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Scheme 6 Synthesis of azepane derivatives.
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5 From ruthenium catalysts, see for example: (a) A. Lopez-Rodrıguez,
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Scheme 7 Synthesis of oxepane derivatives and assumed elimination
mechanism.
allows it to be presumed that the expected cyclization process
occurs first and the titanabicycle 41 intermediate undergoes an
irreversible competitive elimination process leading to diol 42.22
Thenceforth, a rapid quenching of the intermediate, after 10 min
as described, is required in order to limit this undesired pathway.
In summary, we have extended the intramolecular low-valent
dialkoxytitanium(II) mediated cyclization reaction of ene–yne sys-
tems to seven-membered ring compounds with moderate to good
yields. In most cases, a very good selectivity in favour of the trans-
isomer was obtained. The conditions developed herein are mild
and easy to set-up, and rely on the use of inexpensive Ti(OiPr)4,
and the cyclic adducts are obtained after a short time (B10 min)
as a single diastereoisomer. Thanks to the conditions established,
we were able to reach various (hetero)cycles i.e. cycloheptane as
well as azepane or oxepane frameworks in a single step. We also
highlighted the crucial role of the free allylic alcohol regarding the
selectivity and the yield of the cyclization reaction. Thanks to the
titanabicyclic intermediate, a complete control of the exocyclic
double bond geometry is also possible in all cases. Finally, we
demonstrated the possibility to access 8-membered rings in
moderate yield through a SN20 mechanism, which could open
the way to the synthesis of larger cycles. Thus, the conditions need
to be optimized and are currently underway in our laboratory.
¨
A. Gudmundsson, A. Ricke, F. Himo and J.-E. Backvall, J. Am. Chem.
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7 From rhodium catalysts, see for example: (a) A. Tap, C. Lecourt,
S. Dhambri, M. Arnould, G. Galvani, O. Nguyen Van Buu,
´ ´
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K. M. Brummond, J. Org. Chem., 2013, 78, 3737–3754.
´
8 From iridium catalysts, see for example: D. F. Fernandez,
´
˜
C. A. B. Rodrigues, M. Calvelo, M. Gulıas, J. L. Mascarenas and
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F. Lopez, ACS Catal., 2018, 8, 7397–7402.
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F. Sato, Tetrahedron Lett., 1995, 36, 4261–4264; (b) K. Suzuki,
H. Urabe and F. Sato, J. Am. Chem. Soc., 1996, 118,
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1997, 119, 10014–10027For a review on Sato’s works, see: (d) F. Sato
and S. Okamoto, Adv. Synth. Catal., 2001, 343, 759–784.
15 Y. Sanogo, R. Ben Othman, S. Dhambri, M. Selkti, A. Jeuken,
Conflicts of interest
´ ´
J. Prunet, J.-P. Ferezou, J. Ardisson, M.-I. Lannou and G. Sorin,
J. Org. Chem., 2019, 84, 5821–5830.
There are no conflicts to declare.
16 For the synthesis of the cyclization precursors, see ESI†.
17 For a complete survey on optimization of the reaction, see ESI†.
18 A. A. A. Quntar, O. Baum, A. Shibli, V. M. Dembitsky and M. Srebnik,
Angew. Chem., Int. Ed., 2003, 42, 4777–4779.
Notes and references
19 For the assumed mechanism and selectivity, see ESI†.
20 For some representative examples, see: (a) W. A. Nugent and
D. F. Taber, J. Am. Chem. Soc., 1989, 111, 6435–6437; (b) H. Urabe
and F. Sato, Tetrahedron Lett., 1998, 39, 7329–7332.
21 (a) T. Nakagawa, A. Kasatkin and F. Sato, Tetrahedron Lett., 1995, 36,
3207–3210; (b) S. Okamoto, D. K. An and F. Sato, Tetrahedron Lett.,
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1 For recent representative reviews on total synthesis, see: (a) L. Li, Z. Chen,
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2 For selected book chapters or articles on the synthesis of seven
membered rings, see: (a) J. Ryan, J. Green, C. Hyland, J. Smith and
C. Williams, Seven-membered rings, in Progress in Heterocyclic 22 Due to its volatility, the resulting TMS-allene was not detected.
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