“…24,30 Despite these challenges, there are scattered reports of forming tetrahydropyran, chroman, and chromanone cores through 1,6-C-H insertion using donor/acceptor or acceptor carbenes. 18,20,[22][23][24][31][32][33] In 2012, Cossy and coworkers demonstrated the use of donor carbenes generated from cyclopropenes in the synthesis of tetrahydropyrans by C-H insertion, 22,31 further expanding the diazo-free insertion work demonstrated by Zhu. 34,35 In the cases where chiral catalysts were used, enantioselectivity was oen moderate; 19,20 it wasn't until 2015 when Hashimoto achieved higher levels of enantioselectivity using Rh 2 (S-PTTL) 4 (up to 97 : 3 er).…”