C. 4 (pts) In the following synthesis steps of salmeterol, a long-acting broncho

Question

C. 4 (pts) In the following synthesis steps of salmeterol, a long-acting bronchodilator, what reagent is used and why? What will be the product if treated with (S) CBS reagent? H OH [1] (R)-CBS H OH reagent [2] H20 four steps PhCH O PhCH2O HO Y (R)-S COOCH COOCH rade nar HO (R)-Salmeterol is a long-acting bronchodilator used for the treatment of asthma. n this example, the (R-CBs reagent adds the new H atom from behind, the same result observed with propiophenone, In this case, however, alcohol A has the R configuration using the rules for assigning p

Explanation / Answer

c. In the above shown synthesis, (R)-CBS adds the H from the least hindered side, that is from below the phase of C=O, to give -OH facing upwards.

alcohol has R-configuration. Priority of groups are, -OH (1) > CH2Br (2) > Ph (3) > H (4)

the next step is done by first deprotecting the -OCH2Ph with H2,Pt/C and then reducing ester by LiAlH4 to primary alcohol.

If instead it is treated with a (S)-CBS, we would expect a product with an (S) configuration, H added from above the plane giving -OH facing down.

d. The major product expected to be formed from reaction of propiophenone with (S)-CBS reagent is, (R)-alcohol. The (S)-CBS adds H from top giving alcohol -OH facing down. The absolute configuration of the compound would be thus an (R).

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