4-Isocyanopermethylbutane-1,1,3-triol (IPB): A convertible isonitrile for multicomponent reactions

Article abstract of DOI:10.1016/j.tetlet.2012.07.064

The synthesis and applications of 4-isocyanopermethylbutane-1,1,3-triol (IPB) as a new convertible isonitrile (isocyanide) for isocyanide-based multicomponent reactions (IMCRs) like Ugi, Ugi-Smiles, and Passerini reactions are described. The primary products obtained from these IMCRs can be converted into highly activated N-acylpyrroles, which upon treatment with nucleophiles can be transformed into carboxylic acids, esters, amides, alcohols, and olefins. In this sense the reagent can be seen as a neutral carbanion equivalent to formate (HO₂C^-), and carboxylates or carboxamides etc. (RNu-CO^-).

Full text of DOI:10.1016/j.tetlet.2012.07.064

Tetrahedron Letters 53 (2012) 5360–5363
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
4-Isocyanopermethylbutane-1,1,3-triol (IPB): a convertible isonitrile
for multicomponent reactions
Ricardo A. W. Neves Filho a, Sebastian Stark a, Micjel C. Morejon a, Bernhard Westermann a,b
,
Ludger A. Wessjohann a,b,
⇑
a Department of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Weinberg 3, 06120 Halle (Saale), Germany
b Martin-Luther-University Halle-Wittenberg, Institute of Chemistry, Kurt-Mothes-Str. 2, 06120 Halle (Saale), Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and applications of 4-isocyanopermethylbutane-1,1,3-triol (IPB) as a new convertible iso-
nitrile (isocyanide) for isocyanide-based multicomponent reactions (IMCRs) like Ugi, Ugi-Smiles, and
Passerini reactions are described. The primary products obtained from these IMCRs can be converted into
highly activated N-acylpyrroles, which upon treatment with nucleophiles can be transformed into car-
boxylic acids, esters, amides, alcohols, and olefins. In this sense the reagent can be seen as a neutral carb-
anion equivalent to formate (HO2CÀ), and carboxylates or carboxamides etc. (RNu-COÀ).
Ó 2012 Elsevier Ltd. All rights reserved.
Received 15 May 2012
Revised 11 July 2012
Accepted 13 July 2012
Available online 28 July 2012
Keywords:
Functional group interconversions
N-Acylpyrroles
Activated carboxylic acids
Activated amides
Ugi reaction
Ugi-Smiles reaction
Passerini reaction
Formate carbanion equivalent
Isocyanides
Isonitrile-based multicomponent reactions (IMCRs) have gained
much attention due to their ability to generate high structural
complexity in a single step.1 Among the IMCRs, the Ugi four-
component reaction (U-4CR) is by far the most explored one.2
The combination of this reaction with other protocols (Ugi
post-modifications) has been extensively applied in the synthesis
of heterocycles, macrocycles, and other compounds of medicinal
interest.3 One of the drawbacks that limit the post-modification
of highly functionalized Ugi products is the poor reactivity of the
terminal secondary amide, which bears the substituent of the
isonitrile component. In order to overcome this problem, a family
of reagents named ‘convertible isonitriles’ has been developed.4
These compounds are able to generate activated amides directly
or upon triggering, allowing their more or less smooth conversion
with nucleophiles. The implementation of these reagents improved
the scope of applications of the U-4CR, expanding considerably its
usefulness, for example for the synthesis of natural products and
its analogs.5 A widely applicable convertible isonitrile must feature
requirements such as: (a) easy preparation from readily available
starting materials, (b) long-term handling and storage stability,
(c) good reactivity in IMCRs, (d) convertibility/activation under
mild conditions, ideally orthogonal to other functional groups
present in the MCR product, and (e) reactivity toward functional
group conversions (FGC) under mild reaction conditions. Most cur-
rent convertible isonitriles fulfill several of these requirements, but
none can fulfill all of them. Therefore there is an ongoing need for
readily accessible and easily convertible isonitrile reagents.
2,4,4-Trimethoxy-butylamine, recently developed by Fukuyama
et. al.,6 can be used as a precursor for cleavable amides 1
(Scheme 1). These intermediates appear to also combine all the
requisites for the concept of convertible isonitriles discussed
above. The amides 1 can be easily converted into N-acylpyrroles
under mild conditions. These compounds provide a large range of
transformation possibilities and can be converted into other func-
tionalities. Despite their high reactivity toward nucleophiles, they
exhibit a pronounced stability during isolation and storage.7 Here-
in we report the synthesis and application of the 4-isocyanoperm-
ethyl-butane-1,1,3-triol (2, abbreviated IPB) as a new convertible
isonitrile for IMCRs.
IPB 2 was prepared in 15 g amounts from 2,4,4-trimethoxy-
butylamine in a two-step procedure and 95% overall yield, avail-
able from bulky tetramethoxypropane.8 The reagent is stable and
it can be stored at À18 °C for weeks without decomposition. How-
ever, IPB 2 turned out to be very reactive in U-4CRs, including such
with functionalized building blocks (Table 1). All products have a
⇑
Corresponding author. Tel.: +49 (345) 5582 1301; fax: +49 (345) 5582 1309.
E-mail address: wessjohann@ipb-halle.de (L.A. Wessjohann).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.064

R. A. W. Neves Filho et al. / Tetrahedron Letters 53 (2012) 5360–5363
5361
O
and benzyl esters (entries 3, 6, and 9). A pyridyl moiety in the
Ugi-components was also compatible with the employed condi-
tions (entry 5). The U-4CR involving 4-acetoxy benzoic acid gave
the deacetylated product in 73% yield (entry 2).
To our disappointment, some of the substrates tested furnished
lower yields in the conversion step when the original Fukuyama
conditions were applied, probably due to partial thermal decompo-
sition of the starting materials. This prompted us to investigate
milder conversion conditions. In this endeavor, it was discovered
that the N-acylpyrroles readily form in the presence of 5% trifluo-
roacetic acid (TFA) in dichloromethane at room temperature with-
in 60 min.3 This procedure resulted in the desired N-acylpyrroles in
OMe
OMe
OMe OMe
CN
R
N
H
OMe OMe
2
IPB
1
Fukuyama amides
Scheme 1. IPB 2 as a precursor of Fukuyama amides 1.
good solubility in standard organic solvents used. The U-4CRs as
well as the conversion into the N-acylpyrroles work excellently
for most of the tested substrates.9,10 The two-step procedure toler-
ates the presence of many protective groups like Fmoc, Cbz, Boc,
Table 1
Reactivity study of IPB (2) in Ugi-4CRS and conversion of the products 3a–11a to N-acylpyrroles 3b–11b
R1 COOH
R2 NH2
R2
N
O
R2
N
O
a
R1
OMe
OMe OMe
R1
b or c
N
H
N
R3 CHO
O
R3
O
R3
3b-11b
2
IPB
3a-11a
Entry
Product
Acid
Amine
Aldehyde
Isolated yields (%)
a: U-4CR
b: N-acylpyrroled
O
O
NH2
NH2
b
1
2
3a,b
4a,b
84
73
98 (90)c
OH
H
H
H
H
O
O
O
OH
88 (92)
65 (70)
AcO
Cl
O
H
N
NH2
O
H
H
3
5a,b
48
OH
O
Cl
F
O
NH2
4
5
6a,b
7a,b
90
48 (n.d.)e
89 (n.d.)e
C15H31
OH
O
O
NH2
91f
OH
O
O
N
NH2
O
H
O
N
6
8a,b
75
66 (70)
OH
O
O
O
O
NH2
F
F
7
8
9a,b
98
48
55 (50)
69 (75)
OH
O
NH2
O
10a,b
OH
O
NH2
O
O
H
N
O
OH
O
H
H
O
9
11a,b
61
66 (n.d.)
O
a
b
c
d
e
f
Reaction conditions: carboxylic acid (1.0 mmol), amine (1.0 mmol), aldehyde (1.7 mmol), and IPB (2, 1.0 mmol), MeOH, rt, 18 h.
camphorsulfonic acid (CSA, 10 mol %), quinoline (10 mol %), toluene, reflux, 30 min.
5% TFA, CH2Cl2, rt, 1 h.
yields in parenthesis correspond to the N-acylpyrrole formation using 5% TFA (method c).
TLC analysis revealed considerable side product formation.
Reflux for 3 h.

5362
R. A. W. Neves Filho et al. / Tetrahedron Letters 53 (2012) 5360–5363
a
b
c
d
e
O
O
O
methoxide as the nucleophile afforded the respective methyl ester
H
N
15 in quantitative yield.11 The formation of 14 and 15 is particu-
larly noteworthy because secondary amides generated from some
other convertible isonitriles can be transformed into primary
amides or methyl esters under harsh acidic conditions only.4b,c,j,k
Saponification of 3b to the corresponding carboxylic acid 16 was
accomplished in the presence of lithium hydroxide at room tem-
perature in 92% yield, i.e. in the overall reaction cascade IPB serves
as a neutral formate carbanion equivalent, adding the –COOH
group to imine (U-4CR).
Ph
Ph
N
Bu
12
13
O
O
O
F
N
N
Bu
NH2
Ph
Ph
Ph
N
Bu
14
15
16
Additionally, the suitability of the pyrrolic carbinol chemistry
for more complex transformations was investigated, based on the
N-acylpyrrole 3b.12 In a sequential procedure, 3b was reduced with
sodium borohydride, followed by the addition of a catalytic
amount of DBU to afford aldehyde 17.13 Although it was possible
to verify the formation of 17 by TLC and ESI-MS analysis, we failed
to isolate it in pure form, probably due to its reactivity.
In view of this result, 17 was generated and reacted further
in situ in order to obtain advanced derivatives. Thus, reaction of
3b with an excess of sodium borohydride gave the corresponding
primary alcohol 18 in 70% yield. In another experiment, 3b was
converted into 17 and trapped by a Horner-Wadsworth-Emmons
(HWE) olefination.14 This procedure afforded the desired alkene
19 in 51% yield. Compounds like 19 are valuable intermediates
and have been employed in the preparation of 3-aza–bicy-
clo[3.1.0]hexane ring systems,15 constrained amino acid analogs,16
papain inhibitors,17 and in the total synthesis of ( )-isocynodine
and ( )-isocyanometrine.18 This clearly underlines the synthetic
utility of the Ugi-4CR/HWE-olefination combination.
O
O
OMe
OH
N
Bu
O
O
3b
N
Bu
O
H
f
Ph
N
Bu
17
18
O
O
O
OH
g
Ph
Ph
N
Bu
(E)
OEt
19
N
Bu
h
O
The reactivity of 2 in other IMCRs was investigated too. The Ugi-
Smiles reaction of IPB, 2-nitrophenol, isobutyraldehyde, and pro-
pylamine afforded product 20 in excellent yield (97%, Scheme 3).
A Passerini reaction involving benzoic acid and isobutyraldehyde,
proceeded well with reagent 2 and resulted in the desired product
21 in 81% yield. Intermediates 20 and 21 were converted into the
respective N-acylpyrroles 22–23 in good yields using 10 mol %
CSA and 10 mol % quinoline in refluxing toluene. Treatment of 22
in the presence of potassium hydroxide gives the corresponding
carboxylic acid 24 in 46% yield. A benchmark challenge is the
saponification of the N-acylpyrrole intermediate 23, because the
Passerini product carries an internal ester, which often is more
hydrolysis sensitive than the amides resulting from Ugi reactions.
A mild procedure previously employed in the hydrolysis of N-acyl-
indoles (DMAP in tBuOH/H2O) works well for the substrate 23 and
affords the desired carboxylic acid 25 in 60% yield.4a,i
Scheme 2. Reactions and conditions: (a) p-FC6H4CH2CH2NH2, MeOH, reflux, 8 h,
64%. (b) piperidine, toluene, reflux, 12 h, 77%. (c) NH4OH, MeOH, reflux, 8 h, 64%. (d)
MeONa, MeOH, reflux, 8 h, 98%. (e) LiOH, THF/H2O, rt, 12 h, 92%. (f) DBU (10 mol %),
THF, rt, yield n.d. (g) NaBH4, MeOH, rt, 3 h, then DBU (10 mol %), NaBH4, THF/MeOH,
rt, 16 h, 70%. (h) NaBH4, MeOH, rt, 3 h, then DBU (10 mol %), (EtO)2POCHCO2Et,
MeCN, rt, 18 h, 51%.
comparable or slightly better yields for most IPB-MCR products
and, most important, it facilitated work-up and purification for
compounds 3b–11b (Table 1). Overall, both methods have their
merits, with Fukuyama’s being less acidic and the TFA-version
allowing lower temperatures and easier work-up.
Finally the conversion of the IMCR-originated N-acylpyrroles
3b–11b with varied nucleophiles into other organic functionalities
had to be tested (Scheme 2). N-Acylpyrrole 3b was chosen as the
model compound for the conversion protocols. Reaction of 3b with
4-fluorophenethylamine or piperidine afforded the respective sec-
ondary and tertiary amides (12 and 13) in good yields. Treatment
of 3b with NH4OH in refluxing methanol afforded the desired pri-
mary amide 14 in 64% yield. The same process employing sodium
In conclusion, a new convertible isonitrile IPB (2) has been
developed which allows mild functional group interconversions
via an activated carboxylic amide intermediate. The reagent can
be prepared in multigram scale from readily available and
NO2 Pr
N
O
NO2 Pr
N
O
o-nitrophenol
OMe
OMe OMe
a
c
i-Pr CHO
n-Pr NH2
2
N
H
OH
OH
24
20
22
b
O
O
PhCOOH
d
e
Ph
O
OMe
OMe OMe
Ph
O
i-Pr CHO
N
H
2
O
O
21
23
25
b
Scheme 3. Reactions and conditions: (a) MeOH, rt, 18 h., 97%. (b) CSA (10 mol %), quinoline (10 mol %), toluene, reflux 30 min 70% for 22, 70% for 23. (c) KOH, MeOH/THF/H2O,
M.W. 110 °C., 30 min 46%. (d) CH2Cl2, rt, 24 h, 81%. (e) DMAP, tBuOH/H2O, 100 °C, 2 h, 60%.

R. A. W. Neves Filho et al. / Tetrahedron Letters 53 (2012) 5360–5363
5363
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C. B.; Kobayashi, Y. J. Org. Chem. 2008, 73, 4198–4204; (f) Lindhorst, T.; Bock, H.;
Ugi, I. Tetrahedron 1999, 55, 7411–7420; (g) Linderman, R. J.; Binet, S.; Petrich,
S. R. J. Org. Chem. 1999, 64, 336–337; (h) Kennedy, A. L.; Fryer, A. M.; Josey, J. A.
Org. Lett. 2002, 4, 1167–1170; (i) Rikimaru, K.; Yanagisawa, A.; Kan, T.;
Fukuyama, T. A. Synlett 2004, 41–43; (j) Pirrung, M. C.; Ghorai, S. J. Am. Chem.
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Lett. 2011, 52, 2209–2211.
affordable starting materials in a short sequence. It has great sta-
bility in handling and storage, and shows good to excellent reactiv-
ity in different IMCRs. The activation/conversion conditions are
compatible with numerous functionalities, and therefore can be
applied to many highly functionalized molecules. The generated
N-acylpyrrole intermediates present a good balance between sta-
bility and reactivity, and can be transformed into other carbonyl
functions in good yields. Sequential procedures involving the for-
mation of a carbaldehyde intermediate made the conversion of
3b into a primary alcohol and olefin possible in reasonable yields.
The IMCR reagent 2 also displays good reactivity in Ugi-Smiles and
Passerini reactions. The compounds generated by these latter
IMCRs were successfully converted into the respective N-acylpyr-
roles and subsequently into the corresponding carboxylic acids
24 and 25 in good yield and chemoselectivity. Thus several of the
constraints found in some of the earlier convertible isonitriles,
with limited stability, reactivity, or limited convertibility do not
apply here.4
5. (a) Schultz, E. E.; Pujanauski, B. G.; Sarpong, R. Org. Lett. 2012, 14, 648–651; (b)
Neves Filho, R. A.; Westermann, B.; Wessjohann, L. A. Beilstein J. Org. Chem.
2011, 7, 1504–1507; (c) Pando, O.; Stark, S.; Denkert, A.; Porzel, A.;
Preusentanz, R.; Wessjohann, L. A. J. Am. Chem. Soc. 2011, 133, 7692–7695;
(d) Chinigo, G. M.; Breder, A.; Carreira, E. M. Org. Lett. 2011, 13, 78–81; (e) Plant,
A.; Thompson, P.; Williams, D. M. J. Org. Chem. 2009, 74, 4870–4873; (f)
Isaacson, J.; Kobayashi, Y. Angew., Chem. Int. Ed. 2009, 48, 1845–1848; (g)
Isaacson, J.; Loo, M.; Kobayashi, Y. Org. Lett. 2008, 10, 1461–1463.
6. Maehara, T.; Kanno, R.; Yokoshima, S.; Fukuyama, T. Org. Lett. 2012, 14, 1946–
1948.
7. Goldys, A. M.; McErlean, C. S. P. Eur. J. Org. Chem. 2012, 1877–1888.
8. For details see Supplementary data.
9. General procedure for Ugi-4CR: To
a stirred solution of a suitable amine
(1.0 mmol) in MeOH (10.0 mL) aldehyde (1.7 mmol) is added and the contents
are stirred for 2 h. Then the carboxylic acid (1.0 mmol) and IPB (2, 0.17 g,
1.0 mmol) are added and stirring is continued for 18 h. The solvent is removed
under reduced pressure and the crude material purified by silica gel column
chromatography to afford the desired product.
Example compound: N-Butyl-N-(2-oxo-2-(2,4,4-trimethoxybutylamino)ethyl)
benzamide (3a): Purified by silica gel column chromatography (methanol/
dichloromethane 3:97). Yield: 84%. Rf = 0.44 (hexane/ethyl acetate 5:5).1H-
NMR (400 MHz, CDCl3) d = 0.77 and 0.94 (t, J = 7.2 Hz, 3H), 1.13 (t, J = 7.2 Hz,
2H), 1.54 (m, 2H), 1.73 (m, 1H), 1.80 (m, 1H), 3.31–3.51 (m, 14H), 4.10 (m, 2H),
4.51 (dd, J = 5.2 Hz, 5.2 Hz,1H), 6.95 (bs, 1H), 7.39 (m, 5H). 13C NMR (100 MHz,
CDCl3) d = 13.4, 13.4, 19.5, 19.7, 30.3, 30.4, 35.0, 41.3, 50.0, 50.7, 52.9, 56.9,
75.9, 101.7, 126.5, 128.4, 129.7, 135.4, 135.4, 169.4, 172.7. HRMS (ESI+) m/z
calcd for C20H32N2O5 (M+Na)+ 403.2209, found 403.2203.
Based upon the results exposed above, we propose IPB (2) as a
very readily accessible, universal convertible isonitrile for use in
IMCRs and other isonitrile based reactions.
Acknowledgments
R.A.W.N.F. and S.S. contributed equally to this work. The
authors thank Dr. Jürgen Schmidt and Dr. Andrea Porzel for HRMS
and NMR support. R.A.W.N.F. is grateful to CNPq (Brazil) for a Ph.D.
fellowship. This work was supported by funds from the state of
Saxony-Anhalt (LSA, WZW-Projekt Lipopeptide).
10. General procedure for the conversion of Ugi products 3a-11a into N-acylpyrroles
3b–11b: Method A (CSA/quinoline/heat): To a solution of a suitable amide like
3a–13a (0.5 mmol) in toluene (10 mL), 10-camphorsulfonic acid (10 mol %)
and quinoline (10 mol%) are added. After 1 min at room temperature the
stirred mixture is refluxed for 30 min. The contents are cooled, transferred to a
separatory funnel and washed with 1 M aqueous HCl (2 Â 30 mL). The acidic
aqueous phase is further extracted with ethyl acetate (1 Â 20 mL). The organic
layers are combined, washed with brine (2 Â 20 mL), dried over anhydrous
Na2SO4, filtered, and evaporated under reduced pressure. The residual material
is purified by silica gel column chromatography to give the desired product.
Method B (TFA/rt): A suitable amide like 3a–13a (0.5 mmol) is dissolved in 5%
(v/v) TFA in CH2Cl2 (10 mL). The contents are stirred for 1 h at room
temperature before evaporating the solvent under reduced pressure. The
residual material is directly purified by silica gel column chromatography to
give the desired product.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.07.
064.
References and notes
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Example compound: N-Butyl-N-(2-oxo-2-(1H-pyrrol-1-yl)ethyl) benzamide (3b):
Purified by silica gel column chromatography (ethyl acetate/hexanes 3:7).
Yield method A: 98%. Yield method B: 90%. Rf 0.38 (hexane/ethyl acetate
5:5).1H-NMR (400 MHz, CDCl3) d = 0.79 and 0.96 (t, J = 7.2 Hz, 3H), 1.15 and
1.38 (m, J = 7.2 Hz, 2H), 1.53 and 1.64 (q, J = 7.2 Hz, 2H), 3.35 and 3.58 (t,
J = 7.2 Hz, 2H), 4.15 and 4.79 (s, 2H), 6.33 and 7.15 (m, 2H), 7.37–7.48 (m, 7H).
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128.4, 129.7, 135.6, 165.6, 172.5. HRMS (ESI+) m/z calcd for C17H20N2NaO2
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