2476
J . A m . Chem. SOC.1984, 106, 2416-2411 -
0 5
\
c
A
Outside
3 c
B
0 4
" :03 D
H2°
500
400
400
700
600
W a v e l e n g t h , nm
500
700
600
Inside
Wavelength, nm
Figure 1. (A) Absorption spectra of 2 X IO-' M DHP vesicle-entrapped 1X M Cd2+and approximately 3.2 X M Rh3+: (-) after
after exposure to H2S (formation of removal of external cations, after 60 min of UV irradiation (formation of Rho). (B) CdS), Fluorescence emission spectra of 2 X lo-' M DHP vesicle-entrapped 1 X lo4 M colloidal CdS, under 330-nm excitation: (-) in the absence (before and after UV irradiation under Ar bubbling) ( - - - ) and in the presence of approximately 3.2 X M Rh'+. (e-)
(-.-e)
lo5 X
10
dihexadecylphosphate
vesicles should not be taken for granted, however. (Figure 3). After 48 h, more than 90% of PhSH was consumed. Sustained hydrogen formation is the consequence of electron transfer from PhSH, presumably located in the D H P membranes, to the positive holes in the colloidal CdS. This, in turn, diminishes undesirable electron-hole recombinations and allows electron transfer to the surface of the semiconductor where Rho-catalyzed water reduction occurs (Figure 4). In the absence of either CdS, Rho, or PhSH, no or very little amount of H2 is produced. Optimization of surfactant vesicle-entrapped catalyst-coated semiconductors is the objective of our current scrutiny. These systems, along with polymer membrane incorporated s e m i c o n d ~ c t o r s , ' ~ J ~ may well provide means for viable solar energy conversion.
initial CRh3+1, M
5
O== =
Figure 4. An idealized model for CdS-sensitized water photoreduction by PhSH in aqueous DHP vesicles. The position of the colloid in the
15
Acknowledgment. Support of this work by the United States Department of Energy is gratefully acknowledged. 1
J
2
4
6
0
lo5X
CMV2+1, M Figure 2. Stern-Volmer plots for the quenching of 2 X IO-) M DHP vesicle-entrapped 1 X lo4 M colloidal CdS by coentrapped MVZ*(0), coentrapped Rh3+(O),and externally adsorbed MV2+ ( 0 ) .
Registry No. Dihexadecyl phosphate, 21 97-63-9; thiophenol, 108-98-5; rhodium, 7440-16-6; cadmium sulfide, 1306-23-6;hydrogen, 1333-74-0. (14) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96. 34. (15) Krishnan, M.; White, J. R.; Fox, M. A,; Bard, A. J J . A m . Chem. Soc. 1983, 105, 7002.
Isolation from Pistacia Resins of a Bicyclic Triterpenoid Representing an Apparent Trapped Intermediate of Squalene 2,IEpoxide Cylization Robin B. Boar,*' Lorraine A. Couchman, Alan J . Jaques, and M. John Perkins
-
10
Department of Chemistry, Chelsea College London S W3 6LX, England Received December 12, 1983 Revised Manuscript Received February 20, 1984
+-
,
20
30
40
Irradiation time, h Figure 3. Hydrogen production in deaerated solution as a function of irradiation time using 350-nm cutoff and water filters at 30 OC. Plotted are the amount of hydrogen produced by a 25-mL DHP vesicle solution and measured in the gas phase (16 mL) by GPLC: 2 X M DHP, 1X M CdS, rhodium coated as described in the text. In the presence of lo-' M PhSH and at pH approximately 7, adjusted before irradiation ( 0 ,0; separate samples, indicating our reproducibility); same sample in the absence of CdS and Rh (+) or in the absence of Rh only (m) or in the absence of PhSH only (0). by varying the initial concentration of CdCI2 from 0 to I X lo4 M. Irradiation of degassed, vesicle-entrapped, rhodium-coated, colloidal CdS by visible light (450-W xenon lamp, 350-nm cutoff M PhSH resulted in hydrogen filter) in the presence of formation, which could be sustained for approximately 48 h 0002-1863 18411506-2416$01.50/0 , ,
I
Following the establishment of the role of squalene and more specifically (3S)-squalene 2,3-epoxide213 in the biosynthesis of polycyclic triterpenoids, we have found that there has been considerable interest and speculation concerning the exact nature of this remarkable biotransformation. The cyclization was originally envisaged as a completely concerted p r o c e ~ s a, ~view supported by the failure to trap or otherwise detect any intermediates. However, the most recent work of van Tamelen and his co-workers (1) Present address: Janssen Pharmaceutica, 2340 Beerse, Belgium. (2) Willett, J. D.; Sharpless, K. B.; Lord, K. E., van Tamelen, E. E.; Clayton, R. B. J . Biol. Chem. 1967, 242, 4 1 8 2 . (3) Barton, D. H. R.; Jarman, T. R.; Watson, K. C.; Widdowson, D. A.; Boar, R. B.; Damps, K. J . Chem. Soc., Perkin Trans. 1 1975, 1134. (4) Eschenmoser, A,; Ruzicka, L.; Jeger, 0.;Arigoni, D. Helc. Chim. Acta. 1955, 38, 1890.
0 1984 American Chemical Society
2477
J . Am. Chem. SOC. 1984, 106, 2477-2479
151 x=o 161 X=H2
171
of (6) (KMn04-H20/PhH-n-Bu4N+Br-; room temperature) gave +33' (c 0.45 in (+)-ambreinolide (7), mp 143-144 OC, DI.[ CHCI,), identical in every respect with an authentic sample." While other partially cyclized triterpenoids have been described, these normally appear to be products of aberrant modes of cyclization" (or laboratory synthesisI2). One example is the aforementioned ambrein, in which partial cyclization has occurred at both ends of the squalene chain. In contrast, the diol 2 is the first6 example of a bicyclic triterpenoid that retains all of the regie and stereochemical features necessary for continued cyclization and occurs in a system that clearly has the enzyme(s) needed for more complete cyclization.
- I , , , , * -
