Basic Introduction

Estrone is the second most common type of estrogen produced by during the childbearing years. Estrone is produced by the adrenal glands and fatty tissue, and is the only type of oestrogen produced after menopause. A small amount of estrone is produced by the ovary.

The Total Synthesis of Estrone

Key requirements for this to be an attractive and versatile synthetic strategy include convenient access to the intermediate A in a highly diastereo- and enantioselective fashion, and, crucially, the viability to easily introduce suitable R1-R4 groups in order to enable the synthesis of structurally diverse steroid targets via a range of suitable cyclization reactions. In this communication we will showcase our approach toward steroid synthesis by the enantioselective total synthesis of estrone 1. [1]

Hence, the corresponding retrosynthetic analysis of 1 involves a B- and C-ring disconnection to give the key intermediate 2. This intermediate was envisioned to be directly accessible by a one-pot process involving a conjugate addition of an allylic phosphonate/phosphonamide 3, to be synthesized from 3-methyl anisole 5, to 2-methyl-2-cyclopentenone acceptor 4, followed by diastereoselective alkylation of the resulting enolate with allyl bromide. Conjugate addition reactions of allylic phosphonates are known to be very diastereoselective, with the allylic double bond configuration being translated into the relative configuration of the sp3-centers of the formed C?C bond.  The desired relative C8?C14 configuration requires a Z-configuration of the phosphonate double bond. An enantioselective synthesis is possible, eg by using a homochiral phosphonamide auxiliary as described by Hanessian. From 2, the C-ring of estrone would be obtained via a ring closing metathesis, followed by a B-ring Heck cyclization.  In general, this approach would allow convenient synthesis of a D-ring intermediate with possible introduction of R1-R3 groups by appropriate choice of starting materials. The R4 group, restricted to an E-vinylic phosphonamide/phosphonate, was thought of as a versatile reactive handle to achieve the B and C ring cyclizations.

Starting from the known dibromide 6, accessible in one step from 3-methyl anisole 5, benzylic displacement with allenyl magnesium bromide followed by one-carbon extension led to the propargylic alcohol 7. Diastereoselective alkyne reduction (>98% Z) was achieved with Zn/BrCH2CH2Br, which proved consistently reproducible even upon upscaling, unlike more conventional methods using poisoned heterogeneous Pd-catalysts. Conversion to the Z-allylic chloride 8 using hexachloroacetone, and final Arbuzov reaction with triethyl phosphite gave 3a in excellent overall yield. The homochiral phosphonamide 3b was obtained by reaction of 8 with homochiral phospholane 9.

The key conjugate addition/allylation sequence was investigated next. Deprotonation of 3a with BuLi, followed by addition to 2-methyl-2- cyclopentenone 4 and final alkylation with allyl bromide, gave 2a as the only observable diastereomer in excellent yield. The conjugate addition/alkylation process with the homochiral allylic phosphonamide 3b proceeded in a slightly lower yield, leading to 2b as the only isolated diastereomer. In both cases, the corresponding nonallylated product 10 was also isolated, as well as, surprisingly, a diallylation product 11. It is believed that the formation of both byproducts originate from the same process, in that the obtained enolate after conjugate addition reaction deprotonates already formed allylation product 2 to give 10 and a new enolate, which is then allylated to give 11. Both byproducts are separable by chromatography. It was important to use a limited excess (1.1 equiv) of BuLi to avoid concomitant bromine- lithium exchange, which led to the formation of the corresponding debrominated products (not shown). Fortunately, the relative configuration of 2a could be unambiguously established by X-ray crystallographic analysis, which confirmed the formation of the desired relative configuration of the C8, C13, and C14 stereogenic centers. With the key intermediates 2 in hand, the subsequent cyclization reactions to give estrone were accomplished. The C-ring closure by ring closing metathesis to the trans-hydrindene ring was investigated first using the racemic phosphonate 2a. RCM reaction using the Hoveyda-Grubbs II catalyst in toluene at 70 °C afforded trans-hydrindene rac-12 in good yield. Small amounts (up to 3%) of debrominated product—separable by chromatography—could be observed (NMR) when the Grubbs II catalyst was used. From rac-12 onward, the estrone synthesis is similar to the Tietze estrone synthesis (in which the C17 ketone was protected as tbutyl ether). The Δ9,11 double bond resulting from the RCM reaction is ideally positioned for the subsequent Heck B-ring closure, which proceeded in quantitative yield under conditions developed by Tietze. This led to 13, with the undesired 9β-configuration. This can be converted to the desired configuration under particular conditions in which the Δ11,12 double bond in 13 is isomerized to its conjugated position, followed by hydrogenation, a process also developed by Tietze. In our case, the isomerization/hydrogenation process from 13 led to a 7:3 mixture of O-methyl estrone 14 and its 9β-epimer, in almost quantitative yield. Presumably, the different C17 functionalization is the reason for the difference in stereochemical outcome compared to Tietze’s report. Our results are in accord with a literature report in which Δ9,11-O-methyl estrone was converted to a 65:35 mixture of 14 and 9β?14 using H2, Pd/C. Estrone methyl ether could be obtained pure by crystallization, and estrone 1 was then obtained by conventional deprotection. An enantioselective synthesis was achieved from 3b. Unfortunately, the RCM reaction only proceeded well after prior hydrolysis of the phosphonamide group to the corresponding phosphonic acid 2c. Starting from 2b, only 6% of 12 was obtained. The thus synthesized enantioenriched 12 was then taken through to O-methyl estrone 14, of which optical rotation compared well with the reported value for authentic estrone methyl ether.

Estrogen's Role in Female Physiology and Metabolism

Estrogens are the primary female sex hormones responsible for the development of the secondary sex characteristics, regulation of the menstrual cycle (in conjunction with progesterone), breast and uterine growth, and the maintenance of pregnancy.[2] Although more than 20 different estrogens have been identified, only three have known clinical relevance: estrone (E1), estradiol (E2), and estriol (E3). Of these, estradiol is the predominant estrogen produced by the ovaries, making it a good marker of ovarian function. Estrogen is also produced in small amounts by the adrenal glands and peripheral conversion of androgens by aromatization to estrone. During pregnancy, estriol, synthesized in the placenta from fetal 16α-OH-DHEA-S, is present in high concentrations in maternal serum. In nonpregnant, premenopausal women, the concentration of estrogen varies in a temporal pattern throughout the menstrual cycle (Fig. 43.2). Estrogen exerts its effects by binding to nuclear estrogen receptors. The receptor–ligand complex binds specific response elements in target genes and induces transcription. In addition to nuclear receptors, there are also membrane-bound G-protein-coupled estrogen receptors (mERs). These receptors act nongenomically through complex and incompletely understood mechanisms. Circulating estrogen is metabolized by the liver to estrone, and subsequently to catechol estrogens (2-hydroxyestrone, 2-hydroxyestradiol, and 2-hydroxyestriol) or estriol. Estrogen metabolites are glucuronidated and sulfated rendering them water soluble facilitating excretion in bile or urine. The majority (97%) of estradiol is bound weakly to albumin (~60%) and tightly to SHBG (~40%) in circulation. Estrogen stimulates the synthesis of binding globulins; thus SHBG concentrations are nearly twice as high in females as in males. Both free and albumin-bound estradiol are considered biologically active, although the clinical significance of this is unclear.

Effects on Musculoskeletal Function

Estrogen has a dramatic effect on musculoskeletal function. Estrogen is known for its relationship with bone, however it directly affects the structure and function of other musculoskeletal tissues such as muscle, tendon, and ligament. [3] Estrogen is the key regulator of bone metabolism in both men and women. Bone is a complex tissue, consisting of a matrix of proteins and minerals that give it the flexibility and strength to support body movement. Bones specialized cells, help to maintain this matrix. Estrogen protects bones by inactivating osteoclast activity. The dramatic fall in estrogen levels at menopause increases bone loss and can lead to osteoporosis, which puts women at greater risk for a hip fracture. 2.  Estrogen acts as a regulator of muscle energy metabolism and plays a role in the maintenance of muscle stem cells (i.e., satellite cells) as well promoting self-renewal and differentiation into muscle fibers. Menopause leads to the cessation of ovarian estrogen production concurrent to the deterioration of muscle function. Estradiol deficiency reduces skeletal muscle mass and force generation in women. 3. Estrogen increases the collagen content of connective tissues. 4. Estrogen in tendons and ligaments decreases stiffness, and this directly affects performance and injury rates. High estrogen levels can make women more prone for catastrophic ligament injury.

References

[1] Fouchert, V., Guizzardi, B., Groen, M. B., Light, M., & Linclau, B. (2010). A novel, versatile D→BCD steroid construction strategy, illustrated by the enantioselective total synthesis of estrone. Organic Letters, 12(4), 1194-1197.

[2] Eyster, K. M. (2020). Structure, function, and relationship of estrogens. In Encyclopedia of Bone Biology (pp. 495-502). Elsevier.

[3] Chidi-Ogbolu N, Baar K. Effect of Estrogen on Musculoskeletal Performance and Injury Risk. Front Physiol. 2019 Jan 15;9:1834.