α‐Carbonyl Radicals for Total Synthesis of Natural Products

Abstract

Intramolecular cyclization of a-carbonyl radicals to the trimethylsilylacetylenic side chain gives bicydie vinylsilane ketones and enones. Application of this cyclization to total synthesis of (±)-modhephene is described. 637 Intramolecular radical cyclization proves highly efficient for the carbon-carbon bond formation. Several excellent reviews have appeared.' We reported two methods using a-carbonyl radical cyclization to synthesize bicyclic vinylsilane ketones" and enones" (eq. I and eq. 2).(1) According to our method, a-carbonyl radicals were generated withtributyltin hydride from the corresponding a-iodo ketones" and n-iodo enones." The tethered acetylenic side chain in6 was introduced by 1,4-addition to cycloalkenone 5. Thus, conjugate addition of organocuprate reagent to cycloalkenone 5 followed by trapping with trimethylsilylchloride gave trimethylsilyl enol ether 6. Iodination of 6 with sodium iodide and m-chloroperbenzoic acid afforded o-iodo ketone 7. Treatment of 7 with 1 3 X=CH2, 0, NR dS R H 2 or (2) 4 tributyltin hydride and AIBN on a slow addition gave bicyclic vinylsilane ketone 8 smoothly. Several examples are shown in Table 1.Cycloalkenones 29 with a~-acetylenic side chain were prepared from 3-ethoxycycJoalkenones 27, eq. 3. 2bVinylogous esters and amides were synthesized by condensation of trimethylsilylpropargyl alcohol or amine 31 with I ,3-cycloalkanones, eq. 4. 2b Iodination 01'29 and 32 according to Mclntosh's" or Johnson's methods gave 33, eq. 5. Treatment of 33a with tributyltin hydride and AIBN on rapid addition afforded cyclized product 34a, eq. 5. Protection of the NH group of 33b with the t-Boc group gave 33c. Radical cyclization of 33c with tributyltin hydride afforded 34c. Six examples appear in Table 2. 0 (11 a,Mg(cHv.-=--TMS 0 00& 28~T MS (3) (2) H,O' 27 29 n~2. 3 TMS 0 HX-CH:!~ThfS 6x J 60 31 (4)