Carbohydrates benzylidene-type acetal

For the combination of two six-membered rings, similar restrictions on the five- and six-membered anellation exist. In the trans fused system, the six-membered ring must be able to adopt a conformation in which the connecting bonds are diequatorially oriented. The most common carbohydrate derivatives belonging to these types of compounds are the cyclic 0-benzylidene acetals, in which the bulky phenyl substituent prefers the equatorial arrangement, and thereby restricts the conformational mobility (Figure 2.33). 

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