“…4). 21,22 In contrast, compounds 15a and 15b afforded the desired β‐lactams 16a 23 and 16b ,24 respectively, in good yields. These results suggested that the degree of stabilization with one phenyl group for the radical intermediate by 4‐ exo‐trig cyclization was insufficient, probably because of free rotation of the phenyl group, and that an additional stabilization group was needed to obtain the desired β‐lactams.…”
Section: ‐Exo Vs 5‐endo Radical Cyclizationsmentioning
confidence: 98%
“…Compound 18a could be converted to a potential intermediate for the synthesis of (±)‐PS‐5 25. Cyclizations of enamides having a chiral auxiliary were applied to asymmetric synthesis of (+)‐PS‐5,21,22,27,28 (+)‐thienamycin22,26,28 and 1‐β‐methylcarbapenem 23…”
Section: ‐Exo Vs 5‐endo Radical Cyclizationsmentioning
This review describes the results of our recent studies on the control of the regiochemistry of radical cyclizations. N-vinylic alpha-chloroacetamides generally cyclized in a 5-endo-trig manner to give five-membered lactams, whereas 4-exo-trig cyclization occurred when the cyclized radical intermediates were highly stabilized by an adjacent phenyl or phenylthio group to afford beta-lactams. The 5-exo or 6-exo cyclization of aryl radicals onto the alkenic bond of enamides could be shifted to the corresponding 6-endo or 7-endo mode of cyclization by a positional change of the carbonyl group of enamides. The 6-endo- and 7-endo-selective aryl radical cyclizations were applied to radical cascades for the synthesis of alkaloids such as phenanthroindolizidine, cephalotaxine skeleton, and lennoxamine. The 5-exo-trig cyclization of an alkyl radical onto the alkenyl bond of enamides could also be shifted to the 6-endo mode by a positional change of the carbonyl group of enamides. The 6-endo- selective cyclization was applied to the radical cascade to afford a cylindricine skeleton. Other examples of controlling the regiochemistry of radical cyclizations and their applications to the synthesis of natural products are also discussed.
“…4). 21,22 In contrast, compounds 15a and 15b afforded the desired β‐lactams 16a 23 and 16b ,24 respectively, in good yields. These results suggested that the degree of stabilization with one phenyl group for the radical intermediate by 4‐ exo‐trig cyclization was insufficient, probably because of free rotation of the phenyl group, and that an additional stabilization group was needed to obtain the desired β‐lactams.…”
Section: ‐Exo Vs 5‐endo Radical Cyclizationsmentioning
confidence: 98%
“…Compound 18a could be converted to a potential intermediate for the synthesis of (±)‐PS‐5 25. Cyclizations of enamides having a chiral auxiliary were applied to asymmetric synthesis of (+)‐PS‐5,21,22,27,28 (+)‐thienamycin22,26,28 and 1‐β‐methylcarbapenem 23…”
Section: ‐Exo Vs 5‐endo Radical Cyclizationsmentioning
This review describes the results of our recent studies on the control of the regiochemistry of radical cyclizations. N-vinylic alpha-chloroacetamides generally cyclized in a 5-endo-trig manner to give five-membered lactams, whereas 4-exo-trig cyclization occurred when the cyclized radical intermediates were highly stabilized by an adjacent phenyl or phenylthio group to afford beta-lactams. The 5-exo or 6-exo cyclization of aryl radicals onto the alkenic bond of enamides could be shifted to the corresponding 6-endo or 7-endo mode of cyclization by a positional change of the carbonyl group of enamides. The 6-endo- and 7-endo-selective aryl radical cyclizations were applied to radical cascades for the synthesis of alkaloids such as phenanthroindolizidine, cephalotaxine skeleton, and lennoxamine. The 5-exo-trig cyclization of an alkyl radical onto the alkenyl bond of enamides could also be shifted to the 6-endo mode by a positional change of the carbonyl group of enamides. The 6-endo- selective cyclization was applied to the radical cascade to afford a cylindricine skeleton. Other examples of controlling the regiochemistry of radical cyclizations and their applications to the synthesis of natural products are also discussed.
“…When heated with tri- n -butyltin hydride and a catalytic amount of AIBN in toluene, 1a gave both β-lactam 2a and γ-lactam 3a (ca. 1:1, 31%), with the reduction product formed predominantly (41%) . In contrast, the more highly activated system 1b was transformed into β-lactam 2b (diastereomeric ratio of 2.7:1, 56%) more efficiently and with only a trace of reduction product and no detectable 3b . , …”
mentioning
confidence: 97%
“…1:1, 31%), with the reduction product formed predominantly (41%). 2 In contrast, the more highly activated system 1b was transformed into β-lactam 2b (diastereomeric ratio of 2.7:1, 56%) more efficiently and with only a trace of reduction product and no detectable 3b. 2,3 The extent to which enamides are prone to undergo uncommon 5-endo radical ring closure has been noted.…”
mentioning
confidence: 98%
“…2 In contrast, the more highly activated system 1b was transformed into β-lactam 2b (diastereomeric ratio of 2.7:1, 56%) more efficiently and with only a trace of reduction product and no detectable 3b. 2,3 The extent to which enamides are prone to undergo uncommon 5-endo radical ring closure has been noted. 1a,4 Although the bicyclic lactams 5 and 6 formed from 4a and 4b are produced with variable stereoselectivities, their tricyclic congeners 7 cyclize to give only 8.…”
A series of homologous R-sulfonamidyl radicals has been generated by reaction of R-halomethyl precursors with tri-n-butyltin hydride under AIBN catalysis. The intramolecular cyclization capability of these highly reactive intermediates has been evaluated. Where possible, five-membered sultams are formed by 5-exo transition states. The longer C-SO 2 and SO 2 -NR 2 bonds have little demonstrable effect on this pathway. In larger systems, however, the 7-endo option predominates over the 6-exo alternative. A preparatively useful route to sultams has emerged from this investigation.
Moderate stereocontrol in 5‐exo radical cyclizations of N,N‐disubstituted (−)‐8‐amino menthol derivatives, promoted by tributyltin hydride and AIBN, was achieved. The stereoselection was explained in terms of hydrogen bond formation in the 1,3‐amino alcohol derivative. The presence of one additional stereocenter at the allylic chain enhanced the stereoselection giving a single cyclization stereoisomer. The cyclization products were easily converted into enantiopure 3‐alkyl‐ or 2,3‐dialkyl‐substituted pyrrolidines.
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