Explain the diastereoselectivity of this reaction.

Molecule 13 was produced in 81% yield as a 10:1 mixture of diastereomers. The favored product involves the hydroxyl group in equatorial position. As seen in the chair conformations below, below the 6-membered ring there many steric interactions because of the large R1 group. Therefore, it is more favored for the ethynylmagnesium bromide to attack the carbonyl carbon from above the ring, which pushes the oxygen into the more stable equatorial position. Therefore, the conformation with the hydroxyl group in the equatorial position is more favored sterically and is also the more stable product. The product with the hydroxyl group equatorial is in 10:1 ratio with the hydroxyl group in the axial positions in which the Grignard reagent would attack from below the ring.

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