Intramolecular acetal formation by primary versus secondary hydroxyl groups

On acid-catalyzed equilibration in aqueous solution, three aldoheptoses, d-glycero-l-manno-, d-glycero-l-allo-, and d-glycero-l-altro-heptose, give 15, 54, and 98%, respectively, of the 1,6-anhydropyranose, a much higher proportion than is formed from the homomorphous aldohexoses. This is another example of the greater tendency of secondary, compared to primary, hydroxyl groups to participate in the formation of cyclic acetals.