Comparative Cyclotrimerisation of Enantiopure vic‐Bromo(trimethylstannyl)bicycloalkenes Derived from (+)‐Camphor, (+)‐Fenchocamphorone and (−)‐Epicamphor: Effect of the Bridgehead Methyl Group on the syn/anti Product Ratios

Abstract

The influence of the bridgehead methyl group on syn/anti diastereoselectivity in the cyclotrimerisation of polycyclic alkenes was investigated. To this end, three different enantiopure vic-bromo(trimethylstannyl)-substituted bicyclic olefins were prepared and used as probes. The effects of different copper(II) or -(I) salts were studied, and most dimeric intermediates were independently synthesised and characterised.

Synthesis, chiroptical properties, and their theoretical simulation of some highly rotating benzotricamphor derivatives

“…The cyclotrimerization of 7 was accomplished with copper(I) 2‐thiophenecarboxylate (CuTC),72 to produce a 6.8:1 syn to anti ratio mixture of cyclotrimers 2 in 70% overall yields (Scheme ). The favorable diastereoselectivity results from the presence of sterically hindered trimethyltin moiety and scarcely hindered ligating functional group in the enantiopure bicyclic olefin, as previously observed with related substrates 73, 74…”

Highly diastereoselective synthesis of a novel functionalized benzocyclotrimer

“…The mixture of diastereomers was dehydrobrominated with potassium tert-butoxide in THF, 26 to afford the alkenyldibromide 8 in good yield (93%) (Scheme 1). The key reagent 9 for the cyclotrimerization was obtained either by proton abstraction of halide 6 with lithium diisopropylamide (LDA) 11,13,[27][28][29][30][31][32][33] or by removal of one of the bromine atoms of the dibromide 8 with nbutyl lithium 30,31,34,35 followed by a trans-metalation reaction with trimethyltin chloride in 96% yield in both cases. The high chemoselectivity of removal of the bromine atom in 8 was surprisingly: indeed, the bromine atoms attached to the aromatic ring remained completely unaffected.…”

A Novel C3‐Symmetric Triol as Chiral Receptor for Ammonium Ions

“…[2h–2k,3i–3l,5f] This reagent afforded good yields of trimers [ 16 (82 %), 17 (86 %), 18 (93 %)], with different syn / anti ratios, according to the protecting groups on the alcoholic moieties (Scheme ). The highest stereoselectivity was obtained with methyl derivative 12 , which gave essentially pure syn ‐ 16 , while MOM‐ and MEM‐protected bornenols 13 and 14 afforded 85:15 and 83:17 syn / anti mixtures, respectively 2k…”

“…The transformation of the hydrazone moiety into bromo olefin 5 was attempted in a first run with the use of pyridinium perbromate as the brominating reagent and pyridine as a proton scavenger, producing a mixture of products, identified by GC‐MS and 1 H NMR as the desired olefin 5 and dibromide 6 , in a 2:1 ratio (Scheme ) 2k. The mixture treated with potassium tert ‐butoxide in DMSO afforded pure 5 in 58 % yield after flash chromatography (Scheme ) 1b.…”