Heterocycles as Moderators of Allyl Cation Cycloaddition Reactivity

Abstract

For the rapid elaboration of polycarbocyclic scaffolds, prevalent in many important families of terpenoid natural products, allyl cations derived from simple heterocyclic alcohols can be used as versatile reaction partners in both (4+3) and (3+2) cycloaddition pathways. Our recent progress in this area is outlined, pointing towards the untapped potential of heterocycles to act as reagents in novel or known but challenging organic transformations.1 Heterocyclic Reagents2 Cycloadditions and Allyl Cations3 Furfuryl Cations in Cycloadditions4 Heterocycle-Substituted Cations in Cycloadditions5 Mechanistic Considerations6 Conclusions and Outlook

Dithioallyl cation (3 + 2) cycloadditions under aprotic reaction conditions: rapid access to spiro-fused cyclopentane scaffolds

“…Lewis acids in combination with dithioallyl alcohol 1 generally gave poor results for the cyclopentannulation of styrene as this gave complex mixtures. The most promising reactions were those with gallium( iii )triflate and zinc( ii )bromide, 14,15 but these still gave low isolated yields for the expected adduct 2a compared to the reference experiment with trifluoroacetic acid (Table 1, entries 1–3). The combination of the silyl ether 12 with gallium( iii )triflate gave a relatively clean reaction, 15 a but the isolated yield was again low (entry 4).…”

“…12 In 2016, our group reported an allyl cation (3 + 2) cycloaddition in which the simple dithioallyl alcohol 1 serves as a versatile three-carbon synthon for cyclopentene assembly (Scheme 1c). 14 An attractive feature of this methodology is that the obtained cyclopentanoids like 2a or 2b can be chemoselectively transformed into a cyclopentene, via hydrodesulfurization. 15 Thus, the dithioallyl alcohol serves as a synthetic equivalent of a ‘naked’ allyl cation, opening up attractive options in synthesis.…”