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8.1.21.4.3.6 Variation 6: Asymmetric Deprotonation, Transmetalation to Aluminum or Titanium, and Addition to Aldehydes

DOI: 10.1055/sos-SD-008-00412

Gawley, R. E.; O'Connor, S.; Klein, R.Science of Synthesis, (20068735.

The aldehyde homoenolate synthons discussed in Sections 8.1.21.4.3.4 and 8.1.21.4.3.5 do not undergo addition to aldehydes with a high degree of stereoselectivity. However, transmetalation from lithium to either aluminum or titanium complexes provides entry to anti homoaldol products with excellent enantio- and diastereoselectivities (Scheme 64).[‌223‌] ZimmermanTraxler-like transition states have been postulated to account for the relative configuration of the major product. Of particular note is the fact that the product E/Z geometric isomers from the aluminum transmetalation sequence have opposite configurations at their two stereocenters. In order to maintain the high enantiomeric purity of the aldehyde hydrolysis product, the E and Z geometric isomers of the products must be separated. Both the absolute configuration and the double bond geometry of the products resulting from transmetalation with chlorotriisopropoxytitanium(IV) (e.g., to give 137) are opposite to those obtained from the transmetalation with diethylaluminum chloride (e.g., to give 136) (Scheme 64).

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