B.R.S.M. The Battlefield Earth of organic chemistry blogs

Cycloadditions are great

This is my attempt at answering Rachel Pepling’s call for posts for her blog carnival over at CENtral Science. The theme is 'Your favourite chemical reaction'.

Update: 23/09/11 - removed some of the more egregious grammatical errors.

It only took me a few seconds of thought to answer the question 'What's your favourite chemical reaction', far less than if I'd been asked about my favourite book, meal or album. It's the Diels-Alder cycloaddition, obviously. I reckon I've done well over a hundred of these by now (including three today), and those two names will certainly appear in the title of my PhD thesis. On social media websites I usually list my interests as rock climbing, mountain biking and cycloadditions.[1]

I guess what I like about this reaction, and why it's a mainstay of total synthesis, is that it's a fantastic way to generate stuctural complexity blindingly fast. It can set up to four stereocenters, and you can even take simple achiral precursors and introduce asymmetry catalytically. But, unlike a lot of reactions which are capable of such synthetic leaps forward, it's also generally very  predictable. One of my favourite parts of the history of this reaction is this quote from the seminal paper by Diels and Alder:

"We explicitly reserve for ourselves the application of the reaction developed by us to [natural products synthesis]"[2]

Nice try guys, but the 1300+ papers published in 2010 on this transformation seem to indicate that this request has been largely ignored by the synthetic community. And why not - it's an all round great reaction, probably the hammer and/or duct tape of the total synthesis toolbox. It's also nice that it's named for an advisor and his PhD student, because the people who actually do the chemistry often don't get the credit they deserve. However, despite all of these things I'm not actually going to write about the Diels-Alder reaction here. The reason is that it's just too well known - we even teach it to first year undergraduates. Everyone knows what it is, and how good it is, and I think this carnival is a great opportunity to dredge up some lesser known reactions and hopefully get people thinking about some chemistry that's new to them.