A Concise Diastereoselective Total Synthesis of α-Ambrinol

(−)-cis-α-Ambrinol is a natural product present in ambergris, a substance of marine origin that has been highly valued by perfumers. In this paper, we present a new approach to its total synthesis. The starting material is commercially available α-ionone and the key step is an intramolecular Barbier-type cyclization induced by CpTiCl2, an organometallic compound prepared in situ by a CpTiCl3 reduction with Mn.


Introduction
Ambergris is a solid that forms pathogenically in the digestive tract of sperm whales (Physeter macrocephalus). Although it has an unpleasant odor when fresh, as it dries and ages it emits a pleasant and subtle fragrance, which is the reason why it is highly prized in the perfume industry. On average, ambergris is present in one out of every hundred sperm whales of both sexes. It has been documented that of the 1933 sperm whales captured between 1934 and 1953, only 19 had ambergris, with a total weight of approximately 1155 kg [1]. Due to the prized properties of ambergris as a fixative and stabilizer in perfumery, its chemical composition has been the subject of numerous studies. Although in variable proportions, the main constituents include a group of steroids with a cholestane skeleton, and the squalene-derived triterpene ambrein (2). In fact, although ambrein (2) has been prepared in vitro by enzymatic cyclization of squalene (1) (Scheme 1a), 13 C isotopic ratio studies suggest that its biosynthesis [2], unlike the co-occurring steroids, takes place with the participation of some bacteria and it is not exclusive to the sperm whale, a result which is in line with considering ambergris as a product resulting from a pathology of the marine mammal [3].
Although ambrein (2) is an odorless compound, the aroma of ambergris is due to products formed through a natural oxidative degradation, possibly as a result of its exposure to air, seawater and sunlight, catalyzed by the presence of copper. This metal could be present because it is in the hemocyanin of the blood of squids, the main food of sperm whales [1]. This oxidative degradation process generates highly appreciated products in perfumery known as ambroxides, substances that are usually obtained by distillation and that usually represent less than 1% by weight of the raw material. These include ambrafuran (3), also called ambroxide (ambrox TM ), and other related compounds such as ambraoxide (4), ambracetal (5), and cis-α-ambrinol (6) (Scheme 1b) [4,5]. Solid phase micro extraction (SPME) and gas chromatography-mass spectrometry (GC-MS) have allowed the identification of some of the more volatile natural components of ambergris, such pristane, a fully saturated acyclic odorless nor-diterpene and γ-dihydroionone (7) [6], which has a similar structure to ambraaldehyde (8) (Scheme 1b) [7]. The stereochemistry of ambrein (2) has been well established by chemical correlation methods [8]. In addition, the configuration of the tri-and tetracyclic natural products (3)(4)(5) derives from the bicyclic part or ambrein (2) (in red in Scheme 1), while bi-and monocycles (6)(7)(8) derive from the left-hand-side monocyclic part of ambrein (in blue) [9]. cis-α-Ambrinol (6) has long attracted the attention of synthetic chemists due to its odor, which has been related to damp earth with a crude civet subnote [9]. It can be easily prepared by acid-catalyzed cyclization of γ-ionone, although in the process, β-ambrinol (9) and other cyclization products (10) are formed (Scheme 2a) [10].

Results and Discussion
As part of our continued effort in the preparation of marine natural products [14][15][16], and due to the fact that all the previously reported synthesis of cis-α-ambrinol (6) rely on an acid cyclization step that affords a mixture of regioisomers which is not easily purified by conventional chromatographic technics, we were interested in developing a concise diastereoselective total synthesis of cis-α-ambrinol (6).
The synthesis of compound 6 was planned according to the retrosynthesis depicted in Scheme 4 through two alternative pathways, both of them using commercially available α-ionone (19) as a starting material. The first approach has as a key step a diastereoselective cyclization of the allylic carbonate (20) using the bimetallic system Cp2Ti III Cl/Pd 0 Scheme 2. Representative racemic or enantioselective syntheses of α-ambrinol from γ-dihydroionone. (a) Acid-catalyzed synthesis [10]; (b) resolution by derivatization [11]; (c) resolution with lipase [11]; (d) biocatalysis and terpene cyclase [12].

Results and Discussion
As part of our continued effort in the preparation of marine natural products [14][15][16], and due to the fact that all the previously reported synthesis of cis-α-ambrinol (6) rely on an acid cyclization step that affords a mixture of regioisomers which is not easily purified by conventional chromatographic technics, we were interested in developing a concise diastereoselective total synthesis of cis-α-ambrinol (6).
The synthesis of compound 6 was planned according to the retrosynthesis depicted in Scheme 4 through two alternative pathways, both of them using commercially available α-ionone (19) as a starting material. The first approach has as a key step a diastereoselective cyclization of the allylic carbonate (20) using the bimetallic system Cp2Ti III Cl/Pd 0 Scheme 3. Enantioselective syntheses of cis-α-ambrinol through a Ti(III)-catalyzed radical process [13].

Results and Discussion
As part of our continued effort in the preparation of marine natural products [14][15][16], and due to the fact that all the previously reported synthesis of cis-α-ambrinol (6) rely on an acid cyclization step that affords a mixture of regioisomers which is not easily purified by conventional chromatographic technics, we were interested in developing a concise diastereoselective total synthesis of cis-α-ambrinol (6).
The synthesis of compound 6 was planned according to the retrosynthesis depicted in Scheme 4 through two alternative pathways, both of them using commercially available α-ionone (19) as a starting material. The first approach has as a key step a diastereoselective cyclization of the allylic carbonate (20) using the bimetallic system Cp 2 Ti III Cl/Pd 0 (Scheme 4a). The second one (Scheme 4b), is based on a Barbier-type CpTi III Cl 2 -catalyzed intramolecular allylation of the chlorinated derivative (22).  The synthetic route based on the bimetallic system Ti(III)/Pd(0) is depicted in Scheme 5. The first step is the regioselective reduction of the conjugated double in α-ionone (19), following a modification of a previously described methodology [17], to give α-dihydroinone (12). Epoxidation of (12) with m-CPBA afforded the epoxide (21) as a diastereoselective mixture cis:trans (86:12) in an 98% yield. Acid treatment of the cis-epoxide (cis-21) gave 1-hydroxy-γ-dihydroionone (23) in a 62% yield. The formation of carbonate (20) was carried out using ethyl chloroformate under basic conditions, yielding the desired compound (20) in an 80% yield. The key step of this synthetic approach relies on a cooperative catalytic method [18,19]. We first tried the reaction using the combination Cp2TiCl/Ni(PPh3)2Cl2, a bimetallic system which has been described as an efficient promoter for the allylation of carbonyl compounds [19], although with little success in our case, as the process proved to be unproductive. However, treatment of allylic carbonate (20) with a source of Ti(III)/Pd(0) [18] (see materials and methods section for details) gave a mixture of the desired natural cis-α-ambrinol (6) (37% yield), trans-α-ambrinol (24) (14% yield), and the monocyclic compound (25) (9% yield).  The synthetic route based on the bimetallic system Ti(III)/Pd(0) is depicted in Scheme 5. The first step is the regioselective reduction of the conjugated double in α-ionone (19), following a modification of a previously described methodology [17], to give α-dihydroinone (12). Epoxidation of (12) with m-CPBA afforded the epoxide (21) as a diastereoselective mixture cis:trans (86:12) in an 98% yield. Acid treatment of the cis-epoxide (cis-21) gave 1-hydroxy-γ-dihydroionone (23) in a 62% yield. The formation of carbonate (20) was carried out using ethyl chloroformate under basic conditions, yielding the desired compound (20) in an 80% yield. The key step of this synthetic approach relies on a cooperative catalytic method [18,19]. We first tried the reaction using the combination Cp 2 TiCl/Ni(PPh 3 ) 2 Cl 2 , a bimetallic system which has been described as an efficient promoter for the allylation of carbonyl compounds [19], although with little success in our case, as the process proved to be unproductive. However, treatment of allylic carbonate (20) with a source of Ti(III)/Pd(0) [18] (see Section 3 for details) gave a mixture of the desired natural cis-α-ambrinol (6) (37% yield), trans-α-ambrinol (24) (14% yield), and the monocyclic compound (25) (9% yield).  The synthetic route based on the bimetallic system Ti(III)/Pd(0) is depicted in Scheme 5. The first step is the regioselective reduction of the conjugated double in α-ionone (19), following a modification of a previously described methodology [17], to give α-dihydroinone (12). Epoxidation of (12) with m-CPBA afforded the epoxide (21) as a diastereoselective mixture cis:trans (86:12) in an 98% yield. Acid treatment of the cis-epoxide (cis-21) gave 1-hydroxy-γ-dihydroionone (23) in a 62% yield. The formation of carbonate (20) was carried out using ethyl chloroformate under basic conditions, yielding the desired compound (20) in an 80% yield. The key step of this synthetic approach relies on a cooperative catalytic method [18,19]. We first tried the reaction using the combination Cp2TiCl/Ni(PPh3)2Cl2, a bimetallic system which has been described as an efficient promoter for the allylation of carbonyl compounds [19], although with little success in our case, as the process proved to be unproductive. However, treatment of allylic carbonate (20) with a source of Ti(III)/Pd(0) [18] (see materials and methods section for details) gave a mixture of the desired natural cis-α-ambrinol (6) (37% yield), trans-α-ambrinol (24) (14% yield), and the monocyclic compound (25) (9% yield). The relative stereochemistry of both isomers cis-α-ambrinol (6) and trans-α-ambrinol (24) was determined with the aid of NOE experiments (Figure 1). Especially relevant is the presence of correlations in (6) between the equatorial CH 3 -13 (δ = 1.24 ppm) and all four hydrogens in CH 2 -7 and CH 2 -9 while in (24), the observed correlations between the axial methyl CH 3 -13 (δ = 1.14 ppm) are only those with the β H (equatorial) of CH 2 -7 and CH 2 -9 in each case.
Although the yield of (6) was not completely satisfactory, the C-C bond formation reaction proceeds with high diastereoselectivity, and with simultaneous formation of the desired trisubstituted double bond. Spectroscopic data for synthetic cis-α-ambrinol (cis-6) were identical to those of the natural compound [20][21][22].  The experimental results obtained in the cyclization of (20) with the Ti(III)/Pd(0) bimetallic system can be fully explained by the mechanism tentatively proposed in Scheme 6. Initially, an oxidative addition of the allylic electrophile carbonate (20) to Pd 0 would give the corresponding ƞ 3 -allyl-palladium intermediate I. Monoelectronic reduction of intermediate I by Cp2TiCl generates a ƞ 3 -allyl-palladium(II) intermediate, which could give the carbon-centered radical intermediate II, while the Pd 0 complex is regenerated. This radical intermediate II could be trapped by a second molecule of Cp2TiCl to form two alkyl-Ti IV species in metallotropic equilibrium (III and IV). β-Elimination of hydrogen in species III would account for the formation of monocyclic diene (25) and Cp2TiCl(H). It has been previously reported that this titanium hydride spontaneously decomposes to regenerate Cp2TiCl and molecular hydrogen [23]. On the other hand, the least sterically hindered alkyl-Ti IV intermediate IV can evolve through two different rotational conformers, IVa and IVb. In one of them, IVa, the carbonyl group oxygen is arranged spatially close to the titanium atom (axial-like orientation), and therefore the intramolecular nucleophilic addition of the alkyl-Ti IV to the ketone favors the diastereoselective formation of cis-α-ambrinol (6) as the main product. However, if the ketone oxygen is located at a Although the yield of (6) was not completely satisfactory, the C-C bond formation reaction proceeds with high diastereoselectivity, and with simultaneous formation of the desired trisubstituted double bond. Spectroscopic data for synthetic cis-α-ambrinol (cis-6) were identical to those of the natural compound [20][21][22].
The experimental results obtained in the cyclization of (20) with the Ti(III)/Pd(0) bimetallic system can be fully explained by the mechanism tentatively proposed in Scheme 6. Initially, an oxidative addition of the allylic electrophile carbonate (20) to Pd 0 would give the corresponding η 3 -allyl-palladium intermediate I. Monoelectronic reduction of intermediate I by Cp 2 TiCl generates a η 3 -allyl-palladium(II) intermediate, which could give the carbon-centered radical intermediate II, while the Pd 0 complex is regenerated. This radical intermediate II could be trapped by a second molecule of Cp 2 TiCl to form two alkyl-Ti IV species in metallotropic equilibrium (III and IV). β-Elimination of hydrogen in species III would account for the formation of monocyclic diene (25) and Cp 2 TiCl(H). It has been previously reported that this titanium hydride spontaneously decomposes to regenerate Cp 2 TiCl and molecular hydrogen [23]. On the other hand, the least sterically hindered alkyl-Ti IV intermediate IV can evolve through two different rotational conformers, IVa and IVb. In one of them, IVa, the carbonyl group oxygen is arranged spatially close to the titanium atom (axial-like orientation), and therefore the intramolecular nucleophilic addition of the alkyl-Ti IV to the ketone favors the diastereoselective formation of cis-α-ambrinol (6) as the main product. However, if the ketone oxygen is located at a greater distance from the titanium atom (equatorial-like orientation, IVb), the interaction between both atoms should be slightly weaker, resulting in a slower nucleophilic addition of the alkyl-Ti IV intermediate greater distance from the titanium atom (equatorial-like orientation, IVb), the interaction between both atoms should be slightly weaker, resulting in a slower nucleophilic addition of the alkyl-Ti IV intermediate (IVb) to the carbonyl group, and further leading to the formation of the diastereoisomer trans-α-ambrinol (24) as a minor product.
The synthetic advantage of this synthetic route is that it could lead to the enantiopure (-)-6 using an enantiopure alcohol (-)-23. This was previously prepared by Serra [10] using a lipase-mediated racemic resolution.
Our second synthetic approach is based on a Barbier-type Ti(III)-catalyzed intramolecular allylation of an allyl chloride, which is summarized in Scheme 7. In this case, we used a starting material α-dihydroionone (12), previously prepared in the other route. Chlorination of 12 with NaClO afforded a diastereomeric mixture of allylic chlorides (22) in a 96% yield. With this substrate in hand, we first tried to induce the intramolecular allylation using an excess of Zn dust, which is known to react with allyl chlorides to form The synthetic advantage of this synthetic route is that it could lead to the enantiopure (−)-6 using an enantiopure alcohol (−)-23. This was previously prepared by Serra [10] using a lipase-mediated racemic resolution.
Our second synthetic approach is based on a Barbier-type Ti(III)-catalyzed intramolecular allylation of an allyl chloride, which is summarized in Scheme 7. In this case, we used a starting material α-dihydroionone (12), previously prepared in the other route. The synthetic advantage of this synthetic route is that it could lead to the enantiopure (-)-6 using an enantiopure alcohol (-)-23. This was previously prepared by Serra [10] using a lipase-mediated racemic resolution.
Our second synthetic approach is based on a Barbier-type Ti(III)-catalyzed intramolecular allylation of an allyl chloride, which is summarized in Scheme 7. In this case, we used a starting material α-dihydroionone (12), previously prepared in the other route. Chlorination of 12 with NaClO afforded a diastereomeric mixture of allylic chlorides (22) in a 96% yield. With this substrate in hand, we first tried to induce the intramolecular allylation using an excess of Zn dust, which is known to react with allyl chlorides to form Chlorination of 12 with NaClO afforded a diastereomeric mixture of allylic chlorides (22) in a 96% yield. With this substrate in hand, we first tried to induce the intramolecular allylation using an excess of Zn dust, which is known to react with allyl chlorides to form organometallic systems which can react with carbonyl groups. However, the reaction led to the formation of the bicyclic product (26), which originated as a result of the formation of a C-C bond between C1 and the carbonyl (C9). We next tried the allylation with two Ti(III) systems, the well-established single electron transfer reagent Cp 2 TiCl, and the halfsandwich titanocene CpTiCl 2 , both prepared by reduction with Mn of the appropriate Ti(IV) species.
The allylation was tested under catalytic and stoichiometric conditions for both systems. The results, summarized in Table 1, show a similar behavior in all cases, although CpTiCl 3 seems to be superior both in terms of global yields and diastereoselectivity, particularly under stoichiometric conditions (Table 1, entry 3). In the light of these results, we decided to check whether the Ti(III)/Pd(0) combination strategy could be performed with the half-sandwich titanocene reagent CpTiCl 2 using the ethylcarbonate (20) as a substrate. Indeed, the reaction proved to be successful ( Table 1, entry 5), leading to a 73% global yield of the cyclic product, and with a 73:27 diastereoselectivity ratio using stoichiometric amounts of the Ti(III) source and catalytic of the Pd(0). ( a ) 2 eq Cp 2 TiCl 2 , 6 eq Mn; ( b ) 0.2 eq Cp 2 TiCl 2 , 6 eq Mn, 6 eq collidine, 3 eq TMSCl; ( c ) 1 eq CpTiCl 3 , 2 eq Mn; ( d ) 0.1 eq CpTiCl 3 , 2 eq Mn, 1 eq TMSBr; ( e ) 2 eq CpTiCl 3 , 8 eq Mn, 0.2 eq Pd(PPh 3 ) 2 Cl 2 .
The diastereoselectivity observed in the cyclization of (22) to give (6) and (24) mediated or catalyzed by CpTiCl 2 can be easily explained by a mechanism similar to the one discussed in Scheme 6, although in this case the radical intermediate II would be formed by the homolytic cleavage of the activated C-Cl bond present in (22).
In conclusion, we have proved that cis-α-ambrinol (6) can be prepared from commercial α-ionone (19) with an overall yield of 46% in only three steps using a stoichiometric amount of CpTiCl 2 for the intramolecular Barbier-type allylation of the chloro-derivative (22). cis-α-Ambrinol (6) can also be prepared in five steps from the same starting material through a Ti(III)/Pd(0) cyclization of the carbonate intermediate (20) with a 35% global yield. Finally, it should be mentioned that this synthesis of α-ambrinol (6) constitutes a new application of the usefulness of CpTiCl 2 as a new monoelectronic transfer reagent, as we [24][25][26] and others [27,28] have previously reported.

General Details
THF was distilled from Na/benzophenone under argon, and in all experiments involving titanocene (III) was deoxygenated prior to use, and oven-dried glassware was used in all cases. NMR spectra were recorded on Bruker Nanobay Avance III HD 300 MHz, and Avance III HD 600 MHz spectrometers. Proton-decoupled 13 C{ 1 H} NMR and DEPT-135 were measured in all cases. When required, NOE 1D, COSY, HSQC and HMBC experiments were used for signal assignation. Chemical shifts (δ) are expressed in ppm and coupling constants (J) in hertzs (Hz). Chemical shifts are reported using CDCl 3 as internal reference. IR Spectra were recorded with a Bruker Alpha spectrometer. Mass spectra were recorded in a Waters Xevo by LC-QTof-MS by electrospray ionization. All reactions were monitored by thin-layer chromatography (TLC) carried out on 0.2 mm DC-Fertigfolien Alugram ® XtraSil G/UV254 silica gel plates. The TLC plates were visualized with UV light and 7% phosphomolybdic acid or KMnO 4 in water/heat. Flash chromatography was performed on silicagel 60 (0.04-0.06 mm). Hard copies of NMR and IR spectra can be found as Supplementary Materials. (12) To a solution of α-ionone (19) (864 mg, 4.5 mmol) in THF (10 mL) was added Ni-Raney (0.4 g). The mixture was stirred under H 2 (1 atm) for 30 min at room temperature in a hydrogenation apparatus. The mixture was filtered through celite, and the solvent evaporated, yielding (12) (873 mg, 4.5 mmol, 100%) as a colorless oil. Spectroscopic data are in agreement with literature values [29].

Preparation of Epoxide 21
To a solution of α-dihydroionone (12) (3.05 g, 15.67 mmol) in anhydrous CH 2 Cl 2 (60 mL) at 0 • C, MCPBA (4.25 g, 17.24 mmol) was added. The mixture was stirred under N 2 and allowed to reach room temperature for 3 h. The reaction was quenched by stirring for 15 min. with saturated NaHCO 3 (30 mL) and another 30 mL of Na 2 S 2 O 3 (10% in water). The two phases were separated and the organic layer was washed with brine, dried over anhydrous MgSO 4 and the solvent was removed in a vacuum to give (21) as a mixture 86:12 cis:trans (3.21 g, 98%). Compound (cis-21) was purified by column chromatography (hexane:EtOAc 9:1) (83% yield from (12)). Colorless oil. Spectroscopic data are in agreement with literature values [17].  (23) To a solution of compound (cis-21) (934 mg, 4.44 mmol) in CH 2 Cl 2 (30 mL) at 0 • C, p-TSA (76 mg, 0.44 mmol) was added. The mixture was stirred for 12 h (0 • C), and then p-TSA (76 mg, 0.44 mmol) was again added and stirred for 4 h at room temperature. The mixture was washed with saturated NaHCO 3 (10 mL × 3) and dried over anhydrous MgSO 4 . The solvent was removed in a vacuum and the residue purified by silica gel flash column chromatography (hexane/EtOAc, 8:2) to afford alcohol (23) (575 mg, 62%) as a colorless oil. Spectroscopic data are in agreement with literature values [30].  (20) In an N 2 atmosphere at 0 • C, ethyl chloroformate (0.74 mL, 7.59 mmol), pyridine (1.54 mL, 18.98) and DMAP (64 mg. 0.51 mmol) were added to a solution of (23) (532 mg, 2.53 mmol) in CH 2 Cl 2 (40 mL). After 10 min, the cooling bath was removed, and the mixture was stirred for 20 h. TIt was then diluted with Et 2 O and washed with HCl (3%) and water. The organic layer was dried over anhydrous MgSO 4 , the solvent was removed in a vacuum and the residue purified by silica gel flash column chromatography (gradient hexane/EtOAc) to afford carbonate (20)
cis-α-ambrinol (6): Colorless oil; spectroscopic data are in agreement with literature values [12]. IR (ATR) v (cm Funding: This work was supported by the University of Almería and Junta de Andalucía (Conserjería de Transformación Económica, Industria, Conocimiento y Universidades) and Fondo Europeo de Desarrollo Regional (FEDER) for the Projects UALFEDER 2020-FQM-B1989, project PY20_01027 and project CEIA3 PYC20 RE 060 UAL, and also for the Horizon 2020-Research and Innovation Framework Programme of the European Commission for the project 101022507 LAURELIN and also by the University of Seville, through the Vicerrectorado de Investigación (Projects 2020/00001014 and 2021/00000422: Ayudas a Consolidación de Grupos de la Junta de Andalucía and Project Politec-Biomat: Red de Biomateriales en la Universidad de Sevilla).

Data Availability Statement:
Data is contained within the article and Supplementary Materials.