Claisen rearrangement acyclic substrates

I.6.3.I.I.2.2. Catalyzed Claisen Rearrangements Organoaluminmn-Promoted Rearrangements Acyclic Substrates... 

Although a chairlike transition state is favored for the Claisen rearrangement reactions of acyclic substrates, this is not always the case with cyclic systems. For example, Bartlett and Ireland independently studied the rearrangement reactions of cyclohexenyl silylketeneacetals and found that there was competition between the chairlike and boatlike transition states11 (Scheme l.VII). Clearly, the -isomer IE gives 8a via a chairlike transition state, whereas the Z-isomer 7Z affords the same product (8a) via a boatlike transition state. 

Generally, Claisen rearrangement of ring-bearing substrates with remote asymmetric centers has led to higher stereoselectivities than in the acyclic series. Kurth and cowoikers used amino acid derived chiral auxiliaries such as (302) for the preparation of enantiomerically pure carboxylic acids. The excellent facial selectivity in these systems was hampered, however, by poor chair vs. boat selectivities. 

The stereochemistry of Claisen systems with an exocyclic double bond ( 1,1 and 6,6 rearrangements) often parallels the rearrangement of 1,1- or 6,6-disubstituted acyclic substrates. 

As already discussed for Claisen rearrangements, Cope rearrangements as self-immolative processes are combined with conservation of chirality. In contrast to the Claisen rearrangements, however, only a few examples of the 1,3-chirality transfer in Cope rearrangements of acyclic systems have been reported in the literature. This is probably due to the fact that the synthesis of the optically substrates is as difficult as that of the products. 

For most acyclic Claisen rearrangement substrates, reactions may be viewed as taking place... 

In addition to its wide substrate scope and functional group compatibility, the Meerwein-Eschenmoser-Claisen rearrangement is marked by high regio- and stereoselectivity, notably in the case of acydic substrates. In fact, the reaction provides some of the best examples for acyclic stereocontrol reported in the literature. 

Since the initial studies, it has become generally accepted that the Claisen rearrangement of acyclic substrates preferentially proceeds via chair-like, cyclic transition states, with the standard placement of the largest groups in the... 

Chirahty transmission can also be induced in thio-Claisen reactions by a chiral center adjacent to the allyl vinyl sulfide core. The group of Metz-ner examined the diastereoselectivity of the rearrangement of acyclic S-allyl ketene dithioacetals bearing a chiral center adjacent to carbon 6. hi these substrates, the sigmatropic shift proceeded smoothly and with modest syn anti diastereoselectivity [74]. In contrast, higher selectivities are foimd for thio-Claisen precursors bearing a chiral center adjacent to carbon 1 mainly due to steric effects and allylic strain [75]. In 1991, the group of Beslin examined the effect of a hydroxyl substituted chiral center attached to C-1 of Z and E S-allyl ketene dithioacetals [76,77] (Scheme 7). Under the reaction conditions, these... 

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