Enantioselective methylalumination of alkenes

Scheme 7-106 Enantioselective methylalumination of alkenes with a chiral Zr catalyst.

Zirconium-Catalyzed Enantioselective Methylalumination of Unactivated Alkenes... 

To further probe the potential synthetic utility of the Zr-catalyzed enantioselective carboalumination of alkenes, the C14 sidechain units of vitamin E, that is, (2R,6R)-2.6.10-trimethyl-1 -undecanol (4) and the corresponding iodide 5, have been synthesized as summarized in Scheme 4.10 [15]. Whereas the enantioselectivity in each methylalumination step must be significantly improved, the four-step synthesis of 4 and 5 summarized in Scheme 4.10 appears to provide one of the most efficient enantioselective routes to the C14 sidechain units of vitamin E [16]. 

Although the application of carboalumination to the synthesis of natural products is still in its infancy, a few preliminary results shown in Scheme 1.50 [167,168,171,172] suggest that it promises to become a major asymmetric synthetic reaction, provided that (i) the singularly important case of methylalumination can be made to proceed with S90% ee, and (ii) satisfactory and convenient methods for enantiomeric and diastereo-meric separation/purification can be developed. In this context, significant increases in ee in the synthesis of methyl-substituted alkanols from around 75 % to 90—93 % achieved through some strategic modifications are noteworthy (Scheme 1.50) [168]. Shortly before the discovery of the Zr-catalyzed enantioselective carboalumination, a fundamentally discrete Zr-catalyzed asymmetric reaction of allylically heterosubstituted alkenes proceeding via cyclic carbozirconation was reported, as discussed later in this section. 

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