Mechanism, neighboring stereochemistry

When the 3-bromo-2-butanol with the stereochemical structure A is treated with concentrated HBr, it yields mestf-2,3-dibromobutane a similar reaction of the 3-bromo-2-butanol B yields (+ )-2,3-dibromobutane. This classic experiment performed in 1939 by S. Winstein and H. J. Lucas was the starting point for a series of investigations of what are called neighboring group efficts. Propose mechanisms that will account for the stereochemistry of these reactions. 

To account for the stereochemical outcome, we propose a mechanism in Scheme 38 that involves a metal-5-fra j -diene cationic intermediate I. A near chairlike transition state structure is proposed to account for the stereochemistry. The states A and B represent the most likely transition states that have the most bulky complexed RCO(BF3) group situated at the equatorial site to avoid 1,3-diaxial steric hindrance. A further comparison between the two stractures indicates that A is the preferred structure because it avoids steric hindrance between two neighboring equatorial substituents, as in B. The presence of an electron-withdrawing group, such as carbomethoxyl, is required for this cyclization. No cycli-zation occurs for other T) -pentadienyl compounds, including that having the substituent CH=CH2 at the C(2)-allylic carbon position. 

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