Chartelline and copper catalyzed decarboxylation

A former mentor-student duo has published successive papers in JACS.

Total Synthesis of Antheliolide A
Chandra Sekhar Mushti, Jae-Hun Kim, and E. J. Corey
http://dx.doi.org/10.1021/ja066336b

Total Synthesis of (±)-Chartelline C
Phil S. Baran and Ryan A. Shenvi
http://dx.doi.org/10.1021/ja0659673

First, Corey publishes a synthesis of Antheliolide and then Baran publishes Chartelline. Corey uses a several nice transformations including a [2+2] ketene formation. In one step, he uses LiOH/MeOH to deprotect a TMS protected alkyne, instead of TBAF, because TBAF will also protect the TBDPS group. Why does LiOH protect only the TBS and not the TBDPS?

Also, in another step, he wanted to convert a (methylated) lactol to a lactone. But he had a sensitive caryophyllene like moeity in the ring, which would have been notoriously sensitive to acid and oxidants. So he takes the indirect way and first converts the OMe to a phenyl seleno, then turns it into a hydroxyl with AgNO3, and then finally uses mild oxidation with TPAP to oxidise the hydroxyl. Neat.

Baran's synthesis is more elegant, and as usual, based on biosynthetic proposals. He does some neat transformations, including a cascade like reaction to effect a nice ring closure.



However, it is the last step which I found interesting, in which he was trying to decarboxylate a vinyl carboxylate. Copper catalyzed decarboxylation failed, but it turned out that the carboxylate was quite sensitive and labile to heat, so simple heating worked; the mechanism involved a proton that seemed to be serving a function akin to copper as noted in the reference below. He also suggested transient positive charge stabilization by chlorine. I know fluorine can do it well; chlorine would be less efficient but could still do it I suppose.



This led to me look up some back references on vinyl decarboxylation. I found out a reference by Theodore Cohen of Pittsburgh (J. Am. Chem. Soc.; 1970; 92(10); 3189-3190) who proposed the following schematic for explaining stabilization of the TS by copper.



He was trying to explain some copper catalyzed quinoline decarboxylations that went back to 1930. I also came upon a 1950 paper by Frank Westheimer (J. Am. Chem. Soc.; 1951; 73(1); 429-435) in which he tried to explain the decarboxylation of alpha-keto acids with copper, a classic reaction which he uses as an enzyme model system.