Formation of some bicyclic systems by radical ring-closure

Athelstan L. J. Beckwith and the Flowering of Hex-5-enyl Radical Cyclization Chemistry. The Adelaide Years

“…Substituents at C-1 (R 1,2 ) and C-6 (R 4,5 ) were consistently found to cause small rate enhancements (1.4-2.4) in 1,5-cyclizations except where there was geminal disubstitution as part of a ring, which caused a very small rate retardation (0.94). [25][26][27][28] In all cases, the amount of competing 1,6-cyclization either remained unchanged or was markedly decreased. In contrast, substituents at C-5 (R 3 ) led to strong rate retardations (e.g.…”

“…The major products are in fact the cis-fused hydrindanes 22 and 23, both of which can arise from either axial or equatorial orientations of the butenyl side chain, in which the angle of attack of the radical centre on the p* orbital and their separation is near optimal, while the trans-fused hydrindane can only be formed from an equatorial orientation which gives a very suboptimal approach. [28] Peroxy Analogues…”

“…Cis-selectivity was in fact seen, and the major radical 46c underwent 1,5-cyclization very readily to give the isomeric pentalenes 47 and 48 (Scheme 15). [28] Geometric constraints imposed by the ring promote the cyclization of 46c and at the same time reduce the distinction between the transition states leading to the two pentalenes, resulting in low stereoselectivity in the second cyclization step. The transradical 46t also cyclized, but at a very much slower rate because of the forced separation of the reacting centres, and mainly in the endo-mode to give the hydrindanyl radical 50.…”

Stereoselectivity in Amidyl Radical Cyclisations: Alkyl Mode Cyclisations

“…In addition both semi-empirical (AM1) and higher level ab initio calculations (DFT 22 ) indicated that the transition state for cyclisation of 6a to 7a was lower in energy than that to 8a (DE= 2.1 (AM1) and 7.0 kJ mol -1 (DFT) respectively). Interestingly the diastereoselectivity of the reactions are significantly better than for related amidyl radical cyclisations proceeding in the acyl mode (de = 10-23%) 10 and for the cyclisation of the 2-methyl-5-hexenyl carbon radical 12 (de=30%) [5][6][7] and the corresponding N-methyl-5-pentenaminium radical 13 (de=34%) 23 . The steric nature of the N-substituent of the amidyl radicals seems to have little controlling effect on the stereoselectivity of the process with both 6a and 6e undergoing cyclisation with similar selectivities (de 54% and 58% respectively).…”

Tin‐Free Radical Carbonylation: Thiol Ester Synthesis Using Alkyl Allyl Sulfone Precursors, Phenyl Benzenethiosulfonate, and CO

“…[16] When a four-component coupling reaction using 12, allyl trimethylsilane, CO, and 2 d was carried out under the same conditions, a 4.7:1 diastereomeric mixture of the desired product 14 was isolated in 83 % yield (Scheme 2). [17] Evidently, the electrophilic alkyl radical generated from 12 failed to undergo carbonylation and reacted with allyl trimethylsilane to yield intermediate 13.…”