# Synthesis of a lactone

I'm solving some synthesis problems for practice and I came across this one:

I thought of going for compound A to B(aldehyde) with this procedure:

1. $$\ce{t-BuOK}$$
2. $$\ce{BH3,THF,NaOH,H2O2}$$
3. $$\ce{PCC,CH2Cl2}$$

For the second path, in order to get to a lactone, I don't know what to do. I see that the carbonyl bond is moved to another carbon. I've learnt a way, in order to get an ester, is to add an alcohol $$\ce{ROH}$$ to an acyl chloride. But I'm so confused.

• What do three arrows signify? Three synthetic steps, or just a very fast transition?:) – andselisk Jun 10 '20 at 11:00
• You have added a 4 carbon unit to your aldehyde. How many steps are you contemplating? – Waylander Jun 10 '20 at 11:00
• @andselisk I really don't know, the exercise just asks to write the reagents which will lead to the wanted compounds. – chemistryluvv Jun 10 '20 at 11:09
• @Waylander Ι don't know where to start, im really confused. The part that confuses me the most, is that there are 4 carbon atoms added, but they are in a ring and i wonder if it is an intramolecular type of thing or not – chemistryluvv Jun 10 '20 at 11:12
• Add a 5-carbon unit via the Grignard reagent of 5-bromo-1-pentene to the aldehyde. Ozonolysis of the double bond to a hydroxy aldehyde that will likely form the hemiacetal. Oxidize with PCC – user55119 Jun 10 '20 at 16:12

Here is a way to make the lactone:

1. To your aldehyde in dry THF at 0 °C, add the commercially available 3-butenylmagnesium bromide. Work up and collect the resulting secondary alcohol.

2. React the alcohol with dihydropyran in dry $$\ce{CH2Cl2}$$ to form the THP ether.

3. React this THP ether with $$\ce{BH3.THF}$$ as you did before, but do an oxidative work up with PDC to get the carboxylic acid according to Brown et al. [1], or do the basic peroxide as before to get the aldehyde, then oxidise that.

4. Treat the product acid with PTSA in MeOH to remove the pyran ether, the lactone will form spontaneously.

Your choice of alcohol protecting group will be driven by the oxidation conditions you use for the borane. THPO is known to be stable to PDC/$$\ce{CrO3}$$ in pyridine.

### Reference

1. Brown, H. C.; Kulkarni, S. V.; Khanna, V. V.; Patil, V. D.; Racherla, U. S. Organoboranes for Synthesis. 14. Convenient Procedures for the Direct Oxidation of Organoboranes from Terminal Alkenes to Carboxylic Acids. J. Org. Chem. 1992, 57 (23), 6173–6177. DOI: 10.1021/jo00049a024.