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and VII

Initial studies focused on a biomimetic synthesis of the tetrahydrofuran core of obtusallenes II and IV (390 and 395).¹⁰² Bromoetherification precursor 429 was prepared via regioselective Sharp-less asymmetric dihydroxylation of a doubly-skipped triene (not shown), to provide diol 429 in 28% yield and 93% e.e. This was then treated with 1.8 equivalents of TBCO to furnish two prod-ucts in a 12 : 1 diastereomeric ratio (Scheme 148). The compounds could be separated by column chromatography and the major diastereomer 430 proved to be crystalline after cleavage of the silyl protecting group. X-ray crystallography showed that this major diastereomer does in fact exhibit anti stereochemistry across the tetrahydrofuran oxygen, as is required for obtusallenes II and IV (390 and 395).

With the successful preparation of des-chloro compound 430, a method for early chloride intro-duction was targeted which should allow for the synthesis of the fully elaborated tetrahydrofuran core of obtusallenes II and IV (390 and 395).¹⁰² A skipped-diene, which was to become chloro-hydrin 431, was prepared by a double Wittig reaction of a bis-phosphonium salt with heptanal.

This provided the desired product in 96% yield and with good selectivity for the all cis isomer.

Monoepoxidation with mCPBA was followed by treatment with trimethylsilylchloride and triph-enylphosphine. Quenching this reaction with TBAF provided a mixture of two chlorohydrin re-gioisomers, which could be separated by careful column chromatography to provide the desired

OH

OH

OTBDPS Br O

OTBDPS OH

H H Br O

OTBDPS OH

H H

anti-430 syn-430

+

429

TBCO CH₂Cl₂

87% 7%

Br O

Cl

H H

C₅H₁₁ 431

Cl

OH

432 76%

TBCO CH₂Cl₂

Scheme 148:A biomimetic synthesis of the tetrahydrofuran core present in obtusallenes II and IV (390 and 395).¹⁰²

product 431. Subsequent treatment of this with TBCO in dichloromethane initiated bromoether-ification, providing the desired anti product 432 as a single diastereomer and in a yield of 76%

(Scheme 148).¹⁰²

With a method having been developed for the biomimetic synthesis of the fully elaborated tetrahydrofuran core of obtusallenes II and IV (390 and 395), attention within the group turned to investigating the proposed conversion of obtusallene II (390) into obtusallene VII (391), and indeed to determine whether the structure of this natural product should be reassigned (from 391 to 422, Scheme 147). Two model systems, 433 and 436, with the core structural motif of obtusallene II (390) were prepared by initial biomimetic tetrahydrofuran formation and subsequent ring-closing metathesis using Hoveyda-Grubbs I catalyst to construct the 11-membered macrocycles.¹¹⁶ Both 433and 436 exhibit the same relative stereochemistries at C-6, C-7, C-9 and C-10 as obtusallene II (390). They also both contain the required (E)-configured alkene at C(12)-C(13), as has been confirmed by X-ray crystallography for both model macrocycles 433 and 436. Indeed, compari-son of these X-ray crystal structures with that of obtusallene II (390) itself revealed them to have essentially the same structure aside from a local change in conformation at C(14)-O-C(4)-C(5), pre-sumably due to the absence of the bromoallene moiety in this portion of the molecule. This small change in conformation was thought to be of little consequence for the purpose of investigating the proposed interconversion.

Once chloride-containing macrocycle 433 had been prepared, it was subjected to the action of NBS in deuterated chloroform with added water (Scheme 149).¹¹⁶ After 20 days the complex mix-ture of products was separated by column chromatography and two compounds with obtusallene VII-type structure were isolated, 434 and 435. It was found that bromide rather than the antici-pated hydroxyl group was present at C-6, presumably due to the formation of molecular bromine in situ by decomposition of NBS.

X-ray crystallography of both compounds revealed that 434 is a diastereomer of 435, differ-ing in the stereochemistry at C-12 and C-13. This must arise from electrophilic bromination on the exo faces of both alkene conformers. Although the yields of these two products was low (8%) and bromide acted as a nucleophile at C-6 instead of water, compound 435 represents the core structure of obtusallene VII with all of the relative stereochemistry correctly set. This compound must arise from electrophilic bromination of the C(12)-C(13) alkene, 5-exo attack by the tetrahy-drofuran oxygen at C-12 to form an oxonium ion, with subsequent fragmentation of the oxonium

O

Scheme 149:Model studies investigating the postulated conversion of obtusallene II (390) into obtusallene VII.¹¹⁶

ion by stereospecific attack by bromide at C-6. This is in line with the hypothesis proposed by Braddock,¹⁷² and importantly the X-ray structure unambiguously places the chloride at C-7 and the bromide at C-13, which supports a reassignment of the structure of obtusallene VII to 422 as opposed to the initially reported 391.

Acetate-containing macrocycle 436 was also prepared in order to encourage conversion to the obtusallene VII-type framework by neighbouring group participation of the acetate moiety.¹¹⁶ Treatment of 436 with NBS in dichloromethane resulted in the formation of compounds 437 and 438after 4 days (Scheme 149). The compounds could be separated by column chromatography and the structure of 437 was elucidated by X-ray crystallography. The structure of 438 was deduced by converting both compounds to the corresponding bis-acetate and comparing the NMR spectra.

As the acetate is present at C-7 in 437 and C-6 in 438 the anticipated neighbouring group partici-pation of the acetate moiety must have occurred. Therefore, after formation of the bromonium ion and trapping in a 5-exo sense, the C-7 acetate must have undergone nucleophilic attack at the C-6 position of the oxonium ion, thereby ensuring an obtusallene VII-like motif. Adventitious water

would have then quenched this charged intermediate to form 437 and 438. Although the macro-cyclic skeleton produced is that of obtusallene VII, the stereochemistry at C-12 and C-13 is inverted to that which is found in the natural product and evidently arises from electrophilic bromination on the opposite face of the olefin to that required. However, this experiment still demonstrated the validity of the proposed formation of obtusallene VII from obtusallene II (390).

In an effort to improve the yields and also the reaction times a further set of conditions were investigated. Chloride-containing macrocycle 433 was treated with NBS in dichloromethane and acetic acid was used as the nucleophile, with a catalytic quantity of TMG¹³²(Scheme 149).¹¹⁶ It was found that two compounds, 439 and 440, could be isolated after column chromatography, in pleas-ing yields of 30% and 13% respectively. It was found that compound 439 exhibits the macrocyclic framework of obtusallene VII and contains halogen substituents at C-7, C-10 and C-13 and also an oxygen-based substituent at C-6, as required in the natural product. Although it was not possi-ble to assign the stereochemistry at C-12 and C-13, it is important to note that again, the chloride substituent is present at C-7 and the bromide at C-13, in contrary to what was initially reported as the structure for obtusallene VII (391).¹⁷⁹ These successful bromonium ion-induced rearrange-ments that place the bromide at C-13 instead of C-7 in accordance with Braddock’s hypothesis,¹⁷² prompted the reassignment of the structure of obtusallene VII from 391 to 422.¹¹⁶

Interestingly, the other compound 440 which was isolated from this third rearrangement reac-tion (Scheme 149) represents a structural motif, which at the time, had not been observed in any natural products isolated from Laurencia species. This compound must arise from 5-exo attack of the tetrahydrofuran oxygen onto the C-12 position of an incipient bromonium ion, as in the case of the formation of obtusallene VII-like 439. However, rather than nucleophilic attack by acetate at the C-6 position of the oxonium ion, acetate instead undergoes stereospecific S_N2 attack at C-9, thus providing the bicyclic structure of 440. This compound has had its structure and all relative stereochemistries confirmed by X-ray crystallography.

The isolation of this different structural motif prompted Braddock to propose that “it may rep-resent the core of an as yet undiscovered natural product from Laurencia species.”¹¹⁶ Recently, Souto reported the isolation of four new halogenated C₁₅acetogenins from Laurencia marilzae (Fig-ure 33).¹⁸⁰ Gratifyingly these compounds were found to have the same core structural motif as compound 440. The isolation of these compounds strongly supports the hypothesis of how the members of the obtusallene family of natural products arise.¹⁷²

O

R = H; Marilzabicycloallene A (441) R = Me; Marilzabicycloallene C (443)

O

Figure 33:The marilzabicycloallene family of natural products.¹⁸⁰

Named marilzabicycloallene A-D (441-444), it was found that at the C-13 position there is either a hydroxyl group or a chloride present, as opposed to the bromide found at this position in 440.

Souto proposes that this arises from epoxidation or chloronium ion formation on the C(12)-C(13) alkene, instead of bromonium ion formation (Scheme 150).¹⁸⁰ Epoxidation on the exo face of the C(12)-C(13) olefin of the major conformer of obtusallene II (390) forms compound 445. This can un-dergo 5-exo attack by the tetrahydrofuran oxygen to provide tricyclic oxonium ion 446, in an anal-ogous way to the rearrangement to form obtusallene VII (422). Finally, stereospecific nucleophilic attack by an oxygen-based nucleophile at C-9 results in fragmentation of the oxonium ion to form marilzabicycloallene A (441) or marilzabicycloallene C (443). Alternatively, electrophilic chlorina-tion on this same alkene conformer of obtusallene II (390) results in the formachlorina-tion of chloronium ion 447. Attack in a 5-exo manner by the tetrahydrofuran oxygen provides oxonium ion 448, which subsequently undergoes fragmentation by chloride attack at C-9, providing marilzabicycloallene D (444).

Analysis of the stereochemistry of the bromoallene moiety and also the C-4 position of marilz-abicycloallene B (442) reveals that it must arise from obtusallene IV (395). Epoxidation of the exo face of the minor conformer of the C(12)-C(13) olefin forms compound 449 (Scheme 150). Again, this can undergo 5-exo attack by the well-placed oxygen of the tetrahydrofuran to form tricyclic oxonium ion 450. Finally, attack by water at C-9 in a stereospecific manner results in the formation of marilzabicycloallene B (442). The structure of epoxide 449 looks remarkably similar to that of 12-epoxyobtusallene IV (396), a natural product recently reported by the same research group (Figure 31).¹⁷³Analysis of the stereochemistry of the epoxide moiety and also that of the bromoallene motif and C-4 position, reveals that 12-epoxyobtusallene IV (396) must arise from epoxidation of the exo face of the C(12)-C(13) olefin of the major conformer of obtusallene IV (395).¹⁸⁰ The recently dis-covered natural product 402¹⁷³must arise from subsequent rearrangement of 12-epoxyobtusallene IV (396) by virtue of the stereochemistry at the C-4 position and also the C(12)-C(13) epoxide being

O

R = H; Marilzabicycloallene A (441) R = Me; Marilzabicycloallene C (443)

O

Scheme 150:Proposed biogenesis of the marilzabicycloallene family of natural products.¹⁸⁰ identical (Scheme 151). Epoxide formation on the bromoallene of 12-epoxyobtusallene IV (396) would provide 404, which after neighbouring group participation by the bromide would furnish bromonium ion 454. This is then ready to undergo the key rearrangement, where a hydride mi-grates from C-1 to C-2, with ring-opening of the bromonium ion, to form α,β-unsaturated 455.

Hydrolysis of the acid bromide followed by methylation of the resulting acid would furnish the natural product, 402.

The only remaining natural product that has not had a biogenesis proposed for it is

obtusal-lene X (401). This compound was also discovered by Souto¹⁷³ and reported in 2011. Analysis of the stereochemistry around the bromoallene motif reveals that obtusallene X (401) must arise from obtusallene IV (395). It is possible to envisage two biogenetic routes to this natural product, one involving electrophilic bromination and the other via 12-epoxyobtusallene IV (396) (Scheme 151).

The (12R,13R)-configuration of the natural product, along with the fact that the alkene present in obtusallene IV (395) is trans reveals that obtusallene X (401) must be formed from a (12R,13R)-configured epoxide or bromonium ion. Therefore either the C-12 or C-13 position must undergo double inversion in order to preserve the (R,R)-stereochemistry. This precludes the formation of the bromohydrin in obtusallene X (401) via direct opening of a bromonium ion with water. Pre-sumably this method of bromohydrin formation could not occur anyway as the tetrahydrofuran portion of the molecule would encumber backside attack onto a bromonium ion formed on the exo

O

Obtusallene IV (395) 12-Epoxyobtusallene IV (396) O

Scheme 151:Proposed biogenesis of 12-epoxyobtusallene IV (396),¹⁸⁰402and obtusallene X (401) from obtusallene IV (395).

face of the C(12)-C(13) olefin.

However, after bromonium ion formation (451) on the exo face of the alkene of the major con-former of obtusallene IV (395), the tetrahydrofuran oxygen could undergo nucleophilic attack at C-13 in a 6-endo sense to provide tricyclic oxonium ion 452 (Scheme 151). Subsequent stereospecific attack by water at C-13 would result in fragmentation of the oxonium ion to provide the required (12R,13R)-bromohydrin of obtusallene X (401). Alternatively, 5-exo attack of the tetrahydrofuran oxygen of 12-epoxyobtusallene IV (396) onto the C-12 position of the epoxide would produce tri-cyclic oxonium ion 453. Subsequent S_N2 attack by bromide at the C-12 position of the oxonium ion would result in the formation of obtusallene X (401) with all of the relative stereochemistry correctly set.

With this proposal for the biogenesis of these newly discovered natural products (398, 401 and 402) along with Souto’s proposed biogenesis for the recently reported marilzabicycloallenes (441-444),¹⁸⁰ and Braddock’s original proposal¹⁷²for the biogenesis of obtusallenes I-IX (390-400), a consistent and tested^{102, 116} hypothesis for the biogenesis of all known members of the wider obtusallene family of natural products has been described. All that remains is to address the dis-crepancies between the reported structures of obtusallenes V, VI, VIII and IX (397, 398, 399 and 400) and the structures that are predicted by the proposed biogenesis (424, 425, 427 and 428).¹⁷²

The reported structures of the natural products obtusallenes V and VI (397 and 398) place a bromide substituent at C-7 and a chloride substituent at C-13. However, Braddock’s proposed bio-genesis requires chloride to be present at C-7 and bromide at C-13.¹⁷² In 2008 Braddock and Rzepa reported the use of GIAO-based density functional prediction to provide evidence to support the reassignment of these structures.¹⁸¹Calculated¹³C NMR chemical shifts for all 15 carbon atoms in the reported structure of obtusallene V (397) were compared to the reported values. It was found that the calculated shifts were in good agreement with those reported, apart from at the C-7 and C-13 positions. These were found to differ by approximately 12-13 ppm from the reported values.

When the same process was repeated for Braddock’s proposed structure for obtusallene V (424), it was found that the discrepancy in chemical shift for these carbons decreased to approximately 0.5-1.5 ppm. As it is these positions that bear the halogen substituents in question, this computational study consequently provides strong evidence for the positioning of chloride at C-7 and bromide at C-13, in line with Braddock’s proposed structure (424). The same affects were observed when the reported (398) and proposed (425) structures for obtusallene VI were subjected to this