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causing upfield shifts on the aforementioned signals was not de-
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The IR, 1H and 13C NMR data on the new compounds presented
in this paper are given in Tables 5 and 6. They provide unambigu-
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frequencies (1732–1757 cmꢀ1) characteristic of strained b-lactams
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As concerns the cis–trans isomerism, the most uncommon phe-
nomenonis thestrikingdifferencein thechemicalshiftsof themeth-
oxy H atoms in Pos. 8, which lie between 3.89 and 4.05 ppm for the
cis compounds 2a–d, 5b–d and 8, and in the interval 3.20–3.32 ppm
for the trans isomers.
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Similarly, the H-9 singlet is significantly upfield-shifted for the
transisomers(5.83 0.02 ppmfor3a–d and6a–d)ascomparedwith
their cis counterparts: 6.84 0.01 ppm (2a–d), 7.18 0.01 ppm (5b–
d) and 7.25 ppm (8).
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These observations can be explained by the anisotropic shield-
ing of the benzene ring [25a] in Pos. 1, which lies near H-9 and the
methyl H atoms in the 8-methoxy group in the preferred confor-
mation, containing the thiazine ring in a boat-like form with
out-of-plane S and C-9b. The other conformation (half-boat-like
form with out-of-plane C-4) is unfavourable because of steric hin-
drance between the 9b-phenyl ring and one of the non-bonded
electron pair on the S atom.
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Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, T. Vreven, Jr., K.N.
Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
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Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G.
Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D.
Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S.
Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
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[17] The studied compounds were racemates. The term cis or trans refers to the
relative position of the 1-phenyl to the 9b-phenyl (10b-phenyl for compound
8) group throughout the text. In Schemes 1–4, C-1 (for 2a–d and 5a–d, 8) is
A further fact in support of the configuration is the 13C NMR
chemical shift of C-9b. This line is upfield-shifted for the more
crowded trans isomers 3a–d and 6a–d (at 64.3–64.8 ppm) due to
the field effect [25c], while it appears in the interval 66.5–
67.7 ppm for the cis compounds 2a–d and 5b–d.
The cis position of the two phenyl substituents in 8 was proved by
DIFFNOEmeasurements. On saturation of the orthoH atoms of oneof
the benzene rings, the analogous ortho H-signal of the other ring
underwent an intensity enhancement. These results confirm the
near (cis) position of these groups relative to the azetidinone ring.
*
drawn in R .
Acknowledgements
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