Transition stale reagents

The only reactions in Table 1 where diastereoselectivity deviates maikedly from the reagent isomeric purity involve the (Z)-7-alkoxyallylboronates (entries 21-26), and then only when a sterically demanding aldehyde (isobutyraldehyde, entries 24-26) or a bulky protecting group is employed. Un r these circumstances it appears that boat-like transition stale (13) becomes competitive with the otherwise favored chair arrangement (12 Figure 7). 

The results obtained with the tertiary 5-hcxenyl probe (see above) indicate that even with tertiary Grignard reagents the alkyl radical has approached the carbonyl in the transit ion stale so that immediate radical recombination in the cage may lake place to a certain extent. The bonding, however, must be weak. 

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