Question

Show how you would accomplish the following syntheses efficiently and in good yield. You may use any necessary reagents.

(a)$\mathbf{acetaldehyde}\to \mathbf{lacticacid}\mathbf{,}{\mathbf{CH}}_3\mathbf{CH}\mathbf{\left(}\mathbf{OH}\mathbf{\right)}\mathbf{COOH}$

(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