From Chiral ortho-Benzoquinone Monoketals to Nonracemic Indolinocodeines through Diels–Alder and Cope Reactions

A one-pot ultrasound-assisted regio and stereoselective synthesis of indenoquinoxaline engrafted spiropyrrolidines

A catalyst free ultrasound-assisted regio-/stereoselective modular approach was accomplished for the synthesis of highly constrained indenoquinoxaline engrafted spiro pyrrolidines from easily available substrates. This one-pot strategy utilizes 1,3-dipolar cycloaddition from a four component reaction of ninhydrin, 1,2-phenylenediamine, β-nitrostyrene and benzylamine or amino acids under ultrasound irradiation. The transformation is mild and operationally simple, providing architecturally complex fused spiro polycyclic heterocycles. This synthesis was confined to follow the group-assistant-purification (GAP) chemistry process, which can avoid chromatographic purifications and use of catalysts and allows easy access to a novel class of spiro engrafted polyheterocyclic scaffolds, which may be beneficial in biomedical research/materials science in the near future. This opens an era for the formation of a single exo product, when compared with reported protocols, by merely switching over reaction conditions to US irradiation.

Chemoenzymatic Total Synthesis of Hydromorphone by an Oxidative Dearomatization/Intramolecular [4 + 2] Cycloaddition Sequence: A Second-Generation Approach

A second-generation approach to the synthesis of hydromorphone by oxidative dearomatization/Diels-Alder cycloaddition was investigated. Detailed analysis of the stereochemical outcome of the [4 + 2] cycloaddition was performed first on a truncated model system as well as on the material leading to ent-hydromorphone. The stereochemical assignments were made by NMR and X-ray methods. The second-generation synthesis of hydromorphone was completed in both enantiomeric series. Improvements in the dearomatization conditions were attained using hypervalent iodine reagents instead of Pb(OAc)₄. Electrochemical methods of oxidative dearomatization were also investigated. New conditions enabling the rearomatization of ring A from the methoxyketal were developed, and a formal synthesis of the natural enantiomer of hydromorphone was completed. Experimental and spectral data are provided for all new compounds.

The quest for a practical synthesis of morphine alkaloids and their derivatives by chemoenzymatic methods

We became interested in approaches to morphine in the early 1990s following our immersion into the new program on the enzymatic dihydroxylation of aromatics. Larry Kwart, a former classmate of one of us at Rice University, who worked with our group at Virginia Tech in the mid-1980s, introduced to us the use of blocked mutants of Pseudomonas putida (Pp39D) for the production of arene-cis-dihydrodiols. Larry had gained expertise in microbiology from a postdoctoral stay with David Gibson, who discovered this unique enzymatic transformation, and he helped us to establish a strong program in chemoenzymatic synthesis that continues to this day. Without his pioneering effort, none of our accomplishments in chemoenzymatic synthesis, including the various approaches to morphine, would have materialized. Here we trace the evolution of our approaches to morphine alkaloids and some commercial opiate-derived medicinal agents. The design features and chronology of our approaches are discussed in a way that allows the reader to appreciate a number of errors that were made in conception as well as in execution. Experience acquired from many failed or less-than-effective attempts has finally led to an "almost reasonable" total synthesis, the key concept being based on our very first but unsuccessful attempt more than two decades ago. The irony of this accomplishment has not been lost on us. Each section of this Account presents a summary of distinctly different approaches to morphine alkaloids. Each ends with a short and philosophical lesson that was (or should have been) learned in the process. We intend for this Account to offer more than the history of a search for the perfect design solution to a synthetic problem. In today's era of rapid and often careless publication of results, it should serve also as a reminder that the success and the integrity of synthetic ventures depends on perseverance, adjustment of strategy, improvements of previous attempts, and serious attention to the quality of experimental data. Although somewhat satisfied with our latest accomplishment in morphinan synthesis, we plan to improve our design in the hope that a six-step synthesis is no longer in the realm of fantasy. With more than 20 years of effort in this area, our continuing involvement may qualify as obsession.

Short chemoenzymatic total synthesis of ent-hydromorphone: an oxidative dearomatization/intramolecular [4+2] cycloaddition/amination sequence

A short synthesis of ent-hydromorphone has been achieved in twelve steps from β-bromoethylbenzene. The key transformations involved the enzymatic dihydroxylation of the arene to the corresponding cis-dihydrodiol, Mitsunobu coupling with the ring A fragment, oxidative dearomatization of the C3 phenol, and the subsequent [4+2] cycloaddition to form ring B of the morphinan. The synthesis was completed by intramolecular amination at C9.