Expansion to Seven-Membered Rings

Synthesis of L-arabinose-based crown ethers and their applications as enantioselective phase transfer catalysts

“…Oximes 1-10 and 26 we prepared as described in article. 2-Iodobenzyl bromide (11a) (AlfaAesar), Pd 2 (dba) 3 (Acros), Xantphos (Acros), Cs 2 CO 3 (Acros), and dioxane (extra dry over molecular sieves, Acros) were used without purification. Diffraction data were collected at -50 °C on a Bruker-Nonius KappaCCD diffractometer using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å).…”

“…1 Several recent chemical review articles were dedicated to the synthesis and transformation of seven membered rings. [2][3][4][5][6] Furthermore, two chapters on the chemistry of seven-membered rings with three heteroatoms at the positions 1, 2 and 4 have appeared. 6,7 3,5,7-Trisubstituted 4,7-dihydro-1,2,4-oxadiazepines have been prepared via a two step method starting from enones using NH 2 CONHOH/MeONa then Me 2 SO 4 /KOH, 8 and the intramolecular condensation of methyl 2-chloro-2-(phenylcarbamoylimidoylaminooxycyclopropyl)acetate in the presence of NaH in acetonitrile leading to 9-chloro-6-phenyl-5,7-diaza-4-oxaspiro [2,6]non-5-en-8-one has also been reported.…”

“…The experimental data suggests that the double cyclization forming two large membered rings can be achieved in the case of 2-bromophenyl amidoxime (for example, compound 10). Our first experiments showed that cyclization of oxime 10 in the system 2-iodobenzyl bromide (11a)/solid Cs 2 CO 3 /Pd 2 (dba) 3 /Xantphos/dry dioxane afforded oxadiazepine 24 as a main product in 32% yield. Bicyclic product 25 was identified by LC-MS and 1 H NMR only in trace amounts because of the side reactions in the synthesis of nine-membered ring (for example oligomerization of intermediate 24 in the presence of palladium catalyst) (Table 1, entry 13).…”

Rearrangement of 4ß,5ß-methylenepregnanes: A simple approach to A-homopregnanes and 5ß-methylpregnanes

“…In particular, the interest in developing expansion reactions for the construction of seven-membered rings, derives from the common occurrence of these frameworks in biologically active natural products and the preparation of synthetic homologues of active compounds. 1 Ionic and radical expansion reactions, usually follow different paths giving different distributions of products, which may be envisaged and promoted by a judicious choice of conditions. This fact has been used, for example, to easily differentiate between radical and ionic enzymatic mechanisms.…”

Stereoselective synthesis of hydroxylated 3-aminoazepanes using a multi-bond forming, three-step tandem process

“…25 In a similar fashion, use of (R)-COP-Cl gave the (S)-enantiomer in 84% yield and in 92% ee. Having demonstrated the asymmetric synthesis of 4, these products now have the potential to be used for the 85 enantioselective preparation of highly functionalised aminoazepanes. …”