Evolution of natural product scaffolds by acyl- and arylnitroso hetero-diels-alder reactions: New chemistry on pipeline

Article abstract of DOI:10.1021/jo8004827

(Chemical Equation Presented) Piperine, a natural product containing a conjugated diene, was reacted with polymer-supported acyl- and arylnitroso dienophiles. The reactions with arylnitroso dienophiles were also carried out in solution. The oxazine rings formed by the corresponding hetero-Diels-Alder reactions were further transformed and novel acyclic as well as heterocyclic derivatives including pyrroles and quinoxalinones were prepared.

Full text of DOI:10.1021/jo8004827

Evolution of Natural Product Scaffolds by Acyl- and Arylnitroso
Hetero-Diels-Alder Reactions: New Chemistry on Piperine
Viktor Krchnˇa´k,*,† Katherine R. Waring,† Bruce C. Noll,† Ute Moellmann,‡
Hans-Martin Dahse,‡ and Marvin J. Miller†
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, UniVersity of Notre Dame, Notre
Dame, Indiana 46556, and Leibniz Institute for Natural Product Research and Infection Biology – Hans
Knoell Institute, Beutenbergstrasse 11a, 07745 Jena, Germany
Vkrchnak@nd.edu
ReceiVed March 11, 2008
Piperine, a natural product containing a conjugated diene, was reacted with polymer-supported acyl- and
arylnitroso dienophiles. The reactions with arylnitroso dienophiles were also carried out in solution. The
oxazine rings formed by the corresponding hetero-Diels-Alder reactions were further transformed and
novel acyclic as well as heterocyclic derivatives including pyrroles and quinoxalinones were prepared.
Introduction
would allow natural product scaffolds to be evolved into new
platforms for drug discovery.
Transformation of natural products represents one of the main
avenues for generating pharmacologically relevant compounds
with novel and sometimes altered biological properties. Typical
chemical derivatization of natural products such as acylation
or alkylation of hydroxy and amino groups results in modifica-
tion of the periphery of the starting molecules whereas the parent
scaffold typically remains mostly unchanged. Although such
modifications resulted in many derivatives with desirable
biological properties, we sought to develop more substantial
transformations of parent molecules under mild and efficient
conditions. If able to be accomplished, such transformations
A large pool of natural product-containing conjugated dienes
are available that offer unique opportunities for atypical
transformation by hetero-Diels-Alder (HDA) reactions. HDA
reactions of diene-containing natural products are attractive from
at least three aspects: (i) considerable transformation of the
parent molecule topography, (ii) introduction of new heteroatom
functionality, as this reaction rigidifies the diene by introducing
a six-membered ring with concurrent formation of two
carbon-carbon/heteroatom bonds, and (iii) compatibility with
a variety of functional groups. The development of the general
methodology for transformation of diene-containing natural
products to previously unknown scaffolds will enhance op-
portunities for drug discovery.
† University of Notre Dame.
‡ Hans Knoell Institute.
10.1021/jo8004827 CCC: $40.75
Published on Web 05/20/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 4559–4567 4559

Krchnˇa´k et al.
for immobilization of individual natural products. Acyl- and
arylnitroso HDA adducts were synthesized from polymer-
supported nitroso dienophiles and dienes following recently
described procedures1,2 that addressed the preparation of cy-
cloadducts on various linkers and allowed release of target
molecules by mild reagents as well as the standard trifluoroacetic
acid (TFA) in dichloromethane (DCM) solution, commonly used
for cleavage from acid-sensitive linkers.
FIGURE 1. Structure of piperine.
We recently described acyl- and arylnitroso polymer-sup-
ported HDA reactions using simple dienes as model compounds
to address the scope and limitations of these transformations
on the solid phase.1,2 We3 and others4,5 also demonstrated that
many complex diene-containing natural products readily undergo
nitroso cycloaddition reactions (for reviews on nitroso cycload-
dition chemistry see refs 6–12). An especially attractive feature
of this reaction is that there are no byproducts since both the
natural product and the entire nitroso reagent are incorporated
into the reaction product, often in high yield under very mild
conditions. This provides opportunities for Modular Enhance-
ment of Nature’s Diversity (MEND).3 Here we wish to report
our results with piperine, 1, a natural product containing a
conjugated diene motif, that further demonstrates the remarkable
versatility of the initial nitroso cycloadduct for use as an
evolvable scaffold for MEND. Both solid phase and solution
phase elaborations are described.
Piperine (Figure 1) is found as a major component in Asian
vine Piper nigrum and it is responsible for the spicy flavor of
peppers. Piperine has been used in traditional medicine and as
an insecticide. Piperine exhibits numerous biological activities,
including inhibition of human P-glycoprotein, CYP3A4,13 and
other enzymes important in drug metabolism.14,15 Piperine
stimulates melanocyte proliferation.16 In addition, piperine
enhances the bioavailability of drugs.17 Bioperine is a marketed
product for increasing bioavailability of various dietary
supplements.
N-H Oxazine. Synthesis of an HDA adduct without any
substitution on the oxazine nitrogen was carried out on a linker
that enabled immobilization of a hydroxamate, the precursor
of the corresponding transient acylnitroso species, and, after the
HDA reaction, release of the cycloadduct by scission of the
acyl substituent from the oxazine nitrogen. The acid-stability
of HDA adducts of piperine was not known and, while piperine
can be considered an electronically mixed diene since it has
both an electron-withdrawing acyl group and an electron-rich
aromatic group on the diene, based on our previous results,18
the presence of an electron-rich aromatic ring on the diene
terminal carbon indicated potential acid sensitivity. Therefore
we carried out the synthesis of the N-H oxazine adduct on a
silyloxy-based linker cleavable by the mild reagent, TBAF.
Briefly, methyl 4-hydroxybenzoate was reacted with an equimo-
lar amount of diisopropyldichlorosilane and the resulting
monosilyl ether was added to hydroxymethyl polystyrene-1%
divinylbenzene support to provide resin 2 (Scheme 1). The
polymer-supported ester, 2, was reduced by DIBAL and the
4-silyloxybenzyl alcohol 3 was reacted with CDI and subse-
quently with hydroxylamine to afford the required intermediate
N-hydroxy carbamate, 4, for subsequent HDA reactions. Oxida-
tion to the corresponding nitroso species was carried out by
treatment with n-Bu4NIO4 in the presence of piperine. The
cycloaddition products, 6, were cleaved from the derivatized
solid support 5 by TBAF and analyzed.
Analysis of the crude product after cleavage by TBAF
revealed the presence of two major components exhibiting
identical mass spectra that corresponded to two closely eluting
regioisomers 6a and 6b in a 7:3 ratio. In all piperine HDA
adducts described in this paper isomer “a” always refers to the
structure that contains the carboxamide residue connected to
the C3 carbon and the phenyl ring attached to the C6 oxazine
carbon. In our previous study of HDA reactions on the solid
phase with asymmetric dienes we observed predominant forma-
tion of one regioisomer. The major isomer contained the bulkier
group on the C6 carbon, irrespective of the electronic properties
of the group.2 Piperine contains two sizable groups on both diene
termini, the electron-rich phenyl group and electron-withdrawing
carboxamide. The fact that both isomers were formed docu-
mented the prevalence of steric effects over electronic effects.
Because the silyloxy linker is acid sensitive, we also released
the isomeric HDA adducts using standard TFA conditions to
address the stability of the new piperine-derived oxazine ring.
In 50% TFA in DCM, the major isomer, 6a, partially decom-
posed. The alkoxyphenyl substituent stabilized carbocation 8
formed by acid-mediated cleavage of the C-O bond caused
decomposition of the oxazine ring. When we exposed the
mixture of two resin-bound isomers 5 to TFA containing the
carbocation scavenger triethylsilane (TES), isomer 6b was found
to be stable, whereas the oxazine ring of isomer 6a was cleaved
and intermediate carbocation 8 was quenched by TES to form
Results and Discussion
To exploit the diversity of natural products containing
conjugated dienes for solid phase syntheses, we chose to
immobilize the dienophile rather than developing specific routes
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Fromm, M. F. J. Pharmacol. Exp. Ther. 2002, 302, 645–650.
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Wiebel, F. J.; Singh, J. Biochem. Pharmacol. 1993, 46, 229–238.
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4560 J. Org. Chem. Vol. 73, No. 12, 2008

Acyl- and Arylnitroso Hetero Diels-Alder Reactions
SCHEME 1. Synthesis of N-H Oxazine from Piperine on a Solid Support with a Silyloxy-Based Linkera
a Reagents and conditions: (i) TEA, DMAP, DCM, rt, 1 h, then hydroxymethyl resin, rt, 1 h; (ii) DIBAL, THF, 1 h; (iii) CDI, pyridine, DCM, rt, 1 h;
(iv) NH2OH·HCl, pyridine, rt, 2 h; (v) piperine, 0.1 M n-Bu4NIO4, DCM, rt, 1 h; (vi) 0.1 M TBAF, THF, rt, 30 min, 13% (yields for all solid-phase
syntheses are based on initial loading of a linker).
SCHEME 2. Preparation of a Piperine-Derived Acyclic Hydroxylaminea
a Reagents and conditions: (i) 45% TFA, 10% TES, DCM, 30 min, 19%.
acyclic product 9 (Scheme 2). The different fates of the
regioisomers upon treatment with acidic conditions are consis-
tent with the structural assignment of the isomers as acid
treatment of 6a would be expected to more readily generate
the highly stabilized benzylic cation intermediate.
N-Acyloxazine. Acylnitroso HDA adducts with N-acylated
oxazine were prepared in an analogous manner. Thus, 4-hy-
droxymethylbenzoic acid was immobilized through a Rink
amide linker and the resin-bound benzyl alcohol was converted
to hydroxamate 10 (Scheme 3). The Rink resin facilitated acid-
mediated cleavage of the product as a carboxamide 11, while
leaving the acyl substituent at the oxazine nitrogen intact due
to the electron-withdrawing effect of the carboxamide group.
LCMS analysis of the crude product showed only one peak;
however, the 1H and 13C NMR spectra of HPLC purified
material revealed the presence of two regioisomers of 11 in an
8:2 ratio.
Arylnitroso HDA Adducts. The arylnitroso dienophiles were
separately prepared from 4-fluoro-3-nitrobenzoic and 6-fluo-
ronicotinic acid immobilized by using two different linkers: (i)
via ethanolamine to a silyloxy linker that would be cleavable
by TBAF and (ii) by acylation of the Rink resin that would be
J. Org. Chem. Vol. 73, No. 12, 2008 4561

Krchnˇa´k et al.
SCHEME 3. Synthesis of N-Acyl Oxazine from Piperine on Rink Resina
a Reagents and conditions: (i) piperine, 0.1 M n-Bu4NIO4, DCM, rt, 1 h; (ii) 50% TFA, DCM, 30 min, 23%.
SCHEME 4. Synthesis of (Hetero)arylnitroso HDA Adductsa
a Reagents and conditions: (i) 0.1 M n-Bu4NIO4, DCM, rt, 1 h; (ii) piperine, DCM, overnight; (iii) 0.1 M TBAF, THF, rt, 30 min, 33% (14a), 13% (14b);
(iv) 50% TFA, DCM, 30 min, 33% (18a), 13% (18b).
SCHEME 5. Fragmentation of N-Pyridyl Oxazine from Piperine
cleavable by TFA (Scheme 4). Fluorine was replaced by
nucleophilic aromatic substitution with hydroxylamine followed
by oxidation to the corresponding nitroso species with tetrabu-
tylammonium periodate prior to HDA reaction with piperine.2
Both dienophiles represent electron-deficient arylnitroso species.
The major differences were the presence of a nitro group ortho
to the nitroso group in HDA adducts 15 and 19 and the presence
of protonatable pyridyl nitrogen in cycloadducts 14 and 18,
structural differences that turned out to have a more profound
effect than initially expected.
spectrum of one isomer revealed the presence of a very intense
fragment at m/z 396.2, in addition to a molecular ion at [M +
H]+ 481.2. This fragment corresponded to acylium ion, 20,
formed by scission of the amide bond (Scheme 5). Because
isomer 14a can stabilize the acylium ion by formation of a five-
membered acylpyridinium ring, 21, we concluded that the
compound that afforded the fragment corresponded to isomer
14a whereas the other isomer 14b did not have any stabilizing
contribution of the neighboring groups.
Cleavage with 10% TFA in DCM afforded a mixture of
isomers in a 74:26 ratio that favored isomer 14a. Increasing
the concentration of TFA to 50% changed the ratio to 64:36
indicating decomposition of isomer 14a. Addition of TES to
N-Pyridyloxazine. Solid-Phase Experiments. Reaction of
the pyridylnitroso dienophiles with piperine afforded, after
cleavage with TBAF, two regioisomers in a 7:3 ratio. The mass
4562 J. Org. Chem. Vol. 73, No. 12, 2008

Acyl- and Arylnitroso Hetero Diels-Alder Reactions
SCHEME 6. Epimerization of One Enantiomeric HDA Adduct in TFA
SCHEME 7. Preparation of Piperine HDA Adducts in Solutiona
a Conditions: (i) 60 °C, 3 h, then rt, 16 h, 86% (27), 73% (28), 64% (29).
TABLE 1. Relative Ratio of Regioisomers As a Function of
the TFA-containing cleavage cocktail did not change the isomer
ratio, although formation of the reduced compound was not
observed.
Catalyst Quantitya
entry
Cu catalyst, equiv
ratio (27a:27b)
HDA adducts 18, without the ethanolamine appendage,
prepared from the 6-fluoronicotinic acids coupled to Rink resin,
were found to behave analogously. Cleavage with 10% TFA
provided the expected two regioisomers. Increasing the con-
centration to 50% TFA caused formation of a third component,
which exhibited a mass spectrum identical with that of isomer
1
2
3
4
5
none
0.20
0.50
0.75
1.0
2:1
4:1
8:1
12:1
16:1
a Reagents and conditions: (MeCN)4Cu(I)PF6, 2-methyl-6-nitroso-
pyridine (1 equiv), rt, 30 min, then piperine (1 equiv), rt, 16 h.
1
18a. The H NMR spectrum contained the same pattern of
proton resonances, but slightly shifted in the 5-7 ppm region,
indicating formation of diastereomers 23a. Thus, apparently the
C-O bond was cleaved in the TFA solution and the initially
formed carbocation 22a recombined to give an oxazine;
however, at this stage the stereochemistry of the C6 oxazine
carbon was lost and a diastereoisomer mixture at the benzylic
carbinol carbon of 23a was formed (Scheme 6).
corresponding to a fragment formed by scission of the amide
bond, analogously to isomer 14a. Unequivocal assignment
of the structure of individual regioisomers was made possible
by X-ray analysis. The major regioisomer crystallized and
its X-ray analysis confirmed the structure as that shown for
27a.
Solution-Phase Experiments. In parallel with solid-phase
syntheses we carried out the HDA reaction with both
components in solution. Reaction of piperine with 2-methyl-
6-nitrosopyridine, 24, 5-bromo-2-nitrosopyridine, 25, and
5-chloro-2-nitrosopyridine, 26, prepared according to Taylor
et al.,19 afforded the HDA cycloadducts as a 2:1 mixture of
regioisomers (Scheme 7). The mixture of two regioisomers
of 27 was analyzed by chiral HPLC and four compounds
corresponding to two pairs of enantiomers (ratio 1:1) from
both regioisomers were detected. Purification of regioisomers
by flash column chromatography was unsuccessful, resulting
in isolation of a mixture. Both regioisomers were isolated
by semipreparative HPLC and characterized by NMR and
MS spectra. The mass spectra of two regioisomers 27a and
27b revealed a fragmentation pattern comparable to MS
spectra of HDA adducts 14a and 14b. Isomer 27a afforded,
in addition to the molecular ion, a strong signal at m/z 323.4,
To determine whether the regioselectivity could be enhanced,
we carried out the HDA reaction in the presence of a copper
catalyst, tetrakis(acetonitrile)copper(I) hexafluorophosphate.20
The crude products were analyzed by normal phase chiral
HPLC. A control sample with no copper catalyst was also
included and showed a 2:1 ratio of the major isomer, 27a, to
the minor isomer, 27b. Chiral analytical HPLC analysis revealed
a 1:1 ratio of each set of enantiomers for the given regioisomer,
as expected (the individual compounds were not isolated). An
increased amount of the copper catalyst enhanced the regiose-
lectivity in favor of the major isomer, while the ratio of the
enantiomers expectedly remained unchanged for each pair. The
results are summarized in Table 1.
Although the nitroso HDA adducts are interesting compounds
per se, and exhibit a range of biological activities,1 they provide
an attractive scaffold for further transformations. Kouklovsky
(19) Taylor, E. C.; Tseng, C. P.; Rampal, J. B. J. Org. Chem. 1982, 47,
552–555.
(20) Yamamoto, Y.; Yamamoto, H. J. Am. Chem. Soc. 2004, 126, 4128–
4129.
J. Org. Chem. Vol. 73, No. 12, 2008 4563

Krchnˇa´k et al.
SCHEME 8. Transformation of HDA Adducts (Scaffold Evolution)a
a Reagents and conditions: (i) Mo(CO)6, reflux 6 h in MeCN:H2O (16:1), 47% (30a), 40% (31a), 38% (30); (ii) MnO2, DCM, 30 °C, 16 h; (iii) TFA,
DCM, 45 min.
SCHEME 9. Synthesis of Dihydropyrroles (Only One Stereoisomer Depicted)a
a Conditions: (i) 10% TFA/DCM, 45 min, 41% (33a), 53% (33b), 36% (34a), 24% (34b), 42% (35a), 15% (35b).
SCHEME 10. Ring Contraction of N-Nitroaryloxazine to a Pyrrolea
a Reagents and conditions: (i) 50% TFA, DCM, 30 min, 19%.
SCHEME 11. Treatment of Model HDA Adduct with
Tin(II) Chloridea
SCHEME 12. Tin(II) Chloride Reduction of Piperine HDA
Adduct on Rink Linkera
a L refers to Rink linker. Reagents and conditions: (i) SnCl2 ·2H2O, NMP,
rt, 2 h; (ii) 50% TFA, DCM, 30 min, 23%.
a
L stands for Rink amide linker. Reagents and conditions: (i) SnCl2 ·
et al.21 have shown that reduction of the N-O bond in HDA
cycloadducts provides an allylic alcohol that can be subsequently
oxidized and cyclized to pyrroles. Accordingly, we reduced the
2H2O, DIEA, NMP, rt, 2 h; (ii) 50% TFA, DCM, 30 min, 21%.
To convert the amino alcohols 30 to pyrroles, we subjected
the major isomer of the N-O bond reduced compound 30a to
MnO2 in refluxing dichlormethane to oxidize the alcohol;
however, the formation of the cyclized product was not
observed. The original paper21 described pyrrole synthesis from
N-Boc oxazines whereas our substrates contained a heteroaryl
substituent. The failure to form the pyrrole derivative with our
substrates suggested structural dependence on the N-substitution
of the amino alcohol toward the transformation.
22
N-O bond in the HDA adducts 21 by Mo(CO)6 to afford
amino alcohols 30 (Scheme 8). The major regioisomer 30a was
isolated and fully characterized. HDA adducts with nitroso
compounds 25 and 26 were also subjected to reaction with
Mo(CO)6 and readily provided the N-O reduced compounds
31 and 32. This chemistry demonstrates that further derivatives
can be made starting from different nitroso species.
An alternative route to synthesize pyrrole derivatives was
initiated by exposure of the N-O bond-reduced cycloadduct
30a to a 50% TFA/DCM. Analysis by LCMS confirmed the
(21) Calvet, G.; Blanchard, N.; Kouklovsky, C. Synthesis 2005, 3346–3354.
(22) Li, F. Z.; Brogan, J. B.; Gage, J. L.; Zhang, D. Y.; Miller, M. J. J. Org.
Chem. 2004, 69, 4538–4540.
4564 J. Org. Chem. Vol. 73, No. 12, 2008

Acyl- and Arylnitroso Hetero Diels-Alder Reactions
SCHEME 13. Tin(II) Chloride Reduction of Piperine HDA Adduct on Silicon Linkera
a L stands for silicon linker (for structure, see 13). Reagents and conditions: (i) SnCl2 ·2H2O, NMP, rt, 16 h; (ii) 0.01 M TBAF, THF, 30 min; 31%; (iii)
50% TFA, DCM, 30 min, 42%.
presence of two components with a mass spectrum correspond-
ing to the 2,5-dihydro compounds 33a and 33b (Scheme 9). 1H
NMR lent further support for formation of 33a and 33b.
Ultimately, X-ray data were obtained for compound 33a as the
TFA salt and confirmed its structure. Encouraged by this result,
amino alcohols 31a and 32a were subjected to a 50% TFA/
DCM solution and 2,5-dihydropyrroles 34 and 35 were obtained,
isolated, and characterized.
N-Nitroaryloxazine. Two regioisomers of the N-nitroary-
loxazine cycloadduct 15 were obtained after TBAF cleavage
from the silicon linker. Treatment of the nitroarylnitroso HDA
adducts with 50% TFA was accompanied by ring contraction
and formation of pyrrole derivative 36 (Scheme 10). The aryl
substituent was not present on the pyrrole ring, but replaced by
a proton, presumably by electrophilic aromatic substitution.
Transformation of oxazine derivatizes to pyrroles in acidic media
has already been reported.23 The presence of TES in 50% TFA
did not afford the reduced compound, but the same pyrrole
derivative was detected in the cleaved sample. The same ring
contraction and formation of pyrrole 37 was observed when
the HDA adduct was synthesized on Rink resin and cleaved by
50% TFA in DCM.
Tin(II) Reduction. Tin(II) chloride dihydrate is a frequently
used reagent for reduction of polymer-supported nitroaromatic
compounds to anilines.24 In a model experiment, we treated
HDA adduct 38 with a solution of tin(II) chloride dihydrate in
NMP and observed formation of one major product, 39,
indicating that both the nitro group and the N-O bond of the
oxazine had been reduced to give an amino cyclohexenol
derivative (Scheme 11). This constitutes a net 1,4-aminohy-
droxylation reaction.
Encouraged by these results, we subjected HDA adducts 19
to reducing conditions. Treatment with 1 M SnCl2 ·2H2O, 1 M
DIEA in NMP for 2 h afforded, after cleavage by 50% TFA,
5H-pyrrolo[1,2-a]quinoxalin-4-one derivative 40 (Scheme 12)
as the major component.
linear precursor, the arylnitroso species was linked to the resin
via a silicon-based linker that facilitated mild cleavage of
products by TBAF. After the Tin(II)-mediated reduction and
TBAF cleavage, we isolated compound 41, resulting from
reduction of both the nitro group as well as the N-O oxazine
bond (Scheme 13). Cleavage with TFA yielded 1,3a-dihydro-
5H-pyrrolo[1,2-a]quinoxalin-4-one derivative 42, unlike the
HDA adducts 19 on Rink linker that afforded pyrrole derivatives.
The cause of N-O bond reduction in the case of the HDA
adduct on silicon linker remains to be explained.
Reduction of the N-O bond by tin(II) chloride was unsuc-
cessful on acylnitroso- and pyridylnitroso-derived HDA adducts.
After overnight exposure to a 2 M solution in NMP, formation
of amino alcohols was not observed. When compared to the
model experiment with cyclohexadiene adduct, 38, the piperine
cycloadducts contained only one six-membered ring, whereas
the cyclohexadiene adduct 38 is a bicyclic ring system. Breaking
the N-O bond in the two-ring system still preserved the
carbocyclic six-membered ring, whereas the same reaction on
the piperine adduct would result in formation of an acyclic
compound. Cleavage of the N-O bond in single ring oxazines
requires more forcing conditions and was typically carried out
by zinc in acid or use of Mo(CO)6 as described earlier.
Compounds prepared on the solid phase were subjected to a
panel of biological assays that included tests for antimicrobial,
antiinflammatory, antiproliferative, and cytotoxic activities. Assay
details are described in the Supporting Information. Antimicrobial
activity was assayed by agar diffusion tests, using a range of
Gram-positive and Gram-negative bacteria, yeasts, and fungi.
Compound 40 exhibited weak activity against Gram-positive
bacteria and inhibited growth of mycobacteria. Compound 40
also exhibited moderate antiproliferative efficacy against K-562
cells and moderate antiproliferative efficacy on Huvec cells.
Antiinflamatory activity in the horseradish peroxidase assay,
comparable to the standard, N-acetylcysteine, was exhibited by
compounds 39 and 41.
The cyclization to quinoxalinone derivative 40 was not
spontaneous and it was induced by TFA, required for cleavage
of the target compound from the Rink linker. To isolate the
Conclusion
Four different acyl- and arylnitroso HDA cycloadducts with
piperine were prepared by using polymer-supported dienophiles.
Related solution-phase chemistry was also performed to dem-
(23) Firl, J. Chem. Ber. 1968, 101, 218–225.
(24) Morales, G. A.; Corbett, J. W.; DeGrado, W. F. J. Org. Chem. 1998,
63, 1172–1177.
J. Org. Chem. Vol. 73, No. 12, 2008 4565

Krchnˇa´k et al.
The cooled mixture was filtered and concentrated to a dark oil.
Flash chromatography (silica gel; eluted with 30% EtOAc/hex)
provided 47 mg (47%) of a yellow solid.
onstrate the utility of the methodology and to confirm product
structures. The type of N-substituent had a remarkable affect
on the results of attempted further modifications of the HDA
adducts. The HDA adducts were further transformed to yield
novel acyclic products as well as heterocycles including pyrrole
and quinoxalinone derivatives.
(Z)-5-(Benzo[d][1,3]dioxol-5-yl)-5-hydroxy-2-(6-methylpyridin-
2-ylamino)-1-(piperidin-1-yl)pent-3-en-1-one (30a). 1H NMR (300
Experimental Section
Syntheses of resin-bound acyl- and arylnitroso dienophiles and
their HDA reactions were described in detail in our previous
papers.1,2 Here we report analytical data of representative piperine
HDA adducts.
(6-Benzo[1,3]dioxol-5-yl-3,6-dihydro-2H-[1,2]oxazin-3-yl)piperi-
din-1-yl-methanone and Its Regioisomer (6). Yield (HPLC purified)
MHz, CDCl3) δ 8.04 (br s, 1 H), 7.26 (dd, J ) 8.4, 7.2 Hz, 1 H),
6.81-6.70 (m, 2 H), 6.43 (dd, J ) 10.5, 6.9 Hz, 1 H), 6.34 (dd, J
) 15 Hz, 4.5 Hz, 2 H), 6.00 (d, J ) 6.3 Hz, 1 H), 5.93 (s, 2 H),
5.76 (dd, J ) 11.7, 3.6 Hz, 1 H), 5.42 (dd, 4, 4 Hz, 1 H), 5.32
(ddd, J ) 22, 11.7, 2.4 Hz, 1 H), 3.78-3.74 (m, 2 H), 3.61-3.44
(m, 2 H), 2.06 (s, 3 H), 1.71-1.57 (m, 6 H). 13C NMR (75 MHz,
CDCl3) δ: 169.2, 156.3, 147.8, 147.0, 138.4, 138.2, 133.8, 127.4,
120.7, 112.2, 108.2, 108.1, 107.9, 101.1, 76.8, 74.1, 49.0, 46.3,
43.9, 26.2, 25.9, 24.7. HRMS (FAB) m/z C23H27N3O4 calcd
410.2080, found 410.2096.
33 µmol (13%) of two regioisomers 7:3. ESI-MS m/z 317.2, [M +
H]+. 1H NMR (300 MHz, DMSO-d6) δ 6.72-6.96 (m, 3 H),
6.02-6.11 (m, 1 H), 6.02 (s, 2 H), 5.96-6.00 (m, 1 H), 5.06-5.20
(m, 1 H), 4.49-4.66 (m, 1 H), 3.39-3.54 (m, 4 H), 1.37-1.67
(m, 6 H). Isomer 6a: 13C NMR (75 MHz, DMSO-d6) δ major isomer
167.3, 147.3, 147.3, 132.9, 128.4, 124.3, 122.2, 108.8, 107.9, 101.1,
75.1, 54.6, 45.8, 42.4, 26.1, 25.3, 24.0. Isomer 6b: 13C NMR (75
MHz, DMSO-d6) δ minor isomer 168.2, 147.2, 146.8, 132.8, 128.9,
124.2, 121.6, 108.7, 108.2, 101.0, 70.1, 57.5, 45.8, 42.3, 26.3, 25.3,
24.0. HRMS (FAB) m/z calcd for C17H21N2O4 317.1501, found
317.1494.
Reduction of the Nitro Group. Resins 19 and 25 (200 mg)
were washed 3× with DMF and 2 mL of a solution of 1 M tin
chloride dihydrate and 1 M DIEA, prepared under nitrogen
bubbling, was added to the resin and the resin slurry was shaken
for 2 h. The resin was washed very thoroughly 7× with DMF and
3× with DCM.
3-Amino-4-[4-benzo[1,3]dioxol-5-yl-4-hydroxy-1-(piperidine-1-
carbonyl)but-2-enylamino]-N-(2-hydroxyethyl)benzamide (41).
Yield (HPLC purified) 9.6 umol (31%). ESI-MS m/z 497.5 [M +
Piperine Cycloadduct 27. Piperine (116 mg, 0.407 mmole) and
2-methylnitrosopyridine (74 mg, 0.610 mmol) were dissolved in 1
mL of MeOH and the resulting mixture was heated to 60 °C for
3 h, then stirred for 16 h at room temperature. The solvent was
removed under reduced pressure. Flash chromatography (silica gel;
eluted with 50% EtOAc/hex) provided 129.5 mg (86%) of 27a and
27b in a 2:1 ratio as a yellow oil, and 63 mg of the product was
purified by prep HPLC (70% MeCN/H2O) to yield 45.5 mg of two
isomers 27a and 27b (29.1 and 16.4 mg, respectively). ESI-MS
m/z 408, [M + H]+ for both isomers.
H]+. 1H NMR (300 MHz, DMSO-d6) δ 7.88 (t, J ) 5.52 Hz, 1 H),
7.10 (d, J ) 1.93 Hz, 1 H), 7.05 (dd, J ) 8.42, 2.07 Hz, 1 H), 6.91
(s, 1 H), 6.85 (d, J ) 0.83 Hz, 2 H), 6.51 (d, J ) 8.01 Hz, 1 H),
5.98 (s, 2 H), 5.66-5.81 (m, 1 H), 5.38-5.53 (m, 3 H), 5.24-5.36
(m, 2 H), 4.62-4.77 (m, 3 H), 3.41-3.50 (m, 2 H), 3.32-3.41
(m, 4 H), 3.26 (q, J ) 5.80 Hz, 2 H), 1.49 (br s, 2 H), 1.30 (dd, J
) 14.78, 7.32 Hz, 4 H). HRMS (FAB) m/z calcd for C26H32N4O6
[M]+ 496.2322, found 496.2330.
(6-(Benzo[d][1,3]dioxol-5-yl)-2-(6-methylpyridin-2-yl)-3,6-dihy-
dro-2H-1,2-oxazine-3-yl)piperdin-1-yl)methanone (27a). 1H NMR
1-Benzo[1,3]dioxol-5-yl-4-oxo-1,3a,4,5-tetrahydropyrrolo[1,2-
a]quinoxaline-7-carboxylic acid (2-hydroxyethyl)amide (42). Yield
(300 MHz, CDCl3) δ 7.51 (dd, J ) 7.5 Hz, 8.1 Hz, 1 H), 7.08 (d,
J ) 1.5 Hz, 1 H), 7.00 (d, J ) 8.1 Hz, 1 H), 6.99 (dd, J ) 7.8, 1.5
Hz, 1 H), 6.81 (d, J ) 7.8 Hz, 1 H), 6.65 (d, J ) 7.5 Hz, 1 H),
6.17 (ddd, J ) 7.2, 5.1, 2.4 Hz, 1 H), 6.07-6.04 (m, 1 H),
5.96-5.95 (m, 1 H), 5.93-5.92 (m, 1 H), 5.38-5.36 (m, 1 H),
3.68-3.49 (m, 4 H), 2.41 (s, 1 H), 1.61-1.58 (m, 6 H). 13C NMR
(75 MHz, CDCl3) δ 168.0, 158.0, 156.7, 148.3, 148.0, 138.4, 131.8,
128.9, 123.4, 122.8, 115.4, 109.5, 108.4, 106.5, 101.3, 53.9, 46.9,
43.8, 26.6, 25.9, 24.9, 24.6. HRMS C23H25N3O4 calcd 408.1923,
found 408.1920.
N-O Bond Reduction of Piperine Cycloadducts 27. A solution
of cycloadducts 27 (100 mg, 0.245 mmol) in 14:1 MeCN:H2O (1.5
mL) was charged with Mo(CO)6 (64.8 mg, 0.245 mmol). The flask
was equipped with a cold-water condenser and heated to reflux.
(HPLC purified) 13.2 umol (42%). ESI-MS m/z 394.3 [M + H]+.
1H NMR (300 MHz, DMSO-d6) δ 10.56 (s, 1 H), 8.06 (t, J ) 5.80
Hz, 1 H), 7.32 (d, J ) 1.66 Hz, 1 H), 7.24 (dd, J ) 8.42, 1.80 Hz,
1 H), 6.82 (d, J ) 7.73 Hz, 1 H), 6.74 (dd, J ) 8.01, 1.66 Hz, 1
H), 6.60 (d, J ) 1.38 Hz, 1 H), 6.52 (d, J ) 8.56 Hz, 1 H),
6.19-6.27 (m, 1 H), 5.99-6.08 (m, 1 H), 5.94 (d, J ) 6.91 Hz, 2
H), 5.65-5.74 (m, 1 H), 4.76-4.85 (m, 1 H), 4.66 (t, J ) 5.52
Hz, 1 H), 3.38-3.48 (m, 2 H), 3.13-3.28 (m, 2 H). 13C NMR
4566 J. Org. Chem. Vol. 73, No. 12, 2008

Acyl- and Arylnitroso Hetero Diels-Alder Reactions
spectrum (75.4 MHz, DMSO-d6) δ 166.1, 165.1, 147.5, 146.7,
134.4, 133.9, 133.3, 127.9, 124.7, 124.0, 121.5, 121.0, 114.9, 113.6,
108.1, 107.1, 101.1, 68.4, 65.5, 59.9, 42.0. HRMS (FAB) m/z calcd
for C21H20N3O5 392.1246, found 392.1265.
X-ray crystallographic equipment was purchased with the
support of the NSF under grant CHE-0443233.
Supporting Information Available: Details of experimental
procedures, spectroscopic data for synthesized compounds, and
copies of NMR spectra. Crystallographic results are deposited
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by the
Department of Chemistry and Biochemistry University of Notre
Dame and the NIH (GM075855). We gratefully appreciate the
use of the NMR facility at the University of Notre Dame and
the skillful technical assistance of Uta Wohlfeld at the HKI.
JO8004827
J. Org. Chem. Vol. 73, No. 12, 2008 4567