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IV. Ring Opening of Specially Designed Acetals

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    24024

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    Search for compounds with more versatile reactivity than that provided by a 4,6-O-ben­zyl­idene group has stimulated development of some specially designed structures.14–18 The acetal 23, which fits into this “specially designed” category, reacts with Bu3Sn· to form the aryl radical 24. The iodine-atom abstraction that generates 24 is the first step in a sequence of radical reactions that culmin­ates in producing the protected glycoside 25 (Scheme 9).14–16 An example of the synthetic usefulness of this reaction is found in the conversion of a tetrasaccharide containing four such protecting groups into one in which each group is transformed into an O-benzoyl group.15 The glycoside 26 is another cyclic benzylidene acetal with an aromatic iodo substituent that undergoes a sequential radical reaction that leads to the corresponding deoxy benzoate 27 (eq 10).17 The reac­tions pictured in Scheme 9 and eq 10 are two more examples (in addition to those shown in equa­tions 2 and 3) where trans-fused rings open to produce primary rather than a secondary radicals. Ring opening of the 4,6-O-benzylidene acetal 28 to give a secondary radical (eq 11) further supports the proposal made for the acetal 11 (eq 4) that for a more flexible, cis-fused ring system the direction of ring opening is controlled by radical stability rather than ring strain at the transition state.

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    This page titled IV. Ring Opening of Specially Designed Acetals is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.