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Magazine Chapter 18: Q51P (page 970)
Show how you would accomplish the following syntheses efficiently and in good yield. You may use any necessary reagents.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Short Answer
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Step by step solution
Synthetic methodology
(a) Acetaldehyde on treatment with potassium cyanide (KCN) gives a cyanohydrin (2-hydroxypropanenitrile) which on further acid-catalyzed hydration forms the product lactic acid.
reaction a
(b) Cyclopentanone on treatment with benzylidenetriphenylphosphorane gives the product as (cyclopentylidenemethyl)benzene.This is known as the Wittig reaction.
reaction b
(c) 3-oxocyclopentancarbaldehyde on reduction with NaBH(OAc)3 (sodium triacetoxy borohydride)gives the desired product as 3-(hydroxymethyl)cyclopentanone. Only the aldehyde group is reduced to alcohol.
reaction c
(d) 3-oxocyclopentanecarbaldehyde reacts with ethane-1,2-diol in presence of an acidic catalyst gives the initial product as 3-(1,3-dioxolan-2-yl)cyclopentanone. In this step the aldehyde group is protected by the formation of acetal. So on further treatment with sodium borohydride (NaBH4) only the ketone is reduced to alcohol to form 3-(1,3-dioxolan-2-yl)cyclopentanol. Lastly, on acid-catalyzed hydration the acetal is hydrolysed to aldehyde to give the desired product which is 3-hydroxycyclopentanecarbaldehyde.
reaction d
(e) 3-oxocyclopentanecarbaldehyde reacts with ethane-1,2-diol in presence of an acidic catalyst gives the initial product as 3-(1,3-dioxolan-2-yl)cyclopentanone. This step is carried out to protect the aldehyde group by formation of acetal which on treatment with triphenyl(propylidene)phosphorane forms another product as (E)-2-(3-propylidenecyclopentyl)-1,3-dioxolane. Lastly, on acid-catalyzed hydration results in the hydrolyses of acetal to give aldehyde. The final product formed is (E)-3-propylidenecyclopentanecarbaldehyde.
reaction e
(f) Cyclohept-4-enone on catalytic hydrogenation gives the desired product as cycloheptanone.The reagent used results in the reduction of only the double bond and not the carbonyl group.
reaction f
(g) Cyclohept-4-enone on reduction with sodium borohydride (NaBH4) gives the desired product as cyclohept-4-enol. In this case, the ketone is reduced to alcohol while the double bond remains intact.
reaction g
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