A mechanistic study on the SHi reaction at tin atoms in a radical cascade reaction

A kinetic study on the intramolecular direct radical substitution reaction on tin atoms was undertaken.

Preparation of optically active bicyclodihydrosiloles by a radical cascade reaction

Bicyclodihydrosiloles were readily prepared from optically active enyne compounds by a radical cascade reaction triggered by tris(trimethylsilyl)silane ((Me₃Si)₃SiH). The reaction was initiated by the addition of a silyl radical to an α,β-unsaturated ester, forming an α-carbonyl radical that underwent radical cyclization to a terminal alkyne unit. The resulting vinyl radical attacked the silicon atom in an S_Hi manner to give dihydrosilole. The reaction preferentially formed trans isomers of bicyclosiloles with an approximately 7:3 to 9:1 selectivity.

Radical-Mediated 5-Exo-TrigCyclizations of 3-Silylhepta-1,6-dienes

Regioselectivity of the sulfonyl radical mediated 5-exo-trig cyclization of 3-silylheptadienyl systems 3a-d has been studied. At low temperature, the reaction of the sulfonyl radical occurs regioselectively at the allylsilane terminus, while a reversal of regioselectivity is observed at 80 degrees C. This general trend has been rationalized on the basis of polar effects and radical stabilization. Thiyl-mediated radical cyclization of dienes 3a, 3c-d, 7 with subsequent sulfur atom transfer was also studied, providing thiabicyclo[3.3.0] skeleton in one step with excellent stereocontrol.

Free-Radical-5-exo-Trig Cyclization of Chiral 3-Silylhepta-1,6-dienes: Concise Approach to the A−B−C Ring Core of Hexacyclinic Acid

[Chemical reaction: See text] Free-radical-mediated 5-exo-trig cyclization of hepta-1,6-dienes 6a-c incorporating an allylsilane moiety was shown to provide at low temperature exclusively the trans-cis cyclopentanes 7a-c in good yield. In contrast, the same reaction with the alkyl analogue 9a led to the formation of all four possible stereoisomers. Interestingly, extension of this sequence of radical processes to alkoxy analogues 12-14 provided the complementary cis-cis stereoisomers with modest to excellent stereoselectivity. It is noteworthy that under such conditions allylsilanes were found to be much more reactive than their alkyl and alkoxy analogues. Beckwith-Houk-type models were proposed that rationalize the stereochemical course of these 5-exo-trig cyclizations. Finally, an illustration of the value of this methodology was proposed with the synthesis of the A-B-C tricyclic unit of polyketide hexacyclinic acid 3a.