrochloride salt), hydroxylamine, and hydro-
chloric acid in a heated sodium sulfate-saturated aqueous media
(Fig. 1). The molecular mechanism for this process is believed to
proceed through an initial glyoxamide 4 which reacts, in turn, with
hydroxylamine to form oximinoacetanilide, 5.5a,b Heating
compound 5 to 90 °C in sulfuric acid affects the ring cyclization
to produce isatin, 1.
Although this process has worked for simple analogs (Fig. 1, 2a),
as the substitution of the precursor anilines increases both in num-
ber and lipophilicity, the facility of this classical reaction to form
the oximinoacetanilide intermediate 5 suffers. For example, when
4-n-hexylaniline (2b) was used as starting material, <5% yield of
intermediate 5b could be isolated. Attempts to modify this process
through the use of co-solvents such as ethanol6a or microwave
heating6b have helped obtain some products not otherwise avail-
able, albeit in moderate yields. The fact that the key reagent, chlo-
ral hydrate, is a regulated substance also impacts the potential to
perform large scale production of the isatins.
loximinoacetic acid 6 (Fig. 2) and an aniline under standard amide-
forming conditions; however, though hydroxyiminoacetic acid (6a)
is known,7 reaction of this compound with anilines failed to pro-
vide product 5. Crystalline benzyloximinoacetic acid (6b) is avail-
able in multi-gram quantities via an extractive work-up
following combination of O-benzylhydroxylamine and glyoxylic
acid hydrate.8 Coupling of 6b to a variety of substituted anilines
led to good yields of the benzyloximinoacetanilides, 7 as shown
in Table 1. The corresponding benzyloximinoacetyl chloride (8) is
also available via treatment of 6b with oxalyl chloride and serves
equally well to produce substituted anilines 7 in the presence of
bases such as triethylamine or diisopropylethylamine in common
organic solvents (e.g., dichloromethane, tetrahydrofuran).
It has been suggested that the ring cyclization step in the Sand-
meyer process involves a dehydration of the oxime to form a-acyl-
nitrile (Fig. 1). In the case of the benzyl analog, rather than loss of
water, the loss of benzyl alcohol would have to take place to form
this same reactive intermediate. When 7a was treated in this man-
ner, the isatin product was formed in a similar yield as for the
unsubstituted oximinoacetanilide, 5. Since no chemical trace of
the benzyl residue was apparent from this modified reaction, it is
presumed that any such alkyl moiety can be used in this approach.
This reaction sequence was carried out at a 10 g scale without
change in the yield of isatin 9 produced (Table 2, entry 1).
When oximinoacetanilide analogs of high lipophilicity (e.g., 7f–
h) were heated in sulfuric acid as per the classical Sandmeyer
route, cyclization was frequently incomplete due to the poor solu-
bility under these conditions. Use of methanesulfonic acid9 as the
media with these three oximinoacetanilides proved to be helpful
in circumventing the problems and served to provide the corre-
sponding isatin products even when little or no product was avail-
able from the sulfuric acid method (Table 2). In general, yields of
An alternative method for production of intermediate 5 or its
equivalent was envisioned involving a coupling reaction of an alky-
⇑
Corresponding author. Tel.: +1 312 413 9440; fax: +1 312 355 5539.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
Klein, Larry L.
