Allylchromium species

In contrast to these results, syn diastereoselectivity is observed in the reactions of (109) and (110) with aldehydes.- ° The stereochemistry in these cases leaves little doubt that it is the (Z)-allylchromium species that is involved, presumably as a result of a destabilizing interaction between the R and bulky R- substituents in the ( )-isomer (Scheme 23). In the case of methallyl derivative (113), however, anti dia-stereoselection is realized in this interesting macrocyclization (Scheme 24). - ... 

Formal intramolecular [4+1]-cycloaddition of chromium aminocarbenes (16) has provided a nice entry to iV-heteropolycyclic compounds." The reaction occurs via a [2+2]-cycloaddition between the carbene and the internal olefin yielding an allylchromium species which equilibrates with the metallacyclohexane that finally provides the polycycle after reductive coupling (Scheme 2). 

This paper will deal with four topics the first one is related to allylchromium reagents, while the latter three refer to the behavior of trialkylaluminum or related species in different situations. The authors main concern here is to describe new reactions useful for selective synthesis. 

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