Stereoselective synthesis uncatalyzed

Tandem intramolecular silylformylation-allylation reaction of diallylhydrosilyl ethers derived from homoallyl alcohols is convenient for rapid, stereoselective synthesis of 1,3,5-triols convertible to more oxygen-functionalized compounds (Scheme 12).142,142a,142b 143 [ he second uncatalyzed allylation step would be facilitated by the formation of a strained silacycle intermediate, which has enough Lewis acidity to activate the formyl group. A similar tandem reaction via alkyne silylformylation has been reported.144... 

In 1985, Mannig and Noth reported the discovery that Wilkinson s catalyst ([RhlPPhjljCl]) is an effective catalyst for hydroboration of alkenes with catecholborane, a reaction that otherwise requires elevated temperatures [32], Extensive subsequent studies on metal-catalyzed hydroborations [8, 11, 33-35] revealed a range of Interesting results. Firstly, hydroboration of cyclohex-2-enols affords regioisomeric products that are complementary to those isolated from the uncatalyzed reaction with 9-BBN (Equation 2) [33, 34). Secondly, the hydroboration of 91 with 9-BBN affords anti product 93, but the Rh-catalyzed reaction leads to preferential formation of syn product 92. This feature of the metal-catalyzed process was used by Evans in the stereoselective synthesis of polyketide building blocks 92 and 93 (Equation 3) [33]. 

The availability of oxepins that bear a side chain containing a Lewis basic oxygen atom (entry 2, Table 6.4) has further important implications in enantioselective synthesis. The derived alcohol, benzyl ether, or methoxyethoxymethyl (MEM) ethers, in which resident Lewis basic heteroatoms are less sterically hindered, readily undergo diastereoselective uncatalyzed alkylation reactions when treated with a variety of Grignard reagents [17]. The examples shown below (Scheme 6.7) demonstrate the excellent synthetic potential of these stereoselective alkylations. 

---