Journal of the American Chemical Society
COMMUNICATION
The thiomethyl group also increases the reactivity of the allylic
isomer toward solvolysis, which gives a thiomethyl hemiketal
followed by elimination of methanethiol. We anticipate that
methyl vinyl ketone produced from the thiomethyl DMAPP
analogue would be unstable during the incubation. A related
solvolysis reaction was seen following isomerization of 3-cyclo-
propyl-3-butenyl diphosphate to its allylic isomer, where σp+ for the
cyclopropane ring (À0.601) is similar to the thiomethyl value.8,20
In this case, the solvolysis product from (Z)-3-cyclopropyl-2-butenyl
diphosphate was sufficiently stable to be characterized.
A considerable body of evidence with analogues of IPP indicates
that substantial positive charge develops at C3 of IPP during the
isomerization to DMAPP. However, the previous data do not
distinguish between a stepwise mechanism that proceeds through a
carbocationic intermediate and a concerted process with a late
transition state for their isomerization reactions. The rapid
exchange of the vinyl protons in 1-OPP with solvent without
concomitant rearrangement to its allylic isomer shows that the
analogue is protonated by IDI-2 to give an intermediate carbocation,
which loses a proton to return to 1-OPP without a mandatory
isomerization. 1-OPP is the first example of a substrate for IDI in
which protonation and isomerization are decoupled. The trends
in reactivity for IPP and other C3 analogues and the exchange
results for 1-OPP are consistent with a common protonationÀ
deprotonation mechanism for their isomerization reactions.
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’ ASSOCIATED CONTENT
S
Supporting Information. Procedures for synthesis of
b
1-OPP and NMR spectra; descriptions of product studies and
mass spectra. This material is available free of charge via the
’ AUTHOR INFORMATION
Corresponding Author
Present Addresses
‡Sapphire Energy, 3115 Merryfield Row, San Diego, California
92121, United States.
’ ACKNOWLEDGMENT
This work was supported by NIH grant GM 25521.
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