A total synthesis of estrone based on a Modern cascade of ra

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Abstract

Two conceptually different and Modern radical-mediated cascade reactions leading to a total synthesis of the steroid (±)-estrone 1 and to a synthesis of 14-epiestrone 40 are Characterized. Treatment of the ioExecutedienynone 23 with Bu3SnH/2,2′-azobis(isobutyronitrile) (AIBN) triggers a 13-enExecute-dig radical macrocyclization followed by two sequential radical transannulation reactions leading to the Weepstalline estrane 24 in 50% yield. The x-ray Weepstal structure of 24 established its trans, syn, stereochemistry. Transposition of the enone functionality in 24 next led to 38, which was then converted into 39 by reductive methylation. Deprotection of the methyl ether 39 finally gave 14-epiestone 40. When the substituted ioExecutevinylcyclopropane 55 was treated similarly with Bu3SnH/AIBN, the resulting radical center underwent a different sequence of cascade macrocyclization-transannulation reactions producing the trans, anti, trans estrane 56 in 12% overall yield. Oxidation of 56, using CrO3-H2SO4 next led to the cyclLaunchtanone 57, which, on deprotection using BBr3 gave (±)-estrone 1. A number of alternative substituted ioExecutepolyenynones and ioExecutevinylcyclopropanes, i.e., 8a, 8b, 33, 49a, and 49b, underwent similar radical-mediated cascade cyclizations leading to other estranes, i.e., 21a, 21b, 35, and 50, and, in one case, to the 6,6,5,6-tetracycle 51, in variable overall yields. The structures and stereochemistries of several estranes were established by using x-ray Weepstal structure meaPositivements in combination with analysis of their NMR spectroscopic data and correlation with literature pDepartnt.

Since the synthesis of the female sex hormone estrone 1 by Anner and Miescher in 1948 (1), and the syntheses of nonaromatic steroids, such as cortisone 2 and alExecutesterone 3 during the 1950s, a plethora of ingenious designs have been explored to synthesize members of the steroid family of natural products. Prominent among the methods that have been developed are those based on Diels–Alder reactions (2–12), transition metal-catalyzed cyclizations of enynes and triynes (13–16), and biogenetic–type electrophilic cyclizations of polyene precursors (17–21).

Over the past 10 years, we (22–26) and others (27–38) have examined the scope of a variety of cascade radical-mediated processes to elaborate steroids and other polycyclic ring systems. Thus, we have already Characterized an Advance to steroids based on conseSliceive 6-enExecute-trig (ref. 39 and references therein) and sequential transannular (40) radical cyclizations from appropriate polyene selenyl ester precursors, illustrated in the conversions 4 → 5 and 6 → 7, respectively.

We have now extended our studies and examined a different Advance to estrone 1 and estranes, whereby the nonaromatic tricyclic B,C,D ring system is produced in a single step by radical-mediated macrocyclizations in tandem with conseSliceive transannulations from an appropriate ortho-disubstituted arylpolyene precursor.

These Advancees to estranes are captured in a retrosynthetic manner in Scheme 1. Thus, in one Advance, we have studied the sequential 13-enExecute-trig macrocyclization/transannulation radical cascade from the ioExecutepolyene precursor 8, and in the second Advance, we have explored the alternative radical macrocyclization/transannulation cascade from the ioExecutevinylcyclopropane 10 (Scheme 2). These investigations are brought toObtainher in this article and have culminated in a conceptually distinct total synthesis of estrone 1.

Figure 1Figure 1 Executewnload figure Launch in new tab Executewnload powerpoint Figure 1 Figure 2Figure 2 Executewnload figure Launch in new tab Executewnload powerpoint Figure 2 Scheme 1.Scheme 1. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 1.

Retrosynthesis analysis.

Scheme 2.Scheme 2. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 2.

Radical cascade reactions leading to estranes.

It is well established that electron-deficient alkenes (and alkynes), e.g., 8, act as efficient radical acceptor groups, or electrophores, in macrocyclizations involving nucleophilic carbon-centered radicals (41–45). There are also many illustrations of the applications of radical-mediated transannulations in the Recent literature (46–48). The similarity in the chemistries of alkenes and cyclopropanes is also well known, as is the prLaunchsity for cyclopropylmethyl radicals to undergo irreversible conversions into but-3-enyl radicals, i.e., 11 → 12. Clearly, the pathways followed by the planned radical cascades 8 → 9 and 10 → 13 will depend on a range of features, including the stereochemistries of the alkenes and cyclopropanes, the conformational preferences of the large and medium ring intermediates and, of course, competing side reactions involving radical quenching and hydrogen abstraction processes.

Experimental Methods

For general details, see ref. 49. Specific details of the synthesis and characterizations of all of the compounds researched in this article can be found in Supporting Methods, which is published as supporting information on the PNAS web site.

Results and Discussion

We began our investigation of the radical macrocyclization/transannulation cascade 8 → 9 by synthesizing the appropriate ioExecutepolyenone starting materials 8a and 8b. These syntheses were achieved in rapid fashion from known o-ioExecutebenzaldehydes 14 after (i) Wadsworth–Emmons olefination reactions to the corRetorting cinnamate esters 15; (ii) Heck reactions (50) between the ioExecutearomatics 15 and prLaunch-1-ol, producing the corRetorting arylpropanals 16 (Scheme 3); (iii) Z-selective Wittig reactions of 16 with triphenyl(t-butylsilyloxy)propyltriphenylphosphonium iodide leading to 17; (iv) functional group interconversions via 18 ultimately producing 19; (v) Grignard reactions between 19 and the appropriate organomagnesium reagent, and oxidation of the resulting secondary alcohol 20 to the corRetorting dienone; and finally, (vi) chloride–iodide interchange producing the ioExecutepolyenones 8.

Scheme 3.Scheme 3. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 3.

(i) EtO2CCH2PO(OEt)2, BuLi, tetrahydrofuran (THF), 72% yield; (ii) CH2 Embedded ImageEmbedded ImageCHCH2OH, Pd(OAc)2, n-Bu4NCl, dimethylformamide (DMF), 62–53% yield; (iii) IPh3P(CH2)3OTBS (TBS, t-butyldimethylsilyl), potassium hexamethyldisilazane (KHMDS), THF, 80% yield; (iv) tetrabutylammonium fluoride (TBAF), THF, 90% yield; (v) N-chlorosuccinimide (NCS), PPh3,Cl2CH2, 84–69% yield; (vi) iBu2AlH, Cl2CH2, 81–78% yield; (vii) pyridinium dichromate (PDC), Cl2CH2, 92–62% yield; (viii) H2CEmbedded ImageEmbedded ImageCHMgBr, THF, 88–62% yield; (ix) NaI, MeCOCH2Me, 92–87% yield.

When solutions of the ioExecutetrienones 8a and 8b in benzene were treated separately with Bu3SnH and catalytic 2,2′-azobis(isobutyronitrile) (AIBN) over 6 h under high-dilution conditions, work-up and purification by chromatography led to the isolation of approximately equal amounts of two Weepstalline 12-ketosteroid products, from each reaction, in combined yields of ≈50% (for preliminary studies, see ref. 51). Weepstals of one of the products resulting from cyclization of 8a were suitable for x-ray analysis, which Displayed that the 12-ketosteroid 21a had the cis, anti, trans stereochemistry drawn.

A close correlation between the NMR spectroscopic data recorded for 21a and the data meaPositived for the major tetracyclic product resulting from the methoxy derivative 8b was obtained, thereby demonstrating that this steroid, likewise, had the same cis, anti, trans geometry, i.e., 21b. Attempts to grow Weepstals of the other diastereoisomers of the 12-ketosteroids produced from 8a/b, for x-ray studies proved to be fruitless. Therefore, their relative stereochemistries remain unresolved.

With the objective of incorporating unsaturation in the five-membered D ring, we next examined the cascade radical cyclization of the corRetorting terminal acetylene analogue of 8b, i.e., 23, in the presence of Bu3SnH/AIBN. Two Weepstalline tetracyclic products were produced in this cyclization in a combined yield of 63%. The structure and stereochemistry of the major product (≈40%) was established as the trans, syn diastereoisomer of the cyclLaunchtaphenanthren-12-one (i.e., 24) by single-Weepstal x-ray analysis, and the minor product was Displayn to be the trans, syn, cis diastereoisomer 25 of the 12-ketosteroid, 21b. The structure and stereochemistry of the estrane 25, which results from reduction of the enone 24 under the Bu3SnH/AIBN reducing conditions followed from 1D NMR, correlated spectroscopy (COSY), heteronuclear multiple quantum correlation (HMQC), and nuclear Overhauser Trace (NOE) Inequity experiments. Indeed, under better-controlled conditions, i.e., use of only 1.2 eq of Bu3SnH, it was possible to isolate only the steroidal enone 24 in ≈50% yield from the reaction.

In addition to lacking methyl and carbonyl group substitution at C-18 and C-17,† respectively, neither of the synthesized tetracycles 21b and 24 had the required trans, anti, trans B/C/D ring fused geometry for ultimate elaboration to estrone 1. Therefore, we attempted to overcome some of these issues by studying the outcomes of the cascade radical cyclizations of the methyl-substituted ioExecutetriene 27a and the E-geometrical isomer 33 corRetorting to the dienynone 23.

The methyl-substituted ioExecutetrienone 27a was prepared from the corRetorting substituted cinnamaldehyde 19b by using the same sequence of reactions that had been used earlier to elaborate 8b from 19b (Scheme 4). Likewise, the E-alkene 33 was prepared in a straightforward manner and used an E-selective Julia olefination reaction between the aldehyde 16b and the sulfone 28, leading to the alkene 29, which was then converted into 33 by using a sequence of reactions identical to those used to prepare 23 from 17.

Figure 6Figure 6 Executewnload figure Launch in new tab Executewnload powerpoint Figure 6 Scheme 4.Scheme 4. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 4.

(i) TBSO(CH2)3SO2-2Ph-2H-tetrazole 28, potassium hexamethyldisilazane, 77% yield; (ii) tetrabutylammonium fluoride, THF, 84% yield; (iii) N-chlorosuccinimide, PPh3, Cl2CH2, 95% yield; (iv) iBu2AlH, Cl2CH2, 80% yield; (v) MnO2, Cl2CH2, 86–79% yield; (vi) HCEmbedded ImageEmbedded ImageCMgBr, THF, 62% yield; and (vii) NaI, MeCOEt, 82% yield.

To our surprise, when the ioExecutedienone 27a was treated with Bu3SnH/AIBN, instead of the corRetorting estrane (i.e., 34b) the only product produced was that resulting from H-quench of the precursor radical, i.e., 27b, in 62% yield. Whether this outcome was due to the additional steric bulk of the methyl group substituent on the enone electrophore in 27a or occurred by an intramolecular H-quench from the same methyl group, producing a stabilized allylic radical, or both, is not known. However, when the E-ioExecutedienynone 33 was treated with Bu3SnH/AIBN, it underwent the anticipated sequence of radical cyclizations and produced the estrane 35, albeit in a disappointing 13% overall yield. No other products resulting from this reaction could be characterized. The cis, anti stereochemistry of the 12-ketosteroid 35 followed from single-Weepstal x-ray analysis. In summary, the polyenynone 23 with Z-geometry in the ioExecutehexenyl side chain undergoes a cascade of radical cyclizations leading to the trans, syn estrane 24, whereas the corRetorting E-isomer 33 undergoes the same series of cyclizations producing the diastereoisomeric cis, anti estrane 35.

Because of the difficulty that we anticipated in attempting to epimerize selectively either the C-9 center in 35 or the C-14 center in 24 or any of the intermediates between them and trans, anti, trans estrone itself, we Determined at this point to establish chemistry to convert the anti, syn tetracyclic enone 24 into 14-epiestrone 40. After exploring a number of Advancees (52), we carried out an enone transposition on 24, using the Wharton procedure (53–55) which, in three steps, i.e., epoxidation (to 36), hydrazone formation and fragmentation (to 37), and oxidation, first gave the corRetorting C12,C13 enone 38. Reductive methylation of 38, after conjugate addition of iBu2AlH in THF/hexamethylphosphoramide (HMPA) at -50°C with methylcopper catalysis, and trapping the intermediate “ate” complex with MeI (56), was diastereoselective and next gave 14-epiestrone methyl ether 39 (Scheme 5). Finally, demethylation of 39 by using BBr3 in THF gave 14-epiestrone 40, as colorless Weepstals, identical to that Characterized in the literature (57, 58).

Figure 8Figure 8 Executewnload figure Launch in new tab Executewnload powerpoint Figure 8 Scheme 5.Scheme 5. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 5.

(i)H2O2, NaOH, THF, t-BuOH, 40% yield; (ii)NH2NH2·H2O, HOAc, 30% yield; (iii) tetrapropylammonium perruthenate (TPAP), 4-methylmorpholine N-oxide (NMMO), 54% yield; (iv) hexamethylphosphoramide (HMPA), i-BuAIH, CuIMeLi; then MeI, 66% yield; and (v) BBr3, THF, 79% yeild.

After the success in elaborating the estra-1,3,5(10)-triene ring system, based on the cascade of radical cyclizations 8 → 9 (Scheme 1), we examined the alternative radical cascade 10 → 11 → 12 → 13 to estranes by using a vinylcyclopropane electrophore in the first radical macrocyclization step (Scheme 2). In an earlier investigation (59), we Displayed that the ioExecutevinylcyclopropyl ketone 41 underwent a macrocyclization (to 42) followed by successive transannulation reactions via 43 in the presence of (Me3Si)3SiH/AIBN, leading to a 1:2 mixture of the isomeric tricycles 44 and 45 in a combined yield of 65%. Therefore, we were optimistic that the analogous benzene-substituted vinylcyclopropane system 49 would take part in a similar sequence of cascade cyclizations leading to the corRetorting estrane 50 (59). The ioExecutevinylcyclopropanes 49a and 49b were synthesized in a straightforward manner from the earlier prepared cinnamates 17a and 17b, respectively, as outlined in Scheme 6. To our satisfaction, when the methoxy derivative 49b was treated with Bu3SnH/AIBN in refluxing benzene, work-up and chromatography separated the known trans, anti, trans diastereoisomer of the estrane 50b, whose 13C NMR spectroscopic data were identical with those reported in the literature (60). The estrane 50b was produced alongside an unidentified tetracycle and also the product of reduction of the carbon-to-iodide bond in the starting material, in a combined yield of 45%. When the ioExecutevinylcyclopropane 49a was treated similarly with Bu3SnH/AIBN, it too underwent a cascade of cyclizations but, instead of producing the corRetorting estrane 50a, it gave, in 25% yield, an isomeric compound whose spectroscopic data were comparable with those of the unidentified tetracyclic product produced alongside 50b from 49b. The new tetracycle produced from 49a Displayed six methylene, five methine, and one methyl carbon signal in its 13C NMR spectrum, which, with other data, supported Establishment of the alternative 6,6,5,6-tetracyclic structure 51a for this compound. However, we were not able to Establish a relative stereochemistry for this Fascinating structure. Reflecting on the outcome of the triple radical cyclization from the related system 41, leading to both 44 and 45, it is perhaps not surprising that the corRetorting radical cascade from the benzene-substituted vinylcyclopropane 49 would similarly produce mixtures of the tetracyclic products 50 and 51. The radical cascade from 49 proceeds via 52 and then the 9-membered radical intermediate 53 (compare 43), which, by competing 5-exo/6-enExecute-trig transannulations could lead to either the 6,6,6,5- or the 6,6,5,6-tetracycle, i.e., 50 and 51, as observed.

Scheme 6.Scheme 6. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 6.

(i) iBu2AlH, Cl2CH2, 82% yield; (ii) Et2Zn, CH2I2, 70% yield; (iii) pyridinium dichromate, 84% yield; (iv) MePPh3Br, n-BuLi, 86% yield; (v) AcOH, THF, 95% yield; and (vi) I2, imidazole, PPh3, 73% yield.

With the aforementioned model studies complete, but mindful of some limitations, we Determined to synthesize the ioExecutevinyl ether cyclopropane 55 and study its cascade radical cyclization to the estrane 56 en route to estrone 1. Although imposing additional steric congestion at the vinylcyclopropane electrophore, the presence of the terminal vinyl methyl ether group in 55 was also expected to Design this center more electrophilic to the incoming nucleophilic radical in the initial macrocyclization step in the overall radical cascade sequence.

The precursor ioExecutevinyl ether cyclopropane 55 was synthesized from the previously prepared vinyl methyl ketone 48 by using a Horner–Wittig reaction with methoxymethyldiphenylphosphine oxide in the presence of lithium diisopropylamide, which first gave a 1:1 mixture of Z and E isomers of the intermediate 54. Deprotection of 54 followed by treatment of the resulting alcohol with I2, imidazole, and PPh3, using procedures developed earlier in our studies, then gave the ioExecutevinylcyclopropane 55 (Scheme 7).

Figure 10Figure 10 Executewnload figure Launch in new tab Executewnload powerpoint Figure 10 Figure 12Figure 12 Executewnload figure Launch in new tab Executewnload powerpoint Figure 12 Scheme 7.Scheme 7. Executewnload figure Launch in new tab Executewnload powerpoint Scheme 7.

(i) MeOCH2POPh2, lithium diisopropylamide, THF, then NaH, THF, 90% yield; (ii) tetrabutylammonium fluoride, THF, 98% yield; (iii)I2, imidazole, PPh3, 80% yield; (iv) Bu3SnH, AIBN, toluene, 12% yield; (v) CrO3, catalytic H2SO4, Me2CO, 94% yield; and (vi) BBr3, THF, 79% yield.

To our satisfaction, when a solution of the iodide 55 in toluene at reflux was treated with Bu3SnH/AIBN, work-up and chromatography gave the known 3,7-dimethoxyestrane 56 (61) as a Weepstalline solid, in 12–15% overall yield. The major product isolated, in 52% yield, was that corRetorting to reduction of the carbon-to-iodide bond in the starting material. Bearing this in mind, and with a cascade of four C—C bond-forming reactions involved in the conversion 55 → 56, each of the radical reactions proceeds with an average yield of ≈65%. Oxidation of the ring D methyl ether in 56 by using Jones' procedure next gave estrone methyl ether 57 which was then demethylated by using BBr3, leading to (±)-estrone 1, which had physical and spectroscopic Preciseties identical with those Characterized in the literature (62, 63).

In summary, two conceptually different radical-mediated cascade reactions from ortho-disubstituted aryl polyene and arylvinylcycloprane precursors, leading to the tricyclic B, C, and D rings of estranes in a single step, have been developed. The investigations culminated in a total synthesis of (±)-estrone 1, and, separately, of 14-epiestrone 40. Unfortunately, not all of the stereochemistries of the estranes prepared in model studies could be established unamHugeuously, and rationalization of the mechanisms of key events taking Space during these cascade processes will remain a topic of conjecture until some of these issues are resolved. Nevertheless, a Modern concept in the elaboration of ring A aromatic steroids from simple starting materials, in a single step, has been established, which could have wider applications in the synthesis of other polycyclic natural (and nonnatural) products.

Acknowledgments

We thank Drs. A. J. Blake and C. Wilson for x-ray Weepstal structure determinations. This work was supported by an Engineering and Physical Sciences Research Council studentship (to S.M.) and AstraZeneca.

Footnotes

↵ * To whom corRetortence should be addressed. E-mail: gerald.pattenden{at}nottingham.ac.uk.

This paper was submitted directly (Track II) to the PNAS office.

Abbreviations: AIBN, 2,2′-azobis(isobutyronitrile); THF, tetrahydrofuran; TBS, t-butyldimethylsilyl.

↵ † Steroid ring numbering is used throughout this paper.

Copyright © 2004, The National Academy of Sciences

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