Monday, June 04, 2007

Asymmetric synthesis of vinylcyclopropanes

Apologies for the sluggish posting. Life is more complicated than usual as I've moved from one research group to another and as many of you will know this essentially means that you are working in two groups for a while. Finalising old stuff, cleaning up and writing papers on one topic whilst trying to start new projects in another lab....somewhat stressful and time consuming. Anyway, enough moaning. As you can see from the previous post I've sacked my fellow bloggers as they weren't blogging. So now it's all down to me (which it was anyway). I've been meaning to post this stuff since I read the paper in late December 2006. I have had a long lasting affair with cyclopropanes, in particular cyclopropane amino acids so I was very pleased to see this paper by Deng et al., DOI: 10.1021/ja056751o. These guys from Shanghai are doing some real cyclopropane magic using some easily obtainable camphor-derived sulfur ylides:The work is very throrough and makes up an 11 page JACS paper (not including any experimental). Many chemists would probably have split this work up in two papers. It's really nice to see these guys decided to stick the whole story in one paper. In brief these guys discover that they can make trisubstituted vinyl-cyclopropanes in high yield, diastereoselectivity and enantioselectivity. Moreover, they can make both enantiomers of cyclopropane selectively by switching from endo- to exo-sulfur ylides. This table from the paper illustrates how sweet this stuff is:Only "problem" here is that they are using stoichiometric sulfur ylide. However, they address this by developing a catalytic ylide cyclopropanation. The yields are not as impressive and the ee's are down to 50-80%. Still pretty cool and I bet these guys are working hard to improve the catalytic system. Finally, they decide to pull off a short and high yielding formal total synthesis of a known cyclopropane amino acid.
Obviously, they are making both enantiomers as well as both enantiomers of a diastereoisomer. And here I'm messing around trying to improve my lousy dr's on the racemic synthesis of the same target. Crap! D!

2 comments:

taitauwai said...

Hi there, just a thought ...
We can get -78C, using dry ice and acetone. Right?
But how do we get -95C?

Daniel Sejer said...

Methanol and nitrogen will do the trick. There's a short entry at Wikipedia on the topic: http://en.wikipedia.org/wiki/Cooling_bath. D!