Grob fragmentation

Grob fragmentation

A Grob fragmentation, named for the British chemist Cyril A. Grob, is an elimination reaction taking place when an electrofuge and nucleofuge are situated in positions 1 and 3 on an aliphatic chain. [Grob, C. A.; Baumann, W. "Helv. Chim. Acta" 1955, "38", 594.] [Weyerstahl, P.; Marschall, H. "Comp. Org. Syn." 1991, "6", 1044-1065. (Review)] ["Strategic Applications of Named Reactions in Organic Synthesis" Laszlo Kurti, Barbara Czako Academic Press (March 4, 2005) ISBN 0-12-429785-4] The reaction product is an electrofugal fragment (carbonium ion, acylium ion), an unsaturated fragment (alkene, alkyne, imine) and a nucleofugal fragment (leaving group such as tosyl, hydroxyl).

The reaction mechanism varies with reactant and reaction conditions with the fragmentation taking place in a concerted reaction, taking place in two steps with a carbocationic intermediate when the nucleofuge leaves first or taking place in two steps with an anionic intermediate when the electrofuge leaves first.

An example of a Grob-like fragmentation in organic synthesis is the expansion of the Wieland-Miescher ketone (1 in "scheme 1") to Thapsigargin. ["Synthesis of the thapsigargins" Steven V. Ley et. al PNAS | August 17, 2004 | vol. 101 | no. 33 | [http://dx.doi.org/10.1073/pnas.0403300101 Abstract and online article] ] [Reaction details: Organic reduction of Wieland-Miescher ketone with sodium borohydride to alcohol 2 followed by functionalizion to mesylate 3 with mesyl chloride in pyridine. Then reduction of enone to allyl alcohol 4 with tri-tert-butoxyaluminum hydride in tetrahydrofuran followed by hydroboration with borane in THF to borane 5 (only one substituent displayed for clarity). The Grob fragmentation to 6 takes place with sodium methoxide in methanol at reflux. A methoxide group attacks boron giving the borate complex which fragments. As each boron atom can hold three substrate molecules (R), the ultimate boron byproduct is trimethyl borate ]

Another example is an epoxy alcohol fragmentation reaction as part of the Holton Taxol total synthesis.

References

ee also

*Eschenmoser fragmentation
*Wharton reaction


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