Synthesis and Unusual NMR-Spectroscopic Behavior of a
Strained Bicyclic Ammonium Salt
and Till Opatz*
Abstract: The first synthesis of a 2-azabicyclo[2.2.0]hexenium
salt is described. It is available from pyridine in four steps. This
strained bicyclic ammonium salt was characterized by
and 2D NMR spectroscopy as well as by XRD. Due to its unusual
One of the possibilities to activate otherwise unreactive C–C
and C–X single bonds is the use of ring strain. Photochemistry
represents a particularly powerful tool to generate strained
compounds, which are not readily accessible otherwise. The
structure elucidation of strained photoproducts can be a chal-
lenging task as not only complex reaction cascades can occur,
which yield products of unexpected molecular connectivity but
particular electronic effects may result in unusual spectroscopic
We became interested in compounds with a 2-azabicyclo-
[2.2.0]hexenium skeleton that can be generated, e.g., through
the photochemical disrotatory [4π] electrocyclization of N-
methoxycarbonyl 1 to carbamate 2 (Scheme 1).
Scheme 1. Dihydropyridine photoelectrocyclization and carbamate reduction.
It is known that carbamate 2 can be reduced to the corre-
sponding N-methyl derivative with LiAlH
The tertiary amine
is however sensitive to oxidation and prone to thermal ring
opening to the corresponding dihydropyridine.
Results and Discussion
We aimed at a way to transform compound 3 into a quaternary
ammonium salt, thus forming a highly strained allylammonium
system. However, treatment of freshly prepared amine 3 (used
without further purification) with iodomethane, benzyl bromide
[a] Institute of Organic Chemistry, Johannes Gutenberg University,
Duesbergweg 10-14, 55128 Mainz, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201701594.
Eur. J. Org. Chem. 2018, 1204–1207
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
NMR spectroscopic characteristics, DFT calculations were per-
formed to additionally support the proposed structure. The
strained cation shows significant hyperconjugative stabilization.
or allyl chloride only yielded complex mixtures, most probably
caused by the nucleophilicity of the quaternary salt's counter-
ion. The desired N-alkylation could ultimately be accomplished
using methyl triflate as the electrophile (Scheme 2).
Scheme 2. Carbamate reduction and amine quaternization.
The confirmation of the structure of 4
by standard 1D and
2D NMR methods had an unexpected outcome and more so-
phisticated experiments such as NOESY and ADEQUATE had to
be used. Interestingly, no
J COSY correlations could be de-
tected between hydrogens 4 and 5 as well as 1 and 6, while
coupling was strong instead (Figure 1).
Figure 1. Part of the COSY spectrum (400 MHz, [D
]DMSO) for compound
The experimentally observed coupling constants for the
contacts were 3.3 Hz (1,5) and 2.6 Hz (4,6), whereas the
plings (1,6 and 4,5) were too weak to be determined from the