Multicomponent Diene-Transmissive Diels-Alder Sequences Featuring Aminodendralenes

Article abstract of DOI:10.1002/anie.201510925

1-Aminodecalins were prepared from acyclic precursors by combining the powerful twofold diene-transmissive Diels-Alder chemistry of [3]dendralenes with the simplicity of enamine formation. On mixing at ambient temperature, a simple dienal condenses with a primary or secondary amine to generate the enamine, a 1-amino-[3]dendralene in situ, and this participates as a double diene in a sequence of two Diels-Alder events with separate dienophiles. Overall, four C-C bonds and one C-N bond are formed. Mechanistic insights into these reactions are provided by means of density functional theory calculations.

Full text of DOI:10.1002/anie.201510925

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International Edition: DOI: 10.1002/anie.201510925
German Edition: DOI: 10.1002/ange.201510925
Domino Reactions
Multicomponent Diene-Transmissive Diels–Alder Sequences Featuring
Aminodendralenes
Siu Min Tan, Anthony C. Willis, Michael N. Paddon-Row,* and Michael S. Sherburn*
Abstract: 1-Aminodecalins were prepared from acyclic pre-
cursors by combining the powerful twofold diene-transmissive
Diels–Alder chemistry of [3]dendralenes with the simplicity of
enamine formation. On mixing at ambient temperature,
a simple dienal condenses with a primary or secondary
amine to generate the enamine, a 1-amino-[3]dendralene
in situ, and this participates as a double diene in a sequence
of two Diels–Alder events with separate dienophiles. Overall,
À
À
four C C bonds and one C N bond are formed. Mechanistic
insights into these reactions are provided by means of density
functional theory calculations.
Scheme 1. Schematic representation of the four-component sequence.
S
tep-economic synthesis necessitates the invention of new
methods for converting simple and readily accessible pre-
cursors into more complex products.[1,2] The rapid generation
of structural complexity is inexorably linked with processes
that form several new covalent bonds. Such multiple single-
bond-forming transformations[1] have several subclassifica-
tions, with those involving successive reactions at sequentially
generated functional groups featuring strongly in current
research endeavors.[3] In addition to maximizing useful
structural complexity gains, a new synthetic method should
ideally be atom-economic,[4] operationally simple, and
robust.[5]
sequence is depicted in stripped-back form in Scheme 1, and
shows that skipped dienal 1 would condense reversibly with
an amine to generate 1-amino-[3]dendralene 2.[11] Steric
effects notwithstanding, this species would be expected to
react with an electron-poor dienophile at the more strongly
activated 1,3-disubstituted 1,3-butadiene unit[12] to produce
“transmitted” semicyclic diene 3, which would in turn react
with a second dienophile to deliver aminodecalin[13] system 4.
Significant structural complexity would thus be generated
from four simple precursors through three consecutive
reactions.
E-Configured trienamine 2a was generated in CDCl3
solution at ambient temperature within 5 minutes, simply by
mixing methylene-skipped dienal 1a with morpholine
(Scheme 2).[14] The new dendralene 2a readily decomposed
upon attempted isolation or standing in solution, thereby
resulting in complex mixtures of products including the two
geometrical isomers of isomeric conjugated dienal 1a’.
Addition of the electron-poor dienophile N-methylmaleimide
(NMM) to a preformed solution of trienamine 2a delivered
endo-cycloadduct 3a very cleanly.[12] Conjugated dienal 1a’
was not converted into trienamine 2a, instead yielding the
products of aza-Michael additions upon exposure to morpho-
line and NMM.[14,15]
Dendralenes[6] are cross-conjugated olefins of significant
value in the step-economic synthesis of complex molecules
owing to their multiple-1,3-butadiene character, which per-
mits their participation in diene-transmissive[7] Diels–Alder
(DA) cycloaddition sequences.[8] Such sequences, which are
amongst the most powerful of all multiple single-bond-
forming processes,[9] are now finding application in step-
economic total synthesis.[10] The dendralenes are invariably
made first and then used separately in a cycloaddition
sequence.[6] If it were possible to unite the preparation of
dendralenes with their cycloaddition sequences in a single,
simple synthetic operation, we reasoned that significant
efficiency dividends would result. Herein, we report the
successful realization of this proposition. The conceptualized
[*] S. M. Tan, Dr. A. C. Willis, Prof. M. S. Sherburn
Research School of Chemistry, Australian National University
Canberra, ACT 0200 (Australia)
E-mail: michael.sherburn@anu.edu.au
Prof. M. N. Paddon-Row
School of Chemistry, The University of New South Wales
Sydney, NSW 2052 (Australia)
E-mail: m.paddonrow@unsw.edu.au
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201510925.
Scheme 2. Generation and Diels–Alder reaction of trienamine 2a.
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The formation of cycloadduct 3a confirmed our expect-
ation that 1-amino-[3]dendralene 2a would undergo a highly
site-selective addition of a dienophile to the more substituted
1,3-butadiene residue. The sequential addition of the dien-
ophile to the preformed trienamine was not necessary, since
optimal yields of products 3a–n were obtained by premixing
skipped dienal 1a and the dienophile, then adding the amine
last. Presumably, it is better to generate the reactive trien-
amine in low concentration and trap it quickly with a dien-
ophile, rather than allow it to increase in concentration. This
one-pot, three-component sequence has a broad scope with
respect to both the amine and dienophile, as demonstrated by
the fourteen examples depicted in Scheme 3. Primary and
secondary, cyclic and acyclic, and alkyl- and aryl- amines are
tolerated, as are carbo- and hetero-dienophiles with a variety
of activating groups and other substituents. As might be
anticipated,[16] the products of endo-cycloadditions were
generally formed. This broad substrate tolerance is consistent
with both a very favorable condensation and a trienamine that
is highly reactive towards dienophiles, presumably due to its
Rawal-type diene[17] character.
Scheme 4. One-pot, four-component sequences with dienal 1. Major
stereoisomer depicted, d.r.>95:5 unless indicated otherwise.
[a] d.r.=91:9. [b] d.r.=71:29. [c] C6D6 solvent used. [d] 6.0 mol equiv
NMM used.
This sequence was extended to a one-pot, four-component
reaction through the implementation of an in situ Diels–
Alder addition of a dienophile to the semicyclic diene
segment of mono-adduct 3 (Scheme 4). Compounds 4a–c
were produced through a highly diastereoselective endo
addition of the dienophile to the face of the semicyclic
diene intermediate 3 lacking the amine substituent. The diene
component of skipped dienal 1a is also amenable to direct
Diels–Alder addition by a dienophile, producing skipped enal
5 (Scheme 4), which upon addition of a second dienophile and
amine gives rise to aminotetralins 7a and 7b, presumably
through the intermediacy of 1-amino-1,3-butadiene 6. Simply
changing the order of addition of the dienophiles and amine
to dienal 1 thus results in the formation of constitutional
isomers 4 and 7, each of which carries five new covalent bonds
and two new rings. Substitution on the precursor is also
tolerated, as demonstrated by the formation of derivatives 4d
and 4e from substituted skipped dienals 1b and 1c.
In the presence of an excess of the dienophile NMM,
dienal 1a reacted with a stoichiometric amount of first-
generation MacMillan organocatalyst[18] 8 to deliver penta-
cycle 4 f as the major diastereomeric product (d.r. = 73:27;
Scheme 5). In a similar manner, Jørgensen–Hayashi organo-
catalyst[19] 9 generated four component product 4g as a single
diastereomer, within the limits of NMR detection. Enamines
derived from amine 8 are known to be very poor nucleo-
philes,[20] so Diels–Alder trapping of the HOMO-activated
trienamine derivative of oxazolidinone 8 with a dienophile is
interesting.
Density functional theory calculations, using B3LYP/6-
31G(d) model chemistry,[14] were carried out in order to gain
mechanistic insights into the origin of the observed p-
diastereofacial selectivity of the first cycloadditions depicted
in Scheme 5.[21] It is instructive, in the first place, to analyze
the two conformations associated with rotation about the
Scheme 3. Scope of the condensation–cycloaddition sequence with
dienal 1a. Major stereoisomer depicted, d.r.>95:5 unless indicated
otherwise. [a] isolated as a 1:1 diastereomeric mixture. [b] 2.5 mol
equiv of amine and dienophile used, acrolein as dienophile, yield of
isolated product after NaBH4 reduction. [c] 0.83 mol equiv amine used.
[d] d.r.=89:11. [e] 30 minute reaction time.
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Figure 2. B3LYP/6-31G(d)-optimized structures 2b-syn and 2c-syn,
Scheme 5. Trapping trienamines of chiral amines as double DA
adducts. [a] X-ray crystal structure of the tertiary alcohol product of
silyl ether hydrolysis.
with the favored trajectories of dienophile approach indicated. H
atoms are omitted from phenyl and some methyl groups and the
dendralene is colored green for clarity.
bond connecting the dendralene and the heterocycle in the
two reactant aminodendralenes 2b and 2c. As shown in
Figure 1, the two conformations are distinguished according
(Figure 2). Stabilizing CH···p interactions between a phenyl
ring and the proximal methyl group in both 2b[9,23] and 2c[22]
results in the aromatic ring partially obscuring the Si and Re
faces of the diene component in the syn and anti forms,
respectively (Figure 2).
À
À
to whether the N C2’ or N C5’ bond partially eclipses the
À
dendralene C1 C2 bond. The syn/anti notation refers to the
À
disposition of the N C5’ bond, that is, the bond involving the
less substituted C5’ atom, with respect to the C1 C2 bond. As
The above analysis leads to a reactant-based explanation
of the observed p-diastereofacial selectivity of the DA
reactions of 2b and 2c, namely that the dienophile preferen-
tially approaches the more exposed Re faces of the syn
conformers of 2b and 2c and the more exposed Si faces of the
anti conformers of these molecules. Because the syn confor-
mer is more stable than the anti form in both 2b and 2c, which
is greater for 2c, it is concluded that Re facial selectivity
prevails in these reactions and that it should be more
pronounced when using 2c as the diene reagent than with
2b. A more rigorous approach is to compare the relative
energies of the transition structures (TSs) for Re and Si
addition modes. The results with B3LYP optimized TSs for
endo addition of NMM to 2b and 2c are presented in Table 1.
=
might be expected from the small dihedral angles between
=
À
C1 C2 and the partially eclipsing N C bond in both the syn
and anti conformations of 2b and 2c (Figure 1), the syn
conformer should be more stable than the anti conformer
because of diminished adverse steric interactions between the
À
dendralene C2 H group and the less substituted C5’ center of
the heterocycle.[22] Indeed, the B3LYP calculations predict the
syn form to be more stable than the anti form in both 2b and
2c, by 9.4 and 12.2 kJmolÀ1, respectively.
In both syn and anti conformers of 2c, one of the OTMS
methyl groups lies substantially over the Si face of the
dendralene C1C2C3C4 diene component of the former and
over the Re face of this diene component in the latter
Table 1: Energies of B3LYP-optimized TSs for endo addition of NMM to
2b and 2c.
Cycloaddition mode[a]
2c
2b
H° G°
H° G°
rel
rel
rel
rel
Re/syn
Si/syn
Re/anti
Si/anti
0 0
0 0
6.2 4.7
16.5 14.7
5.6 5.1
25.2 24.3
19.9 21.2
10.8 11.0
[a] See Figures 1 and 2 for definitions.
The results of these calculations clearly predict a strong
preference for Re-face addition to the syn conformations of
the two aminodendralenes and indicate that Re addition on
the anti conformations is an unimportant pathway to Re-
based product formation. The Si/anti channel is the near
exclusive source of Si-based product from 2c, whereas both
Si/anti and Si/syn channels are contributors to the Si-based
product in the case of 2b. The relative free energy data in
Table 1 indicate that p-diastereofacial selectivity in the DA
reaction with NMM is stronger for aminodendralene 2c than
Figure 1. Schematic of the two trienamine conformations for 2b and
À
2c with respect to rotation about the C1 N bond, together with the
À
B3LYP/6-31G(d) dihedral angles between the dendralene and N C
bonds of the heterocycle, and their relative enthalpies, Hrel (298 K,
kJmolÀ1). Note the similar steric clash in the higher-energy anti
conformations.
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for 2b. In fact, a Boltzmann distribution calculation gives
a total Re-based product/Si-based product ratio of 99:1 and
76:24 for 2c and 2b, respectively. These ratios are in
reasonable accord with the experimental values of > 95:5
and 73:27, respectively (Scheme 5).
The main structural features of the reactant aminoden-
dralenes, as discussed above, are essentially retained in the
respective Diels–Alder transition structures (TSs), as exem-
plified by 2c-endo-Re-syn-TS and 2c-endo-Si-anti-TS
(Figure 3). A noteworthy feature of these TSs (and of those
Scheme 6. Tricycle synthesis through one-pot, three-component
sequences featuring an intramolecular Diels–Alder (IMDA) reaction.
Major stereoisomer depicted, d.r.>95:5 unless indicated otherwise.
[a] mono-adducts 11a and 11 f were isolated before being subjected to
intramolecular cycloaddition. [b] 2.5 mol equiv of amine and dienophile
used, acrolein as dienophile, yield of isolated product after NaBH4
reduction. [c] d.r. =93:7.
Figure 3. B3LYP/6-31G(d)-optimized TSs for the Diels–Alder cycloaddi-
tion of the NMM dienophile to aminodendralene 2c. H atoms are
omitted from phenyl and methyl groups and the dendralene is colored
green for clarity.
agents. This study demonstrates the diversity of multicompo-
nent transformations with only one amine group at a specific
position of the simplest possible dendralene structure. Evi-
dently, the possibilities for step-economic synthesis with
aminodendralenes are vast.
not shown in the Figure) is the high degree of bond-forming
asynchronicity between the two developing bonds between
NMM and the dendralene diene component. Thus, the
forming bond lengths in 2c-endo-Re-syn-TS are 2.896 and
2.083 Š (Dr= 0.81 Š) and for 2c-endo-Si-anti-TS they are
2.905 and 2.072 Š (Dr= 0.83 Š). The shorter forming bond
involves C6 of the central double bond of the aminodendra-
lene and this has the effect of conferring stabilizing penta-
dienyl radicaloid character on the dendralene component.
This large degree of bond-forming asynchronicity is a general
characteristic of dendralenes in their DA addition reac-
tions.[24]
Acknowledgements
This work was supported by the Australian Research Council.
We thank Dr Emily Mackay and Ms Natalie Shadwell (ANU)
for synthetic assistance. MNP-R acknowledges that the
computational component of this research was undertaken
with the assistance of resources provided at the NCI National
Facility through the National Computational Merit Alloca-
tion Scheme supported by the Australian Government.
Tethering the dienophile to the amine (as in 10a–c)
permits the second cycloaddition (11!12) to be realized in an
intramolecular[25] fashion (Scheme 6). The benefits of
employing this tactic include: 1) the generation of additional
structural complexity with complete orientational regioselec-
tivity, 2) attainment of products with complementary stereo-
selectivity to the intermolecular process,[26] and 3) the ability
to deploy nonactivated dienophiles (10a). Substituted dienals
1d and 1e also participate in the tricyclization sequence, thus
confirming the robust character of this new method.
Keywords: density functional calculations · Diels–
Alder reactions · domino reactions · multicomponent reactions ·
polycycles
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3081–3085
Angew. Chem. 2016, 128, 3133–3137
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[26] The dienophile is constrained to approach the semicyclic diene
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NMM-derived succinimide ring blocks endo approach of the
-CO2Me group, thereby resulting in the formation of exo-adduct
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[14] See the Supporting Information for full details.
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[27] CCDC nos. 1429160 – 1429173 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre.
Received: November 25, 2015
Published online: January 28, 2016
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