Synthesis of B-Ring-Modified Steroids through BF3-Promoted Rearrangement/Substitution of 6β-Hydroxy-5,19-cyclosteroids

Taming nonclassical carbocations to control small ring reactivity

Prediction of the outcome of ring opening of small organic rings under cationic conditions can be challenging due to the intermediacy of nonclassical carbocations. For example, the solvolysis of cyclobutyl or cyclopropylmethyl derivatives generates up to four products on nucleophilic capture or elimination via cyclopropylcarbinyl and bicyclobutonium ions. Here, we show that such reaction outcomes can be controlled by subtle changes to the structure of nonclassical carbocation. Using bicyclo[1.1.0]butanes as cation precursors, the regio- and stereochemistry of ring opening is shown to depend on the degree and nature of the substituents on the cationic intermediates. Reaction outcomes are rationalized using computational models, resulting in a flowchart to predict product formation from a given cation precursor.

Cyclopropylcarbinyl cation chemistry in synthetic method development and natural product synthesis: cyclopropane formation and skeletal rearrangement

In this Review, the underrecognized utilities of the cyclopropylcarbinyl cation chemistry are summarized in cyclopropane synthesis and skeletal rearrangements, and their applications in natural product total synthesis are highlighted.

Rapid Access to Arylated and Allylated Cyclopropanes via Brønsted Acid-Catalyzed Dehydrative Coupling of Cyclopropylcarbinols

A regioselective protocol for the synthesis of cyclopropyl derivatives that relies on Brookhart acid-catalyzed dehydrative coupling over substituted cyclopropylcarbinols without rearrangement is reported herein. The reactions proceed promptly at 25 °C with only 2.0 mol % catalyst loading and produce the cyclopropyl derivatives in excellent yields. This method is well tolerated with a vast range of cyclopropylcarbinols including aliphatic cyclopropylcarbinols, where no elimination product was obtained, demonstrating the protocol's utility. Further, the Hammett correlation suggested the formation of a cyclopropylcarbinyl cation followed by a coupling reaction. An extremely effective gram-scale reaction has also been demonstrated with a high turnover number.

Absolute structure of (3aS,5S,7aS,7bS,9aR,10R,12aR,12bS)-7b-hy-droxy-4,4,7a,9a,12a-penta-methyl-10-[(2'R)-6-methyl-heptan-2-yl]-2,8,9-trioxo-octa-deca-hydro-benzo[d]indeno-[4,5-b]azepin-5-yl acetate from 62-year-old crystals

The structure of the title compound was determined using single crystals obtained more than 60 years ago at the Facultad de Química, Universidad de la República. The chemical structure of the compound, now confirmed by X-ray diffraction, was determined spectroscopically and was relevant in the determination of the structure of lanosterol and other triterpenoids in the early 50′s.

The structure of the title compound, C₃₂H₅₁NO₆, was determined from 62-year-old crystals at room temperature and refined with 100 K data in a monoclinic (C2) space group. This compound with a triterpenoid structure, now confirmed by this study, played an important role in the determination of the structure of lanosterol. The mol­ecules pack in linear O—H⋯O hydrogen-bonded chains along the short axis (b), while parallel chains display weak van der Waals inter­actions that explain the needle-shaped crystal morphology. The structure exhibits disorder of the flexible methyl­heptane chain at one end of the main mol­ecule with a small void around it. Crystals of the compounds were resistant to data collection for decades with the available cameras and Mo Kα radiation single-crystal diffractometer in our laboratory until a new instrument with Cu Kα radiation operating at 100 K allowed the structure to be solved and refined.

Facile Access to Bridged Ring Systems via Point-to-Planar Chirality Transfer: Unified Synthesis of Ten Cyclocitrinols

Bridged ring systems are found in a wide variety of biologically active molecules including pharmaceuticals and natural products. However, the development of practical methods to access such systems with precise control of the planar chirality presents considerable challenges to synthetic chemists. In the context of our work on the synthesis of cyclocitrinols, a family of steroidal natural products, we herein report the development of a point-to-planar chirality transfer strategy for preparing bridged ring systems from readily accessible fused ring systems. Inspired by the proposed pathway for biosynthesis of cyclocitrinols from ergosterol, our strategy involves a bioinspired cascade rearrangement, which enabled the gram-scale synthesis of a common intermediate in nine steps and subsequent unified synthesis of 10 cyclocitrinols in an additional one to three steps. Our work provides experimental support for the proposed biosynthetic pathway and for the possible interrelationships between members of the cyclocitrinol family. In addition to being a convenient route to 5(10→19) abeo-steroids, our strategy also offers a generalized approach to bridged ring systems via point-to-planar chirality transfer. Mechanistic investigations suggest that the key cascade rearrangement involves a regioselective ring scission of a cyclopropylcarbinyl cation rather than a direct Wagner-Meerwein rearrangement.

Intramolecular cyclopropylmethylation via non-classical carbocations

Cyclopropyl-cyclopropyl rearrangement can be achieved selectively by intramolecular trapping of cyclopropylmethyl carbenium ions with an internal nucleophile. This can be exploited as a useful method for the introduction of a cyclopropyl group into complex molecules using readily accessible disubstituted cyclopropane intermediates.