Synthesis of a 14β-Hydroxysteroid Using the Transannular Diels-Alder Strategy

A three-step route to a tricyclic steroid precursor

2-Alkyl cyclohexenones are useful intermediates for organic synthesis. The Wittig reaction of a series of aldehydes with (cyclopropylmethyl)triphenylphosphonium bromide delivered the corresponding alkenyl cyclopropanes. UV irradiation in the presence of Fe(CO)5 converted the alkenyl cyclopropanes to the 2-substituted cyclohexenones. This approach enabled a three-step synthesis of the tricyclic core of estrone methyl ether.

Formal Enantioselective Synthesis of (+)-Estrone

A formal total synthesis of (+)-estrone (4% overall yield; ca. 12 steps) could be achieved via the Torgov diene. An asymmetric allylic substitution is the key step for the construction of the chiral quaternary carbon center of a synthetic intermediate which was converted in four steps to the Torgov diene. [reaction: see text]

Cascade radical-mediated cyclisations with conjugated ynone electrophores. An approach to the synthesis of steroids and other novel ring-fused polycyclic carbocycles

A cascade radical-mediated Diels-Alder reaction with the iododienynone 16b produced the tricyclic ketone 17 (22%). By contrast, treatment of the substituted furans 36 and 47 with Bu(3)SnH-AIBN, instead led to the tetracycles 44 and 58 respectively, rather than the anticipated oestranes, i.e. 38 and 48. In a separate study, attempted cascade radical-mediated cyclisations from the ortho-aryl substituted iododienynones 72 and 73, leading to the ring-D aromatic steroid 7, instead gave the macrocyclic ketone 76 or the novel bridged tricycles 77/82, respectively, depending on whether benzene or heptane was used as solvent in the reactions.

Synthetic Approach Toward the Total Synthesis of Kempane Diterpenes via Transannular Diels−Alder Strategy

Total syntheses of two new (+/-)-kempane derivatives 30 and 47 were achieved with transannular Diels-Alder reaction (TADA) serving as the key step for the stereoselective formation of tricyclic [6.6.5] system 3. This synthetic approach also reveals that the geometry of the C3 group of such a tricyclic system plays an important role for the formation of the seven-membered kempane skeleton.

A total synthesis of estrone based on a novel cascade of radical cyclizations

Two conceptually different and novel radical-mediated cascade reactions leading to a total synthesis of the steroid (+/-)-estrone 1 and to a synthesis of 14-epiestrone 40 are described. Treatment of the iododienynone 23 with Bu(3)SnH/2,2'-azobis(isobutyronitrile) (AIBN) triggers a 13-endo-dig radical macrocyclization followed by two sequential radical transannulation reactions leading to the crystalline estrane 24 in 50% yield. The x-ray crystal 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 iodovinylcyclopropane 55 was treated similarly with Bu(3)SnH/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 CrO(3)-H(2)SO(4) next led to the cyclopentanone 57, which, on deprotection using BBr(3) gave (+/-)-estrone 1. A number of alternative substituted iodopolyenynones and iodovinylcyclopropanes, 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 crystal structure measurements in combination with analysis of their NMR spectroscopic data and correlation with literature precedent.

A cycloaddition cascade approach to the total synthesis of (-)-FR182877

An asymmetric synthesis of the cytotoxic natural product, (-)-FR182877 (1), has been achieved. Chirality for the entire structure was established using two (4R)-4-benzyl-2-oxazolidinone-mediated boron aldol reactions. A 19-membered macrocarbocycle was synthesized by the coupling of two fragments using a regioselective Suzuki coupling (17 + 23 --> 26; 84%) and macrocyclization of a beta-keto ester (30 --> 31; 77%). Oxidation of 31 triggered a sequence of stereoselective transannular Diels-Alder reactions (32 -->34; 63%) forming four new rings and seven new stereocenters in the pivotal construction event. This pentacyclic intermediate was subsequently transformed to (-)-FR182877. Semiempirical calculations of the transannular Diels-Alder cycloaddition cascade were carried out to determine the origins of asymmetric induction.