Construction of Halofunctionalized Indenes via a Cascade Prins‐Nazarov Cyclization Promoted by Dual Roles of BX₃

Abstract

Halofunctionalization of various unactivated arylalkynes to the corresponding 1H‐indenes in the presence of a particular class of carboxaldehydes and boron trihalides (BX3, X=F, Cl, Br, I) is described. A diverse array of halofunctionalized indenes substituted with a heterocycle has been synthesized regioselectively with BX3 as a promotor for the carbocyclization and a source of X− for halogenation. This reaction proceeds via a formal halogenative [4+1] cycloaddition between arylalkynes and carboxaldehydes promoted by boron trihalides to generate halofunctionalized indenes. The usefulness of the halofunctionalized indenes was demonstrated by their conversion to other derivatives via coupling, nucleophilic substitution, and oxidation.

BF3-Catalyzed Intramolecular Fluorocarbamoylation of Alkynes: Synthesis of 3-Fluoromethylene Oxindoles and γ-Lactams

“…While the identity of the initial BF3X -species remains unclear, any Lewis adduct of BF3 (e.g., 1a•BF3) can serve as a fluoride source. For example, Lewis adducts of aldehydes, 31 imines, 32 and hypervalent iodine reagents 33 with BF3 have been previously reported to liberate fluoride. It is also possible that trace amounts of HBF4 present in commercial BF3•OEt2 can initiate this process.…”

A Tandem Prins/Friedel‐Crafts Cyclization Strategy for the Stereoselective Synthesis of Hexahydro‐2H‐benzo[g]chromenes

“…The Prins cyclization has emerged as a versatile method for the construction of the tetrahydropyran ring system in fused oxygen heterocycles [7] . By using different nucleophiles to capture the carbocation intermediates, a variety of tandem Prins procedures [8] could be rationalized, for example, by the following Friedel‐Crafts [9] or Nazarov cyclization [10] …”

“…[7] By using different nucleophiles to capture the carbocation intermediates, a variety of tandem Prins procedures [8] could be rationalized, for example, by the following Friedel-Crafts [9] or Nazarov cyclization. [10] In this regard, Nagumo and coworkers have successfully synthesized hydrocyclopentafurans from disubstituted homocinnamyl alcohols and arylaldehydes (Scheme 1A). [11] This reaction pattern was well established, because Nagumo group tactfully involved aryl disubstituted alkenes as starting materials, and thus the carbenium ion intermediates generated by the first Prins step would be stabilized by the adjacent benzene ring.…”

Polycyclizations of Ketoesters: Synthesis of Complex Tricycles with up to Five Stereogenic Centers from Available Starting Materials

“…Finally, 6 is formed through a BBr 3 assisted substitution. 63 Other domino halocyclizations using boron trihalides have been reported in for instance indene functionalizations 64 and polycyclic aromatic hydrocarbon synthesis. 65 The versatility of above prepared derivatives was demonstrated in the following sequence of transformation on dibromo derivatives 6.…”

“…Finally, 6 is formed through a BBr 3 assisted substitution . Other domino halocyclizations using boron trihalides have been reported in for instance indene functionalizations and polycyclic aromatic hydrocarbon synthesis …”