There are two scenarios and in either case, we start with 5-oxohexanal. A reagent or sequence of reactions "A"
helps convert the substrate into 6-hydroxyhexan-2-one. On the other hand, a reagent or sequence of reactions "B"
converts the starting compound into 5-hydroxyhexanal. A and B are:
A: 1 eq. NaBH4 followed by H3O+;
B: 1 eq. HOCH2CH2OH, followed by NaBH4 plus H3O+
Aldehyde is easier to reduce than a general ketone. This could be taken advantage of:
Using 1 equivalent of a hydride donor will selectively reduce the aldehyde.
To the reduce the ketone group selectively, the higher reactiviy of aldehyde could
be put to use. A protective diol could help form an acetal at the aldehyde position.
This could then be reduced as only the ketone part gets reduced. Hydrolysis
does 2 things - work up + hydrolysis of acetal