raphy-mass spectrometry (GC-MS) analyses of the reaction
products revealed the presence of several phytene isomers:
(Z)- and (E)-phyt-2-ene (2 and 3, respectively, in a 1/2 ratio),
as well as two yet unreported compounds (Z)- and (E)-phyt-
3-ene (4 and 5, respectively, in a 1/2 ratio). The residual
sulfur compounds are composed of a mixture of (E)- and
(Z)-phyt-2-ene-1-thiols (1 and 6, respectively, in a 2/1 ratio)
as well as of the corresponding polysulfide cross-linked
dimers 7 (Figure 1).
m-CPBA. The unseparable diols obtained after opening of
the epoxyalcohols by titanium(IV) isopropoxyde6 were then
cleaved by sodium periodate to yield two unsaturated
aldehydes 8 and 9. After separation, aldehyde 8 was reduced
to the corresponding alcohol and subsequently chlorinated.7
Finally, the substitution of the chloride by methylmagnesium
iodide yielded (E)-phyt-3-ene 5, which was fully character-
ized using MS and NMR spectroscopy. Particularly, the
nuclear Overhauser effect observed between methyl 4′ and
methylene 5 confirmed the stereochemistry of the double
bond. To obtain 4, 5 was isomerized photochemically in the
presence of diphenyl disulfide.8 Pure 4 was isolated by
reverse-phase HPLC from the mixture9 (4 and 5; 1/2 ratio)
and characterized by MS and NMR.
When (E)-phyt-2-ene 3 was reacted with hydrogen sulfide
under our simulation conditions (60 days), it was quantita-
tively recovered and no isomerized phytenes could be
detected by GC and GC-MS analyses. This rules out the
possibility that (E)-phyt-2-ene 3 is the initial product formed
by reduction of (E)-phyt-2-ene-1-thiol 1 and that further
double-bond isomerization and migration reactions account
for the presence of the phytene isomers in the reaction
products. Consequently, the formation of the mixture of the
phytene isomers resulting from E/Z isomerization and
migration of the double bond from initial position 2 to
position 3 must be directly related to the nature of the
reaction intermediates.
Figure 1. Desulfurization of (E)-phyt-2-ene-1-thiol 1 by H2S yields
(Z)- and (E)-phyt-2-ene (2 and 3, respectively) as well as (Z)- and
(E)-phyt-3-ene (4 and 5, respectively). Mixtures of (E)- and (Z)-
phyt-2-ene-1-thio1 1 and 6, respectively, as well as the correspond-
ing polysulfides 7 are also recovered.
By analogy with experiments on the replacement of nitro
groups by hydrogen induced by thiolates,10 the reduction of
phyt-2-ene-1-thiol 1 could be related to SRN1 reactions
involving radical intermediates (Scheme 2). Hydrogen sulfide
is, indeed, known to be a good hydrogen atom donor, while
thiols are able to induce SRN1 reactions.10
Furthermore, it has also been shown that carbon-sulfur
bonds can be cleaved under SRN1 conditions.11 Thus, we
propose in the first step of the reaction the formation of a
radical anion by a single-electron transfer (SET) from
hydrogen sulfide ions. Further loss of a sulfhydryl group by
a heterolytic cleavage of the carbon-sulfur bond would then
yield a delocalized radical 10.
Separation of the phytene isomers by GC was difficult
and only obtained using a capillary column coated with a
poly(ethylene glycol)-bonded phase.5 The structural assign-
ment of the two unknown phytenes 4 and 5 was made
possible by comparison of mass spectral data and chromato-
graphic behavior with synthetic standards. In this regard,
compound 5 was synthesized following the procedure
described in Scheme 1. A commercially available mixture
of (E)- and (Z)-phytol was first oxidized to epoxides by
Scheme 1. Synthesis of (Z)- and (E)-Phyt-3-ene 4 and 5a
Quenching of radical 10 by abstraction of a hydrogen atom
from an H2S molecule would give (E)-phyt-2-ene 3 exclu-
sively and no phyt-1-ene, in agreement with the fact that
radicals generally abstract hydrogen by the less substituted
carbon when delocalization is possible.12
Formation of (Z)-phyt-2-ene 2 might be explained by
isomerization of the allylic radical 10 to allylic radical 11,
(5) J&W DB-WAX, 30 m × 0.254 mm, film thickness ) 0.15 µm.
(6) Morgans, D. J.; Sharpless, K. B.; Traynor, S. G. J. Am. Chem. Soc.
1981, 103, 462.
(7) Hwang, C. K.; Li, W. S.; Nicolaou, K. C. Tetrahedron Lett. 1984,
25, 2295.
(8) Moussebois, C.; Dale, J. J. Chem. Soc. C 1966, 260.
(9) Du Pont Zorbax ODS 250 × 9.4 mm, 8 µm; MeOH-H2O 94:6 v:v;
5 mL/min.
(10) Kornblum, N.; Carlson, S.; Smith, R. J. Am. Chem. Soc. 1978, 100,
289. Kornblum, N.; Carlson, S.; Smith, R. J. Am. Chem. Soc. 1979, 101,
7086.
(11) Cheng, C.; Stock, L. J. Org. Chem. 1991, 56, 2436. Rossi, R.;
Bunnett, J. J. Org. Chem. 1973, 38, 1407. Rossi, R.; Bunnett, J. J. Am.
Chem. Soc. 1974, 96, 112.
a Reaction conditions: (i) m-CPBA/DCM; (ii) Ti(i-OPr)4/DCM;
(iii) NaIO4/EtOH-H2O; (iv) LC separation and then DIBAH/THF;
(v) TsCl, DMAP, TEA/DCM; (vi) CH3MgI/Et2O, (vii) hν, PhSSPh/
hexane.
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Org. Lett., Vol. 5, No. 9, 2003