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References
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Figure 1. Crystal structure of pyrroloquinoxaline 7a.
5. (a) US Patent 3,813,392; Chem. Abstr. 1974, 81, 49700; (b)
US Patent 3,891,643; Chem. Abstr. 1975, 83, 147502; (c)
US Patent 3,892,746; Chem. Abstr. 1975, 83, 147503; (d)
US Patent 4,087,527; Chem. Abstr. 1978, 89, 109587; (e)
US Patent 4,075,206; Chem. Abstr. 1978, 88, 190901.
6. All new compounds had spectra in accord with the
assigned structures, and gave satisfactory combustion
analyses or high resolution mass spectra. Data for 7a: mp
120–123°C. Found: MH+ 311.1763. C19H22N2O2 requires
MH 311.1759; wmax (Nujol)/cm−1 2944, 2881, 1735, 1697,
1524, 1463, 1390, 1248, 1172, 1098, 1051, 916, 732; lH
(CDCl3; 300 MHz) 1.45 (1H, dtd, J 2.5, 11.6, 13.5 Hz,
9-CHH), 1.76 (1H, dddt, J 2.1, 6.0, 11.6, 13.7 Hz, 8-
CHH), 2.12 (1H, m, 8-CHH), 2.29 (1H, m, 9-CHH), 2.49
(1H, ddd, J 7.5, 10.0, 12.5 Hz, 2-CHH), 2.66 (1H, dddd, J
2.1, 5.8, 11.2, 16.8 Hz, 7-CHH), 2.86 (1H, dd, J 6.4, 16.8
Hz, 7-CHH), 3.02 (1H, ddd, J 2.3, 3.9, 12.5 Hz, 2-CHH),
3.13 (1H, d, J 13.2 Hz, PhCHH), 3.28 (1H, br. d, J 10.6
Hz, 9a-CH), 3.76 (3H, s, OCH3), 3.85 (2H, m, 3-CH2),
4.25 (1H, d, J 13.2 Hz, PhCHH), 7.12 (1H, s, 5-CH),
7.23-7.37 (5H, m, Ar-H); lC (CDCl3; 75 MHz) 22.4, 22.5
and 27.9 (C-7, 8, 9), 44.9 and 49.7 (C-2, 3), 50.6 (OCH3),
57.3 (PhCH2), 59.7 (C-9a), 113.8 (C-6a), 117.0 (C-6), 124.3
(C-5), 127.2, 128.4, and 129.0 (Ar-CH), 130.2 (C-9b), 138.4
(Ar-C), 165.9 (CO); m/z 311 (MH+, 10%), 283 (10), 282
(41), 191 (39), 161 (6), 91 (100), 49 (21), 43 (17).
Figure 2. Crystal structure of cycloadduct 6a.
presumably arising from thermodynamic control of the
prototropic shift (Scheme 3). In addition, an X-ray
crystal structure was obtained for the primary cycload-
duct 6a9 and showed (Fig. 2) that the cycloaddition had
indeed proceeded in the endo-mode that we have con-
sistently observed in earlier reports on dihydroimida-
zolium ylides;1–3 NOE studies further support this
stereochemical assignment.
We have thus demonstrated formation of the rare
hexahydropyrrolo[1,2,3-de]quinoxaline ring system as a
secondary product of intramolecular dihydroimida-
zolium ylide cycloaddition. Exploitation of both the
primary and secondary processes is under investigation.
Acknowledgements
We thank the Open University for a studentship
(P.M.J.L.), the EPSRC for its support of the National
X-Ray Crystallography Service Centre (Southampton),
and the EPSRC National Mass Spectrometry Service
Centre (Swansea) for some MS data.
7. c.f. Refs. 1 and 3 for related ring-opening–lactam forma-
tion.
8. Prepared from (2-benzylamino)ethylamine and ethyl benz-
imidate hydrochloride: Howard, K. J. Ph.D. Thesis, Uni-
versity of Nottingham, 1995.