Allylboration as the Initiating Process

With reagent IIS, however, the corresponding allylboronate intermediate 116 is thought to favor transition structure 117 in which die a-substituent is in a pseudo-axial orientation to escape steric interactions with the bulky tetraphenyl dioxaborolane (Equation 57). This way, a Z-configured allylic alcohol unit of opposite stereochemistry is obtained in product 118. Such steric control had been demonstrated before by the work of Hoffman and co-workers on a-chiral aUylboronates (Section 6.3.1.3). The usefulness of this powerful tandem aUylation/allylation strategy was demonstrated with several examples of both types of 1,5-dioIs, 114 and 118. 

Kennedy and Hall have reported a one-pot allylboration/lactonization using the tetra-substituted 2-alkoxycarbonyl aUylboronates 17 (Section 6.2.1.2) [39]. These allyl-boronates react with aldehydes, thermally and under Lewis acid catalysis, to first provide the hydroxy-ester intermediate 119 (Equation 58). This initial product cyclizes under the reaction conditions by lactonization to afford a-exomethylene y-lactones 120. This facile cyclization is probably a manifestation of the gem-dialkyl effect. In- 

The stability of allylboronates and the high level of diastereoselectivity in their additions to carbonyl compounds and imines represent very attractive attributes in organic synthesis. The examples in this chapter clearly show that recent advances in the preparation of allylboronates will help in furthering their applications. The development of efficient catalytic enantioselective allylboration methods and the invention of more powerful and more elaborate tandem reaction processes constitute two emerging areas for further development. 

Denmark, N. G. Almstead, in Modem Carbonyl Chemistry J. Otera (Ed), Wiley-VCH, Weinheim, 2000, pp 299-402, Chapter 10. 

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