After observing the excellent selectivity for ether formation in the reaction of model substrate 131with NBS in water, the same conditions were used for the first attempted cyclisation of epoxy-diene 160. It was thought that the hydrophobic effect would encourage the largely lipophilic molecule to undergo bromonium ion-assisted epoxide ring-opening, in the same way as in the case of cyclooctene epoxide 131. Epoxy-diene 160 was stirred in water for approximately 20 min-utes, prior to the addition of catalytic TMG and NBS. The concentration of epoxy-diene 160 was kept relatively low at 0.01 M, in an effort to discourage intermolecular attack of the C-4 alcohol
OH dibromohydrins
10%
Scheme 77:Distribution of products after treatment of epoxy-diene 160 with NBS in water.
onto a bromonium ion formed on a second molecule of 160. After stirring for 3 days at room temperature, the reaction was worked-up and the crude mixture was observed to comprise 9 com-ponents by TLC. These comcom-ponents were successfully separated by column chromatography, and the compounds subsequently identified to be those shown in Scheme 77.
All of the compounds produced in this reaction were found to be epoxide-containing, by virtue of the1H NMR spectra containing two signals at 2.9-3.3 ppm which were found by HSQC spectra to correlate to signals at approximately 55 ppm in the13C NMR spectra. It would be anticipated that for the medium-ring ethers, signals for the α-protons will appear at a higher chemical shift of approximately 3.4-4.5 ppm, and the13C NMR spectra will contain signals at approximately 70-90 ppm due to the ether. Therefore, the desired cyclisation via bromonium ion-assisted epoxide ring-opening with water was unsuccessful.
OH O
OH Br O
Br
H2O
OH O
OH Br
OH O
Br OH
(2 diastereomers) (2 diastereomers)
224
225 160
4
13 12
13 12
13 12 6
9 15 12
Scheme 78:Mechanism for formation of bromohydrins 224 and 225.
The compounds isolated in the greatest yield from the reaction were bromohydrin regioisomers 224and 225. These bromohydrins result from bromonium ion formation on the C(12)-C(13) olefin and subsequent ring-opening by nucleophilic attack of water at either C-12 to give 225, or C-13 to give 224 (Scheme 78). Two diastereomers of each regioisomer were produced due to formation of the bromonium ion on opposite faces of the olefin, and stereospecific ring-opening. It was possible to separate the four compounds by preparative HPLC to allow characterisation, but so far it has not been possible to fully assign relative stereochemistry within each isolated diastereomer. Inter-estingly, bromohydrin 224 which results from nucleophilic attack by water at C-13 was isolated by HPLC in a yield of 87 mg (13%), whereas only 18 mg (3%) of bromohydrin 225, which results from attack on the bromonium ion at C-12, was isolated.
The identity of the bromohydrin regioisomers was elucidated using a range of NMR experi-ments. COSY spectra confirmed that it was in fact the C(12)-C(13) olefin that had been attacked,
Figure 16:Use of the bromine isotope effect in13C NMR to prove the identity of the bromide-bearing carbon.
as opposed to the C(9)-C(10) olefin, which remained intact in these compounds. In the case of regioisomer 224, analysis of the HSQC spectrum of one of the diastereomers, in conjunction with the COSY spectrum showed that the C-13 carbon had a chemical shift of 75.0 ppm, whereas the resonance for the C-12 carbon appeared at a shift of 60.6 ppm. A chemical shift of 75.0 ppm is typ-ical for a secondary alcohol, which demonstrated that the hydroxyl is present at C-13. The signal at 60.6 ppm is in the same region in which the primary alcohol in the molecule would be expected to resonate. Analysis of the DEPT-135 spectrum confirmed that the signal at 60.6 ppm in the13C NMR spectrum was in fact due to a CH or CH3 carbon, whereas the signal at 60.5 ppm is due to a CH2 carbon. Finally, the presence of bromide at C-12 was confirmed by the observation of a bromine isotope effect on the signal corresponding to this carbon (Figure 16). Enhanced resolution of this area of the spectrum showed two signals of equal intensity split by 2 ppb, due to the two naturally occurring isotopes of bromine. HRMS confirmed the proposed molecular formula of the compound. The structures of all four compounds with a bromohydrin present at the C(12)-C(13) position were elucidated using this process, and bromine isotope effects were observed in all but
one case, confirming the identity of the bromide-bearing carbon.
There was also a 4% yield of what is believed to be bromohydrin regioisomers 226 and 227.
The1H and13C NMR spectra of the crude mixture of compounds looks very similar to that of the crude mixture of bromohydrins 224 and 225, and the Rf value was very similar. However, it has not proven possible to separate the compounds by preparative HPLC in order to aid identifica-tion. These bromohydrins will have been produced by bromonium ion formation at the C(9)-C(10) olefin, followed by nucleophilic ring-opening by water at C-9 or C-10 to give 227 or 226 respec-tively. There can potentially be two diastereomers of each regioisomer formed, due to formation of the bromonium ion on different faces of the olefin. It is interesting to note that there was a signifi-cantly greater yield of bromohydrins formed from the C(12)-C(13) olefin, compared to those from the C(9)-C(10) olefin.
OH Br O
Br
OH Br O
Br
OH O
Br Br
160 228
13 12 13
12
13 12
Scheme 79:Mechanism to show the reaction of epoxy-diene 160 with molecular bromine.
Also isolated from the reaction mixture in a yield of 1%, was dibromide-containing 228 as a mixture of two diastereomers. This compound is produced by the reaction of epoxy-diene 160 with molecular bromine, which is formed via radical decomposition of NBS in the presence of light. Bromonium ion formation would have occurred on either face of the C(12)-C(13) olefin and then stereospecific nucleophilic ring-opening by the bromide anion at either C-12 or C-13 would have led to the formation of the two diastereomers (Scheme 79). Analysis of the COSY spectrum confirmed that it was the C(12)-C(13), rather than the C(9)-C(10) olefin that was attacked.
The HSQC NMR spectrum showed that the signals for both the C-9 and C-10 carbons were at a chemical shift of approximately 60 ppm in the13C NMR spectrum. As demonstrated previously, this appears to be a typical chemical shift for carbon atoms bearing secondary bromides in these compounds. Finally, the ammonium CI+ mass spectrum of the compound showed three signals for the molecular ion at m/z 372, 374 and 376, in a 1 : 2 : 1 ratio which is as would be expected for a compound containing two bromine atoms, due to the two naturally occurring isotopes of bromine.
HRMS supported the proposed molecular formula for the diastereomers.
Four diastereomers of tetrahydrofuran 229 were produced in the reaction and they were
iso-yield of 2%. These compounds were presumably produced from bromohydrin 224, by attack of the C-13 hydroxyl onto an incipient bromonium ion formed on the C(9)-C(10) olefin (Scheme 80).
Cyclisation presumably occurred in a favourable 5-exo manner to furnish the tetrahydrofuran 229.
Four diastereomers of this compound will have arisen due to the fact that there are two diastere-omers of bromohydrin 224 produced, and each of these is capable of undergoing cyclisation onto a bromonium ion which can be formed on two faces of the C(9)-C(10) olefin. Consequently, each bromohydrin diastereomer gives rise to two diastereomers of tetrahydrofuran 229.
OH O
OH Br
OH O
OH Br Br Br
O Br
H H Br
OH O
5-exo 224
229 (4 diastereomers)
13 12 10 9
13 12 10 9 13
12 10 9
Scheme 80:Mechanism for the formation of tetrahydrofurans.
The identity of the compounds was again elucidated largely using a combination of COSY and HSQC NMR spectra. 1H and13C NMR spectra showed that the compounds were not alkene con-taining, but from the TLC of the reaction mixture it appeared that the compounds were not dibro-mohydrins, as they were less polar than the monobromohydrins that were isolated. Ammonium CI+ mass spectrometry also confirmed that a second water molecule had not been incorporated to form dibromohydrins. However, the compounds did appear to contain two bromine atoms, by virtue of there being three signals present for the molecular ion, in a 1 : 2 : 1 ratio. In the COSY NMR spectra, the H-14 protons were found to couple with protons reponsible for a multiplet at approximately 4 ppm. This was found to correlate to signals at 87-88 ppm in the13C NMR spectra, which would be a typical chemical shift for an α-carbon in a cyclic ether. The protons responsible for this multiplet were found to couple with protons that produced a signal at approximately 3.8 ppm in the1H NMR spectrum. This correlated to signals at 46 ppm in the13C NMR spectrum, which were confirmed to be for CH or CH3 carbons by DEPT-135, as opposed to CH2units. This would be consistent with a carbon bearing a secondary bromide as part of the tetrahydrofuran.
The proton attached to this carbon was found to couple to a methylene unit, which subse-quently coupled to a proton which had a chemical shift of approximately 4 ppm. HSQC NMR spectra showed that this proton is directly attached to a carbon that appears in the ether region of the13C NMR spectrum. Finally, this proton was found to couple to another proton which had a chemical shift in the same region of the1H NMR spectrum. This proton was found to be attached
to a carbon that had a chemical shift of approximately 54 ppm, which would be consistent for the second bromide-bearing carbon in the molecule. Analysis of the COSY and HSQC NMR spectra shows coupling through to the epoxide and finally the C-4 primary alcohol, as expected.
Interestingly, tetrahydrofuran 229 comprises the core structure of laureepoxide (230), which is another natural product isolated from Laurencia species.134 Laureepoxide (230) presumably arises via bromohydrin formation, followed by ring-opening of an incipient bromonium ion in a mech-anism similar to that which is proposed in Scheme 80. Analysis of the stereochemistry reveals that laureepoxide (230) would be produced from (9E,12E)-configured skipped epoxy-diene 231 (Scheme 81). This has been the only product isolated from Laurencia species to date that contains both an epoxide and a tetrahydrofuran. It is possible that this is due to the geometry of the C(9)-C(10) olefin. Epoxy-dienes containing a (Z)-alkene at the C(9)-C(9)-C(10) position are proposed to be the biogenetic precursors to the halogenated medium-ring ethers, and it is believed that the (Z)-alkene gives some conformational constraint. In laureepoxide (230), the (E)-alkene at the C(9)-C(10) posi-tion would prevent attack of the epoxide onto a bromonium ion formed at the C(12)-C(13) olefin.
Therefore, this route to form medium-ring ethers is not available, and bromohydrin formation, followed by cyclisation to a 5-membered ring ether occurs instead.
O O
H Br
Br
H O
1 4 9 6
15
R S 1
4 6 15 9
R S
R S
R S
Laureepoxide (230)
231
12
12
Scheme 81:Laureepoxide (230) and its proposed biogenetic precursor.
There was also an unidentified epoxy-diene compound isolated in a yield of 25 mg, as well as a 10% yield of a mixture of dibromohydrins. As there was no observation of medium-ring ether formation in this reaction, this could be due to the presence of an alcohol in the epoxy-diene start-ing material 160. It was anticipated that by conductstart-ing the reaction in water, the hydrophobic effect would have a large impact in encouraging nucleophilic attack of the epoxide onto a bromo-nium ion. However, the presence of the alcohol may have rendered the molecule not sufficiently lipophilic. Consequently, hydrogen-bonding of the alcohol with water may have restricted the abil-ity of the molecule to cyclise. In order to overcome this, there are two potential solutions: firstly the alcohol could be converted into something that is less able to interact with the water, and sec-ondly, the reaction could be conducted in a solvent that is less able to form hydrogen-bonds with
the alcohol.
3.4.2 Treatment of Epoxy-Diene 160 with Electrophilic Bromine in THF with Water as the