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The electrophile (n-PrI must attack from an axial position in order to interact with the pi orbital of the C=C bond. If it attacks from the same face as the t-Bu group, it will form a twist boat conformation which is unstable. If it attacks from the opposite face, it can form a chair where the t-Bu remains equatorial (stable) and the n-Pr is axial. The chair-like transition state is the most stable option.