Can hemi-acetal rings form under basic conditions? I recently encountered a problem on a test.  It was a standard synth problem, "Provide reagents to produce y from x".  The starting material was DHA and the ending molecule was a branched pentose. (I included an image of the solution I came up with).  In my solution I use acetaldehyde under basic conditions to form the pentose.
My question is would pentose form under basic conditions or do I need to acidifying at the end?  Would acidifying just encourage the formation of side products?  Is there some other way to get this pentose as a primary product that I am not thinking of?
Using base means that the reaction will stop at the hemiacetal and acetal formation would not take place. Acidifying at the end might encourage acetal formation.