Simultaneous Application of Arylmethylene Acetal and Butane Diacetal Groups for Protection of Hexopyranosides: Synthesis and Chemoselective Ring‐Opening Reactions

The reductive cleavage of 4,6‐ O ‐arylmethylene acetals of hexopyranosides is an effective method for the regioselective formation of a benzyl‐type ether at either the C‐4 or the C‐6 hydroxyl group. The compatibility of this method with Ley's butane diacetal (BDA) protection was studied. 4,6‐ O ‐(2‐Naphthyl)methylene, benzylidene, and p ‐methoxybenzylidene acetals were introduced to 2,3‐ O ‐BDA‐protected gluco‐ and galactosides, and the bis‐acetalated products were subjected to reductive acetal openings with different reagents. LiAlH 4 –AlCl 3 reduced the 4,6‐acetals with complete chemo‐ and regioselectivity to give the corresponding 4‐ O ‐ethers. The use of Et 3 SiH–BF 3 · Et 2 O led to a mixture of differently reduced products, because transformation of the 4,6‐acetal into the related 6‐ O ‐ether and reduction of the BDA system into a butane‐2,3‐diyl group took place competitively. BH 3 · NMe 3 –AlCl 3 selectively cleaved the 4,6‐acetal ring to give the 6‐ O ‐ethers as the major or exclusive products.