Enantiodivergent Cyclization by Inversion of the Reactivity in Ambiphilic Molecules

Synthesis of Bridged Oxabicycles via Cascade Reactions involving O-Acyloxocarbenium Ion Intermediates

Compared to related electrophilic species, O-acyloxocarbenium ions (AOIs) have been much less utilized in organic synthesis due to the lack of an efficient formation method. Here, we present a facile and simple approach for the generation of AOI from ester and acetal groups. Based on our AOI system with a pendant nucleophile, we obtained a unique bridged bicyclic system via an epoxonium-like transition state. The proposed mechanism is based on experimental and computational studies.

Litosetoenins A-E, Diterpenoids from the Soft Coral Litophyton setoensis, Backbone-Rearranged through Divergent Cyclization Achieved by Epoxide Reactivity Inversion

Litosetoenins A-E (1-5), five new ring-rearranged serrulatane-type diterpenoids with a common tricyclo[3.0.4]decane core, along with a known diterpenoid glycoside (6), a related known diterpenoid (7), and four known sesquiterpenoids (8-11), were isolated from a Balinese soft coral Litophyton setoensis. Spirolitosetoenin A (5a) and isospirolitosetoenin A (5b), featuring an unprecedented spiro[4,5]decane core, were obtained after treatment of compound 5 with HCl in methanol. The structures of new compounds were elucidated by extensive spectroscopic analysis, quantum mechanical nuclear magnetic resonance approach, and chemical methods. A plausible biosynthetic pathway involving an unusual divergent biogenesis was proposed.

Intramolecular Nicholas Reaction Enables the Stereoselective Synthesis of Strained Cyclooctynes

Cyclic products can be obtained through the intramolecular version of the Nicholas reaction, which requires having the nucleophile connected to the alkyne unit. Here, we report the synthesis of 1-oxa-3-cyclooctynes starting from commercially available (1R,3S)-camphoric acid. The strategy is based on the initial preparation of propargylic alcohols, complexation of the triple bond with Co₂(CO)₈, and treatment with BF₃·Et₂O to induce an intramolecular Nicholas reaction with the free hydroxyl group as nucleophile. Finally, oxidative deprotection of the alkyne afforded the cyclooctynes in good yields. Notably, large-sized R substituents at the chiral center connected to the O atom were oriented in such a way that steric interactions were minimized in the cyclization, allowing the formation of cyclooctynes exclusively with (R) configuration, in good agreement with theoretical predictions. Moreover, preliminary studies demonstrated that these cyclooctynes were reactive in the presence of azides yielding substituted triazoles.

Perrhenate Esters as Intermediates in Molecular Complexity-Increasing Reactions

Allylic alcohols form perrhenate esters upon reaction with Re2O7 or HOReO3. These species undergo nonstereospecific and nonregiospecific alcohol-transposition reactions through cationic intermediates. Sequencing these nonselective processes with reversible trapping by electrophiles results in cyclization reactions where regio- and stereocontrol are dictated by thermodynamics. The cationic intermediates can also be utilized as electrophiles in intra- or intermolecular dehydrative reactions with nucleophiles. These processes serve as the basis for applications in catalytic syntheses of a wide range of heterocyclic and carbocyclic structures that often show considerable increases in molecular complexity. This Account describes a sequence of events that started from a need to solve a problem for the completion of a natural product synthesis and evolved into a central element in the design of numerous new transformations that proceed under mild conditions from readily accessible substrates.

1 Introduction

2 Exploratory Studies

3 Application to Spiroketal Synthesis

4 Reactions with Epoxides as Trapping Agents

5 Development of Dehydrative Cyclizations

6 Bimolecular Reactions

7 Spirocyclic Ether Formation

8 Conclusions