Can sodium borohydride reduce hemiacetals? In the synthesis of Panacene by Canesi et al. [1], a step involves the installation of an $\ce{-OH}$ group on a double bond, using the oxymercuration-demercuration procedure. However, the product happens to be a hemiacetal, as shown in the picture. With a 78% yield, it seems that the hemiacetal is not reactive towards the added sodium borohydride. Is this true in general, that hemiacetals will not be reduced to diols by $\ce{NaBH_4}$? What about $\ce{LiAlH_4}$?


References

Cyrille Sabot, Didier Bérard, Sylvain Canesi, &quot;Expeditious Total Syntheses of Natural Allenic Products via Aromatic Ring Umpolung,&quot; Organic Letters 2008, 10(20), 4629-4632 (DOI: 10.1021/ol801921d).
The major purpose of $\ce{NaBH4}$ in the second step of given reaction sequence is the demercuration, which stops at the hemiacetal as the applied condition is the limit (at $\pu{0 ^\circ C}$ for $\pu{1 min}$).
In general under usual conditions (at $\pu{25 ^\circ C}$ in ethanol for for an hour or two), $\ce{NaBH4}$ reacts with hemiacetals and reduce them to corresponding diols. For instance, $\ce{NaBH4}$ reacts with glucose, which is a hemiacetal in aqueous solutions, to give sorbitol, the corresponding diol (Ref.1). The reaction has been done in water and at room temperature ($\pu{20-25 ^\circ C}$) for $\pu{1-2 h}$. Thus, running the given reduction in $\pu{0 ^\circ C}$ for shorter time make sense.
Also, although lithium aluminum hydride ($\ce{LiAlH4}$) is too vigorous a reagent for reduction, under carefully controlled conditions, it can be used to convert lactone to lactol (cyclic hemiacetal such as monosaccaride) without reacting further to give the corresponding diol. However, excess $\ce{LiAlH4}$ would give the corresponding diol after reacting further with lactol (Ref.2,3):


References:

M. Abdek-Akher, J. K. Hamilton, F. Smith, &quot;The Reduction of Sugars with Sodium Borohydride,&quot; J. Am. Chem. Soc. 1951, 73(10), 4691–4692 (DOI: https://doi.org/10.1021/ja01154a061).
2.Glen E. Arth, &quot;Reduction of Lactones to Hydroxyaldehydes with Lithium Aluminum Hydride,&quot; J. Am. Chem. Soc. 1953, 75(10), 2413–2415 (DOI: https://doi.org/10.1021/ja01106a040).
Robert A. W. Johnstone, &quot;Chapter 1.11: Reduction of Carboxylic Acids to Aldehydes by Metal Hydrides,&quot; In Comprehensive Organic Synthesis - Selectivity, Strategy and Efficiency in Modern Organic Chemistry, Volumes 8: Reduction; Ian Fleming, Volume Editor; Barry M. Trost, Ian Fleming, Editors-in-Chiefs; Pergamon Press: Oxford, United Kingdom, 1991, pp. 259-282 (ISBN: 978-0-08-040599-1).