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The cis-trans geometry of a nine-membered lactam significantly depends on the N-substituents; N-acyl-1-aza-2-cyclononanones (1a-c) exist as cis form; in contrast, N-Z- 1-aza-2-cyclononanone (1d) exists as trans form both in the crystal and in solution.
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Yamada et al. (2002) studied this question.
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