[DocOG97]

Below is a picture of the two reactions I am struggling with. Both LiAlH4 and NaBH4 are pretty strong reducing agents, though LiAlH4 is able to reduce esters and carboxylic acids while NaBH4 cannot. For these reactions, we are supposed to determine whether the diol is generated in both cases. However, because the reagents contain a hydroxyl group my thought process is that both NaBH4 and LiAlH4 will deprotonate the hydroxyl group. After deprotonation, the molecule will undergo an intramolecular reaction, where the product is shown in parentheses. Is this correct?

[pgk]

In both cases, the cyclic hemiacetal is formed. But:
1). Hydrolysis with diluted acid (LiAlH4 reduction), favors the hemiacetal ring opening and 1,4-butanediol is finally formed.
2). Isopropanol (NaBH4 reduction) has lower pka value than a primary butanol and theoretically cannot open the hemiacetal ring. But in practice, the hemiacetal ring will finally be open during the reaction working up with an aqueous medium.
Hint: Please, do not confuse the stability of the said cyclic hemiacetal with the stability of carbohydrates cyclic hemiacetals (pyranoses and furanoses) that are stabilized by intramolecular hydrogen bonds.