Allylboration as the Terminal Process

Inspired by the pioneering work of Vaultier and co-workers on the two-step carbo-cyclic [4+2] cycloaddition/allylboration [78] and the one-pot variant by LaUemand [79], Tailor and HaU described the first tandem aza[4+2]cycloaddition/allylboration three-component reaction [129]. Thermal reaction between hydrazonobutadienes 91, N-substituted maleimides, and aldehydes provides polysubstituted a-hydroxyalkyl-piperidines 94 via the allylboronate intermediate 92 and the proposed allylboration transition structure 93 (Equation 48) [130], A variant of this process, an aza[4+2]cy-cloaddition/aUylboration/retro-sulfinyl-ene, has been applied to the total synthesis of palustrine alkaloids [131]. 

Recently, the corresponding variant for constructing a-hydroxyalkyl pyran derivatives has been described. This three-component reaction involves 3-boronoacrolein pinacolate, ethyl vinyl ether, and aldehydes as components [132,133]. By making use of Jacobsen s chiral Cr(iii) catalyst 96 in the first step (i.e., the inverse electron demand Diels-Alder cycloaddition), the overall process provides the desired products 

98 in high yields and over 95% diastereo- and enantioselectivity (Equation 49) [133]. 

This hetero-Diels-Alder step marks the first catalytic enantioselective preparation of a C-chiral aUylboronate (i.e., 97). Key to this process is that the possible self-aUylbo-ration between 95 and 97 is avoided by the higher temperature required in the allylb-oration step. An application of this three-component reaction to the synthesis of a mosquito oviposition pheromone was also described. 

As described in Section 6.2.2.6, simple allylboronates can be elaborated into more substituted ones using olefin cross-metathesis (Table 6.1) [82]. In the work of Goldberg and Grubbs, treatment of pinacol allylboronate 2 with various olefin parmers (104) in the presence of catalyst 63 smoothly leads to formation of a 3-substituted allylboronate, the cross product 105 (Equation 51). This new boronate is not isolated but rather is treated directly with benzaldehyde to give the homoallylic alcohol product 106 in good yield. 

---