Dearomatizing spirocyclization of thioureas, ureas and guanidines

C(sp3)-H Oxidation of Spiroguanidine-Containing Cyclopentanes

A C(sp3)-H oxidation reaction for spiroguanidine compounds with 5-membered rings was developed. The introduction of a cyclopentane-fused aminomethylpyridine as a directing group to the amide linkage of the cyclic guanidine moiety, using H2O2-urea in trifluoroethanol, enabled oxidation at the spiro β-position of the cyclopentane ring. This allowed selective introduction of a hydroxy group at the spiro β-position and was applicable to cyclopentane compounds with various substituents, including those challenging to functionalize with common oxidation methods.

Investigation of Dearomatizing Spirocyclizations and Spirocycle Functionalization En Route to Spirocalcaridines A and B-Some Trials and Tribulations

Spirocalcaridines A and B are among the most challenging members of the marine invertebrate-derived Leucetta alkaloids. Approaches to the construction and elaboration of the highly compact spirocyclic core are described. The synthesis of tricyclic guanidine via tandem oxidative amination dearomatizing spirocyclization (TOADS) using hypervalent iodine set the stage for total synthesis via the migration of the C4/C8 double bond to the C4/C5 position, followed by oxidation. The undesired but not surprising propensity of the spirocyclic cyclohexadienone to undergo rearrangement to the phenol hindered the desired olefin migration. Furthermore, initial efforts to install the oxidation sequentially, first at C5 and then at C4 in the complete carbon skeleton, were fraught with unforeseen challenges and unusual outcomes. In addition, the scope and limitations of hypervalent iodine-mediated tandem oxidative dearomatizing spirocyclization on various substrates were explored. Urethanes and thiourethanes underwent spirocyclization with an excellent yield, whereas the reaction with allylic substrates and species lacking the p-methoxy substituent did not proceed. Attempts to prepare other guanidine precursors are briefly discussed.

Electrochemical Oxidative Dearomatization Strategy for Accessing Spiro[4.5]dienones and Derivatives

Electrochemical dearomatization has been recognized as an attractive tool for the rapid construction of structurally diverse molecules. The designed methodology encompasses an eco-friendly and efficient electrochemical approach to synthesizing spiro[4.5]dienones under mild reaction conditions. Furthermore, detailed mechanistic studies strongly bolster our hypothesis and emphasize the role of HFIP in the mechanism. The protocol is scalable and showcases a broad substrate scope with tolerance toward numerous functional groups. Henceforth, this strategy can be deployed as an alternative and sustainable tool for accessing spiro[4.5]dienones.

Enantioselective Total Synthesis of (+)-KB343

A concise total synthesis of the complex guanidinium toxin KB343 is reported traversing through an unusual sequence of chemoselective transformations and strategic skeletal reorganization. The absolute configuration is confirmed through an enantioselective route, and the structures of all key intermediates and the natural product itself are unassailably confirmed through X-ray crystallographic analysis.

Synthesis of Altenuene Backbones through Iodine(III)-Participated Umpolung Diesterification and Insights into the General [1,5]-H Shift in para-Dearomatization of Phenols via Quantum Chemical Calculations

Through PhI(OAc)₂-oxidized dearomatization and diesterification of 3'-hydroxy-[1,1'-biphenyl]-2-carboxylic acids, a series of polycyclic compounds possessing an altenuene backbone were obtained in moderate to good yields. The Umpolung diesterification reaction was completed under mild reaction conditions without an additional nucleophilic reagent. This work offers a concise method for the synthesis of diverse natural altenuene analogues. The mechanism was proposed, and the [1,5]-H shift was studied in isomerization from the ketone-form structure to a phenol employing computational studies.