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A cyclic ketone can only exist as a trans-enolate. This enolate reacts with the aldehyde to give the anti aldol product via a chair transition state with the aldehyde substituent equatorial.
M. Hirama, T. Noda, S. Takeishi and S. Itô, Bull. Chem. Soc. Jpn., 1988, 61, 2645–2646.