Vilsmeier

[GC_MS]

If you have access to POCl3 and DMR, try Vilsmeier-Haack. As discussed on Rh's page (various formylation procedures), yield is quantitative. I can back this statement, as it indeed gave these results for me in the past   .

[moo]

I found the article while looking if there were any preparations of the aldehyde not involving POCl₃ or other such reagents, but thanks for the tip.