DEVELOPMENT OF CARBON-CARBON BOND FORMING REACTION USING ALPHA-HYDROXYSILANE

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Optically active (1-hydroxy)trialkylsilane (α-hydroxysilane) group has attracted significant interests in view of its potential utility in a diastereoselective functionalization of the α-substituted vinylic CC double bond. In this report, we wish to describe: (1) highly stereoselective synthesis of vinylsilane-containing α-amino acid and α,α-disubstituted α-amino acid via a [3,3] sigmatropic rearrangement of α-acyloxysilane, and (2) acid-catalyzed rearrangement of α-hydroxysilanes.(1) A vinylsilane-containing α-amino acid and α,α-disubstituted α-amino acid having two contiguous asymmetric carbon centers at their α and β positions have been synthesized in an optically active form by ester-enolate Claisen rearrangement of the α-acyloxysilane as the key step. The present study demonstrated the potential usefulness of the α-hydroxysilyl group as an excellent chirality transferring group in the [3,3] sigmatropic rearrangement.(2) We investigated cationic rearrangement of several α-hydroxysilanes. Treatment of both optically active syn-α-hydroxycyclopropylsilane and its hydroxy group epimer (anti form) under aqueous H_2SO_4 underwent rearrangement via a common α-silyl cation intermediate to give a mixture of the ring-opened vinylsilane and two cyclopropylsilanes, the tandem [1,2]-CC bond migration products, in an optically active form. On the other hand, the acidic treatment of E- or Z-α-hydroxy alkenylsilane was each accompanied with partial racemization to give an allylic rearrangement product via a preferential syn-facial S_N2' reaction, respectively. Both α-hydroxyalkynylsilane and α-hydroxyalkylsilane were inert to the acidic condition; however, treatment of α-mesyloxyalkynylsilane under aqueous H_2SO_4 gave the mixture of rearranged products which included an optically active allene. Comparisons of the reactivities of these α-hydroxysilanes under acidic conditions are also disclosed.